Low RF output

MMDVM_HS Hat hardware
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G4AON
Posts: 57
Joined: Tue Apr 24, 2018 5:34 pm

Low RF output

Post by G4AON » Thu Jan 31, 2019 4:27 pm

Checking the output power on my Spectrum Analyser, it seems very low at -35dBm. It’s one of the Chinese eBay ones and is set for 434.800

I removed the capacitor to the on-board antenna without any noticeable difference. Altering the RF output under Pi-Star Config makes no difference either.

While there is sufficient output to work around the house, I would have expected nearer to 0dBm or perhaps more.

Any ideas?

Thanks Dave
ID51E plus 2
ID5100E
TYT MD-UV380
Chinese MMDVM on a Pi Zero W
DVAP on a Pi 3B
Shark OpenSPOT 2

G8SEZ
Posts: 52
Joined: Fri Apr 13, 2018 8:26 pm

Re: Low RF output

Post by G8SEZ » Thu Jan 31, 2019 5:15 pm

If memory serves someone mentioned this sort of problem before, almost certainly because the RF output network has been built with incorrect component values.

It should be fixable the correct values can be found fairly easily. But that's letting someone in China off the hook.
--

Brian G8SEZ

G4AON
Posts: 57
Joined: Tue Apr 24, 2018 5:34 pm

Re: Low RF output

Post by G4AON » Thu Jan 31, 2019 5:19 pm

Thanks Brian

This one has an LCD display that makes access to the L/C output parts awkward.

Should the Pi-Star config alter the RF O/O on the MMDVM boards? My changes had no effect...

73 Dave
ID51E plus 2
ID5100E
TYT MD-UV380
Chinese MMDVM on a Pi Zero W
DVAP on a Pi 3B
Shark OpenSPOT 2

G8SEZ
Posts: 52
Joined: Fri Apr 13, 2018 8:26 pm

Re: Low RF output

Post by G8SEZ » Fri Feb 01, 2019 6:39 pm

Yes, I know the arrangement, mine are the same.

I think the output level is controllable using the expert menu and MMDVMhost, but I forget the details.

Even with low power I would expect to see some variation on the SA screen as this is adjusted, so maybe you're tweaking the wrong parameter.
--

Brian G8SEZ

G4AON
Posts: 57
Joined: Tue Apr 24, 2018 5:34 pm

Re: Low RF output

Post by G4AON » Fri Feb 01, 2019 6:45 pm

Thanks Brian

I’ll check on Monday, but I think it’s the correct parameter. I also have a Pi 3/DVAP. I’ll check that and see what effect the RF parameter has.

It would be useful if someone with an RF millivolt meter or spectrum analyser and an MMDVM would check theirs. Maybe many have flea power levels of RF?

73 Dave
ID51E plus 2
ID5100E
TYT MD-UV380
Chinese MMDVM on a Pi Zero W
DVAP on a Pi 3B
Shark OpenSPOT 2

G4AON
Posts: 57
Joined: Tue Apr 24, 2018 5:34 pm

Re: Low RF output

Post by G4AON » Thu Feb 07, 2019 11:29 am

Problem sorted...
This Chinese copy MMDVM board had incorrect parts fitted in the "T" network filter. I removed the components (C27, C28, L1, L2 and C1). Then bridged directly from the junction of C4/C5 to the SMA socket side of C27.

While it means the board is running without filtering and may be open to other frequencies blocking it, it works fine at my QTH on a small "rubber duck" helical antenna. The output on my spectrum analyser is fairly clean, when set to 0dBm out the 2nd harmonic is 35 dB down and 3rd is 20 dB down. Given the 1mW (0dBm) output into a small helical antenna, it's not really an issue.

Thanks to Dave, G3XOU, for info on the problem.

73 Dave
ID51E plus 2
ID5100E
TYT MD-UV380
Chinese MMDVM on a Pi Zero W
DVAP on a Pi 3B
Shark OpenSPOT 2

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