dlj04(at)josephson.com
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 Posted: Sat Apr 26, 2014 5:15 am Post subject: Avionics-List: Garmin SL-30 poor VOR reception |
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This question is being discussed both here and on the avionics list.
Commercial RG-58A, mil spec RG-58 (MIL-DTL-17 M17/28-RG058) such as
Belden 9203, RG-142 and RG400/LMR400 are all rated between 3.3 and 4.9
dB per 100 ft at 100 MHz -- around 1 dB for a practical airplane
installation. Belden 9203 RG58 is among the lowest loss and is mil QPL
qualified. Cable loss is not really a factor in a VOR/LOC/GS
installation; well made RG58 cable is fine. Cheap RG-58 "type" cable of
unknown impedance and bend resistance, who knows.
Original RG-58 (solid center conductor, solid PE dielectric) was just
fine for decades with receivers far worse than the Garmin. In my
experience, connectors are the problem nine times out of ten. Commercial
grade crimp connectors from a manufacturer that also supplies military
grade connectors (Amp, Amphenol, Kings) crimped with the
manufacturer-specified or mil-spec tool checked for the correct crimp
dimensions by someone experienced with this process is the key. Original
military soldered connectors are also OK but only if the installer is
experienced enough to get the dimensions right without melting the
dielectric.
Quick test, after you've confirmed that the cable has continuity end to
end and isn't shorted: grab the cable in one hand and the body of the
connector or the crimp sleeve in the other. Using moderate force, can
you rotate the connector body with respect to the cable? If so, cut it
off and try again, it is not terminated correctly. This isn't the cause
of all problems but is usually diagnostic of whether it was crimped
right in the first place.
The tests Bob suggested are good. A nav receiver will work well -- not
optimally for IFR but well enough -- on a comm antenna.
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