More SWR fun

In article writes:
I've been having radio transmission problems on my Dittel FGS-2T off
and on since I put it in. Some people say I'm over-modulate, some say
I'm under-modulated. I've got a factory boom mike and a factory wiring
harness.

Given varying reports, one might want to find out why they vary and
what is really happening. Measurement probably beats random reports.

I finally got one of those MFJ-259B SWR analyzers to see if
it's an antenna problem. At 123.4 MHz the SWR reads 2.4. It bottoms
out at around 1.7 at 131 MHz.

Doesn't seem good. If that is really the case, I would be concerned
that the best is 1.7, and that it is so far from where you want to be.


Question: Should I worry about an SWR of 2.4?

it seems worth investigating. It could be causing RF floating around in
the aircraft, which could be coupling into the microphone input.

If so, how could I
possibly tune a built in antenna?

Depends on the antenna. What do you mean by "built-in"? Is it not on the
outside of the aircraft? It may be reacting to its environment.

Also, anyone have any experience in
fixing modulation issues in a radio?

Yes. If that is really a problem, see a good avionics shop.

Second issue.

I have a couple of magnetic mount ground station antennas. They both
show a SWR of around 2.1 at 123.4 MHz and a minimum SWR of 1.1 in the
low 130's.

Question: Should I go get longer whips for these to better tune them
for the glider freqs?

Perhaps. Some allow you to loosen the screw holding the whip and
tightening it with the whip a little longer.


Note to all of you trying to tune these kinds of antennas - Having
them magnetically mounted to a steel roof makes a huge difference in
the readings.

The ground plane is coupled through the base of the magmount. Where
the antenna is on the roof or what is around it can cause the SWR to
change.


If you have something to transmit, it might be very important that
someone hear it clearly -- a good avionics shop would be the place to go.

Alan

On Jan 10, 11:16*pm, (Alan) wrote:
In article writes:

If so, how could I
possibly tune a built in antenna?

* Depends on the antenna. *What do you mean by "built-in"? *Is it not on the
outside of the aircraft? *It may be reacting to its environment.

By built-in I mean molded into the vertical fin. At least that's where
I think Schleicher puts them.

Question: Should I go get longer whips for these to better tune them
for the glider freqs?

* Perhaps. *Some allow you to loosen the screw holding the whip and
tightening it with the whip a little longer.

I'm already at the max length allowed by the set screws.

I'll see if there's a good shop at the local airport(s).

9B

On Jan 11, 2:40*am, wrote:
I'll see if there's a good shop at the local airport(s).


You may get lucky, but you may spend a lot on a "by the hour" bill and
get nothing in return.

Under and overmodulation of the radio is most likely due to an
incompatible microphone or an incorrect adjustment of the modulation
level pot. You can easily tell if you have an over or under
modulation problem. Undermodulation will give a weak but clear voice
but overmodulation will give a loud and distorted voice. If loud and
distorted just speak quieter and see if the distortion goes away. If
it does then adjust the modulation pot. If weak voice increase the
pot adjustment.

If someone listens to your transmission do they hear a difference in
voice quality when you use a different antenna at the glider ? (you
can plug in a mag base whip for the test)

First thing I would check with the glider antenna is the quality of
the coax connector. Was it a crimp on one installed by the factory or
was it installed after delivery. It's unlikely to be an antenna
problem.

One reason for audio distortion may be that your radio is slightly off
frequency. You can find out by doing a zero beat test using 2 other
radios, one transmitting and the other receiving both. But try this
test last if nothing else works.

If you can get in touch with a local radio amateur who is also a
glider pilot you should have all the expertise and equipment you need
to find your problem.

Andy

The antenna will only have the best SWR at the frequency it was tuned for,
ie the frequency at the center of the antennas range.

I recently tested the built in antenna of a Shemp Hirth Ventus and at
130Mhz it gave an SWR of 1.3 which is an acceptable reading.

Hope this is of some help,

Paul