Re: Quarter-wave whip earth plane

Joerg <news@analogconsultants.com> wrote:

On 3/10/25 8:33 AM, Liz Tuddenham wrote:

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 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.

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.

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.

 

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.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk