[amsat-bb] [Video] Preamp test in front of radio / satellite ground station build update

[Hasan al-Basri]

Hi John,
You referenced my old article in Amsat so I thought I would make some
observations that might help you. First, to evaluate the effect of the
preamp on FM you don't use the S-Meter at all, it will tell you have gain,
but tell you next to nothing about noise floor. What you need to do is find
a weak signal on FM, that has some noise on it that is caused by the weak
signal, not by another noise source. Do this with no preamp connected. Get
a feel for how much noise is present. Now, connect the preamp and what you
should hear is the background noise decreasing (in other words, better
quieting on the FM signal). If this happens, then you are benefiting from
the preamp. If this doesn't happen, then it is not helping, but we aren't
sure why. I will post more info in reply to your message on the amsat list.

Let's look at a better way to evaluate your preamp. Do these steps, it will
get you in the ballpark, without fancy test equipment:

0. No preamp connected.
1. Turn the agc off on your radio, if you can. If not, we can live with it.
2. Tune your radio to one of the satellite downlink frequencies like 145.733
     (upper sideband, noise blanker off)
3. Disconnect your antenna from the radio.
3a. If you have a 50 ohm dummy load, connect it to your antenna. If not,
skip this step and leave your antenna connection open (no antenna)
4. Turn your RF Gain to Maximum. Turn your audio up to a reasonably high
level so you can clearly hear the noise.
5. Take notice of "how loud" the noise is
6. Now, connect your antenna (replacing the 50 ohm load, or replacing the
lack of an antenna.

Question: Did the noise level jump up markedly (a lot) when you connected
the antenna? (compared to the 50 ohm load or compared to no antenna)?

If it didn't, your rx sensitivity is poor. If it did, you can at least hear
your ambient noise level with your antenna and existing radio front end.

The good news is, if your ambient noise level is low, then you are doing
very well. If it is high, then your ambient noise level is artificially
killing some of your receiver sensitivity and a preamp won't be of much
help.

Now, let's evaluate the preamp:

Do the same procedure above, with the preamp connected, i.e.,

Take notice of the noise level with the 50 ohm resistor (or open antenna)
Resistor is better, but we use what we have. Set your audio volume so you
have a good "feel" for how much noise is from the resistor or open circuit
antenna.


*Now connect the antenna...did the noise jump up considerably , i.e. is the
noise from the antenna/preamp overriding the internal noise of your radio's
front end?*

If it jumped up, again, that's good.

To evaluate whether or not your local ambient noise is ruining the
effectiveness of your preamp or not, without fancy equipment you can do
this:

Have someone generate a weak signal on 2m FM, such that you have some
noticeable noise on the FM signal (with no preamp connected, but with your
antenna connected).

Note how much 'background' noise there is on that deliberately weakened FM
signal. Now, turn on or connect your preamp. Did the signal get quieter? If
it did, the preamp is helping. If it didn't, it isn't and one of two things
is taking place:

1. Your radio front end has as low a noise figure as your preamp. VERY
UNLIKELY if that is an ARR 2m GASFET preamp.

2. Your ambient noise is so high that the improved noise figure / noise
floor of your system is being masked by the external noise your antenna is
picking up.

Hope this helps,

73, Hasan, N0AN


On Tue, Jan 28, 2020 at 2:58 PM John Brier via AMSAT-BB <amsat-bb at amsat.org>
wrote:

> Here is a little update on my satellite ground station build project
> and a demonstration of a test I did with my Advanced Receiver Research
> 2 meter preamplifier.
>
> https://youtu.be/BNai203eUnI
>
> 73, John Brier KG4AKV
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