Page 1 of 2

Power on 2M

Posted: Wed May 08, 2019 2:18 pm
by radiogareth
I've been experimenting now I have a proper power meter and spectrum analyser.
Using one of the cheap chinese VHF 100 watt amps ... 1776288254.
Driving it with Portsdown at 0% attenuation, one of these 'amps' ( ... ctupt=true which are a single device on a reworked board - 2 watts dissapation maybe but not RF!) 125KS and tweaking the bias for power and symmetry, I get the attached waveforms and 44 dBm = 25 watts.
It looks 'good to go' to me or am I being hopeful?
2019-05-08 14.39.39.jpg
2019-05-08 14.39.39.jpg (81.73 KiB) Viewed 2302 times
2019-05-08 14.40.10.jpg
2019-05-08 14.40.10.jpg (121.41 KiB) Viewed 2302 times
2019-05-08 14.42.12.jpg
2019-05-08 14.42.12.jpg (127.63 KiB) Viewed 2302 times

Re: Power on 2M

Posted: Wed May 08, 2019 2:33 pm
by radiogareth
And the REALLY good news is that it seems to be happy on 71MHz too, delivering +40 dBm :-)
Mind you, it might need a filter ;-)
2019-05-08 15.28.16.jpg
2019-05-08 15.28.16.jpg (100 KiB) Viewed 2297 times
Gareth, G4XAT

Re: Power on 2M

Posted: Wed May 08, 2019 4:12 pm
by radiogareth
Sticking an open 1/4 wave of RG 174 on the output cleaned that right up. Amp draws 1.6 amps @28 volts so 45 watts in for 20+ out. Am I in cloud cuckoo land? Bias is 0.3 amp.
Time to fix a fan, box and add coaxial switching.
2019-05-08 17.05.44.jpg
2019-05-08 17.05.44.jpg (114.1 KiB) Viewed 2289 times

Re: Power on 2M

Posted: Wed May 08, 2019 4:34 pm
by g8gtz
Looks good however not sure about power out figures
45 watts in for 20+ out.

Preume you mean DC in! What drive in is needed?


Re: Power on 2M

Posted: Wed May 08, 2019 4:55 pm
by radiogareth
Noel, Yes, DC input. From memory the first amp delivers around 0dBm. The China amp has shown itself to be VERY lively, giving 120+ watts of CW for very low drive 1-2 watts CW. MRF 186 ex cellphone service AFAIK.
Planning the box/switching so SA off and portable batteries charging. Will check later. Note to self, WRITE STUFF DOWN!!

Re: Power on 2M

Posted: Thu May 09, 2019 12:39 pm
by radiogareth
Output from my little 'anodized box' amp is +20 dBm so the PA is doing great guns. Surprised it stable TBH but looks fine so far. No doubt when boxed it will become a power oscillator.....

Re: Power on 2M

Posted: Fri May 17, 2019 8:54 pm
by radiogareth
So I persuaded a local friend of mine Martin, G4FKK, to get involved with DATV. We both now have 20 watts on 146.5 and are separated by 8 miles, non line of sight. For comparison, using beams each end and 5 watts of NB FM, we exchanged 5-9.
We did not manage DATV though, neither with 125SR nor 333SR. Martin is using a decent RTL dongle on a Portsdown, I'm using a MT V2 straight in. We can both see the beginnings of signals but not enough to lock. More signal required, but how much more?
I'm guessing that some form of preamp for my MT would be a good idea, given it was designed to operate with a BIG LNB signal, maybe the same for the RTL, or is that at its limit anyway perhaps ie more gain may not improve matters unless well-filtered?

Suggestions welcome, other than 'move house' ;-)


Gareth G4XAT

Re: Power on 2M

Posted: Fri May 17, 2019 8:57 pm
by M5TXJ
Gareth, don't forget the ERP limit on 2M,
73 Dave.

Re: Power on 2M

Posted: Fri May 17, 2019 9:12 pm
by radiogareth
Yes, noted, we both have 'feeder losses' - it was a bit of a patch-lead fest for this tryout!.

Re: Power on 2M

Posted: Sat May 18, 2019 8:04 am
by g8gtz
Hi Gareth,

You will definetely need a pre-amp and filtering in front of a MiniTiouner - the normal LNB that it was designed to work with has ~55dB of gain and a low noise front end.

There are a number of solutions but the best I found was the GM3SEK design DG8 pre-amp which has a decent NF plus on board filtering but the source of kits is no longer available.

An alternative could be this unit made by Anthony M0UPU for the HA balloning market - it has a low noise device followed by a SAW filter which should give protection from paging / band 2 but not from packet and local repeaters ... duct_id=89

Alternativly there are loads of low noise pre-amps on ebay but you MUST MUST use a filter - they will appear to work on the bench but not on the aerial- I know, I've been there - see and

Re G4FKK - if he is using the leanDVB in the Portsdown he will instantly be 8 dB down as there is no FEC implmented and the above about filtering also applies. He maybe be better off trying the s/w decoder developed by Marko92 on the Oscar 100 chat - apparantly it's got a challenging UI and not sure anyone has used it for anything other than Oscar100 but reports say it would be better than LeanDVB. Available from here:

Hope this helps...

Noel - G8GTZ