Dropping power output during TX…

[MW0XDT]

Hi Guys,

I’m very happy I waited for the G2, it’s been a learning curve, despite being a seasoned SDR user, but well worth the time spent!

However I have noticed that during transmit on SSB that my output gradually drops. Driving 40w into my linear, I’ve witnessed a 10w drop, during a 2 minute over. This equates to maybe 100w lost from my linear output, which is annoying with prevailing conditions. I’ve had the G2 for 4-5 months now and didn’t notice this issue earlier, but then I have been like a ‘rabbit caught in the headlights’ (!) so maybe I just missed it!

I wonder if anyone else has witnessed the same?…and if not, any ideas on what could be causing this?…or recommended tests to carry out?

Thanks in advance!

Rich

[GW0NTM]

Hi Rich

I see exactly the same thing with my G2. I tested it with a short tick power cable, a good PSU into a dummy load, as I wondered if it was voltage related, but it doesn't appear to be.

Best
Robin

[w-u-2-o]

This seems more like a thermal issue. While the RF power amp bias is produced by a precision, low-drop-out voltage regulator, the bias is not temperature compensated, i.e. the bias required by the RD100HHF FETs to maintain a certain gain changes with the temperature of the FET. As the FET warms up during a long transmission, output power will change. This is normal behavior for an RF power transistor.

Unfortunately there has never been an Apache design that has included a temperature compensated bias supply.

TI makes an excellent part for digital control and temperature compensation of bias voltage. Perhaps we'll see it in the next design. For now all you can do is to try to keep the temperature of the G2 as stable as you can.

[MW0XDT]

Thanks Robin & Scott, glad it’s not just my G2!

I wonder if I can add some additional air cooling to help, although the 120mm case fan is doing a fine job I’m sure. Either way it’s a relatively small issue, only highlighted as I’ve been using more power than usual due to the solar activity and have noticed that final output fading.

Thanks again guys, still loving the G2 and hoping those waiting patiently get theirs soon

73 Rich