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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.

Edit

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://imgur.com/a/og9AsII

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.

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  • $\begingroup$ Hm, how do you do your RX/TX switching? technically, your bandstop filter doesn't have to part of your TX signal chain. $\endgroup$ – Marcus Müller Jan 13 at 17:32
  • $\begingroup$ The band-stop filter uses low voltage SMD components, it'll burn if I use it for transmission. $\endgroup$ – Aldoir Jan 13 at 17:44
  • $\begingroup$ exactly, so keep it out of your TX chain! $\endgroup$ – Marcus Müller Jan 13 at 17:45
  • $\begingroup$ The point is: My helical filter is great for TX/RX on 140-150Mhz, but it deeply attenuate Airband, so I have to switch to this bandstop to listen to airband. A wideband helical filter would solve my question, but it's not possible I believe. $\endgroup$ – Aldoir Jan 13 at 17:48
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    $\begingroup$ Not something I'm proud of, but it works :) imgur.com/a/0YKwsqN $\endgroup$ – Aldoir Jan 13 at 19:06
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The basic idea is a high-pass filter to separate FM broadcast from Aircraft band. You need quite a wide passband, including aircraft, up to top of 2M ham band. So start with a 5-element high pass of the form:

schematic

simulate this circuit – Schematic created using CircuitLab
Then add some traps to take out more of the FM broadcast band, without impacting the passband. Somewhat Chebyshev-like. If you've got some tools like VNA, it can be hand-tweaked. I did the tweaking in LTSPICE, but probably gave up too soon: Chebyshev stopband voltage across R3 (output)

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  • $\begingroup$ Thanks so much for your time @glen_geek. I made my mind for a elliptic high pass + low pass and matched my expectations. I updated the OP to show the results. $\endgroup$ – Aldoir Feb 23 at 23:12
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In my opinion either a ¼ λ ground plane antenna or a ½ λ J-pole antenna, cut for the centre of the Airband, would be a better choice for reception of ground stations.

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  • $\begingroup$ Welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Mar 16 at 13:58
  • $\begingroup$ Thank you very much, rclocher3! $\endgroup$ – vu2nan Mar 16 at 16:39

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