[R-390] Antenna Noise

Fri Aug 20 12:09:43 EDT 2004

Hi

Absolutely true.

The only case that can be made for a super pre amp on a good efficient  
antenna is up in the UHF / moon bounce area. Even a good antenna will 
have a significant amount of thermal noise if it's pointed at ground 
level targets.

Not all antennas are good efficient antennas.

Sixty feet of coax buried three feet down in the back yard terminated 
in 50 ohms will act as an antenna. It's not an efficient antenna, but 
it is an antenna. It will pick up signals and they can be heard on the 
receiver. Since the efficiency of this kind of antenna is quite low it 
will not have as much atmospheric noise out of it as a matched dipole. 
It's source impedance may be similar to the source impedance of the 
dipole, but it's noise will be lower. Of course the signal is also 
lower so who knows which antenna is better.

There are a number of these sorts of antennas out there. Most of them 
are quite small compared to a dipole and are resistively loaded in some 
way to improve directivity. The impedance match on these antennas does 
not imply that they have the same power capture as a properly matched 
short dipole.

In the case of a low efficiency antenna like this you might need a 
lower noise front end even down at some fairly low frequencies. I'm not 
suggesting that we all go use our coax feed lines as antennas, only 
that it has been done. It is a special case but it does come up from 
time to time.

	Take Care!

		Bob Camp
		KB8TQ

On Aug 20, 2004, at 11:32 AM, WF2U wrote:

> 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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