I was thinking the user could just screw on coaxial low pass filters.
For instance, the VLF-xxxx line from Minicircuits are available from 80
MHz to 6 GHz, at about $22 each. An output filter bank would be doable,
but it would probably be 0.4 dB insertion loss per switch as S band, so
it adds up pretty quick. Peregrine makes some SOI switches which are
essentially DC coupled and have P1dB=40 dBm, so that may be an option,
along with relays.
In band filtering may be too specific, and get costly as frequency
Ha ha, yes, burning things out would be a problem. Can’t protect from
some user error. Even TX/RX monopoles with one meter separation may
blow that 2nd stage amp on the WBX card. As far as the amplifier
itself, these 48V GaN parts have 160V breakdown, so running them at ~30
VDC rails should make them pretty robust. Allot of GaAs parts breakdown
at only 2.5 times the rail. Conversing with a guy from Nitronex, he
said 37 dBm with a 30 VDC rail should be feasible without any matching
at the output.
Efficiency would be poor, probably at most 20% at P1dB, but that would
be the trade off for that much bandwidth. I figure true SDR (with the
computer) is already inefficient to begin with. Heat could be taken
care of with a processor cooling fan. There could be space left for the
user to add matching to shape up the drain waveforms, with user
adjustable gate bias, just limited enough so that nothing would be
cooked. Maybe instructions of where to solder parts for 2m, 70cm, ham
I have had good use with the RMS detectors from Linear Tech and Analog
Devices. Typically 0.2 dB variation over 5% bandwidth when used in an
analog ALC loop. There may have to be a LUT to flatten the detection
form VHF to S band. They should be pretty insensitive to modulation
bandwidth as long as it is a fraction of the RF bandwidth, and the video
filtering is low enough. Maybe not a true ALC, but I/O available to
feed back to the SDR board. Linear Tech now has a detector with built
in ADC and 3 wire interface. It would be rough power detection as not
much coupler directivity could be had for that bandwidth, really non at
all; just a resistive tap with some equalization to flatten the
I can see how marketing as a radio would be an issue. It would have to
be test equipment, yet, anyone can freely buy a multi-KW, multi-GHz PA.
I’m not sure how the FCC views that stuff. ITAR would be a concern to
with the GaN; could be a show stopper. Anyway, just a thought for a
project. I can key up repeaters now directly from the WBX card, but I
have a 10 dBi antenna 20 feet up. 5 watts would allow desktop
experimentation, again, given nothing is blown up.