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Showing posts from February, 2016

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

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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 dou…

Duplexer isolation: limits of instrumentation

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Duplexer isolation: limits of instrumentation Introduction With high isolation duplexers, about 100 dB, conventional spectrum analysers and tracking generators are beyond their capability of about 80 dB. The rejection of a high isolation duplexer can be checked using a RF signal generator and spectrum analyser.
High isolation duplexers. I have a couple of sets of four cavity 70 cm pass reject duplexers. The two cavities give about 70 dB rejection, which is ok but more would be better for RX. A third cavity can be added for greater isolation.

Each cavity is individually tuned with a spectrum analyser and tracking generator. The two cavity duplexer can then be assembled and tested. A nice plot with about 70 dB rejection at 438.1 MHz and a pass of 433.1 MHz.




Adding the third cavity results in noise at the rejection notch. The spectrum analyser cannot show the full depth of the notch; it does not have enough dynamic range.



RF generator and spectrum analyser A RF generator can be used ins…