Robert L. Nuckolls, III wrote:
>
> Have un_subscribed for a few weeks . . . had to be out of
> town for ten days and now have a LOT of work to do to get
> ready for OSH.
Enjoy OSH !
> /These simple antennas we are using are quarter wave antennae
> /which if cut to that length (1/4 wave length) result in a 50 ohm
> /impedance.
>
> Not true. A 1/4 wave vertical over a ground plane generally
> runs in the neighborhood of 35 ohms plus some other complex
> impedances. . . .
>
Well now..................IF the ground plane is a flat sheet then you
are right but in practice isn't the ground plane usually the fuselage
which curves downwards and so presents virtually the drooped radials
which give a groundplane antenna a 50 ohm impedance ?
> /As far as the ground plane goes it is there to provide the quarter
> /wave antenna a reflecting surface so it can have an imaginary
> /second element going the other direction (like a di-pole).
> /The ground plane should be at least 1/4 wave length in radius from
> /the mounting point of the antenna (that is a 1/2 wave lengths diameter)./
>
> Generally true but not very useful information for airplanes.
> Some composite structures permit a series of 22" radials to be
> laid out under the comm antenna but they never lay flat and have
> to curve down the side of the fuselage.
Exactly - giving a drooped radial groundplane with a fifty ohm
impedance ! But would you put an antenna on the outside of a GRP
aeroplane ? Better to locate it inside the structure and save drag !
>
> /The bit about not being allowed to cut the co-ax is only true
> /if you have a mismatched impedance on the antenna. etc........
> This is a mix of partial truths some of which were significant
> in the early days of solid state transmitters.
Really ! My ham transmitters, solid state and valve all think that
impedance matching, SWR, feeder length and so on are all still true and
as important as they were when Marconi began the whole thing ! (Actually
Tesla was there first)
Taken as a whole,
> the information is not applicable in contemporary installations.
Well, it came from another e-mail list devoted to experimental aircraft
and it looks applicable to me ! It certainly is in accordance with
the texts on my bookshelf !
> Check the installation manual for any radio you buy. If it were
> necessary to diddle with coax lengths, they would say so.
I wonder ? Don't they just say that you should commect to a
fifty
ohm impedance antenna ?
Further,
> given the bandwidth of the communciations channels (118 to 135 Mhz)
> you will have an antenna be pretty close at some intermediate frequency
> and progressively worse as you move away from it's design frequency.
Of course - but lets make it as good a match as possible at your chosen
frequency.
>
Unless you have access
> to an can use some pretty good test equipment (antenna analyzer)
> to optimize the length, cut it (the antenna) to formula and forget it. If
you
> were to look at this antenna with test equipment, you'll find that
> it's never a perfect match (1:1 SWR) at any frequency and can be
> as poor as 2:1 at the band edges. If King Radio doesn't worry
> about it, you don't have to either.
And of course you can accept the output power degradation which solid
state transcievers automatically inflict when they see a high SWR !
Why not borrow a SWR meter from your friendly neighbourhood ham (and
maybe the ham as well !) and check your SWR - get it as close to 1:1 as
possible !
Regards,
Martin
9Y4TAM
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