FM Trap Placement

RedPenguin

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I am trying to get to the bottom of this.

I have 2-3 FM stations about 1.5 miles away that I believe are interfering with my setup.

Posts and Google Results that I have seen, claim the trap goes before the amp if using one like me.

I had a RS FM Trap and now an RTL-SDR one but I'm waiting on my SMA adapters for the latter.

I am getting confusing results though, putting on antenna end or input if amp seems to still have interference.

Putting it on amp output seems to work more but for my scanner with the most frequencies, I had to resort to putting the RS FM Trap directly on it's antenna cable for now.

Lately my reception is way better but seems like more interference came along with it.
 

vagrant

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Antenna > Filters > Amp > Radio
Put your coaxial cable somewhere between two of those. This allows the amplifier to push the signals you want. Still, if you are getting better results with the filter between the amp and radio, then go with that. My guess is the filter is not only reducing the FM signals, but attenuating your amp that may be pushing too much.

How much dB is your amp pushing out? If it is pushing 20 dB that is probably not going to do you any favors. 10 dB is probably enough and your issue of multiple FM transmitters so close is a big issue.

P.S., the RTL-SDR FM filter works, but you will loose some of the lower Aircraft frequencies. If you don't monitor around 118 MHz, then those filters should work well enough.
 

RedPenguin

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Still, if you are getting better results with the filter between the amp and radio, then go with that. My guess is the filter is not only reducing the FM signals, but attenuating your amp that may be pushing too much.

How much dB is your amp pushing out? If it is pushing 20 dB that is probably not going to do you any favors. 10 dB is probably enough and your issue of multiple FM transmitters so close is a big issue.
I think you hit the nail on the head.

I tried my 10dB amp before that has a 4 splitter as I have 4 scanners.

That seemed to make reception worse, with or without filter.

I decided to try one more time with my 25 or 28dB one and I was going to use my 0-20dB variable attenuator but was having trouble finding it.

So in the mean time as the some of the channels will be silent until tomorrow, I threw the current 4 splitter (4 radios) on another 4 splitter as that obviously would attenuate.

I still can't seem to get 453.300 from doing this strange breakup as show in the image.

It sounds excellent then does that what looks like EFI or something then great again.
 

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Ubbe

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...I tried my 10dB amp before that has a 4 splitter as I have 4 scanners.
That seemed to make reception worse, with or without filter.

I decided to try one more time with my 25 or 28dB one and I was going to use my 0-20dB variable attenuator but was having trouble finding it.
Those high gain preamps usually has a very bad strong signal performance, if they aren't in the $500 range or higher.
Could you state the preamps brand and model types? They could be totally unsuitable for scanner use.

The RadioShack FM trap filters has some bad quality issues that could make them to attenuate FM broadcast too little and/or have a high attenuation at all frequency bands. Wait until you can test with the RTL-SDR filter and the variable attenuator.
What scanner are the picture taken from and how where the setup done at that point?

The most important tool would be the variable attenuator. If you attenuate the signal by something like 6dB and the interference also attenuates just by 6dB then the problem are before the attenuator up to the antenna. If the interference attenuats more than the signal then the problem lay between the attenuator and scanner, or the scanner itself.

/Ubbe
 

RedPenguin

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One amp is the GE Pro 24db and the other is the RCA VH140R.

That picture is from one of the two BC75XLTs and that was with the antenna to the GE Amp then to a 4 splitter which fed two 75XLTs, a BR330T, and a BCT15.
 

RedPenguin

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Well I completely redid the wiring from the feedline to the scanners themselves after even another antenna produced the same result.

I figured maybe something is loose somewhere.

I also checked audio cables by unplugging them and plugging them back in. I also have the variable attenuator to 6dB or so and no FM Trap so far.

It seems so far to have fixed it but I will have to give it some hours to verify.

I know I had to do this once before where it seems when you experiment with the setup, it's best to rewire after.

I think sometimes when I adjust stuff, possibly a wire gets loose or something. I seen nothing that visually looked loose except one audio cable ever so loose but I checked every wire.
 
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iMONITOR

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PAR Electronics or Stridsberg see some good FM-Notch filters. The Radio Shack filter might be causing a lot of loss.

Normally you want to install the notch filter before the preamp (on the input), otherwise the preamp will be amplifying the interfering FM signal too!
 

kruser

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PAR Electronics or Stridsberg see some good FM-Notch filters. The Radio Shack filter might be causing a lot of loss.

Normally you want to install the notch filter before the preamp (on the input), otherwise the preamp will be amplifying the interfering FM signal too!
When RS closed its stores, I purchased several of their FM Traps for pennies on the dollar.
I found them to be all over the place as far as what range they actually attenuated. Several did a very good job at wiping out the AM Civil air band and a couple were even way worse and centered around 150 or 160 MHz!
Only a few actually attenuated the FM broadcast range. I was able to pop them open and found the coils were not potted or supported by anything so something like a simple drop onto a hard surface would move the coils and re-tune the trap.
The ones that did trap the FM broadcast band were filled with what appeared to be hot glue so they could not move.
And a sweep at 600 MHz and higher showed many were attenuating signal by -10 dB and worse so those were no good if you also monitored 7 or 800 MHz.
The ones that were tuned correctly worked fine for older scanners that had intermod or overload problems.
 

Ubbe

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Normally you want to install the notch filter before the preamp (on the input), otherwise the preamp will be amplifying the interfering FM signal too!
A good amplifier that can handle strong signals can have the filter at its output, and will overcome any filter losses. The relationsship between the wanted signal and the interfering broadcast signal will still be the same at the scanner, but having the filter between scanner and amplifier will increase the wanted signal by the same amount the filter loss are. Having the filter at the amplifiers input would attenuate the wanted signal by the filter loss value and when the signal are lost in the noise it cannot be recovered.

Those TV amplifers are of low quality and would need the filter on their input. It could also be other strong signals from cellular towers or pagers that also could overload them and make them useless if not more filters where added. Better to use a high quality amp. Something like a MiniCircuit type ZX60-P103LN+
There are others less costly if you don't need the metal box. GPIOLabs

/Ubbe
 

RedPenguin

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Obviously I have to do more testing but I am beginning to wonder if a 3-way CFL Bulb is causing my issue.

I just randomly thought of how some people complain about LED/CFL then I thought, I do seem to notice the interface when I am in the room obviously with the light on.

Obviously I have to give it a few hours to really know, but instantly after turning it off, 453.150 seemed better and 453.300 seems better.

I feel like I am on to something as I did akways wonder EFI/RFI as it just seemed strange that it would be perfect reception then like a wavy-pattern cut-out instead of static like I'd expect if the antenna or something was failing.
 

RedPenguin

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Well it turns out the CFL Bulb may have been a wild goose chase but it maybe helped anyway

I walked around with the 330T to see if anything is maybe causing EFI/RFI and didn't seem to find anything.

Yet I did notice that 453.300 seems to even come through with a rubber ducky which is interesting as it didn't seem to years ago. I switched that radio to a standard then maybe an RH77CA to see how that works.

So it looks like I may only have to use an amp of any sort on the 453.475 scanner as oddly that frequency, while only like 15 miles away is very delicate to receive.
 

RedPenguin

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Strange, when the the two 75XLTs aren't on the same antenna, way better reception. I have the 453.475 on the old antenna setup, while 453.150 & 453.300 are on the Diamond RH77CA.

So far all 3 sound great, so it looks like probably not the antenna setup. Maybe some ground loop or something.

It seems that a thin wire around the BNC of both 75XLTs and a screw of the PC Case, seems to stop bleedover between soundcards and seems to stop the wavy-breakup.

Originally, I only had the wire around one 75XLT which stopped bleedover but not the interference.
 
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RedPenguin

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Well I finally got to the bottom of it, with great reception all around.

It seems like only the scanner with 453.475 needs the dipole & amp so that's by itself.

453.300, 453.150, and the rest work great with nothing but the Diamond RH77CA with no amp. 453.150 actually is terrible on amp compared to Diamond.

Some frequencies such as 453.350 & 453.650 seem like you could practically use a paperclip as an antenna so they are good.

It seems like the main trick is grounding where a wire that connects to the BCT15 & the BC75XLT (one with the Diamond) to a screw on the PC Case solves bleedover and humm.

So far the reception is excellent for all frequencies. The Diamond picks up 475 also but has more static and noise than the dipole & amp.
 

RedPenguin

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It actually seems like it was mainly a grounding issue.

I tried one more time if the one 75XLT on the amped antenna and it worked flawlessly except one breakup. Yet the FM Trap got rid of any more breakups so far.

I wanted to test one more time as I had some slight breakup on 453.375 with the Diamond even though it's not a far signal.

The real trick that worked was, taking 4 pieces of bell wire and twisting them together with a 5th piece that goes to a screw on PC Case, and wrapping 1 around each scanner.

That immediately got rid of all bleedover and apparent de-sensitizing of the scanners.
 
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