Last month I bought a "surprise box" of surplus (radio)electronics. Well, the content was visually known and the box contained at least one wanted item. In between the heavy remaining stuff there were two VHF filters: one centered at 162 MHz and the second at 137 MHz.
At the time being I have no real uses for such a receive filter, but it is a very useful support to experiment with the NanoVNA, and use both of it ports. Educational!
After a little soldering and a bunch of coax adapters the setup looked like this:
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| Test setup and wiring. |
I intended to retune the filter into the 2m HAM band. At the first glance the NanoVNA showed it is an elliptic bandpass filter. With a mix of awe and patience, adjusting the 4 screws following some fuzzy algorithm the bandbass centre moved down to 144 MHz:
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| Response at the desired frequency. |
I am sure there must be a scientific way to tune these filters. I settled for: the two inner capacitors set the frequency, while the outer two match the impedance. With the obvious slight cross-interaction.
I stopped when I reached a 1.2:1 SWR at 144.300 MHz together with the least attenuation. The diagram matches the theory of elliptic filters. I am unsure of the best insertion loss of -4.06 dB (including the cable and coax adapters) as the label said -3 dB at 162 MHz. But I've got another one to retune and I will pay more attention.
By the way. These filters should be symmetrical, so the pattern should look the same if input and output are swapped.
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| View of 30 MHz span. A pretty elliptic response. |
Which use for a receive VHF bandpass filter about 10 MHz wide in 2021? It should help to reduce out-of-band noise when looking for weak signals, but mind the 3-4 dB insertion loss. I will try it in front of an RTL-SDR and a wideband scanner to see/hear if it makes a difference.
