Quarter-wave whip earth plane

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.

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

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).

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.

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

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:

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.

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.

"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.

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.

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'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.

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

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!

Dave AE6EO

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

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

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.

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

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.

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.

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

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.

Check for materials available from art supply stores.

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

RL

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.

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

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.

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

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.

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

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

Is this it?

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