Tight 70cms filter question
Posted: Wed Jan 12, 2022 10:26 am
In pursuit of a really 'tight' (narrow) passband filter for 70cms DATV on 437MHz (there is so much RF rubbish floating around my location I need it!) I've experimented with adapting a cavity notch removed from a popular diplexer that came from the surplus market.
I added hairpin probes, lengthened the coil and grounded one end. The results are pretty good other than the insertion loss.
I initially set up the filter with a eBay 'noise source' which has three stages of amplification after the actual noise source - so pretty high level. I measured about a -3dB loss which I felt was, for the passband (750kHz) entirely acceptable.
However, when doing further tests using much lower level signals such as might be arriving at an aerial, I found the loss was nearer 12dB! Yikes!
So, some questions....
1. Will any filter with such a narrow passband have a high insertion loss? (My understanding is yes, it will...)
2. Is there anything I can do to improve (reduce) the insertion loss? Its coil is silver-plated copper, the probes are just tinned copper.
3. Does a high level input cause some 'filter bypass' which might explain why my earlier tests showed only -3dB insertion loss.
4. I'm happy to build and machine stuff - does anyone have a favourite 'go-to' 70cms BPF design?
5. The pre-amps I'm using are small SMA-ended eBay 0-6GHz 20dB units, which do indeed work adequately at up to 6GHz. The filter loss is therefore reasonably acceptable, but I appreciate that distributed gain and sequential filters may better achieve my objectives.....
Pic shows the actual unit tested. Comments welcome.....
Gareth
I added hairpin probes, lengthened the coil and grounded one end. The results are pretty good other than the insertion loss.
I initially set up the filter with a eBay 'noise source' which has three stages of amplification after the actual noise source - so pretty high level. I measured about a -3dB loss which I felt was, for the passband (750kHz) entirely acceptable.
However, when doing further tests using much lower level signals such as might be arriving at an aerial, I found the loss was nearer 12dB! Yikes!
So, some questions....
1. Will any filter with such a narrow passband have a high insertion loss? (My understanding is yes, it will...)
2. Is there anything I can do to improve (reduce) the insertion loss? Its coil is silver-plated copper, the probes are just tinned copper.
3. Does a high level input cause some 'filter bypass' which might explain why my earlier tests showed only -3dB insertion loss.
4. I'm happy to build and machine stuff - does anyone have a favourite 'go-to' 70cms BPF design?
5. The pre-amps I'm using are small SMA-ended eBay 0-6GHz 20dB units, which do indeed work adequately at up to 6GHz. The filter loss is therefore reasonably acceptable, but I appreciate that distributed gain and sequential filters may better achieve my objectives.....
Pic shows the actual unit tested. Comments welcome.....
Gareth