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://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.

  • $\begingroup$ Hm, how do you do your RX/TX switching? technically, your bandstop filter doesn't have to part of your TX signal chain. $\endgroup$ Jan 13 '20 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 '20 at 17:44
  • $\begingroup$ exactly, so keep it out of your TX chain! $\endgroup$ Jan 13 '20 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 '20 at 17:48
  • 1
    $\begingroup$ Not something I'm proud of, but it works :) imgur.com/a/0YKwsqN $\endgroup$
    – Aldoir
    Jan 13 '20 at 19:06

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:


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)

  • $\begingroup$ 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 '20 at 23:12

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.

  • $\begingroup$ Welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Mar 16 '20 at 13:58
  • $\begingroup$ Thank you very much, rclocher3! $\endgroup$
    – vu2nan
    Mar 16 '20 at 16:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.