Working on 4M capability

[radiogareth]

After extracting my 4 element Jaybeam for 4M from garage loft and treating it to decent coax, new end-caps and a bit of grease on the bolts I moved on the the actual TX capability. Using my Portsdown, checked output on my analyser and looking good. Thought I'd try it across the shack before building the 4M brick amp. Using a Spectrum comms upconverter carefully adjusted to give 0.2uV on SSB SDR. Into my MT, settings checked but its very reluctant to lock and give a picture. Tried it on 2M for a reality check, almost instant lock.

Yet to try my Lime SDR in place of the Portsdown output, but wondering if anyone else has this 'issue'?

I'm planning on building a RF switch block/box using some NOS RS Components coax relays that I've collected over the years, with the Portsdown switching board controlling them and the PA power supply. Is there any way to provide some additional protection for the MT input, or am I being paranoid? Dead NIMs are not much use.....

Thanks

Gareth

[G4FRE]

Have you tried looking at the output of the spectrum upconverter on your analyzer? MT does not appreciate having lots of signals present at its input (eg the local oscillator.) I had to use a bandpass filter at the IF frequency of my microwave modules upconverter before MT was happy. I now use a 71/437MHz upconverter (and a 51/437MHz upconverter) and had to fit 437Mhz helical filters before MT was happy

Dave
G4FRE

[radiogareth]

Next step then, thanks for the pointer. There is a bit of filtering (two caps and an inductor) and its all a bit 'close' to the desired.
Gareth

[g8vpg]

Bad choice of upconverter. Two of us down here made the same mistake. The second harmonic of the LO is only 4MHz away from the desired output. Minitioune really doesn't like this sort of thing. It can be made to work with tight filtering and traps. The Pye Europa front end filter blocks are very good if you can find them. However I switched to a mixer with 437MHz LO and this is much better.
73 Shaun.

[radiogareth]

OK, that makes further sense as I could get lock, but only with a huge signal into the converter. I wonder is a coax 1/4 wave stub would be good enough/far enough away to kill it. Failing that, anyone got a link to a good 4 to 70 converter kit?
Fire up the analyser time
Gareth

[M5TXJ]

Hi folks.

I've done a fair bit on 4m as my mentor for getting through my RAE 40 years ago was Bill G3JYP (now SK and sadly missed) one of the 4m stalwarts. To get on the band I built a Cray which was state of the art at the time whereas my mate Seamus G7ITT bought a Spectrum transverter. Both Seamus and myself live about the same distance from Bills QTH, roughly 5 miles. Bill used to run full 160W to a 7 element Quagi and when beaming at each other inadvertently during contests I could easily work 5KHz away from Bill with no issue, for Seamus whichever way Bill was beaming the whole band was wiped out. It didn't take long for Seamus to dump the Spectrum.

73 Dave.

[radiogareth]

I wonder if anyone has thought of using the Lime Mini/LimeUSB as an up-converter? The Pluto can't RX &TX at the same time, but the Lime can.
Software to do it is beyond me, not least it will probably need Linux....
I.e 71MHz in 437MHz out for instance....
SHort term I'm impressed by the performance of the RTL dongle and LeanDVB in the Portsdown, but I understand its several dB down on what a correctly fed MT could decode.

Gareth

[g0mjw]

Yes - I did this some time ago using Gnuradio. It works but you need to get the levels right. A very simple flowgraph will suffice.

Mike

[radiogareth]

Interesting. I wonder if it would run on a RPi Zero?
Mind you, integrating it with 20 watts of RF (**feeder losses**) is going to need some care....
Gareth

[radiogareth]

Managed my/our first DATV contact yesterday on 4M with Martin, G4FKK. Using the Portsdown LeanDVB RX and 250KS we managed two way P5
As an experiment I asked Martin if he could try his Pluto direct to aerial as he was already using it to monitor his own transmission. Running with SDRAngel he reported almost instant lock and we also managed 500KS pictures. Without any filtering (other than a 5 element beam) or preamps I am impressed by this. Certainly worth exploring as a suitable RX for 4M as it saves messing with upconverters. There's even a how-to video on setting up SDRAngel on youtube here: https://www.youtube.com/watch?v=Bb8S_J9peD8

xat1.jpg (284.97 KiB) Viewed 3200 times

Thanks for the software

LeanDVB.PNG (640.44 KiB) Viewed 3194 times

Gareth.