[TowerTalk] antenna FS measurements

[Jim Lux]

> The LPL technique, of holding S meter constant and varying TX power, will
> eliminate that.
> So would turning off the AGC and digitizing the audio.
>
> If anyone finds a program to support the latter, I'd like to know about
it.


There are a number of the spectrogram programs that would allow you to
accurately measure the amplitude of a signal coming in from the sound card.
I don't know that they will log it to a file (which would be nice). Spectrum
Lab from DL4YHF (Wolfgang Büscher) has a function in its scripting language
which will return the average power in a frequency range, which can then be
logged.

There are some (free) programs that do simple audio capture and filtering
(SOX does captures, for instance) and there's probably a utility that
returns the average power in the digitized audio file.

You'd still have to deal with the AGC issue though.  Your basic ham receiver
just isn't designed for measurements.

However, here's an idea.  Go get a cheap shortwave receiver (should be
<$30-40) for which the schematic is available, and then cut the necessary
traces to disable the AGC.  Run the audio output to the sound card.  You
won't have absolute calibration, but you can probably get fairly good
relative calibration (because it is mostly determined by the sound card, in
this case). Unfortunately, the cheap SW receivers are AM, so there's a
slight problem there (no BFO), but its possible that something like a volt
meter on the speaker might work well.  For autologging, get one of those $25
DATAQ demo modules that hooks up to the serial port that 12 bit digitizes
voltages and writes them into an Excel spreadsheet.


I'll also note that the PCR1000 does have a fairly well calibrated "S-meter"
(that is, it returns the received signal strength in dBm), however, it would
be in the entire CW filter bandwidth.  The calibration is quite good (better
than 1dB) over a pretty wide dynamic range. I don't know about the adjacent
channel rejection, though, I've only casually looked at it by running a lab
signal generator into the antenna port through precision calibrated
attenuators.

Perhaps some sort of automated PSK31 capture/log program? The detection
bandwidth for psk31 is fairly narrow.