"scratchy" radio transmission; update

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While it was a busy day on the airways (radio wise) I had the same
result; ATC said scratchy but readable at 15 miles. Tried twisting my
arm around the back of the seat to use the rear PTT but it was too
painful while wating for the frequency to open up. I did try calling up
another aircraft at an airport 55 miles away and they reported 'loud
and clear' (this was on 122.9). I can't believe it's a frequency range
thing on two different radios and different antennas.

Jim

[UltraJohn]

wrote:

> While it was a busy day on the airways (radio wise) I had the same
> result; ATC said scratchy but readable at 15 miles. Tried twisting my
> arm around the back of the seat to use the rear PTT but it was too
> painful while wating for the frequency to open up. I did try calling up
> another aircraft at an airport 55 miles away and they reported 'loud
> and clear' (this was on 122.9). I can't believe it's a frequency range
> thing on two different radios and different antennas.
>
> Jim
Maybe the FAA needs to look at their receivers! or they could have a local
strong signal causing desense.
John

[COLIN LAMB]

If the antenna is mounted on the top of your metal aircraft, you could be
shielding the transmissions below you, while having good coverage
horizontally.

Colin

[RST Engineering]

The easy way to see if you have "holes" or voids in your antenna pattern is
to get some goodly distance away from a constant transmitter source (atis or
awos) where the received signal has some noise in it. THen put the airplane
in a SLIGHT bank (say, half standard rate) and see if there are any deep
nulls in the pattern. Deep meaning that the signal disappears completely.
THen cut the distance in half (6 dB) and see if the null is still there.
Half again (another 6 dB) and try again. Most metal aircraft have 15-20 dB
nulls in quite a few spots in the pattern, but more than 18 dB (half, then
half, then half again) calls for some investigation.

Jim




"COLIN LAMB" > wrote in message
k.net...
> If the antenna is mounted on the top of your metal aircraft, you could be
> shielding the transmissions below you, while having good coverage
> horizontally.
>
> Colin
>

[ELIPPSE]

One of the things that will cause nulls in the supposed
omni-directional pattern of an antenna is antenna currents on the
shield. This comes about whenever the coax lead, on non-metal aircraft,
does not come away perpendicular from the antenna, or a balanced
antenna is fed unbalanced from a coax. It is always wise in these
installations to place one or two clamp-on ferrites, such as those
Radio Shack sells, to the coax as close as possible to the antenna.
Another way is to form a balun by running the coax up one leg of the
antenna through a tube or a piece of sheet aluminum covering the coax
and attached to the leg. Leaky coax and connectors will also add to the
over-all pattern, giving additions and cancellations. As Jim pointed
out, these nulls, which are frequency-sensitive, will occur as various
aspect angles around the plane. It's interesting that you can have a
signal running on the outside of a coax that is different from the one
on the inside! Skin-effect and internal coupling, you know! Will
wonders never cease! Paul