Quarter-wave whip earth plane

[Liz Tuddenham]

I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.

There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.

The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.

Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

[Joerg]

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.

I don't see any "non-invasive" solution, assuming you don't even want a
pointy contact screw into the metal because maybe the van is fairly new.
That would also rule out sanding down the thickness of the paint :-)

Another option might be to just live with the asymmetry and use a common
mode choke at the antenna to muffle the imbalance. That would have to be
secured against wind, of course, but at least you don't have German
autobahn speeds in the UK.

No chance to bolt to something conductive up there? I guess the roof
rack channels on most modern verhicles aren't conductive anymore.

One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.

[Liz Tuddenham]

Post by Joerg

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.

I've made it from an assortment of light alloy tubes and rods (the rods
were machined down between centres on a large lathe so they can slide
into the tubes). There's quite a lot of adjustment but I wonder how
much that will affect the terminating impedance of the co-ax (it can be
tuned to be resistive but it may not not necessarily be 50 ohms).

Post by Joerg
I don't see any "non-invasive" solution, assuming you don't even want a
pointy contact screw into the metal because maybe the van is fairly new.
That would also rule out sanding down the thickness of the paint :-)

It's not that new but it is 'tidy' and I would like to keep it that way.
It is also used as a 'stealth' camper, so the mounting has to be easily
removeable so as not to attract attention when parking up for the night.

Post by Joerg
Another option might be to just live with the asymmetry and use a common
mode choke at the antenna to muffle the imbalance. That would have to be
secured against wind, of course, but at least you don't have German
autobahn speeds in the UK.

My intention is to use it portable but not mobile. The transciever I am
designing doesn't lend itself to mobile operation.

Post by Joerg
No chance to bolt to something conductive up there? I guess the roof
rack channels on most modern verhicles aren't conductive anymore.

There are quite substantial roof bars made from galvanised steel tube
but they have pieces of rubber under the mounting clips, so they don't
puncture the paint and make metallic contact with the metalwork of the
van. You can just see the rearmost bar in the picture of the exterior
cowl at:

http://www.poppyrecords.co.uk/Van/vanconversion.htm#COOKER

I had wondered about a large sheet of very thin flexible alloy, which
could be retained by a further 4 magnets, to give much greater
capacitance - but there are two problems with that idea:

1) Stowing the sheet in the van would present problems.

2) For good 2-metre contacts the van needs to be parked on a hilltop
and hilltops tend to be windy. Manipulating the sheet would be
difficult and it may not stay on the roof for very long.

Post by Joerg
One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.

That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.

[Ralph Mowery]

Post by Liz Tuddenham

Post by Joerg
9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.

I've made it from an assortment of light alloy tubes and rods (the rods
were machined down between centres on a large lathe so they can slide
into the tubes). There's quite a lot of adjustment but I wonder how
much that will affect the terminating impedance of the co-ax (it can be
tuned to be resistive but it may not not necessarily be 50 ohms).

Post by Joerg
I don't see any "non-invasive" solution, assuming you don't even want a
pointy contact screw into the metal because maybe the van is fairly new.
That would also rule out sanding down the thickness of the paint :-)

It's not that new but it is 'tidy' and I would like to keep it that way.
It is also used as a 'stealth' camper, so the mounting has to be easily
removeable so as not to attract attention when parking up for the night.

You are probably overthinking it. Just use a 1/4 wave whip and adjust
for lowest SWR. should be close to 19 inches for 2 meters. The
impedance is going to be about 36 ohms if a perfect ground plane. That
should be low enough not to burn out he finals of the transmitter and
the short coax with that low SWR wo't effect the range.

[Joerg]

Post by Liz Tuddenham

Post by Joerg

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.

I've made it from an assortment of light alloy tubes and rods (the rods
were machined down between centres on a large lathe so they can slide
into the tubes). There's quite a lot of adjustment but I wonder how
much that will affect the terminating impedance of the co-ax (it can be
tuned to be resistive but it may not not necessarily be 50 ohms).

I wouldn't worry too much about that. For example, I have an 80m/30m
double dipole at the far end of the house. Easily 150ft of coax. Being
the cheapskate I used leftover TV coax which in the US is 75ohms instead
of the 50ohm I would really need. The SWR isn't stellar but still under
1.5 at resonance. Works fine.

Post by Liz Tuddenham

Post by Joerg
I don't see any "non-invasive" solution, assuming you don't even want a
pointy contact screw into the metal because maybe the van is fairly new.
That would also rule out sanding down the thickness of the paint :-)

It's not that new but it is 'tidy' and I would like to keep it that way.
It is also used as a 'stealth' camper, so the mounting has to be easily
removeable so as not to attract attention when parking up for the night.

Just affix an MI5 sticker on the rear bumper and nobody would dare come
near your van :-)

There are also quick-release 90-degree "dropper mounts" that people use
to drive tall vehicles into their garages without shearing off the
antenna. Then the antenna would lay flat on the van's roof while down.

Post by Liz Tuddenham

Post by Joerg
Another option might be to just live with the asymmetry and use a common
mode choke at the antenna to muffle the imbalance. That would have to be
secured against wind, of course, but at least you don't have German
autobahn speeds in the UK.

My intention is to use it portable but not mobile. The transciever I am
designing doesn't lend itself to mobile operation.

Wow, making your own 2m/4m rig? Hats off!

Post by Liz Tuddenham

Post by Joerg
No chance to bolt to something conductive up there? I guess the roof
rack channels on most modern verhicles aren't conductive anymore.

There are quite substantial roof bars made from galvanised steel tube
but they have pieces of rubber under the mounting clips, so they don't
puncture the paint and make metallic contact with the metalwork of the
van. You can just see the rearmost bar in the picture of the exterior
http://www.poppyrecords.co.uk/Van/vanconversion.htm#COOKER
I had wondered about a large sheet of very thin flexible alloy, which
could be retained by a further 4 magnets, to give much greater
1) Stowing the sheet in the van would present problems.
2) For good 2-metre contacts the van needs to be parked on a hilltop
and hilltops tend to be windy. Manipulating the sheet would be
difficult and it may not stay on the roof for very long.

Yeah, that would flop about wildly when it's windy. But looking at the
rear doors I notice very sturdy hinges where the sections on the side of
the van will have a solid chassis contact. That is where hams sometimes
mount antennas over here, especially big HF antennas. Not only would you
solve the grounding issue but you'd also have a solid mounting point
without drilling into the vehicle.

For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.

Post by Liz Tuddenham

Post by Joerg
One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.

That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.

Or maybe ask a friend who has a bigger lathe.

[Liz Tuddenham]

Joerg <***@analogconsultants.com> wrote:

[...]

Post by Joerg
looking at the
rear doors I notice very sturdy hinges where the sections on the side of
the van will have a solid chassis contact. That is where hams sometimes
mount antennas over here, especially big HF antennas. Not only would you
solve the grounding issue but you'd also have a solid mounting point
without drilling into the vehicle.

That's a brilliant idea which had never crossed my mind. I had already
been thinking along the lines of tubes extending telescopically sideways
from inside the roof bars as supports for vertical poles for public
address loudspeakers but I couldn't think of an easy way to hold the
bottom of the pole to stop it tilting. Universal mounts on the door
hinges would be much better.

Post by Joerg
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.

I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses. (The engine and transmission is reputed to outlast three
sets of bodywork.) Perhaps a hole could be drilled through the hinge
strap to take a bolt - that could be removed later and the hole might
not be noticed. I can do my own welding, but that is much harder to
disguise.

Post by Joerg

Post by Liz Tuddenham

Post by Joerg
One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.

That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.

Or maybe ask a friend who has a bigger lathe.

I have a contact in a heavy engineering factory (I sorted out one of
their CNC machines a few years ago) but another approach might be to
drill a lot of smaller holes and buy a lot of smaller magnets. A sheet
of polythene damp-proof course material or adhesive-backed neoprene
sheet across the whole underside would prevent scratching

[Joerg]

[...]

Post by Liz Tuddenham

Post by Joerg
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.

I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses. (The engine and transmission is reputed to outlast three
sets of bodywork.) Perhaps a hole could be drilled through the hinge
strap to take a bolt - that could be removed later and the hole might
not be noticed. I can do my own welding, but that is much harder to
disguise.

Even the best vehicles occasionally become smashed in an accident such
as a head-on collision or a T-bone. Then the whole chassis is toast.
Someone will take out the engine and transmission but one of the door
hinges could become yours.

Post by Liz Tuddenham

Post by Joerg

Post by Liz Tuddenham

Post by Joerg
One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.

That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.

Or maybe ask a friend who has a bigger lathe.

I have a contact in a heavy engineering factory (I sorted out one of
their CNC machines a few years ago) but another approach might be to
drill a lot of smaller holes and buy a lot of smaller magnets. A sheet
of polythene damp-proof course material or adhesive-backed neoprene
sheet across the whole underside would prevent scratching

That sounds like a lot more work than using one of the hinges.

[Jasen Betts]

Post by Liz Tuddenham

Post by Joerg
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.

I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses.

Possibly still worth a look. Rear door hinges are likely to survive a front
collision. even a rear collision. Genuine parts are also an option.

[Liz Tuddenham]

Post by Jasen Betts

Post by Liz Tuddenham

Post by Joerg
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.

I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses.

Possibly still worth a look. Rear door hinges are likely to survive a front
collision. even a rear collision. Genuine parts are also an option.

The fixing screws for those door hinges are torqued up beyond the
capabilities of any of the tools I have in my kit. The ease of mounting
and demounting a magnetic base still appeals to me - and now I have
taken delivery of a VNA, it will be a lot easier to see where I am going
wrong and what I need to do to put it right.

Using the existing quarter-wave whip on the cooker top as my 'infinite'
plane (the weather outside is too cold to experiment on the van roof)
has already shown up the folly of a series capacitor. I can now
separate tuning effects from feed-point impedance errors, whereas
previously I was floundering about in the dark. I need to do a crash
course in interpreting Smith Charts.

[Jeroen Belleman]

On 3/15/25 10:23, Liz Tuddenham wrote:
[Snip!]

Post by Liz Tuddenham
I need to do a crash
course in interpreting Smith Charts.

I suppose it's enough to know that you want the curve to be
as close as possible to the centre of the chart at the working
frequency. That's where the load impedance matches the
characteristic impedance.

A Smith chart is a chart of the voltage across a Wheatstone
bridge with a funny grid mapping that back to the value of one
of the bridge impedances. It was handy as a graphical aid to
calculate impedance transforming networks and more, but
nowadays computers can do that much better and much faster.

Jeroen Belleman

[Tauno Voipio]

On 15.3.2025 11.23, Liz Tuddenham wrote:
I need to do a crash

Post by Liz Tuddenham
course in interpreting Smith Charts.

The NanoVNA will show the reflection attenuation or VSWR
if you're intimidated by the Smith chart.

The Smith chart shows the both the amplitude and the phase
of the measured reflection. The coordinates are the resistive
and reactive components of the reflection scaled to the system
reference impedance (here: 50 ohms). The coordinate lines are
circles for easy handling of the reflection. Traces of constant
SWR are circles centered on the 50 ohm center point.

The VNA has very small coaxial connectors (SMA), so you'll
need adapters to more conventional feed line connectors.
I prefer adapters with a piece of cable between the connectors,
to protect the small connectors from mechanical stresses.

[Liz Tuddenham]

Post by Liz Tuddenham
I need to do a crash

Post by Liz Tuddenham
course in interpreting Smith Charts.

The NanoVNA will show the reflection attenuation or VSWR
if you're intimidated by the Smith chart.

Not so much 'intimidated' as 'outdated'. I passed an exam in 1969 which
included them - and haven't used one since.

Post by Liz Tuddenham
The Smith chart shows the both the amplitude and the phase
of the measured reflection. The coordinates are the resistive
and reactive components of the reflection scaled to the system
reference impedance (here: 50 ohms). The coordinate lines are
circles for easy handling of the reflection. Traces of constant
SWR are circles centered on the 50 ohm center point.

It gives me a huge amount of information which I am sure is exactly what
I need if only I can get my brain around how it realtes to what I am
doing with the hardware. I need to watch the trace on the screen and
see what makes it rotate, expand, contract and turn back on itself so
that I have a feel for how I can make it do what I want.

It is a bit like the description of learning to fly a helicopter: Waggle
the stick about and see what happens - if you want that to happen, that
is how you must waggle the stick.

Post by Liz Tuddenham
The VNA has very small coaxial connectors (SMA), so you'll
need adapters to more conventional feed line connectors.
I prefer adapters with a piece of cable between the connectors,
to protect the small connectors from mechanical stresses.

It came with a set of short extension leads and I have connected a set
of BNC adaptors to the far ends of those leads so as to remove any
strain they would have imposed on the connectors on the side of the
instrument. I have already standardised my R.F. interconnections with
adaptors on each piece of equipment to convert them to 50-ohm BNC, so my
learning curve shouldn't be complicated by intermittent connections from
botched adaptors.

This evening I've also made up a couple of BNC to croc-clip adaptors for
testing circuits on the bench. For tuned circuits I have an Edometer
that ges up to 100 Mc/s, but the VNA will cover frequencies much higher
than that.

[Jasen Betts]

Post by Liz Tuddenham

Post by Jasen Betts

Post by Liz Tuddenham

Post by Joerg
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.

I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses.

Possibly still worth a look. Rear door hinges are likely to survive a front
collision. even a rear collision. Genuine parts are also an option.

The fixing screws for those door hinges are torqued up beyond the
capabilities of any of the tools I have in my kit.

For undoing car door hingers you want a manual impact driver, starting
price about 5 pounds, but you can pay much more, or even use a powered
impact driver instead.

Post by Liz Tuddenham
The ease of mounting
and demounting a magnetic base still appeals to me - and now I have
taken delivery of a VNA, it will be a lot easier to see where I am going
wrong and what I need to do to put it right.
Using the existing quarter-wave whip on the cooker top as my 'infinite'
plane (the weather outside is too cold to experiment on the van roof)
has already shown up the folly of a series capacitor. I can now
separate tuning effects from feed-point impedance errors, whereas
previously I was floundering about in the dark. I need to do a crash
course in interpreting Smith Charts.

Basically it's a bull's eye target and you try to hit the middle.
Close is often good enough.

There's also this thing you can play with:

https://www.will-kelsey.com/smith_chart/

Put in your measured paramters at the top and then experiment with
the filter building blocks further down the page and view the
transformed result at the bottom.

[legg]

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )

You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.

Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)

Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity.
Capacitance increased by a factor of 25 (using numbers above).

RL

[Joerg]

Post by legg

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.

Just be careful if you meant copper tape. I've had that slightly "bleed
through" over several years. The paint it was affixed to had a minor but
seemingly permanent discoloration. Could be a bummer if Liz ever wants
to sell the van.

Post by legg
Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity.
Capacitance increased by a factor of 25 (using numbers above).

"L" is actually an idea. What if Liz made a series inductor that
resonates with the 120pF and form a series resonant circuit? Of course
it would have to be a diffent inductor if she changes bands and a switch
up there in the typical UK weather won't be so great.

[Liz Tuddenham]

Joerg <***@analogconsultants.com> wrote:

[...]

Post by Joerg
"L" is actually an idea. What if Liz made a series inductor that
resonates with the 120pF and form a series resonant circuit? Of course
it would have to be a diffent inductor if she changes bands and a switch

That was a possibility I had considered but the thought of having to
tune it put me off.

Post by Joerg
up there in the typical UK weather won't be so great.

I tend to be a fine-weather camper but the weather can change quite
suddenly and the radio would be more likely to be used during an
enforced stay indoor because of a long wet spell.

[Joerg]

Post by Liz Tuddenham
[...]

Post by Joerg
"L" is actually an idea. What if Liz made a series inductor that
resonates with the 120pF and form a series resonant circuit? Of course
it would have to be a diffent inductor if she changes bands and a switch

That was a possibility I had considered but the thought of having to
tune it put me off.

That is where the NanoVNA comes in super handy :-)

Post by Liz Tuddenham

Post by Joerg
up there in the typical UK weather won't be so great.

I tend to be a fine-weather camper but the weather can change quite
suddenly and the radio would be more likely to be used during an
enforced stay indoor because of a long wet spell.

When I lived near Aberdeen for a little, you could walk home from the
pub in balmy weather, go to bed and in the morning wake up to howling
winds and driving rain.

[Liz Tuddenham]

Post by legg

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.
Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity.
Capacitance increased by a factor of 25 (using numbers above).

Perhaps a sheet of aluminium cooking foil would be a quick and easy way
to test it.

[legg]

Post by Liz Tuddenham

Post by legg

Post by Liz Tuddenham
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?

Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.
Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity.
Capacitance increased by a factor of 25 (using numbers above).

Perhaps a sheet of aluminium cooking foil would be a quick and easy way
to test it.

Check for materials available from art supply stores.

Aluminium will 'write' on surfaces pdq. It's oxide is also brittle
and abrasive.

RL

[Dave Platt]

Post by Liz Tuddenham
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.

I'm not at all sure that this "balancing" capacitor is going to help
matters more than it hurts. It will probably increase the series
(capacitive) reactance of the antenna system as a whole, raising
the SWR and making life tougher on the radio.

Post by Liz Tuddenham
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything.

I'd recommend leaving out the "balancing" capacitor, and simply
lengthening the rod element a bit. This would add some inductance
to the equation, and should help compensate somewhat for the
capacitance which appears in series with the "ground plane" leg
of the system.

Post by Liz Tuddenham
if the latter, is there a solution
that doesn't involve butchering the van roof?

Starting comments:

It could help you a lot if you can get access to a NanoVNA
(they're cheap) and "sweep" the actual impedance curve in
the frequency ranges you're interested in. This would give
you an accurate picture of what you're actually dealing with.

I'd be concerned that the magnets may not (depending on their
composition and thickness) be particularly conductive to RF...
there might be a lot of resistive loss there,

Possible fixes:

Bigger/thinner magnets? Get the real antenna base closer to the
roof metal that way.

Minimizing the gap in the RF path between coax and roof: Take the poly
disks off of the magnets temporarily. Get a sheet of thin copper
foil, and mount a layer of it on the bottom of each magnet, with a tab
out to the side. Solder some copper braid to this tab, and bring the
braid up to where your coax shield terminates, and solder there.
Then, put the poly disks back on the magnets (or use some
single-adhesive-side Teflon tape... thinner is better!). The closer
you can get the "real" RF ground path to the roof, the better.

You're likely to be dealing with a couple of confounding factors
in trying to get minimum SWR on this antenna. In theory a
quarter-wave radiator over an infinite ground plan will give
you a feedpoint Z of around 35 ohms (not a great match to
50-ohm coax, but most ham radios these days can handle that
sort of SWR without complaining). The roof metal, steel
plate, and magnets are probably not great RF conductors and
will add some resistive loss, which will push the feedpoint
Z upwards somewhat. So, if you compensate for the capacitive
parasitic by lengthening the quarter-wave radiator a bit, you
might end up fairly close to 50+0j to make your radio
quite happy with the situation.

Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

A lot of hams around here operate VHF vehicle-mobile with a simple
wire whip with a magnetic-mount base only an inch or two across.
These don't provide a terribly-good "50 ohm" load to the radio... but
it's often a lot closer to 50 ohms than the miserable "rubber duck"
antennas with which the hand-held radios are being sold!

73,

Dave AE6EO

[Joerg]

On 3/10/25 7:11 PM, Dave Platt wrote:

[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1

Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)

Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

[Phil Hobbs]

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.

The real ones are sold on Tindie.

Cheers

Phil Hobbs

[Joerg]

Post by Phil Hobbs

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.

I got mine at one of the big ham radio dealerships here in the US. Cost
me around $50 plus tax a couple years ago so it has to be the lowest
grade version but it's perfectly fine for tuning antennas and stuff.

[Phil Hobbs]

Post by Joerg

Post by Phil Hobbs

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one.  They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.

I got mine at one of the big ham radio dealerships here in the US. Cost
me around $50 plus tax a couple years ago so it has to be the lowest
grade version but it's perfectly fine for tuning antennas and stuff.

Probably so. The one I have ($180ish) has a nice low noise floor and
good stability. The main thing I miss compared with a boat anchor VNA
is the calibration doohickeys.

Cheers

Phil Hobbs

[Joerg]

Post by Joerg

Post by Phil Hobbs

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one.  They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.

I got mine at one of the big ham radio dealerships here in the US.
Cost me around $50 plus tax a couple years ago so it has to be the
lowest grade version but it's perfectly fine for tuning antennas and
stuff.

Probably so.  The one I have ($180ish) has a nice low noise floor and
good stability.  The main thing I miss compared with a boat anchor VNA
is the calibration doohickeys.

True. Still, my beloved HP3577A has become hardcore unemployed since I
have the NanoVNA.

These new devices are totally silent, small and run on a little battery
for an hour. And fit in a shirt pocket.

The boat anchors produce a robust "TUNGGGG" in their mains transformer
when turned on, the ceiling lights in the room briefly flicker and then
a jet engine style whoosh starts when the fans spool up. The occasional
sheet of note paper goes flying. Then you have to let them idle for
10mins or so to self-calibrate and reach temperature. And when you use
them a lot you'll see a spike on the electricity bill :-)

[john larkin]

On Tue, 11 Mar 2025 08:55:42 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.
Cheers
Phil Hobbs

Cool. We're designing some RF switch thingies and those RF/sinewave
jocks will expect their silly S-parameters.

Is this it?

https://nanorfe.com/nanovna-v2.html

We'll start with some DC-to-3GHz stuff, so this looks right. If we
move on to 40 GHz, we'll probably send it to a test lab.

[Phil Hobbs]

Post by john larkin
On Tue, 11 Mar 2025 08:55:42 -0000 (UTC), Phil Hobbs

Post by Phil Hobbs

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...

I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.
Cheers
Phil Hobbs

Cool. We're designing some RF switch thingies and those RF/sinewave
jocks will expect their silly S-parameters.
Is this it?
https://nanorfe.com/nanovna-v2.html
We'll start with some DC-to-3GHz stuff, so this looks right. If we
move on to 40 GHz, we'll probably send it to a test lab.

Yup, that’s them.

Cheers

Phil Hobbs

[Liz Tuddenham]

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did).

A member of a local radio club had one (or something similar) which he
used to check a sleeve dipole I had made, it was most impressive. I
definitely think it is something I should buy and it may offset some of
its purchase price by the savings in scrapped alloy tubing.

[Liz Tuddenham]

Post by Joerg
[...]

Post by Dave Platt
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.

+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did).

I have one on order.

[Liz Tuddenham]

Post by Dave Platt

Post by Liz Tuddenham
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.

I'm not at all sure that this "balancing" capacitor is going to help
matters more than it hurts. It will probably increase the series
(capacitive) reactance of the antenna system as a whole, raising
the SWR and making life tougher on the radio.

I've taken it out and cut the elements a bit shorter (they were too long
to start with), I can get VSWRs well below 1.5 on 144 Mc/s and 70 Mc/s
with two different elements. There is a lockable sliding joint so I can
trim the length with the NanoVNA I have just bought. The tuning of the
144 Mc/s quarter-wave whip is very broad and uncritical but the 70 Mc/s
whip is quite sharp (perhaps because of the capacitive effect in the
'earth' plane).

I experimented with ways of increasing the capacitance to the van roof
using aluminium cooking foil, but the difference it makes is not worth
the bother of cutting a more substantial capacitor plate from sheet
aluminium.

[...]

Post by Dave Platt
It could help you a lot if you can get access to a NanoVNA

It has made a huge difference, I can see all my mistakes instead of
blundering around in the dark.