What Kind of Filter Should I Get?

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policefreak

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Ok so here is where I am at...I live about 20 miles Southeast of the Philadelphia area and am just out of range of a bunch of 800 MHz systems (Philadelphia P25, Montgomery County PA Smartzone, Cherry Hill NJ PD, Delaware State, etc). Last summer I invested in a small 800/900 terrawave yagi antenna I can mount onto a desktop (I really don't have access to an upper attic). I pointed it towards the Northwest and got some marginal improvement on my PSR 800 scanner. I then took a GRE Super Preamplifier, cranked it up all the way, and stuck it on top of the scanner and between the coax from the yagi, and I got very satisfying results. The only thing I discovered was slight overload from the NJSP tower about 3 miles to the NW (The direction I had to point the antenna). During the fall I noticed that overload got a bit worse once all the leaves came off the trees. Then one day I noticed everything was completely overloaded in the 700 and 800 band to the point it was unlistenable with this set up. If I took the preamp off, the desired 800 MHz signals were too weak. If I put the amp on and turned it on, it created a lot of overload static. If I turned it up all the way, and used the attenuator feature on my PSR 800 some of the signals were a bit better but most were for the most part uncopyable. I went to the site of that NjSP tower with an 800 antenna and the preamp and noticed that I could still get other 800mhz signals in over that signal. I also noticed driving around my vicinity with the amp on turned up and an 800 MHz antenna on my PSR 800 that I was getting lots of overload. But if I headed about 1/2 mile in any direction it would go away and I could hear signals again. I also noticed about a mile to my Northwest there was a new complete dead zone in the 700/800 MHz range due to what I believe is a cell tower. And I really think that is my problem. Now it's summer and the overload has backed off a bit with leaves on the trees but I still am trying to get more out my scanners and set up in the 800 band.
So all of this to say, is there a good filter that works specifically to block out signals in the cellular band that would help me in my situation? I can't take a lot of loss in the band I am trying to listen to, and I don't need a lot of loss in the cellular band, just enough to allow some of these other signals to overcome the stronger cell signal which appears to be coming from about a mile away. Any suggestions? Do you need anymore info?
 

KC8ESL

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Where is your preamp sitting in reference to feedline, antenna, and radio?

If your pre sits at the radio, all it will do is amplify noise that is there. If it is directly under the antenna, it will amplify signals that will degrade down the feedline and you'll have something usable after 150 ft of coax.
 

policefreak

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Ok. Its a small 13 element 800/900 mhz yagi antenna sitting on a shelf in 2nd floor bedroom connected to only 6ft of coax which goes to the preamp which is connected to the scanner. I did try putting the preamp between the antenna and the coax and I got nothing but static. I tried the preamp at all different levels. So I wonder why.
 

KC8ESL

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How much adjustable gain is there? With 6 ft of coax you're probably overloading the front end of the radio.

I'm betting that if you used 25 or 50 ft of coax with the preamp inline you'll have results.

Try this: yagi--> 6 ft coax-->preamp--> 50 ft coax --> (optional dc injector here)--> radio

Only thing is, if you've tuned (gain-staged) the preamp correctly, it should react as the following...

Yagi --> 6 ft coax --> radio

If the signal isn't at the antenna, a sensitive preamp won't pull magic out of the dirt.
 

W3DMV

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You would normally use a notch filter for that type of problem. The good ones (RF Lambda) and
many others can get expensive. Usually available in tunable and fixed ranges.

The GRE preamp is a little noisy and very broadbanded which adds to the problem. If
you are only using this set up for 800 Mhz, you might want to look at using a good single
band preamp. I use low noise preamps made by Advanced Receiver Research. I think
their in Ct. They have a web page of their products and work well. You could get them to
tune the preamp for 851-860 Mhz, this reducing some of the cell band overload.
Good luck. Those type of problems can be tuff...
 

zz0468

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What you need is a bandpass filter for 850-869 MHz. You could try these guy: PAR Electronics | Filters

Or you could try your hand at ebay, but you'd need to know what you're looking for. The DB Products flat-pack trunking duplexers have excellent filters that will reject the cell site signals.

The transmit side of this duplexer would be perfect: Antenna Specialist DB Products 868 901 MHz Duplexer System | eBay

For the adventurous, the perfect filter can be removed from an old 800 MHz Motorola Spectra. They're cheap and easy to find, especially in the parts of the country that have rebanded. It takes a bit of technical skill to extract the filter, and wire it up, but it makes for a fairly uncrunchable receiver for the 800 MHz band.
 

KC8ESL

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I'll agree with the band-pass filter idea. If all you want is a small segment of spectrum, it would be the thing to do.

A cheap 1/4 wave notch filter might still be -3dB right where you want full signal strength - right in your pass-band.

D L 4 X A V - jo 43 xn - h a m b u r g If you're interested and have the skills, try this out. - FYI having a SpecAn with tracking generator makes this a very easy project. Not many people have access to this type of stuff though.
 

nr2d

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The thing to keep in mind is the insertion loss due to the bandpass filter. In my experience a bandpass filter my quite some of the noise BUT the insertion loss, about 4- 5 Db is unacceptable.

As for the preamp if you put it at the antenna or back at the radio you are still amplifying not only the signals you want but everything else on the band including noise.

Low loss feed line and a higher gain antenna would help you a lot. Also putting the antenna up higher and outside will also help.
 

KC8ESL

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So if he used the preamp at the antenna with the lossy bandpass filter at the radio... He could gain stage it correctly as if nothing were on the coax either helping or hindering the signal, which is exactly what he wanted in the first place.

Yes, antenna outside is the best idea yet but if you need a compromise, it would in theory work to do as I mentioned above.
 

kayn1n32008

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So if he used the preamp at the antenna with the lossy bandpass filter at the radio... He could gain stage it correctly as if nothing were on the coax either helping or hindering the signal, which is exactly what he wanted in the first place.



Yes, antenna outside is the best idea yet but if you need a compromise, it would in theory work to do as I mentioned above.

Better off to use low loss coax/hard line and put the preamp behind the filtering, that way it is not amplifying the unwanted signal


Sent from my iPad using Tapatalk
 
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KC8ESL

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For a scanner setup it doesn't matter where the filter is inline, it blocks it or before the pre just as well as it blocks it after. I say again, gain stage correctly.
 

zz0468

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The thing to keep in mind is the insertion loss due to the bandpass filter. In my experience a bandpass filter my quite some of the noise BUT the insertion loss, about 4- 5 Db is unacceptable.
I wouldn't rule it out, if it would help reception. Refer to the dbm to microvolt conversion chart: http://wa8lmf.net/miscinfo/dBm-to-Microvolts.pdf

If your receiver threshold is .2 uV that's -121 dBm. Add 5 db loss, and your receiver system threshold becomes -116 dBm, or .355 uV. Now inject those two signal levels into a receiver and compare. I would submit that, in an FM or P25 receiver, in actual practice, you won't notice that 5 db at all. And if it results in 60 db of attenuation of strong cell site signals that overload the receiver, the effective improvement in receiver performance will vastly outweigh the 5 db insertion loss.

As for the preamp if you put it at the antenna or back at the radio you are still amplifying not only the signals you want but everything else on the band including noise.
Putting the preamp in front of the filter would be pure foolishness. The best thing to do would be to put the filter and preamp at the antenna, with the filter first. That sets the noise figure for the system, and then feedline loss becomes irrelevant. You can get a 0.5 db NF preamp, put it behind a filter with 5 db loss, and have a system NF better than 6 db, which is about what the barefoot scanner is likely to be. What you gain is a reasonably sensitive receiver that's immune to out of band overload.

BTW, a good filter will have considerably less loss than the 4-5 db mentioned.
 

zz0468

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For a scanner setup it doesn't matter where the filter is inline, it blocks it or before the pre just as well as it blocks it after. I say again, gain stage correctly.
It does matter. The filter MUST be in front of the preamp, or the preamp will be vulnerable to overload from out of band signals.
 

policefreak

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Thank you for all your replies.My apologies for delay in response. I just had a chance to try preamp near the antenna with no difference in results. Next step is filter.
 
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