[hfsig] Review of the QRPi TX Shield for Raspberry Pi

[John C. Peterson KD6EKQ]

Greetings all WSPR enthusiasts,

I got my QRPi board about a week ago, and it's a very nice piece of work.
Hopefully the demand will improve so that another production run will
happen.

I have experimented with James Peroulas' WsprryPi transmitter software
for the Raspberry Pi for almost 2 years. I discovered right away just
how much out of band noise that PWM synthesis can create, even with only
10 mW of RF output. Running a barefoot RPi without any filters and just
a stubby (well below resonant) length of wire for an antenna, my OTA
broadcast TV reception was totally trashed, and my desktop computer
even froze a couple times with really strange errors I had never seen
before. Yikes! They weren't kidding when they said filters were required.

Transmitting at frequencies outside of the 20m design band (at low power)
and checking radiated power with a portable SWL receiver suggests the
filters on the QRPi board are doing a great job (and none of the RFI
problems I described above). The advertised higher RF power output is
apparent as well as I seem to be getting more spots with everything
else equal.

On a related note, I'm using an MFJ-971 QRP tuner for matching into
a dipole. The built-in F/B wattmeter is not sensitive enough to the
very low power levels, so I used a portable SWL receiver to crudely
measure radiated RF power and tuned around to maximize that.

To summarize, the QRPi shield is a great value. Together with a Raspberry
Pi, it provides a very low cost entry point to transmitting WSPR on the HF
bands.

-- 
John C. Peterson, KD6EKQ
mailto:jcp at eskimo.com
San Diego, CA U.S.A