Friday, 31 October 2014

144MHz masthead preamp

Most of today was spent building a144MHz masthead preamplifier. I have not used a masthead preamp on 144MHz previously, but for MS and especially EME, a preamp is essential and I really wanted to see how much difference it would make.

The active preamp stage uses a PGA 103+ PHEMT on a WA5VJB PCB. 

G4SWX and I have previously developed a 'better' 130MHz High Pass Filter that has a deep notch in Band2 to suppress FM strong signal effects and a 150MHz Low Pass Filter at the output to suppress high frequency signals reaching the following receiver.
A preamplifier in a box is of little use without a method of powering it at masthead and switching the antenna between transmit and receive.
I chose ( eventually) to use one of the larger Schubert ABS enclosures. These are now available from G3NYK in the UK.
I use a Relcomm SDL 12V coax relay with three N connectors. The relay is rated to 1500W at 150MHz
Power for the preamp and relay is fed over the receive coax and a homebrew bias tee. The bias tee can be seen on the right in this photo. The preamp is in the tinplate box In the centre.

Following my usual masthead practice, the relay is mounted with the three N connectors through the box lid.
N connectors are used to connect to the preamplifier input and output. A short low loss coax cable connects the relay normally open port to the preamplifier input.

The numbers?
Noise figure 0.70dB
Gain 23dB
These numbers are with the relay and two short lengths of coax in front of the 0.5dB preamp stage.


Sunday, 26 October 2014

144MHz transverter and the lunar probe 4M

I have been busy rewiring the 144MHz transverter module into a previously used Vero LBX case. The temporary version I was using hadn't got the frequency locking or PA wired in.
I've spent several days re-wiring it so that I had these additional facilities available when required.

The basis of the transverter is the Anglian with the PA board using an RA08H1317M PA module and my ZLPLL 116MHz LO that can be locked to my 10MHz shack standard.
Now that I've finished the wiring I want to connect it back into the 144MHz antenna and see how it performs compared to the K3 internal 144MHz transverter on the same antenna.

I have been monitoring the 4M lunar probe on 145.980MHz as it makes its way towards the moon and lunar fly-by. Currently the probe is about 31 hours from fly-by (which will happen below our horizon) and now almost as far away as the moon. 

As my 2m yagi cannot be elevated I am stuck with monitoring when the spacecraft is within the main lobe or first sidelobe of my antenna. Even so, for several hours before AOS and LOS the spacecraft can be copied successfully and the JT65B modulation decoded much of the time.
My last decode was about 14:00Z today. It should become copiable again sometime after about 17:30 today. 
By then I should have the new transverter plumbed in and able to try it out on a VERY weak signal!

Thursday, 23 October 2014

Wrong again!

Oh dear, I do seem to get myself into some awkward situations.
In all innocence I earlier tweeted a picture of a commercial video coder/modulator. I was told in good faith that it should be suitable for use within the new 146-147MHz ham band allocation, when frequency translated from its current 70MHz. I should have followed my inititial instincts that it wouldn't be suitable when using QPSK modulation, even at rates significantly below 1Msymbols/sec. 

It seems I innocently walked into an ongoing debate between some parts of BATC and the RSGB. 

It can be done, but it seems that what was being proposed would be unsuitable in view of the difficulties of achieving a suitable SPD at the band edges and consequent potential interference to safety of life services operating above 147MHz and our fellow satellite enthusiast just below 146MHz.

Whilst not wishing to discourage anyone from using DATV in the new band ( or indeed 10 or 6m) it looks like more effort needs to be put into solving the spectrum occupancy issues. This is what radio amateurs are good at.

It seems to me that the current wish to use a number of free ex-commercial coder/modulators may be clouding the real issues. 

It should be added that I am referring to fast scan digital TV. Slow scan variants look fine but may not offer anything new or innovative and hence not enhance our case to keep 146-147MHz in the future, or indeed give us a bit more spectrum in this region.

I hope these issues can be overcome and a real solution found.

Finally, these are my personal views and do not represent those of any organisation.


Sunday, 19 October 2014

144MHz transverter construction

Now that I have finished my 'run' of talks on the Anglian, I have stripped out my temporary homebrew 144MHz transverter unit, ready to add in the PA and the 116MHz GPS locked LO.
Both these units were used as demo pieces for my talks.
I have mounted a small (40mm) diameter fan to the rear panel so as to blow or draw some cooling air across the PA components as the PA will be mounted to one of the case side walls. There will be no external heat sink on the case. At 4W PEP out and class AB operation, the aluminium case wall should be sufficient to dissipate the heat of the PA with quite acceptable temperature rise inside the case.
I will post a picture of the finished transverter on here later this week ( optimistic?).


Saturday, 18 October 2014

More on the Anglian

I have almost cleared the backlog of orders for the Anglian 144MHz transverter, thanks to sterling efforts by G7OCD to get the boards assembled.
If anyone is interested in one of the transverter kits, featured in DUBUS 3/14 and covered in my RSGB Convention talk, I am now able to take more orders.
The cost of the transverter has deliberately been kept low, and assembled boards offered, in order to allow more amateurs to build their own high performance 144MHz transverter, rather than relying on commercial 144MHz black boxes with their (usually) indifferent performance!


Thursday, 2 October 2014

144MHz set up

I have now added a Gemini 2 to my 144MHz set up. It is currently being driven by my K3 and internal K144VX transverter.
I will be migrating the 144MHz over to my Anglian transverter after the RSGB Convention. I don't have a spare Anglian 8W PA whilst my demo one is slated for show at my Anglian talk!
The Gemini gives close to 300W saturated output for 5W input. 
It is small and convenient and sits well on the operating bench, next to the K3.