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QRP 50 MHz Amplifier advice - Printable Version

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QRP 50 MHz Amplifier advice - VK2KRR - 17-08-2019

Having recently got hold of a Zachtek desktop WSPR transmitter that will cover 50 MHz band, I wonder if anyone can give recommendations for a power amplifier? https://www.zachtek.com/product-page/wspr-desktop-transmitter

I've measured the output at about 80 mW. Which is not much. 

It would be handy to find a device that could lift that 80 mW up to at least 5 or 10 Watts. I dont have time to build something, unless its in ready to solder up kit form. Something plug and play, good quality would be the best bet.

Any thoughts on where I could find something for this job? Its only to be used for 50 MHz, not the other bands.


RE: QRP 50 MHz Amplifier advice - VK3LU - 17-08-2019

Best of luck finding one Leigh!
I trawled the internet and found no stand alone ones. There are some hf plus 6m ones but that is pointless.
There are single band ones for 2m and upwards. Weird. The only ones you might find are kits but some kits don't come with heatsinks.
Perhaps there is an opening for some enterprising people!!

Cheers
Nev
VK3LU


RE: QRP 50 MHz Amplifier advice - VK4ADC - 18-08-2019

Leigh

The 6M PA on my Pi-based beacon is a PA section off an old Philips FM828E - i.e. the low band VHF version - with changed coils to drop it to 50MHz.  It is basically a 25W Class C FM design so is fine for WSPR, and mine has a resistive PI attenuator at the input plus a potentiometer in the power supply feed to the first stage to adjust output power - ie mine is set to 5W.

The PA section is screwed on to the back of the rest of the 828 housing with 4 Allen-head screws and after you unsolder he wires from the ceramic feed-through caps and the miniature coax , it simply comes away in your hand.  Discard all but the PA section.

I already had the 6M conversion details but I think you can find them on the web these days. Something like "FM828 6M conversion" or similar should find them. The output LPF definitely needs to be re-done to attenuate 2nd harmonics at 100MHz...  Just keep the PTT relay wired / activated all the time and with a few milliWatts in = watts out.  Big enough heatsink so runs almost dead cold.

There are probably a few of these still around - for a song.  You could probably find something newer (low band VHF) along the same line.

Doug


RE: QRP 50 MHz Amplifier advice - VK5TM - 18-08-2019

If you decide to go the 828 route, details in AR mag Feb 1997 page 42.

PA mod details in the right hand column on that page about half way down.

You can see it on Will's site here https://www.armag.vk6uu.id.au/1997-feb-AR.html


RE: QRP 50 MHz Amplifier advice - OZ2M - 18-08-2019

Not a direct help but taking the long way and from the recent UKSMG Six News:

For the RFzero multipurpose Arduino RF platform, http://rfzero.net , beacon applications there is a need for a 13 dBm/20 mW to 25 W PA for 6 m. The RFzero output level was designed to match the input level required by the Mitsubitshi FET power modules (RA series). But due to lack of time and priorities in general we never got around to do this so Steen, OZ5N, designed a 6 m PA that is in use by three beacons. But again, due to limited resources, Steen cannot undertake the task of maturing this design at least for the time being.

So here is a proposal to anyone who is willing to convert a Mitsubitshi RA30H0608M to perform well on 6 m. We have a handful of RA30H0608M on cracked substrates, easy to fix, that we are willing to donate and send to someone who is capable and willing to do the modifications needed and subsequently do the documentation. We do have some ideas for the modifications. We know that Elecraft used to use the RA30H0608M in their 6 m transverter design. But the second harmonic is -5 dBc or 6 W which is way too much energy to waste on a beacon. But otherwise the RA30H0608M would be ideal.

So, if you are the one, we are looking for you to do many beacon owners, and thus 6 m people, a huge favour.

Bo


RE: QRP 50 MHz Amplifier advice - VK7HH - 19-08-2019

I used a KL70 PA. Gives me a nice 50W output.

[Video: https://www.youtube.com/watch?v=8blnTZW5OR8]


RE: QRP 50 MHz Amplifier advice - VK5ZD - 19-08-2019

There are numerous sellers of 3W amplifiers on eBay for around $20 which would probably be sufficient for WSPR.


RE: QRP 50 MHz Amplifier advice - OZ2M - 20-08-2019

The 2 W to 3 W amplifiers found on sites like eBay are so and so when it comes to ruggedness as discussed here:

https://groups.io/g/RFzero/topic/pa_2w/32132960?p=,,,50,0,0,0::recentpostdate%2Fsticky,,,50,2,0,32132960

In short: don't run them at 13,8 V but better somewhat lower, say 12 V, and only into a fairly resistive load.

Bo


RE: QRP 50 MHz Amplifier advice - VK5PJ - 23-08-2019

Leigh,
  I know your dislike of building stuff but if you look at the Minkits site, there is a Mitsubishi module, PCB and components.  Even an option to purchase the board built (HF amp bits here : https://www.minikits.com.au/RA-HF-Amplifier ).

This is the same sort of boards used by the various beacon projects around the place.  They are generally quite robust for general use.  Given the source of the driver, I would be adding a filter after the P.A ( https://www.minikits.com.au/LPF7-54M ) the same one I have used on the ill fated vk5rbv 50 MHz section.  I still do not know what went wrong there but think it was the ZLPLL that has stopped working, not the Mitisi module P.A

Regards,
Peter.


RE: QRP 50 MHz Amplifier advice - OZ2M - 23-08-2019

I have measured four different RA30H0608M, from different batches, subject to 50 MHz, from a signal generator "without" a second harmonic, and the second harmonic on all four the module were around - 5dBc/6 W. Thus I cannot confirm the -25dBc found on the Minikits page. Of cause a low pass filter can attenuate the second harmonic to the required level. However, it does not remove the electricity bill of generating 6 W RF on 100 MHz which may be an issue for a beacon - at least here in Denmark where 1 kWh costs ~0,5 AUD.

EI0SIX uses a RA30H0608M but with some hefty filtering of the second harmonic. Please ask Tim, EI4GNB, for more details.

The harmonics generated by a stage is not necessarily related to the harmonics at the input to the stage. An example is the 6 m 25 W PA by Steen, OZ5N, mentioned above. This PA can be driven with 13 dBm on 50 MHz and 13 dBm on 100 MHz simultaneously, and it will produce the very same second harmonic on the output as if there was no second harmonic present on the input. So it all depends on the specific PA and how wide-band it is.

Bo