Saturday, 5 May 2018

Cavity filters for Earth Moon Earth communications

Cavity filters for Earth Moon Earth communications- working draft


Re-inventing wheel? Been done, but not well explained or understood.

Google "Cavity filter LNA" or "cavity LNA" Good. Impedance matching AGO design detail

Cavity filter for EME

I haven't seen a cavity filter used in EME, but few know about them. It could be quite desirable to have a pass band cavity filter very close to the antenna and before the broadband low noise amplifier.

The LNAs are usually mounted at the antenna, but the loss of a couple of metres of coax (>-0.3 db?) to a cavity filter (>-0.5 db?) may be justified to give 30 db of out of band noise attenuation. This would greatly help in reducing overload of the LNA by such noise. With a filter it may/should be possible to use much more gain in the LNA. Further, two cavities could be used in series to give 60 db of noise attenuation.

QSOs are usually done when the moon is near the horizon. As such, the antenna will receive a lot of terrestrial noise. The cavity filter will reduce some of such noise.

A high Q 70cm pass band filter is a little taller than a quarter wave length, 250mm, and up to 200mm diameter to give high selectivity. The same cavity can be tuned to its third harmonic to work on 23cm with high Q and selectivity, and possibly at higher bands.

I tried a pass-reject filter as the selectivity is higher than pass-band, but the out of band dips around the pass, but rises significantly a little way from it, making them unsuitable. The reject frequency is immaterial in this application.

Cavity filter and harmonic response

Cavity filters are essentially a quarter wave antenna, the resonator, in a box, the cavity. At resonance they produce a peak or a notch depending on how they are coupled to a driving signal.

The Q of the filter is very high, but varies with design and quality. The larger the diameter of the cavity, the larger the surface area and a higher Q. With silver plating the Q is increased over aluminium, copper or brass. The design of the coupling also effects Q. However, these are all compromises with cost and other factors. Typically the cavity is aluminium, the resonator silver plated and the coupling a copper loop with an N or BNC connector.

For a pass band filter there are two loop couplings for in and out. The fundamental response is at the quarter wave length of the resonator.

As with most antenna, the resonator is resonate on odd harmonics. This harmonic response can be used to filter at high frequencies, but with the simpler mechanical construction of lower frequencies. I will show how a 2m filter can be used on 70 cm and 23 cm. I can't go to a higher band because of the limit of my spectrum analyser, but this post is to show the general principle.

I had a 2m passband cavity at hand, but not a particularly good one, so results are indicative, but surprising good in this application.

The first photo shows the fundamental response at 145 MHz, better than 30 db, with about 0.5 insertion loss.

The next shows the third harmonic at about 415 MHz, still impressive. The exact multiplier seems to be effected by the cavity's geometry. It is just a matter of re-tuning to the desired frequency.

Even the ninth harmonic on 23 cm is still very good. The insertion losses and artefacts are likely influenced by the coupling design, UHF not N connectors and the cable. Further, I did not normalise the analyser for this shot. Again this is indicative. It would be better to use a 70cm filter on its third harmonic for 23cm and perhaps above.

Cavity filters in EME use.
One effect of a highly resonant antenna, like the magnetic loop of

The cavity filter as used

The 2m cavity filter I used, not the best even for 2m. Pass band filters aren't selective enough for the 2m 600 kHz amateur repeater split. However, they can be enough for 70cm with the wider split. It is possible that this filter came from a 70cm or commercial repeater. I bought four as surplus.

The coupling with its nasty UHF connector. The loop seems small for 2m and may have been used on its third harmonic. However, coupling design is not particularly critical at 2m or 70cm. The earthed long side is close to the resonator. The rotation of the coupling influences selectivity and insertion loss. For high selectivity and higher losses, still about 0.5 db, I adjusted it to be closest to the resonator.

The top of the cavity showing the placement of the coupling loops and the adjustment screw. The rod is invar to minimise temperature changes. However, in this application it is not critical as the peak is quite broad at the resonant frequency. The filter looks very selective in the photos, but I am using a wide span.

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