PAs for Portsdown

Discussion about this major DATV Project. See also https://wiki.batc.tv/The_Portsdown_Transmitter
G3JZF
Posts: 10
Joined: Tue Jul 11, 2017 6:43 pm

PAs for Portsdown

Post by G3JZF » Thu Oct 19, 2017 11:30 am

Guys I have a GH Engineering PA unit PA1 3G18 which is a FM PA block with a Motorola M68719.
Is it of any use as a PA for the Portsdown TX?

I also have a Microwave Modules MML 432/100 which is a Linear Amp for 70cm. Can anyone suggest a suitable amplifier that would drive it?

Any help would be much appreciated.

John G3JZF Sutton Coldfield

g0mjw
Posts: 357
Joined: Sat Sep 20, 2014 9:15 am

Re: PAs for Portsdown

Post by g0mjw » Thu Oct 19, 2017 6:02 pm

I doubt it is linear enough for DATV. Best check with other users as it might be. However if not with a few modifications you may be able to replace the RF module for one from the RA series.

Please don't use the MML amplifier on 70cm on DATV. All the ones I have tested have not been linear enough and have unacceptable spectral regrowth (Shoulders) for use on 70cm. The biasing circuit was not designed for it. You might be able to modify it by increasing the bias current but its unknown if that would work. If it does you won't get any more than 10W out so the Portsdown might be sufficient to drive it directly.

Mini-kits do a nice RA module based amplifier that would give you 5-10W. Higher power is currently difficult. This is because DATV requires very linear amplifiers. On 2m I get 5W out of a 70W device, running it at 3A bias current.

Needless to say, anyone using DATV probably needs a spectrum analysis to check their transmissions. Fortunately, an RTL Dongle will do fine for this. Maybe a TX spectrum plot is another one for Dave's Portsdown ToDo pile.

Mike

G3JZF
Posts: 10
Joined: Tue Jul 11, 2017 6:43 pm

Re: PAs for Portsdown

Post by G3JZF » Sun Oct 22, 2017 12:20 pm

Mike

Many thanks for your reply - all comments noted.

I dug out a RA07H4047 that I had had by me for some time. I set it up according to the data sheet and it worked first timer. With a 10dB attenuator after the Portsdown and a dummy load on the output it appears to be working well with a perfect testcard on my Minitiouner. I now need to get my RTL dongle working, build a better attenuator system and sort out power supplies and filters. Hopefully, I am now on my way.

I do thank you for your interest and will shout out if I need more help.

vy 73

John

g3zgz
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Joined: Thu Jul 16, 2015 1:36 pm

Re: PAs for Portsdown

Post by g3zgz » Wed Oct 25, 2017 3:09 pm

I'm also looking at using the Mitsubishi modules for 2m and 70cm

Would you recommend I go for the 30watt or 60watt versions of the modules, and why?

Thanks

Dave

G3ZGZ

radiogareth
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Joined: Wed Jan 06, 2016 9:46 am

Re: PAs for Portsdown

Post by radiogareth » Fri Nov 10, 2017 3:59 pm

Would one of these be good/great for DATV? http://g4fev.atspace.com/dolphinpa.htm

Bought one a while back to add to the 'play-pile'. Untouched at the moment....

g0mjw
Posts: 357
Joined: Sat Sep 20, 2014 9:15 am

Re: PAs for Portsdown

Post by g0mjw » Fri Nov 10, 2017 4:21 pm

radiogareth wrote:Would one of these be good/great for DATV? http://g4fev.atspace.com/dolphinpa.htm

Bought one a while back to add to the 'play-pile'. Untouched at the moment....
Yes - I would think so, but don't do the bias changes as you will need it as linear as possible.

Mike

radiogareth
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Joined: Wed Jan 06, 2016 9:46 am

Re: PAs for Portsdown

Post by radiogareth » Fri Nov 10, 2017 4:25 pm

Yes, I'd leave that part 'as-is' as far as I can recall it was designed to be ultra-linear anyway. What a wise buy :-)

G8GKQ
Site Admin
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Re: PAs for Portsdown

Post by G8GKQ » Fri Nov 10, 2017 6:13 pm

Hi

I use a Nokia dolphin PA on 70cms. No bias modifications and it will give a clean 30W of DVB-S.

Dave, G8GKQ

radiogareth
Posts: 15
Joined: Wed Jan 06, 2016 9:46 am

Re: PAs for Portsdown

Post by radiogareth » Fri Nov 10, 2017 7:04 pm

Thats very good news. And me quiet for several days ;-)
What are you driving it with?

G8GKQ
Site Admin
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Re: PAs for Portsdown

Post by G8GKQ » Sat Nov 11, 2017 8:25 am

I use an MHW710 amplifier and feed about 1 watt into the Dolphin's driver stage.

Dave

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