Looking at ways to reduce 533, 545, and 575 MHz signals to allow reception of 491 MHz signal that is about 20 db lower level and in the same direction (directional antenna won't help). Thinking a stub, or probably multiple stubs, might be appropriate but no experience with them. Any advice is welcome.
Notch filter to reduce adjacent channel level due to front end overload
- Thread starter AC7C
- Start date
Probably a better solution as a home made notch may not be narrow enough to not cause degradation at the desired frequency.
I would go with the low pass filter with the roll off at 520 MHz. Also, with my bearcat scanner on my discone, bypass any amplification and try an in line variable attenuator. That helps reduce overload. I am 200 yards from a major telecoms/cell/and repeater installation.
I do use a stub for FM BC, but note, it will also notch multiples of the 102 MHz it is tuned for.
Stubs are fine except for that problem.
I do use a stub for FM BC, but note, it will also notch multiples of the 102 MHz it is tuned for.
Stubs are fine except for that problem.
Ubbe
Member
If the interfering signals are only 20dB higher in level you could simple attenuate the whole antenna signal until your front-end can cope with it, or your front end are really bad. Use a variable 20dB attenuator. If trying coax stubs, it doesn't cost anything, then cut to 490MHz and start cutting small snippets off the coax until you start to hear enough of the 490 signal. Or continue to cut until the 490 signal starts to degrade again.
The better quality of the coax the more narrow the notch will be. I usually use RG6 that have a decent quality, you'll have to go to LMR400 to improve on that and will not be as easy to cut. It will also notch at 3x the frequency so no issue on the 700-900MHz range but will always do some smaller attenuation on all frequency bands.
/Ubbe
You are correct, the front end design is not the best. It's a network TV tuner. The manufacturer does acknowledge that. All my actual TVs do fine. I've tried attenuation but that degrades the signal I want. I'm getting a NanoNVA so I can tune some stub traps. Should be an interesting project.
Ubbe
Member
Before I had that type of measuring tool I just calculated the length needed, using the coax data to see its delay factor, and selected a steady signal a bit lower in frequency that I cut to and began to cut small pieces of the coax to get a feel for when the max notch where reach for that lower frequency and when it passed that frequency. Then focused on the real frequency higher up that needed to be notched using the experience from the lower frequency notch. As RG6 costs almost nothing I just took a new length if I cut too much.
I use F connectors where the coax are screwed in so I can reuse the connector and also could fine tune the frequency by unscrewing the coax a couple of turns to make it longer and tune back to a lower frequency. As the end of the coax should be open it creates a max SWR where all signal bounces back to the signal source and if the stub are a 1/4 wavelength the signal travels a 1/2 wavelength and arrives at the source at an opposite phase and cancels out that frequency. The less the coax attenuates the more equal the SWR signal level will be to the source signal and do a better cancellation.
Use a T-connector and measure from the center of that T to get the length of a coax stub. You can practice on any steady continuous signal you have at any frequency to get your calculations correct. Maybe you'll need all the signal level from the 490MHz frequency so perhaps cut a bit higher in frequency for the 550MHz ones as it might take just 10dB off those to get the front-end working and not attenuate too much at 490MHz. I used to have a strong TV transmitter at 475MHz that made receiving 469MHz difficult but worked alright when using a RG6 coax stub to reduce the TV signal level by 10dB or so.
/Ubbe
