Monday, 15 February 2016

Three cavity notch RX filter for 2 m with 1.6 MHz RX/TX spacing

Three cavity notch RX filter for 2 m repeater with 1.6 MHz RX/TX spacing

Introduction

A three cavity notch filter was constructed and tested. The RX filter has over 80 dB of TX rejection.

Construction

The club has a 2 m repeater that is to operate on the new 1.6 MHz RX/TX spacing, as opposed to the usual 600 kHz spacing. The existing high/low pass reject cavities could not accommodate the wider spacing, limited to about a 1.1 MHz spacing.

The club has five 1968 band pass filters, old but well made. A pair of them had been used with a phasing harness to act as notch filters, but the rejection was not high enough for TX reject on RX. Two are probably adequate for the TX filter to reject spurious noise at the RX frequency.

I decided to try and make a three cavity notch RX filter, necessitating a new phasing harness. Each of the cables are a quarter wavelength, with those going to the cavities a little shorter to allow for the probe inside the cavity. I used LMR-400 ultraflex for the double screening, plus I had some at hand. RG-214 is preferred but it is very expensive (~$25 per m).

My first attempt was to use soldered tee joins, but I had trouble with shorts. I think the aluminium Mylar film melts when soldering the braid. I changed to N connectors and tee joiners. That was worked.


Results

The RX filter gives over 80 dB TX reject, which is adequate.



I also measured the SWR to see where the minimum was, expecting it to be at the TX frequency, however it is quite some distance away. Previously I had thought that the RX and TX filters would need separate RX and TX antenna, as I thought the RX notch would be a short-circuit for the TX, but the SWR measurement suggests otherwise. I need to investigate this further.


Conclusion

An effective repeater RX notch filter can be made from three band pass cavities and a phasing harness. Further work is needed to see if a common antenna can be used.


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