[R-390] Antenna Noise

[Bob Camp]

Hi

We (I) keep talking about the noise of the radio going up as the 
antenna is attached. That's all well and good and it's an excellent way 
to check a receiving system. Like everything there are a few things you 
need to be careful of on any radio and a few things specific to the 
R-390's.

The first thing to watch out for is a front end that is going in and 
out of oscillation. This is not a problem on a well designed radio, but 
not all radios are well designed. It can be an issue on a radio that's 
broken. Generally the effect is dramatic, the noise goes from "lots" to 
"none" when fairly small changes are made. The front end gain goes 
pretty much to zero db when it starts to oscillate....

If you have a radio with a very hot front end then it may have a 
significantly different noise output with 25 ohms on the antenna 
terminals than with 100 ohms. To be absolutely accurate you should 
first measure the noise with the correct resistor on the antenna 
terminals and then check it with the antenna. If you are working on a 
moon bounce 0.4 db noise figure pre-amp for your R-390 then this is 
something to watch for.

An R-390 and most of the radios of it's era have a tuning adjustment on 
the front end of the radio. Modern radios seem to have forgotten just 
how neat a thing this is. Unfortunately this makes the antenna test a 
little more difficult on the older radios. To be totally accurate you 
need to first peak up the radio with the resistor on the antenna input 
and then peak it up with the antenna. The proper comparison is between 
the two peaked conditions.

On a radio with a good AGC you can have a case where the AGC comes in 
and makes this all a bit hard to check. Most noise measurements are 
made with the AGC turned off for this reason. As an alternative you can 
monitor the AGC voltage and see what's going on. Of course to do this 
you need a radio that was nicely designed to have the AGC voltage show 
up on a easy to get to terminal strip on the back, or better yet a nice 
Navy installed jack on the front panel.

IF filters do not all have the same insertion loss. If you have a radio 
with significant differences between filters this can be an issue. One 
example would be a 4 KHz filter that has the foam inside turning to 
sludge. In that case you need to check with the filter you will be 
using. On a normal radio with well matched filters the measurement will 
be lots easier with the widest bandwidth filter you can switch to. That 
way the noise through the filter will swamp any audio chain hum and 
noise as much as possible.

So now we have a really complicated way to do something that was nice 
and simple. What you used to be able to do in about 10 seconds now 
takes the better part of an hour and requires a spread sheet to 
interpret the results.  Well maybe not. The human ear is a marvelous 
thing. You can pretty quickly tell the difference between antenna noise 
(static pops and crashes) and thermal noise (no pops). Once you get 
used to the difference it's easy to spot a problem. So much for all the 
theory junk above and back to a  quick and easy check.

	Take Care!

		Bob Camp
		KB8TQ