Filter hacking for 2395 ISS TV

[g4hiz]

The topic 'Evanescent Mode Filters' (http://www.g4jnt.com/evanfilt.pdf) is possibly one to look up.

Jen

[G7VVF]

What about this SAW filter, would this help with suppressing a 4g signal
www.aliexpress.com/item/1005007918391623.html Its specs are quoted as

Center frequency: 2450 MHz
1dB bandwidth: 100 MHz
Tape interpolation loss: < 2.0dB
Out-of-band suppression: > 45dB@2.2GHz
> 45dB@2.65GHz
Impedance: 50 Ohms
Maximum load power: 20dBm(100mw)
Interface: SMA (outer screw hole)

It's centered on 2450 and the bandwidth is 100mHz which would mean 2400Mhz is at the edge of the filter, however the plot appears to show it has a bit more bandwidth to the LF side, so 2393 might be within its pass band allowing reception of HAMTV on 2395MHz. Its hard to be accurate when the span on the plot is 3GHz.
Duncan
G7VVF

[g0mjw]

All good stuff, just need to find some alloy tubing of the required size.
Gareth

Yes, it made from 40mm x 30mm box section 200mm long. The inside is 21.5 by 31.5 mm so the wall thickness is 4.25mm, odd size, but I doubt it matters much if it is slightly different. You could weld something up out of tin plate I expect, or copper/brass sheet might be better.

Closest I could find was this https://www.aluminiumwarehouse.co.uk/co ... gular-tube

But it is £13.46 a metre. Also a little narrow but I expect that could be compensated for.

Mike

[G3YKI]

1) To emphasise and illustrate the point that cascading two filters will probably not give the result that a simplistic view might lead you to expect.

Filters are not lossy like attenuators; they operate by reflecting the power that is not transmitted to the output. In the stopband the interconnecting cable sees nearly 100% reflection at each end and will behave like a low loss resonator. At certain frequencies, it is capable of matching the two filters together, thereby cancelling the mismatch loss effect of both. The following simulations show this.

3 element Butterworth bandpass filter for 2.3 to 2.4 GHz.

export1.png (6.03 KiB) Viewed 14 times

Two such filters connected by 30cm of cable.

export2.png (7.03 KiB) Viewed 14 times

2) A 3 element 0.1dB Chebychef BPF 5MHz wide can give 10 db attenuation at 2390 and 2400 MHz. No need for more poles than that to get some useful suppression. The issues will be stability (rigidity and maybe thermal) and loss. Mike's filter shows .75 dB with about 20MHz wide. Reduce that to 5MHz and the loss will be ~3dB.
To maintain 1dB requires unloaded Q of 13000 which is unlikely with a copper resonator, never mind aluminium.

3)The commercial filters are sophisticated designs for specific purposes. I think the adjustments are intended to allow for design and manufacturing tolerances. I would have very little hope that a 100MHz wide filter can be adjusted to 5MHz. Good Luck!