Filter needed or not?

[topgun1986]

It's been awhile since my last post but it's great to be back on here.
I am looking to "remodel" my shack and have a question regarding a filter. How did you know installing one in your feed line, say before a Multicoupler would improve your scanner's reception?


I mainly monitor public safety in the 150-160 MHz area and would like to concentrate on that frequency range for now. Is there a way I could "trace down" certain pager systems and other culprits for my area so I know what I am up against and which filter would work best for my area?

 

[parnass]

Are you experiencing out of band interference? I wouldn't add a filter in the feedline until and unless you are plagued with interference from out of band signals. Try your setup without a filter first.

Good filters are expensive. Even good filters can attenuate the signals you want to receive. In addition, each device you add to your system increases the complexity and chances for failure (i.e., reduces reliability).

 

[topgun1986]

Honestly, I am not sure what would qualify as out of band interference. I have read about others adding some type of notch filter specific to their needs and having great results. My radios (3-Pro 163s & 1-Pro 197 all connected to different antennae) just seem to scan for long periods of time. It seems that for the scanners and antennae I have scanning in such a short radius (10-20 mile) that I should be hearing more. Sometimes I do hear the WX (162.500) in the background on some transmissions.

 

[zz0468]

...It seems that for the scanners and antennae I have scanning in such a short radius (10-20 mile) that I should be hearing more. Sometimes I do hear the WX (162.500) in the background on some transmissions.

If you're hearing weather broadcasts superimposed on other signals, that's symptomatic of an intermod problem. Since intermod can occur in multiple places, you need to try to determine if it's occurring in your receiver, or if it's happening externally. If it's external, there's little to nothing you can do - the receiver sees it as a legitimate signal, and will process it accordingly and deliver the result to your speaker.

If it's internal, there are several steps you can take to try to eliminate it.

If your scanner has an attenuator, try applying it during a transmission where you hear wx and other audio at the same time. If it goes away, it's likely that the intermod is internal to your receiver.

If the interference gets a bit weaker, along with real signals, then it's external, and your options as a scanner owner are limited to non-existent.

If you determine with the attenuator that the interference is generated in your receiver, you can either operate with the attenuator on, or try getting a notch filter for the WX frequencies. These people are the place to go: PAR Electronics

NOAA weather transmitters are notorious for contributing to intermod problems because they frequently run with several thousand watts ERP.

In regards to your original question, I suspect a lot of guys install filters without knowing what's really going on, if anything. Properly done, selecting a filter requires at the least, a spectrum analyzer. Otherwise it's guess work. It's possible to make educated guesses, but that usually takes some industry experience. Typically, a hobbyist type scanner listener will determine the need for a filter by hearing things on frequencies that they're known to not belong on. That's all you have to work with, so run with it.

 

[topgun1986]

I will try the attenuator on my Pro 197 next chance I get.

Reading up on this problem, others have had issues with pager systems causing problems with overloading the scanners front end. Perhaps there are a few in my area causing some problems.

I do not have a true spectrum analyzer but I do have a RTL dongle and SDR Sharp I recently set up on a laptop in my shack. It will take some more studying/reading on my part but I might be able to use that software to help find some of the offenders. I've read lots of great things about PAR Electronics and will more than likely be purchasing one of his filters soon to help with the 150-160 MHz area I mainly scan.

Thanks for info!

 

[BodePlot]

Attempting to do "interference" abatement without a highly sensitive lab grade spectrum analyzer is like trying to shoot fish while blind folded. You really need to borrow one or invite a friend who has one.

First you have to determine the EXACT cause of the "interference". It could be a spurious emission from a problematic transmitter (in our out of band); it could be intermod (from the mixing of several local signals), or; it could just be your receiver (poor selectivity, poor sensitivity, excessive NF in the front end, etc).

The appropriate remedy is determined by the exact type of "interference" experienced. And as mentioned by another poster above, any filter (even those properly applied) will also attenuate desired signals to some degree. Be sure you really need a filter before applying one. This is another case where more isn't always better.

If your enjoying your hobby time and getting some experimental enjoyment, that's great. But I wouldn't recommend spending considerable $$ on manufactured filter product(s) unless you have really identified the specific type and source of the "interference".

Good luck and enjoy the scientific persuit!

 

[PlumCreek]

Filters

Topgun,

I do not know if you got your interference problem solved or not. After purchasing a WS1095 scanner, I experienced a lot of interference that I did not have with the PS500 except occasionally. Using AntennaSearch.com, I found several high power paging and cell phone transmitters about 1-6 miles away as well as a high power NOAA transmitter further away but coming in very strong to my location. There are also TV and FM stations not far away. Attenuation efforts did not work. Upon talking to Dale at Par Electronics (, he recommended a bandpass filter since I was interested in the 450-460 MHz range.

In a few days Dale got back to me with the info on the bandpass filter, and, yes, a filter can be expensive but the satisfaction of having the WS1095 receive properly more than outweighed the cost. I ordered the bandpass filter and now I am a happy camper. When the interference tries to sneak in, the filter knocks most if it out. The transmissions are now loud and clear. The filter is a UHF Bandpass for 450-460MHz. Dale uses a simulator to design the filters to customize them for his customers.

You might want to contact Dale and discuss your situation with him and I am sure he will make a suggestion as to how to proceed and what you may need to make your scanner listening more enjoyable.

Happy scanning!

 

[sparklehorse]

A quick way to figure out if de-sense caused nearby transmitters is your problem is to try one of the band pass filters sold at scanner master:

BPF-VHF-PN Band Pass Filter

or

BPF-VHF Band Pass Filter

They're not exactly cheap, but they are cheaper than any of Par's filters, and they are very effective. I tried one of these on my Whistler 1065 and it made a world of difference on VHF-Hi public safety band. Of course it limits your scanner to receiving only that band, but if that's all you're interested in then you're golden. If it solves your VHF problem, but you want to use your scanner for other bands, then at least you know it's a de-sense issue and you can begin to figure out what notch filter, or filters, you'll need. In my case I needed all of these:
FM notch, two pager notch filters, TV Channel 8, and TV Channel 10. All from Par Electronics. Expensive yes, but they work great, with minimal signal loss. The GRE scanners are infamous for de-sense problems on VHF-Hi, so if the OP is in an urban area then it's quite likely the issue.

.

 

[PlumCreek]

sparklehorse,

Yes Par's filters are costly but remember his filters are custom made for the scanner's particular experience with interference. The mass marketed ones are less expensive due to manufacturers making substantial numbers of filters. Many of the mass marketed filters will not solve all of the scanner's interference problems.

And you hit the nail on the head about Par's filters working great with littler signal loss. My scanner listening is much more pleasant now that I can hear the signals loud and clear.

Happy Scanning!