Found this schematic on the web and decided to print it for testing "one of these days"...well it was now.
It works when tested in the shack with the HT about 2m from the receiving antenna but I didn't tested in real life condition since there's not much UHF activity around. Anyhow, don't expect a super performance but an acceptable for the complexity it shows...
The coil was wound on a plastic wall plug about 6mm diameter with 22 turns (the wire ended before the 24 one on the shcematic) just because ceramic coil formers are rare in the shack... only have one from an old antenna tuner but diamenter is 10 times bigger than neded.
Had to put an 82p cap in the emitter because the oscillator was not starting.
I had no OA5 diode so I guess the OA90 used should be similar. Probably will work with others Ge of similar type.
It should be enough for local coms monitoring... with the advantage or disavantage that due to the mixer nature (and diode capacity) you can still receive in the "normal" frequency, eg: if the receiver is at 144Mhz, you receive that (let's call it "fundamental") and 144+300 and 144+20 and 144+80.... etc etc...that's good in one way and bad in the other... life is not perfect!
Can't see well in the video well but the handy TX frequency is 436.650, the "receiver" is at 136.600 Mhz, so the "oscillator" 15th harmonic is at: 300.050 Mhz. That, makes the fundamental at: 20.0033 Mhz.... I measured 20.0029 Mhz... removing the 3 errors (receiver, transmiter and frequency counter) and it's everything as expected...on frequency!
I am happy with this experiment! Will try to improve a litle bit on the schematic.
Can't remember exactly if this was the original article but the image is similar from here: http://nandustips.blogspot.com/2011/01/uhf-on-vhf-rig_06.html