I want to share the same antenna for VHF (TX/RX 144-148 MHz) and for Airband RX-only (116-136 MHz).
I live on a very congested area where filter is a must, so I'm looking for the best solution for my needs:
- Wide bandwidth (~30 MHz)
- Transmit capable (> 20W power handle with low return loss/SWR)
Currently I use a DIY helical filter for TX and a low-power LC filter for airband, but switching them everytime is very combersome. I like the high "Q" of the helical filter, but its bandwidth is very limited (~5 MHz) and I believe they cant be streteched to cover such bandwidth.
My primary source of interference are strong FM stations in the neighborhood, so I need a high Q, sharp filter with a good degree of attenuation between 108 and 116 MHz.
Any advice in the best filter design? My simulations shows a LC band-pass filter (Butterworth/Chebyshev) would meet my "sharpness" requirements, but they tend to have high return loss (SWR) as well, which could compromise my TX requiriments.
I have some soldering skills and a NanoVNA, tuning the helical filter wasn't a big deal.
I finally had time to build a LC filter, looks like it's still the best option to go.
First attempt was building a band-pass filter, but it showed to by tricky to tune. Then I went by a highpass + lowpass 5th-order Elliptic as calculated by https://rf-tools.com/lc-filter/. to allow individual tuning.
The results can be seen on thes imgur post: https://i.stack.imgur.com/IbhgP.jpg
The overall results are:
- -11dB on 110Mhz (very important to attenuate strong FM station on 107Mhz)
- -40dB on FM band
- 1,5dB insertion loss beteen 117-150Mhz
- SWR < 1:1,5 between 130-150Mhz
Since I had no high voltage variable capacitors in hand, I had to find the correct capacitor values using low voltage ones, then replacing by fixed value 1KV ceramic capacitor. As you might imagine that was a very time consuming exercise. The good thing is that there is no moving parts on the filter.
Sharpness on the lower end is good. On the upper side I didn't push too much the sharpness, since looks like there is a tradeoff between sharpness and return loss. I wanted good SWR and the upper band is not a big source of interference, so a left it a bit soft.
Results are very good. Identical noise compared to a commercial airband filter I had in hand, but extended to 150Mhz. Completely blank airband spectrum, no sign of FM stations spurs.
I wont post schematic or component values because this is a trial and error process. If you want to build something like this, you must have a VNA. Take the values calculated by rf-tools.com as a starting point then tweak to suit your needs.