Filter hacking for 2395 ISS TV

[radiogareth]

From a construction point of view a comb filter does not need a tap feed into the 'first digit' (of an interdigital) so that makes reliable repeatable replication (3 'R's'??) easy by virtue of using pieces of wire, tubing, brake pipe etc basically anything you can solder to a centre pin.
Slots would indeed be possible, but software simulation would surely be a better way these days?
Gareth

[G3YKI]

I may try adding additional screws in between the posts. That is supposed to adjust the coupling. Might be worth a try.

Colin.

Yes, I have seen many filters that use that technique and it is easy to do. I believe from experiments on a similar filter that it increases the coupling, but I could be wrong. ( I cannot see any intuitive reasoning one way or the other) I have also seen (and made) filters where the resonators are partly screened by dividing walls. This, I am sure reduces the coupling and would enable considerable reduction the overall length. It may not easy to do in in practice in your tube filter.

Having said all that, your filter is probably doing what you need. If it passes the signal you want, whether or not the rest of the passband has the text book flat top response becomes an academic exercise.

[g4eml]

Yes, from what I can find, tuning screws between the posts increase the coupling. Some texts recommend deliberately spacing the posts a little too wide and then using the coupling adjust to get the final result. It would probably be better if a design didn't need them as it makes the alignment more complicated.


Colin.

[g0mjw]

The spacing on my filter is much less. 19mm between the input/output rod and the first post, then 38mm between the posts, which are interdigitally arranged. The in/out posts span the waveguide so are 32mm but approx 25mm long at 10mm dia. The other posts are about 8mm dia. Hart to tell how long but it looks like about 3mm of gap at the end. The tuning screw goes into the post so it must have a hole in it.

Given it is shorter, it might be a better route size wise.

Mike

[g0mjw]

Many years ago, before NanoVNAs, I asked Bert Moddermann to make me a 13cm filter. It is fairly small, 110mmx40mmx20mm and excellently CNCd as usual from Bert. Here is the response.

13cm-5pole.jpg (93.16 KiB) Viewed 408 times

It has 5 tuning screws

Cavity width 31.8mm, length 103mm, depth 16mm, rods 6mm dia 27mm long, spacing 19mm, 21mm, 21mm, 19mm, tap 7mm up from ground. It uses M4 tuning screws, A2 stainless !!!

13_cm_filter_image.jpg (1.16 MiB) Viewed 408 times

The loss is low, so I assume the Q is also low. It is not what we want as the bandwidth is 150MHz or so. About right for covering the band, before we lost so much of it.

Mike

[g4eml]

I added some additional coupling adjustment screws between the 70mm spaced poles.

Winding in the two outer coupling screws effectively increased the coupling of the two outer pairs of resonators. So probably similar to an untuned spacing of 65 70 70 65.

The filter now tunes up with a reasonably flat passband and good return loss. The overall bandwidth is a bit wider at 22MHz as would be expected with higher coupling. Increasing all of the spacings should bring the bandwidth back down again but that would involve a lot of re-drilling.

IMG_20251110_154553.jpg (131.35 KiB) Viewed 377 times

Smaller cross section tube should reduce the resonator spacing and hence the total length.

I will now wait until I have some of the smaller tube and repeat the process based on what we have learnt.

Colin G4EML

[radiogareth]

Having competed the metalwork for the PA boxes for GB3JT and GB3OO I made two sets of resonators for the 10 & 20MHz wide filters. My 40x40 tube arrived today (slow delivery) but more 40x40 and two pieces of 40x30 ordered today is already due for delivery tomorrow. Much better
Plan is to build a stock pair of filters and learn how to tune them and see if effects like 'near the edge' cause decoding issues.
Can't help but wonder what thinner rods would do, like 6.25mm and if the diameter of the feed in and out probe makes any difference....
The two 'spares' will be saved for 'developments' or improvements.
The supplier (https://www.aluminium-online.co.uk/alum ... ular-tube/) also offers WG14 sized tubing 11/2"x3/4"x16sgw tubing if anyone fancies trying the WG based design. Should have bought a piece for myself....
Gareth

[g0mjw]

I will order some channel and have a play. I don't need more than the filter I have, but I could do with one for 23cm where I have noise issues that I am fairly sure are from the direction they peak up, due to our local 100kW TV transmitter mixing in my preamp. If instead they are the second harmonic, I will find out, but I suspect the pre-amp because there doesn't seem to be a problem with it off, though I can't hear very well with it off. I think it is very likely that the protection diodes are the cause, but it might also be at the radio end. If so it will be easier to install.

Locals also have this noise problem on 13cm. I don't have 13cm up at the moment. It wasn't a problem for me back when I did, I had the 5 pole 150 MHz wide filter I showed earlier inline, which did cut out the 2.1 GHz base stations, but that was before 4G/5G was deployed in the adjacent band, so it would not be as effective now. Maybe I will need it after all if I ever get back on 13cm.

Mike