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dsvs(at)ca.rr.com
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 Posted: Wed Feb 22, 2012 8:59 pm Post subject: Com and Nav Radio and Antenna Questions |
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nuckolls.bob(at)aeroelect
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| Posted: Thu Feb 23, 2012 7:09 am Post subject: Com and Nav Radio and Antenna Questions |
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>Once you have an MFJ259 in hand, you can both
>trim overall length of the Archer design to
>desired center frequency,
Shorter for higher center freq? If I need longer can I just rivet on an
extension?
Yes. But before you start tweaking the antenna,
Do a plot of SWR vs frequency over your range
of interest (108-118 MHz). If it's under 3:1
everywhere, the $time$ you spend on making
it better will probably not produce observable
increase in performance.
>you could also adjust
>the gamma-match and tuning capacitor for optimum
>performance too.
Is this done by just increasing or decreasing the plate size where the
center conductor attaches? Is this checked with the
Percent match function on the MFJ259B?
Yes. When I build this kind of antenna from
scratch, there are THREE adjustments. Over-all
LENGTH sets the center frequency. POSITION
of the gamma-match tap on the antenna sets
the impedance and VALUE of the capacitor
cancels the reactance of the gamma match
strut.
Bob no doubt did some experiments to finalize
these adjustments into a cookie-cutter antenna
without adjustments.
Degradation of performance due to proximity of
two antennas is not fully explored with an antenna
analyzer. The analyzer can only optimize the
antennas ability to accept energy from or deliver
energy to the feedline. You can hang a 50 ohm
resistor on the end of that feedline and produce
the appearance of a 'perfect' antenna while a
resistor has no useful qualities as an antenna.
A 'real' antenna will couple your feedline to the
'ether' but there are other variables that include
radiation patterns and radiation resistance that
probably have a more profound effect on performance
than just the SWR measurement.
It's REALLY hard to do comparative antenna work
on an airplane without a test range. Flying circles
in the sky while listening/talking to another facility
is useful but anecdotal . . . you can't plot comparative
data for two installations based on what you hear.
The bright star on the horizon is just that . . .
you're not going to talk to anyone over the horizon
no matter how good the antenna is. The energy
propagation needed to talk 150 miles is exceedingly
small . . . this means that an antenna can be a
very poor performer in terms of the best we know how
to do and still adequate to the task on our airplane.
This is more of an opportunity to broaden your own
been-there-done-that horizon than to fine-tune your
ship's proposed combination of antennas.
Bob . . .
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