Hi Leif. I do understand that the impedance change into the antenna could
cause the gain to change, but I still think (from experience) that it is a
worthwile test. At my home location now, When it is very quiet, I can see a
db or so less noise from the antenna than from the load [note]. That is
where I want to set my gain level because I want my system to perfrom best
when I can hear the best. Overload problems can be handled with post preamp
attenuation when necessary.
73, Russ K2TXB
[note]: At other times, and in certain directions I can see up to 10 db
increase in noise compared to the resistor. Then I have to wait for the
moon to move to a quieter location - that is easier than moving my station
to a quieter location instead, hi hi.
[mailto:email@example.com] On Behalf Of Leif Asbrink
Sent: Sunday, January 27, 2013 6:58 AM
Subject: Re: [Moon-Net] FT-736R vs TS-2000
Hi Russ, Alex and all,
Hello Alex. I agree with what you are saying and that
brings up an
issue that I think should be reviewed. You and Leif, and probably
others, are telling people that turning off the head end preamp should
result in about
16 db noise drop, but you do not specify how to control excess antenna
Correct. I advocate this test for making sure that the noise floor
originates in the preamp/ antenna and not in other blocks further down the
signal path. It is a general rule valid on all bands.
I always write this has to be done with the antenna towards quiet sky.
It is NOT a test for the antenna performance. That is another thing and I do
not think the comparison with a 50 ohm resistor is safe. That is a valid
test only if one knows that VSWR is very good or that the amplifier gain
does not change much on source impedance changes. I think it is more safe to
study galactic sources and to map the noise floor in different directions.
More safe but far more time consuming....
I think you should tell people to install a 50 ohm
load at the preamp
input before making that test. That will assure that there is
sufficient gain, and they are not adding excess antenna noise to the
There is no point in having 16 dB noise reduction on a 50 ohm dummy
one has 25 dB noise reduction on the antenna.
In such a case one should reduce gain to get a better dynamic range.
To compare with a 50 ohm dummy to get the real antenna temperature is a
separate test as I see it. If impedances are well under control it becomes a
reliable test (radiometer) and one sould observe about 1 dB less noise from
the antenna than from the resistor on 144 MHz maybe nearly 1.5.
Many people live in areas with terrible noise on 2
meters these days
and I have heard many stories by people who cannot understand why they
cannot hear EME signals even though they are easily being heard by
others. Sometimes it is hard to get them to understand what they must
do to measure how much excess noise is in their environment.
Installing a second antenna relay, with the input switched to load
during transmit, when antenna relay is unpowered, allows the gain
measurement we are speaking of, and also allows one to compare received
against the noise generated by the load.
Yes, One thing is to design the Rx to fit the antenna.
Another thing is to ensure that the antenna is good.
One has to do both things:-)
If one is lucky, the load will produce more noise than
but most of us must live with the opposite condition much of the time.
Very rarely I am able to see less noise from the antenna than from the
That means that your rx noise floor is higher than necessary most of the
time. In case you have dynamic range problems due to fellow 144 operators
you could reduce gain without any loss at all until you reach about 16 dB
for the preamp test.
Gain reduction does of course not solve the antenna/environment problem, but
it could solve other problems...
Leif / SM5BSZ
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