I have a fm radio station a couple blocks down the street. I have a St-2 antenna up on side of tower and this radios station kills my 800 mhz like when I trying to listen to IL Starcom or if I search 800mhz I keep picking up this darn radio station (wish I had some dynomite) what filter would help this. My St-2 has 75 ohm coax with F connector on the end to connect to the scaner
FM filter for 800mhz
- Thread starter kb9hgi
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I don't know that I'd recommend those old FM traps.
I bought a dozen of them for real cheap when all the local RS stores where closing.
At least 50% of them seemed to do nothing so I threw them all on an analyzer.
I found the ones that did not help reduce FM BCB signals were centered far away from the center of the FM radio band.
Many were centered more in the middle of the civil air band so using one when trying to monitor aircraft did not work so well.
I had at least two that were centered more around 151 MHz.
I opened up the ones way out of band and found that tweaking all the dangling coils inside the housing could bring them back to the FM broadcast band again. Usually just moving a coil closer or further from another coil did the trick and sometimes you had to open or close the turns on one (or more) of the coils.
I then opened a couple that were working correctly and immediately found that they had been fixed with what appeared to be hot glue once they were tuned. ALL the ones that were way out of band just had free hanging caps and coils in them with nothing but the wire leads and solder holding it all in place! It was the ones with nothing fixing the coils in their physical places that were way out of tune. Most likely simple handling during packaging and shipping or being dropped knocked them out of tune very easily. It id not take much at all to make a huge difference in the center frequency where they notched the deepest.
RS used to put a letter or date code stamp on many of their products. I checked all these FM traps and the DOM did not seem to matter as to if they were fixed with a hot glue substance or not. It was probably a difference in where they were built.
While I had some of them open, I went ahead and changed out the F connectors to BNC jacks so I did not need to use more adapters. Sweeping them again showed the were still on the center of the FM radio band even though they are still 75 ohm devices in a pure 50 ohm environment.
With these findings, I never bought another one and threw away most of the ones I had already bought. I just never could trust them and did not feel like taking the time to sweep them for other ranges they may attenuate signals so in the trash they went.
I know for a fact that at least one of them that was working correctly for the FM broadcast range was also knocking down signal in the 850 MHz range as I also lost signal from some of the weaker sites I monitored.
I've since stuck with better products such as the trap made by PAR Electronics. They cost more though. The old saying that you get what you pay for definitely applied here.
I do still have a couple of the cheap RS traps in use but only on radios used for fairly strong signal work.
kruser - I couldn't have said it better.
The RS units that I tested were all the same - the notch was usually well above the center of the fm band, which meant for many, only the insertion loss of the filter itself was really doing the trick. I retuned them as well but got real tired of doing that and in many cases it still wasn't enough.
It may have been *just enough* insertion loss to stop noticeable desense, but stopping there, most ops using these don't know the joy of a much lower noise floor when a quality filter is used.
KB9HGI - don't fool around with junk near a flamethrower. This one is a classic:
I haven't tested this filter, but it is interesting that it is dual purpose FM broadcast AND 0-20 mhz highpass filter - the HPN-30118:
No personal experience with it, but I brought out the idea of using highpass filter in another thread targeting flamethrower AM broadcast and common-mode issues with long transmission lines and poor rf grounds. I'd like to see someone test one of these...
The RS units that I tested were all the same - the notch was usually well above the center of the fm band, which meant for many, only the insertion loss of the filter itself was really doing the trick. I retuned them as well but got real tired of doing that and in many cases it still wasn't enough.
It may have been *just enough* insertion loss to stop noticeable desense, but stopping there, most ops using these don't know the joy of a much lower noise floor when a quality filter is used.
KB9HGI - don't fool around with junk near a flamethrower. This one is a classic:
VHF-FM Broadcast Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts
PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave
www.parelectronics.com
I haven't tested this filter, but it is interesting that it is dual purpose FM broadcast AND 0-20 mhz highpass filter - the HPN-30118:
HPN-30118 Combined Notch Filter | Scanner Master
HPN-30118 Combined Notch Filter helps eliminate radio interference and scanner front-end overload.
www.scannermaster.com
No personal experience with it, but I brought out the idea of using highpass filter in another thread targeting flamethrower AM broadcast and common-mode issues with long transmission lines and poor rf grounds. I'd like to see someone test one of these...
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Ubbe
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Ubbe
Member
To me that indicates that only the ones with hot glue where actually tuned and the ones without glue where somehow never tuned. Maybe a worker in china found a way to increase his pay by skipping a stage in the process and also got his picture on the wall as employee of the month as a result of the high number of filters he managed to produce.
/Ubbe
I must have been lucky. All the ones I've swept have been spot on.
vagrant
ker-muhj-uhn
Here is a recent thread that lists various FM filters and their performance.
forums.radioreference.com
FM bandstop filter comparison
I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...
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