My missing 6 dBs

Rob O
Posts: 2
Joined: Thu Jan 07, 2021 8:25 pm

My missing 6 dBs

#1

Unread post by Rob O »

Hi,
Thanks for a great s/w tool, I have been able to get tx as well as rx to QO-100 working and learned a lot along the way.

My rx set up is:
pluto -> 1m patch lead ->bias T -> 25m satellite coax -> sat finder -> 1m patch lead -> 4 channel Octagon lnb -> 80cm Technomate Mesh Satellite Dish.
I think the sat finder adds about 12 dB gain but does little else. The dish is on a scaffold pole in an old garden umbrella stand so I can move it fairly easily (this is relevant!).

I tend to check the CW beacon signal level when I switch on, just as a system sanity check. I set the rx to use manual gain 40 dB, so that I dont get caught out by AGC. I have 2 Octagon lnbs. On 23/1/21, I measured -55.3 and -56.2 dBm and on 13/4/21, after nearly 3 months of no use, I measured -54.8 and -56.0 dBm. So fairly stable. I was using sdr console v3.0.26.

On 20/4/21, I downloaded and installed v3.0.27. Towards the end of April, I moved the dish about 4m. After peaking the CW beacon, the highest level I measured was -60dBm.

In early May, I re-installed v3.0.26. No change - levels still ~-60dBm. However, I think I have read on the main website that reverting to an older version is not guaranteed to work and I didn't do a full un-install new, install old, so it seems quite possible that newer (v3.0.27) libraries may have been lurking and been used by the older (v3.0.26) application?

Today, I have moved the dish back to its original position and measured the level -62.4 dBm (strangely low), and then returned it to its "new" position, again measuring -60.8 dBm (vs -54.8 dBm).

Is there any known software related reason that could account for these missing 6 dB?

AFAIK, I haven't changed any other rx settings and with 30+ dB SNR, I dont think the rx filter bandwidth should matter (I dont think I've changed it anyway).
Is there a common mistake I could have made?
Has the iio library updated ADC scaling values?

6 dB is always a suspicious number for me.

I would be grateful for any helpful suggestions, thank you.

73s
Rob,
M5RAO

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