[R-390] Antenna Noise

[WF2U]

Re.: low noise front ends - it is good to keep in mind, that the ambient
atmospheric noise in the HF band (up to 30 MHz) averages 8 dB.  Going out of
the way on HF with attaching sub - dB noise figure preamps is a complete
waste of effort, as it's not going to reduce the 8 dB noise you're going
input to your receiver even under the most idealized conditions. Antenna
mismatch from the lowest noise input complex impedance at  a particular
frequency and individual receiver is going to contribute additional noise.
As we all know, the best noise figure is not at the best impedance match
anyway...
Fractional dB preamps are only valuable and necessary  in the higher
frequency bands - VHF, UHF and microwave - where the ambient noise is under
a dB or less.

My 2 cents' worth...

73, Meir WF2U
Gowensville, SC


 -----Original Message-----
From: 	r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Bob Camp
Sent:	Friday, August 20, 2004 11:05 AM
To:	R-390 HF Receiver List
Subject:	[R-390] Antenna Noise

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



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