I had mentioned already that I replaced my spoke (not spook!) antenna for 2m with a weird contraption for 21MHz - which now even yielded DX in SSB. To get back onto VHF/UHF again, I needed a diplexer.
If you never opened one: Basically you have two Butterworth Tee filters, a high pass for VHF etc and a low pass for HF. Originally I tried two fifth-order filters, but the enclosure was probably a bit too small.
So I fell back to third-order filters and changed a few more things - and the diplexer seems to work fine. I used a relatively high crossover frequency of about 80MHz, so components would be smaller. (Just use an on-line calculator for this.) Originally I wound the inductors onto bamboo chopsticks, but later switched to a 6mm drill bit, to make them shorter. The capacitors in the high pass actually consist of a varicap with a 16pf ceramic capacitor in parallel. You can not see the 16pf caps, they are beneath the varicaps.
HF did not need any adjustments, on VHF one only needs to adjust the varicaps for best SWR. It helps btw to have an LC meter to measure the coils' true inductance after soldering them into place. As you can see, I did not use any PCB. What you can btw also see here is my RF screwdriver - I could not find the set I did not use for about ten years, but I also did not want to go out and try to buy a new set, so I made my own - again, with a bamboo chopstick. And if you wonder how I got the holes into the enclosure: Step drill...
Crossover attenuation seems to be close to 30dB. Nothing sensational, but considering that 100W HF only result in a little more than 100mW on the VHF port, I think this is acceptable. Now I only hope the component dealers around here have restocked on RG213 (or 8D here in Asia) PL connectors. I already plundered two of them, and last time I checked both were still out of connectors. I will need a few to get diplexers and antennas set up.
So what is next? Perhaps reviving my old LDG ATU? (CPU is dead, but the rest should still be working...)
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