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. We're moving our website to a new server
and will start adding parts catalog pages soon. Will re-join
the Europa list after OSH . . . in mean time, if there are
specific items I can help with, someone might e-mail copies
of the message to nuckolls@aeroelectric.com. We'll be shutting
of the Compuserve account in the not too distant future.
In the mean time, here's a bit I picked up on just before I
left town about ten days ago. . . .
/Just a short note on antennae. The co-ax must indeed be of
/the same impedance as the output of the radio (in this case 50
/ohm).
True . . . don't use RG-59 "television and video"
coax it's 75 ohm.
/Now not only must the impedance of the cable match that
/of the radio so too must that of the antenna match that of
/the cable (each time you have an impedance mismatch you get
/a reflection of the signal which results in a standing wave
/in the cable and that much less of you signal going out into
/the air).
Also true but something over which you have little control
unless you're going to buy the test equipment to do a really
tight matching job on the antenna and then it becomes not-so-
simple . . .
/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. . . .
/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)=2E/
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. BTW, the 1/4 wave value
for Comm antennas is in the neighborhood of 22" . . . half wave
would therefore be 44" . . .
/The bit about not being allowed to cut the co-ax is only true
/if you have a mismatched impedance on the antenna. Along the
/length of the co-ax the resistive and reactive components of the
/impedance change as the wave travels down the cable (that is
/the standing wave which gives you the SWR you may have heard
/about). If you have a mismatch then you have this situation and
/can reduce the SWR by cutting the co-ax to the exact length to
/give you an impedance transformer between the 50 ohm of the radio
/and the ?? ohm of the antenna. But if you stick to the 1/4 wave
/length and the ground plane then your antenna impedance is only
/resistive and your cable can be any length you wish./
This is a mix of partial truths some of which were significant
in the early days of solid state transmitters. Taken as a whole,
the information is not applicable in contemporary installations.
Check the installation manual for any radio you buy. If it were
necessary to diddle with coax lengths, they would say so. 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.
/For a fast calculation of a quarter wave length use L234/F where
/L is the length of a 1/4 wave in feet and F is the frequency in MHz.
/So for 121.5 MHz the 1/4 wave antenna should be 1.9259 feet long.
This is as good an approximation as any. Unless you have access
to an can use some pretty good test equipment (antenna analyzer)
to optimize the length, cut it 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.
Bob . . .
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