Suggestions on attenuating a specific frequency...

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gmclam

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I went and raised my discone and the reception is even better than before (I also home ran LMR-400 equivalent). But now an old ugly 'friend' has popped back up. I am picking up an AT&T paging signal which broadcasts from very close to me on 152.840 MHz. After filtering AM and FM broadcast signals, it is the strongest signal from my antenna.

I built a 'stub filter', but it is NOT a notch filter and reduces too much of the spectrum. I looked at a PAR filter, but it affects +/- 5 MHz. That would mean signficant signal loss from between 147.84 to 157.84 MHz. Most of the stuff I monitor (except CHP) is in that band, and so that is not a good solution.

I did a search on eBay just to see what was available, and found nothing that was in this frequency range. We have had some discussions on this topic before, but I am unable to find them.

I'd like to hear some ideas. Surely others have run in to this issue. If I could attenuate the offending signal a mere 10dB, I would be happy. 20dB of attenuation would put it well below the signals I typically receive.

Thank you in advance.
 

fineshot1

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I went and raised my discone and the reception is even better than before (I also home ran LMR-400 equivalent). But now an old ugly 'friend' has popped back up. I am picking up an AT&T paging signal which broadcasts from very close to me on 152.840 MHz. After filtering AM and FM broadcast signals, it is the strongest signal from my antenna.

I built a 'stub filter', but it is NOT a notch filter and reduces too much of the spectrum. I looked at a PAR filter, but it affects +/- 5 MHz. That would mean signficant signal loss from between 147.84 to 157.84 MHz. Most of the stuff I monitor (except CHP) is in that band, and so that is not a good solution.

I did a search on eBay just to see what was available, and found nothing that was in this frequency range. We have had some discussions on this topic before, but I am unable to find them.

I'd like to hear some ideas. Surely others have run in to this issue. If I could attenuate the offending signal a mere 10dB, I would be happy. 20dB of attenuation would put it well below the signals I typically receive.

Thank you in advance.
Par will make a notch filter for a specific frequency that has a deep notch and not too wide
banded - call them and ask.
 

slicerwizard

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I am picking up an AT&T paging signal which broadcasts from very close to me on 152.840 MHz.
You always have the option of pointing a highly directional antenna at the offending transmitter and combining its signal (suitably attenuated) 180 degrees out of phase with your main antenna.
 
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N_Jay

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I doubt you could get a second antenna tuned and matched.

Build a shorted stub, but make it 1 full wave not 1/2, Use good cable (Maybe hard line).
After you get it tuned by trial and error (or proper test equipment) make a good solid short.
 

gmclam

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Par will make a notch filter for a specific frequency that has a deep notch and not too wide banded - call them and ask.
It looks like the best I could get is something that would reduce 150.54 to 155.070. That is still a chunk of what I monitor. I'd be willing to give up half of the attenuation (20dB instead of 40dB) to get back half of the signal (151.54 to 154.070). Do you know if that is available? Model number/etc?
 

gmclam

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You always have the option of pointing a highly directional antenna at the offending transmitter and combining its signal (suitably attenuated) 180 degrees out of phase with your main antenna.
N_Jay said:
I doubt you could get a second antenna tuned and matched.
Getting it matched is one thing. I'd be concerned that it would stay that way. Since I first encountered this problem well over a year ago, the offending signal has not always been at the same level.

N_Jay said:
Build a shorted stub, but make it 1 full wave not 1/2, Use good cable (Maybe hard line). After you get it tuned by trial and error (or proper test equipment) make a good solid short.
The stub I built (1/2 wave) is made of very good coax (not hard line) that has been swept to 2300MHz. Adding the stub attenuated the offending signal, 152.8 MHz, by 18dB. But it also attenuated NOAA 162.400 MHz by 15dB. Do you think a full wave stub will make that big of a difference? What do you think about multiples of the tuned frequency (for example 305.6 MHz or 458.4 MHz)?
 

fineshot1

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It looks like the best I could get is something that would reduce 150.54 to 155.070. That is still a chunk of what I monitor. I'd be willing to give up half of the attenuation (20dB instead of 40dB) to get back half of the signal (151.54 to 154.070). Do you know if that is available? Model number/etc?
Not sure where you got your info from but it seems the SYM series is what would serve you best.

The SYM152HT seems like it would be your best fit. You could insert it inline easily. You will loose
some vhf spectrum but it won't be 4 or 5 Mhz - more like about 2 or 3 at the most.

Here is a cut & paste from the PAR web site:

"The SYM series of filters are a 3 stage symmetrical design that achieves very fast recovery from the notch. Typical -3dB points are +/- 1.5% of the notched frequency- or about +/- 2.25MHz with a 150MHz notch. Typical notch depth is -35dB."
 

zz0468

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A cavity notch filter is what you need. A pass cavity can be modified, if that's all you have to work with.
 

gmclam

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Not sure where you got your info from but it seems the SYM series is what would serve you best.
I got my information from the Par Electronics website. The filter that looks to be the closest is VHFSYM152HT.

The SYM152HT seems like it would be your best fit. You could insert it inline easily. You will loose some vhf spectrum but it won't be 4 or 5 Mhz - more like about 2 or 3 at the most.
The filter shows a curve from 150.24 to 154.77, with about a 3dB drop at those points. That is a span of 4.53 MHz.

Here is a cut & paste from the PAR web site:....
"The SYM series of filters are a 3 stage symmetrical design that achieves very fast recovery from the notch. Typical -3dB points are ... about +/- 2.25MHz with a 150MHz notch.
+/-2.25MHz = 4.5 MHz total!!!! Not "2 or 3 at most"!!!!
 

gmclam

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A cavity notch filter is what you need. A pass cavity can be modified, if that's all you have to work with.
I suspect those are quiet pricey. Regardless, can you point me to some place I might find one? Thanks.
 

fineshot1

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I got my information from the Par Electronics website. The filter that looks to be the closest is VHFSYM152HT.

The filter shows a curve from 150.24 to 154.77, with about a 3dB drop at those points. That is a span of 4.53 MHz.

+/-2.25MHz = 4.5 MHz total!!!! Not "2 or 3 at most"!!!!
Its only -3dB, its not like you are going to totally loose receive across that small portion of spectrum.

OK - forget my suggestion and see what else comes up. Hopefully someone else can help you out.
 

gmclam

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Its only -3dB, its not like you are going to totally loose receive across that small portion of spectrum.
I am not concerned about the 3dB of loss at the + or - 2.25 MHz points.

Par shows a graph of what to expect with this filter, and what concerns me is the loss I will have between +/- 1.0 MHz and 2.25 MHz from the center of the notch. I don't need 30+dB of attenuation and I am willing to give up some (? half) of that attenuation to narrow the notch. I know this is asking a lot from an LC circuit, that's why I am asking.
 
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N_Jay

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You need a cavity to get the filtering you want.

You will never get an LC circuit with a high enough Q
 

gmclam

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If you patient, Cellwave VHF cavity notch filters show up on Ebay in the $40 to maybe $75 range. A cavity notch will also work at odd multiples, so your going to loose something at 450MHz.
prcguy
I was hot to resolve this issue over a year ago when I first figured out what it was. Then it quieltly went away. Now it reared its ugly head. I'll have to persistently keep looking for one of these.

Should I expect these cavity filters to be already tuned to a specific frequency? Or are they tunable? Or? It's one thing to wait for something to pop up, but if I also have to wait for a specific frequency, that could be a very long shot.

Anyone have sources for these besides eBay? THANKS to ALL!
 

zz0468

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Should I expect these cavity filters to be already tuned to a specific frequency? Or are they tunable? Or? It's one thing to wait for something to pop up, but if I also have to wait for a specific frequency, that could be a very long shot.
They're tunable, and you're going to have to tune it. It'll take, at minimum, an accurate signal generator, and a receiver.

Anyone have sources for these besides eBay? THANKS to ALL!
Ebay is going to be the cheapest and most readily available source. They're available new, but will cost several hundred dollars.
 

gmclam

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They're tunable, and you're going to have to tune it. It'll take, at minimum, an accurate signal generator, and a receiver.
Perfect! I have the means and equipment to tune it. I wasn't sure if these were built for specific frequencies or not. I'll check the links provided and see what I can dredge up.
 

gmclam

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