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Saturday, 6 April 2019

70cm DATV - Portsdown again!

Well,

You may remember a while ago I built a Portsdown 2019, well today I've been thinking about how to get that on the air on 70cm (437MHz).

John, G4BAO does a nice driver amp kit for this frequency, and I have built one here today:


I've built and tested it as per his instructions, and then built a fairly simple circuit to switch the bias line which needs +12V on PTT. The schematic below is built on the veroboard you can see in the image above and takes a ground on TX line from the Portsdown and uses that to switch the VCC to the amp, turn on an LED and also provide a ground on TX line out (to go to the PA perhaps).


I've done some very initial tests and the spectrum analyser image below is created with a 30dB attenuator in line:


Without the driver amp in line, the output was measured at -29dBm (so about +1dBm without the attenuator) and then -6.59dBm with the driver amp in line. I make that a gain of about 23dB which seems bang on the money.


The driver amp is now completed in the box you see atop the Portsdown.

Good, egh?

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