An ‘opening’ to Sweden on 13cm prompted a re-evaluation of my 13cm transverter. Ideally I needed more power. Initially I looked at discrete devices with a view to building a PA but then I was pointed in the direction of someone who had a couple of small PAs for sale. I made them, what I considered was a reasonable offer for the smaller of the two, a 10W amp. My offer was ignored, obviously not enough. Ah well! . . . . Good job really, because a few days later, I discovered the Spectrian single-board 60W PA . . . . and within a week I had obtained one!
It has to be said that these boards are beautifully made. They appear to be constructed on a ceramic filled laminate, which is bonded directly onto a heavy copper heat-spreader which itself is gold-plated. These boards run from a single 26V supply and are only about 14% efficient . . . . But that is acceptable for a Class-A amplifier.
Spectrian 60W PA, 18dB gain
I decided straight away that the way to integrate the PA into the transverter was to adopt the same ‘back-to-back’ approach as employed in my 300W HF Linear. The Spectrian board provides a means of monitoring the individual Drain currents, and also the substrate temperature. However, although I intended to make use of the current monitor voltages from the outset, I initially opted for a mechanical thermostat to switch on any fans. The latter turned out to be an un-wise decision. The basic transverter had no problem in driving the amplifier up to a combined Drain-Current of 14A. At 14% efficiency this equated to a power output of 50W for 365W DC input, implying that 315W was thus expended as heat! The heatsink got very hot, very quickly (no fans yet).
As luck would have it, we had another opening to Scandinavia and I had my second ever 13cm QSO, this time with Carl, SM6HYG who gave me a 59+++ report . . . . So I was rather chuffed! The problem with the mechanical thermostat is that there tends to be a 15 degree hysteresis involved, meaning that if the switch closes at 35 degrees C, it will not open again until the temperature has dropped to 20 degrees C. Well, It never gets that cold in the shack! So the fans never went off once switched on. I was not comfortable with using a 60 degrees thermostat so I decided to make use of the on-board temperature output voltage.
The photograph on the right shows the final build of the amplifier with the fan control PEC taped to the front panel PEC. Note the copper sheets at either end of the PA board. I’m not sure what the preferred method for getting RF on and off the board is. Such is the heat dissipation quality of the copper spreader, that it is virtually impossible to directly solder to the ground-plane, thus I devised a method whereby the coax braid is soldered to a copper sheet which is screwed down at the appropriate point adjacent to the board. The Coax inner is then soldered to the PCB track and copper strips, 1mm thick are screwed down so that they overlap the edge of the board so as to give a solid RF grounding. This creative approach appears to work rather well too.