Thursday, February 2, 2012

Propeller WSPR Beacon Power Amp Results

This morning I finished up my beacon power amp by putting in the low pass filter.  I had a bit of a struggle with that however, as I had no capacitors that would work for the 5 pole filter in hand.  I ended up with some rather significant changes in values in my substituted capacitors.  The filter modeling still showed that I would barely meet a 40 dB supression of the harmonics, so some additional work is needed here before I can call this complete.

The final breadboard looks like this:

The Propeller Beacon with it's outboard low pass filter is putting in a nice clean signal to the input.

Checking the output with the spectrum analyzer, I can see that the harmonics are 40 dB down or better (but just barely).

So, hooking this up to the matched antenna and measuring the power output, I got found that I was putting out a solid 2 watts.

This is pushing things quite a bit and the transistor finals were getting quite warm.  I backed off the bias on the driver until the power dropped to about 500-600 mW.

It seems happy at this level and the entire amplifier draws about 160 mA at this setting at 12 V.

So, we let a WSPR transmission happen on the next even minute and were immediately rewarded:

So, I am quite pleased with the initial results.  I have (subsequently) changed my power setting reported to correctly reflect my new power level.  More work to be done to clean up the harmonic supression to have some more margin.  I am seeing some fluctuation in SWR that appears to be related to the effects of harmonics, so more work to do, but for now we are on the air at around 27 dBm.

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