MMDVM HS HAT not receiving weak signals

MMDVM_HS Hat hardware
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db9cr
Posts: 4
Joined: Thu Apr 22, 2021 11:24 am

MMDVM HS HAT not receiving weak signals

Post by db9cr »

Hello,

According to Spec, the ADF 7021 can receive well in the -100dBm range. My pi-star is showing a different behavior: When my radio is close to the MMDVM, everything is fine. BER is well below 0.3%. The RSSI is indicated in the range of 45-50. When I increase the distance, in the range of about -70dBm the BER increases to 5% and more. If I increase the distance more, the signal is no longer detected.

To track that down, I created the following test setup:
[Radio] --- [Attenuator -40dB on Antenna plug] --- [Rubberducky] -------------------- [MMDVM HS DUAL HAT with the two supplied antennas.]

Results:
When the radio is within few centimeters around the antenna, I get around -50dBm signal. In ca 1m distance, I get around -70dBm, according to MMDVMHost log. Further away, MMDVMHost is not picking up a signal, so I can't tell RSSI in greater distance.

With Pi-Star, in close distance, everything is fine. In ca 1m distance, the BER rises and quickly the signal drops out when I increase the distance further.

I tried the same with MMDVMcal, and the weird result is, that in mode "b" the MMDVM HS DUAL HAT can receive the signal with BER below 0.5% from a distance of up to 3 meters and probably more.
I already tried using a simplex HAT - same behavior.

So my question: Why is MMDVMcal showing a low BER even on weak signals, while MMDVMHost can't receive below -70dBm and in more that 1m distance?

Remember - this is a heavily attenuated test setup to reproduce the problem without taking a walk ;-) In reality without attenuator on the radio, the problem starts in a distance of several hundered meters, even if connecting the MMDVM to my big Diamond station antenna.

Thanks in advance
DB9CR Christian
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