70MHz QRO amplifier

For the UKAS contests in 2015-16, I was using a very old A200 amplifier which would only produce around 35W, ok for the AR section and absolutely fine running off the car battery.  However, once I had bought my generator, I was able to pull an unfinished project out of the cupboard – the SSPA.  I had bought this module from Hellenic RF Lab in Greece a few years earlier and put together the metalwork but just hadn’t got round to powering up.

The module is supplied fitted with an MRF6VP11 device, quite capable of the legal limit.  At full steam, the efficiency is around 80% - so 200W of heat to be dissipated.  I used the Marston Ultra-Fin heatsink fitted with the recommended Papst 3312 fan – this combination should have a thermal resistance of 0.08C/W implying a temperature rise of only 16C.  I have used four long screws through the heatsink to hold the fan in place.  Annoyingly, the fins of the heatsink flex when drilled so it looks a little untidy under the fan.  Don’t forget the finger guard on top of the fan.  The fan doesn’t seem to create a massive airflow (compared with fans used to blow a 4CX250) but all seems to work ok.

View inside amp

The output connector was screwed directly to the heatsink / groundplane and with the high RF currents, it seemed like a good idea to maintain this.    I’ve used a Perancea CFL3 case which has a fixed base and removable lid.  I made a cut-out in the bottom of the case so the heat spreader sits flush onto the heatsink.  Cutting out this rectangle was a bit painful with my limited tools (hacksaw and flat file) and I plan to use the version of the box with two lids for my next amplifier.   The output N connector is screwed directly to the end wall, then the heat spreader is screwed down to the heatsink.  When supplied, the input was also fitted with an N-connector which I removed and replaced with a short length of RG174 and a BNC connector for the input feed.

DC at 50V (and 20A) is fed in via a pair of chunky feed through capacitors and I’ve used phono connectors for the bias volts and fan 12V supplies.  The fan supply is fed back out on an SMC connector, it was easier to use a small screw-on connector to prevent the possibility of the cable loom pulling the wires off the fan.  I’ll write up the details of the main PSU in another blog entry.

The module arrived from Greece with the bias being fed by a power resistor from the 50V rail but I wanted to separate this out so I could turn the amplifier on and off using the PTT line.  I installed a little jumper link so I can select 50V or PTT.  The gate needs to be adjustable between 0V and 3V to set the standing current and there is a multi-turn potentiometer into the gate feed and this also allows the bias to be turned to zero for initial testing.

Testing

I was fortunate to be able to use work’s 50V, 20A, current limited power supply so I could do initial testing on the amp at a protected 2A and then turn everything up carefully.  The transistor data sheet indicates it should be biased to a few hundred mA and with less than a Watt of drive should give lots of power and indeed it did.  However, the power output started to fall off after a few seconds.  I realised the heat spreader was nice and warm but the heatsink was cold.  Although the spreader and heatsink were screwed together they were not connected thermally.  A quick layer of heatsink compound soon sorted that out and the amp is now temperature stable.  I also found that turning the standing current up to around 1A improved the gain significantly and probably did wonders for the linearity.

So after a quick tidy up and covers on, the amp was ready for use

Completed amplifier

Back again, time to catch up

March 2017   A little aside - back in May 2016 there was a technical article which I thought was rather poor with a 'construction' which I thought was a complete waste of time.  So much so, that I wrote to the editor and told him so. The editor said they were always looking for technical / practical articles so I offered an article based on my UKAC exploits.  I wrote the words, got up to speed with Express.sch for the circuit diagrams and took a few new pictures.  I submitted in November and to avoid any copyright issues, I haven't put anything on this blog since.

Three email reminders sent later, nothing from RadCom.  No acceptance, no decline, nothing.


So off we go again.







This is the normal operating set up for G4HGT/P with the KX3 sat on the control box, sat on the 144MHz transverter, all on the front seat of the car.

The transverter was built in the late 1980's with a BF981 front end.  Inside the pretty case is a good old diecast box providing a solid screen.  One day I really must take it into work and make some measurements, then decide if I need to bring it into the 21st century.  For now, the covers are staying firmly in place.
















Inside the control box are 5 functional blocks:  12V regulators, RF switch, 28V inverter, sequencer and audio amps

The KX3 is a pretty expensive box so getting the polarity of the DC feed wrong and when rushing to get the station on-air could be disastrous.  So I've used a low-voltage dropout regulator to add a little protection.  The second regulator feeds the audio amplifiers.  This can also be switched off by the sequencer if I ever get RF breakthrough

RF switch - I need a sinple way to split the I/O of the KX3 into separate TX and RX lines. This box uses a G4JNT design pin-diode switch / attenuator from 1994.  The RF output from the KX3 at 28MHz is turned down to the minimum setting, roughly 100mW, then the potentiometer in the switch box sets the drive level into my transverters at around -10dBm

28V inverter - I have a 28V RF relay so i needed an inverter.  This little module was bought off eBay for less than a tenner - no point in trying to design my own.

Audio amps - probably a little overkill at the start, I could have just plugged my headphones into the KX3 but the audio out seemed a little low and with little LM380 amps available on eBay for pennies, I now have plenty of audio plus a combined audio if I ever need to peak the beam when outside the car

Sequencer - antenna relay switch, transverter switch, power amp switch and +12V feed for a preamp.  A stripped down version of a W1GHZ design

22 June:  So a week after writing my last entry, Giles Read, the RadCom technical editor, got back in touch and the article started moving in the right direction again.  It is now in for final tidy up and should be published in a couple of months.