2008-10-19
VHF Wavemeter
Very similar to the HF version, just designed around a hairpin of 4 mm copper wire instead of a solenoidal coil. The ~60 pF gang of the polyvaricon is used alone to resonate the hairpin. An additional 120 pF of capacitance can be switched in parallel to implement the lower band.
Construction is upon a square slab of rather thick PCB material that Kevin VK2ZKB was kind enough to give me for my experiments. Also from Kevin came the smaller pieces of PCB that the hairpin is soldered to. He has tooling to make short-work of cutting up PCB, a task for which I keep promising myself that I'll purchase some better equipment for at some point. He and I have been discussing cutting thin strips of PCB the right geometry to implement strip lines for higher frequency work. It took a lot of heat to solder that thick chunk of copper to the PCB, challenging the super-glue used to attach the pads to the main board.
The circuit is otherwise identical to the HF instrument, power supply is the same 3V lithium cell inside the black plastic box which also houses the meter. There is no off-switch or any means provided to turn it off. It tunes 49 - 57 MHz for 6 metre coverage, and 76 - beyond 150 MHz for FM broadcast band and 2 metre coverage.
I use it mainly as a selective signal strength meter looking for stray currents in antennas. It isn't a very useful instrument in terms of absolute frequency resolution, but relative RF power and frequency ball-parking it works fine for.
I successfully made a coil for the HF meter which covers a similar range but with somewhat less sensitivity. Here is a video of me demonstrating mutual inductance coupling between tuned circuits at 2 metres using the older wavemeter.
I must admit I kinda like the HF unit better, it is a neater package and is more flexible, but this fixed-coil wavemeter covers the FM broadcast band and 6 metres with better bandspread. It would be excellent for people making FM wireless microphones (bugs) for both working out their rough TX frequency and tuning them up for best output. I wish I had made one this sensitive back in the days when I was an FM broadcast pirate. I did build a VHF wavemeter long ago, but it was a passive one and needed a fair signal for a reasonable deflection, I also then lacked the instruments to calibrate it properly.
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