Solent Club for Amateur Radio & Television

Thursday, February 23, 2012

Last Updated on Friday, 07 October 2011 18:03 Written by M6PJD

A cheap two band digital driver:

The objective in my new portable rig was to build a linear drive train which would be compatible with the Comtech 23cm exciter and also be capable of directly driving a 70cm RA30H4047M1 30 Watt PA. I chose the Mini-Kits' Gali-84 because of its performance and this has proved to be a good choice as I have oodles of drive in hand.

I decided to experiment by driving the 2 filters in parallel (therefore without any need for switching) using series tuned circuits, this idea depends on the small amount of capacitance of the non-resonant filter not unduly affecting the resonant one - and by Jingo it worked!

Another advantage of this method is that the series tuned circuit of the band not in use bypasses any ‘sproggies’ on that band.

The SR exciter delivers up to 1.7mW on 23cms (I believe the DigiLite with two MAR6s delivers a similar amount, so this too could be used) - I run the SR at around the half Gain setting to reduce potential distortion.

The Gali-84 has about 25dB gain on 70 and 23dB on 23cms with over 21dBm output, so not only is a pair of these adequate to drive an RA18 23cm brick but one Gali-84 delivers enough to drive an RA30 70cm brick to its maximum linear power of 14 Watts, I therefore built the following:

“Portable” A/D Transmitter Block Diagram'

This is my grotty looking but effective ‘Junk Box Special’

built on a Comtech Tx-sized piece of PCB to fit my card rack – Total cost was $5.85 AUD (£3.70) plus postage for the Gali-84, the remainder came from my junk boxes (the sub-PCB was used because of laziness)!

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