In article Andy writes:
On Oct 13, 9:17=A0pm, SD wrote:
I am hoping someone can help with a problem I am having in
mySZD55.while thermaling some transmissions are lost as I turn.it
seems to recieve fine no matter if I am turning or not.it's an older
avionic dittle 720c.how do I diagnose if it's antenna related or just
a bad radio?
Thanks, Dennis Vreeken. aka 4Q

If you recieve ok with a given aircraft attitude and heading but do do
not transmit at the same attitude and heading then the law of
reciprocity rules out an antenna problem. An antenna system works
just the same for receive and transmit.

True for most VHF systems, but not true in general.

The reciprocity will provide the same ratio of received power to
transmitted power, but will not guarantee the same signal/noise in
the received signal.

This is easily seen on HF, where atmospheric noise is substantially
more than the front end noise of the radio. The poor antenna receives
the signal attenuated by (for example) 20 dB, but also attenuates the
noise by 20 dB. The resulting signal/noise ratio as heard by the radio
is pretty much the same as when heard with an excellent antenna. However,
the 20 dB attenuation on transmit with the poor antenna means that the
signal received at the other end will be 20 dB down with unattenuated
noise, so that end has a 20 dB s/n loss of receive performance.

Normally the aircraft radio will not have as much atmospheric noise,
but I have heard precipitation static and ignition noise being pretty
bad.


Alan