[Video] Preamp test in front of radio / satellite ground station build update
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.
73, John Brier KG4AKV
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 [email protected] 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.
73, John Brier KG4AKV _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
On 2020-01-28 14:56, John Brier via AMSAT-BB 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.
Hello John,
As others have said, FM is not ideal for this testing.
Ideally, you'd want to run SSB and feed that into your rig. Then, take the audio output and put it into Spectrum Lab or similar.
For a weak CW signal (less than 1 S unit), you should see a peak at the signal just above the noise. This ensures that AGC is inactive, if it can't be disabled.
Now, switch in the LNA. The CW tone should jump by the gain of the preamp. Ideally, the noise floor will not change at all. If it does, then the preamp is not a low-noise type or something else (overload) is going on.
Even with low-loss coax, putting the LNA at the antenna is the right way to do it. Use the following calculator to experiment with this lineup:
https://www.pasternack.com/t-calculator-noise-figure.aspx
(Noise-dB / Gain-dB)
Case 1: LNA (1/20) -> Coax (2.5/-2.5) -> Rig (6/90) == 1.2dB Case 2: Coax (2.5/-2.5) -> LNA (1/20) -> Rig (6/90) == 3.6dB
You're giving up 2.4dB of NFdB for the convenience of putting it indoors. The numbers above assume LMR-400 at 100' loss.
https://www.rtl-sdr.com/tutorial-on-properly-positioning-a-preamp-lna-in-a-r...
Local noise sources, unless they are incredible broadband, won't impact the usefulness of a quality LNA. The thing to look for here is linearity, which is indicated by the 3rd order intercept (IP3) of the LNA. Most modern designs have eye-popping linearity and noise figures as compared to the lowly, ancient U310-based amplifier.
https://www.maximintegrated.com/en/design/technical-documents/tutorials/5/54...
--- Zach N0ZGO
More budget friendly, especially the mini versions: http://www.shf-elektronik.de/en/index.htm
Got the Mini 70 and am very satisfied with it. Can also be ordered from Wimo in Germany.
Hans
BX2ABT
On 1/29/20 6:50 AM, Zach Metzinger via AMSAT-BB wrote:
On 2020-01-28 14:56, John Brier via AMSAT-BB 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.
Hello John,
As others have said, FM is not ideal for this testing.
Ideally, you'd want to run SSB and feed that into your rig. Then, take the audio output and put it into Spectrum Lab or similar.
For a weak CW signal (less than 1 S unit), you should see a peak at the signal just above the noise. This ensures that AGC is inactive, if it can't be disabled.
Now, switch in the LNA. The CW tone should jump by the gain of the preamp. Ideally, the noise floor will not change at all. If it does, then the preamp is not a low-noise type or something else (overload) is going on.
Even with low-loss coax, putting the LNA at the antenna is the right way to do it. Use the following calculator to experiment with this lineup:
https://www.pasternack.com/t-calculator-noise-figure.aspx
(Noise-dB / Gain-dB)
Case 1: LNA (1/20) -> Coax (2.5/-2.5) -> Rig (6/90) == 1.2dB Case 2: Coax (2.5/-2.5) -> LNA (1/20) -> Rig (6/90) == 3.6dB
You're giving up 2.4dB of NFdB for the convenience of putting it indoors. The numbers above assume LMR-400 at 100' loss.
https://www.rtl-sdr.com/tutorial-on-properly-positioning-a-preamp-lna-in-a-r...
Local noise sources, unless they are incredible broadband, won't impact the usefulness of a quality LNA. The thing to look for here is linearity, which is indicated by the 3rd order intercept (IP3) of the LNA. Most modern designs have eye-popping linearity and noise figures as compared to the lowly, ancient U310-based amplifier.
https://www.maximintegrated.com/en/design/technical-documents/tutorials/5/54...
--- Zach N0ZGO _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
participants (4)
-
Hans BX2ABT -
Hasan al-Basri -
John Brier -
Zach Metzinger