70cm 250W Amplifier from secondhand DVB-T PA Part One

When I last posted in June I mentioned I had bought a secondhand DVB-T PA from a German eBay seller well I've finally got around to putting it all in a box and getting it up and running.

The PA module is simple to modify and in fact I found during an internet search a website by a Dutch amateur PA0EHG who had already modified one of these amplifiers and I followed his instructions which worked perfectly.

The PA is coupled with a LPF again from eBay which looks like it's a based on a design by Jim W6PQL the specifications where 2nd order harmonics -35dB and 3rd order -55dB with an insertion loss less than 0.15dB.

When measured with the VNA I found the 3rd order harmonic weren't rejected as much as specified so I expect it might need another coaxial stub filter to drop the 3rd order harmonic a little more.

70cm eBay LPF filter response.

I put the LPF in a tin box bought from Alan G3NYK though I had to buy a bigger box than was required and trim it down to size using tin snips.

To monitor the amplifier I was going to roll my own controller board but again Jim W6PQL has what he calls the ultimate amplifier controller board which at $50 for a kit I thought was a bit of bargain and a lot less hassle than making my own even if half the features of the board aren't being used.

I stuck all this in a rather large Hammond box so its ended up not being a small amplifier but you go with what you have laying around the shack.

I painted the case in acrylic paint and though the result was good looking I don't hold out much hope for it's long term durability.

Front Panel

Reat Panel

Top View

Internals

The amp requires a quite hefty PSU 31V 17A but more on that in later post.

First EME QSO.

I've just spent a couple of days listening to the moon and getting nowhere using WSJT-X.

When I use WSJT-X with the K3s I have a hell of time trying to the get the RX levels right with the K3s LIN OUT set to 1 the PC Microphone level is set at 0.4dB the RX level indicator in WSJTX is still at 60dB and I haven't a clue why, anyway the upshot is I only had a couple of decodes.

Anyway I switched to MSHV and low and behold multi decodes within 60 minutes of using it, yippie!

So with half an hour before bed I noticed K9MRI in EN70IU and thought what the heck I'd give it ago not expecting much with such a small antenna array and low and behold we completed.

I'm now off to party it's been a hard slog getting that first QSO via EME and I'm ecstatic or should that be over the moon?

First EME contact with K9MRI.

Another change of the 2m antenna system.

Though the 2 x 9 Element Powabeams I put up last year worked well I really wished I'd installed a system that had elevation and so I've now replaced the Powabeams with a small array 4 x 6 Element LFA from Innovantennas.

Justin also provided the H frame as well as a new boom for the 70cm 12 element LFA I had so it could be rear mounted in the centre of the frame.

While I was buying the antennas I also bought a SPID RAS rotator with Rot2Prog controller but more about that in a later post.

The new array took around five days to put but mainly down to the fact I hadn't ever installed this type of system and so it took a lot longer than it would if I was to get it all done again.   The main time consumer is in the fact I don't have a tilt over mast and so everything has to put up with the aid of an Aluminium tower.   

The array tuned up quite well with each beam within a half a dB of RL.

Top Left      Z 50.6, X 3.7, SWR 1.07, RL 28.6

Top Right     Z 52.6, X 3.2, SWR 1.08, RL 28.1

Bottom Left   Z 53.0, X 2.8, SWR 1.08, RL 28.1

Bottom Right  Z 51.9, X 3.6, SWR 1.08, RL 28.2 

The whole array including coupler and 16m of Ultraflex 10 gave an SWR of 1.1:1

Justin kindly send me a theoretical  plot of the array at 8m high with a spacing of 2.2m horizontal and 2m vertical separation.

Theoretical Azimuth Plot.

You can see from the plot the gain is quoted as 17.36dBi with a F/B of 23dB and beamwidth of 24 degrees.  

I took my own plot using the GB3VHF beacon which is near to me and was about to plot the results out by hand when I did a search for a blank azimuth polar plot and instead came across a great piece of software by Bob Freeth G4HFQ called PolarPlot  this small piece of software saved me a good amount of plotting everything myself and I highly recommended it.

I measured the gain as being 18.48dBi with a F/B of 22dB and a beamwidth of 24 degrees so in good agreement with Justins theoretical calculations.

Azimuth plot as actually seen.

The rear lobe are a bit weird but then there isn't much space between the antennas and surrounding builds, hills, slopes, overhead wires but even so it looks very good.