Filter hacking for 2395 ISS TV

[G3YKI]

G4EML said
"The 10MHz BW design from the S53MV article is actually shorter which seems a bit strange. The pole spacing is shorter but they are using 8mm diameter resonators instead of 10mm in 40 x 30 tube so maybe that reduces the coupling."

I think the width of the tube is the main influence. Although you may have heard of "evanescent waveguide" as a particular class of filters, there is no firm dividing line between such classifications. One way to look at the filter you have would be to say the coupling is a result of evanescent waves in the rectangular tube. The reduced width increases the cut off frequency of the waveguide and hence the rate at which the 2.4 GHz field decays along the tube. Then it is easy to see that the coupling between resonators will be reduced.
The reduced diameter posts probably just keep them in proportion and the Z0 in the low loss region.

[g4eml]

OK, that makes sense now, thanks.

Colin.

[g0mjw]

All interesting, but I have still not seen a specification of what bandwidth and attenuation characteristics we need. ISS is on 2395 MHz. The adjacent mobile band stops at 2390 MHz. Wifi starts at 2412 MHz. So for one thing, I don't think 2395MHz is ideal as the centre frequency, probably 2400 MHz would be better. 10 MHz bandwidth would though, put that right on the edge and lead to phase distortion, so perhaps 2398MHz.

Given the strength of mobile signals vs Wifi, and noting WiFi was already there when ISS TV was last operating, we probably mostly care about the attenuation below 2390 MHz. However, to get (say) 20 dB of attenuation only 5/8 MHz away is challenging. What is actually needed in practice? I know that's a variable, but can we settle on some spec? Then we can design a filter to suit.

Mike

[radiogareth]

With nothing technical to back it up (like I did not realise that phase shift through a filter might impede the decoding of the ISS signal) I'd suggest

1. As much attenuation as possible at 2390 compatible with minimal phase issues and at least some curtailment of the WiFi band.
By 'as much as possible' an ideal target might be the dB difference as seen by Phil at CAT25 Part 1 where the 5G mast was 100m away and it somewhat dwarfed the ISS signal.
2. Minimum insertion loss (although as Jen commented earlier with a hot pre-amp [G4DDK e.g.] the filter can come after the first pre-amp)
3. Uses easy to find standard alloy tubing sizes and hardware (like tuning screws and resonator rod diameter (stock sizes).
4. Repeatable by building in a home garage/workshop with hand tools
5. Tunable with e.g Satsagen or NanoVNA_V2
6. How to set up and use the nanoVNA_V2 to tune up the filter
7. 3-D printed end-plugs
8. Two fixing hole SMA sockets, or even single hole panel mount for greater ease of a home build

Unrelated to the above, how does a longer (more elements) filter differ from a shorter pair in series?
Gareth

[g0mjw]

Unrelated to the above, how does a longer (more elements) filter differ from a shorter pair in series?
Gareth

No cable in between, whole thing tuned in one go etc etc.

Maybe have a look here and have a play https://rfdesigntools.pythonanywhere.co ... ter_design:

Screenshot 2025-11-08 111516.png (168.51 KiB) Viewed 39 times

Here are some playing with parameters, e.g. more ripple and more poles. So more poles = steeper cut-off. More ripple = steeper cutoff. But more ripple might not be great for the signal and more poles means more metalwork and possibly a bit more loss.

Screenshot 2025-11-08 112619.png (55.42 KiB) Viewed 34 times

Screenshot 2025-11-08 111653.png (55.85 KiB) Viewed 39 times

Screenshot 2025-11-08 111727.png (50.9 KiB) Viewed 39 times

Screenshot 2025-11-08 111836.png (51.01 KiB) Viewed 39 times

This is not the filter we are building, but the principle is the same.

Mike

[g4eml]

A 5 pole version should be practical but will be quite long in the 40mm square tubing. It may still be worth trying the 40 x 30 tubing even though it is a bit harder to obtain.

Colin G4EML