How do I know if my scanner is going into "desense"?

[LIScanner101]

The whole selection process of bandpass/notch filters for scanners is mystifying to me because from what I gather from my limited understanding of the subject you have to know what the source is in order to get rid of it AND to improve the scanner’s performance. I’ve seen posts where people have said their scanner has gone into desense due to local high power hospital pagers, FM stations etc. They don’t always say HOW they know the source, but they say they install filters (such as PAR’s) and they suddenly start hearing signals they never even knew were there, OR their reception of previous weak signals was tremendously improved.


In my case, I would love to improve my overall scanning experience but I honestly don’t even KNOW if I have any offending signals, so I don’t know if my scanner is doing the best it can under any filtering scenarios, OR if it really could benefit from the use of filtering.

How does one even go about testing to see if there is any desense? What if the signals I’m trying to pick up are just too weak to pick up, and no filter in the world would make any difference? Do I try scanning a certain band of signals and take note of any “data like” sounds?

I would really appreciate any help in understanding if I even have a desense problem to begin with!

 

[WA0CBW]

Desense is caused by a strong adjacent channel signal making your receiver less sensitive. You can be listening to a signal fine and when the offending adjacent transmitter comes on your signal gets noisy or disappears altogether. You may or may not also hear "bleed over" from another channel. It usually occurs when trying to listen to a weak signal. If the desired signal is very strong it may over ride the offending frequency. Again it is caused by strong signals near your receiver on a different frequency. The solution can be to filter out the offending frequency before it gets to your receiver front end. Typical offenders are paging and broadcast stations. If you don't know what the offending frequency is a spectrum analyzer connected to your antenna may display the offending signal.
BB

 

[nd5y]

Desense is caused by a strong adjacent channel signal making your receiver less sensitive.

It doesn't need to be an adjacent channel. Strong signals in another band can desense cheap poorly designed receivers like scanners.

 

[WA0CBW]

Tom,

Yes, that would fall into the category of living in the backyard of a high powered transmitter. My experience with scanners would indicate desense occurs mostly with adjacent channel or in band transmitters. The exception is living in the backyard of a broadcast station (usually FM). I find I get more desense with my 396XT when working around commercial business band and public safety band equipment than I do with broadcast stations unless I pull up to the broadcast tower.
BB

 

[LIScanner101]

Thanks for the info folks. Maybe I’m going about it the wrong way but I’m still not clear on this issue:

I guess if someone is actively listening to, say, a VHF-hi frequency and then all of sudden they hear these squawks and squeaks then that would obviously be a pager or some other offending signal. The user could note it, then search for the offending signal up or down from the desired/target frequency and HOPEFULLY catch it “in the act” again. That’s a pretty clear-cut case. But I often see people writing things like “my scanner is going into desense”. To me, that looks like there is some kind of CONTINUAL signal that is swamping the scanner’s front end and causing it to be less sensitive in one, many or all bands/frequencies, problem is, it’s unclear as to how they know their scanner’s potential if the offending signal is ALWAYS there. In this kind of case, where would one start? How would one even know if their scanner was being desensed? Intuitively I would imagine that if you are trying to listen to Podunk PD on 155.155MHz and they are only 2 mile away, yet you can barely hear them then that might be a clear-cut case of general desense. But in my case, most of my VHF hi’s come in pretty clearly with some exceptions. Are those “exceptions” simply a matter of weak repeater output, low repeater antenna height or factors directly associated with the transmitter?

Said in another way: Why shouldn’t I just go out and buy an FM trap, and a few traps for 152MHz, 158MHz and whatever else PAR makes for paging frequencies and just hook them up in series? Aside from the cost, wouldn’t it be best just to assume it’s there to begin with? Too bad PAR doesn’t have a “loaner” program for these things…!!

 

[nd5y]

I guess if someone is actively listening to, say, a VHF-hi frequency and then all of sudden they hear these squawks and squeaks then that would obviously be a pager or some other offending signal.

If you hear tones, data or other transmissions you are hearing getting intermod, not desense.

When intermittent signals like voice communications or paging cause desense, the signal you are trying to listen to will suddenly disappear or be very weak and noisy while the interfering station is transmitting, then return to normal when the interfering station stops transmitting.

If your receiver is being desensed by something continuous like a FM or TV station, you won't be able to tell unless you try different filters, change locations, change antennas, or consistently have better reception all the time with the attenuator on.

Desensitization (telecommunications) - Wikipedia, the free encyclopedia
Intermodulation - Wikipedia, the free encyclopedia

 

[WA0CBW]

There are several things you can look for such as turning on the attenuator and the signal gets stronger. Or the "S" meter shows full bars yet the signal is weak. Driving around with the scanner and noticing an unusual relationship of signal strength and distance from the site. Some of it relies on understanding RF propagation through experience. Knowing the terrain between you and site, the power level, the frequency band, and the equipment can give you some hints if you should or should be hearing. Of course a spectrum analyzer can give you a picture of the RF in your area as well. If you have the right test equipment you can determine how much signal it takes to desense your scanner. Generally it takes a pretty strong signal. Of course it should be obvious if you live in the backyard of a strong transmitter. I would also guess that some people don't really know but if "it worked for the other guy it should work for me" mentality. Also desense is usually associated with a receiver in a repeater. On a scanner I would characterize it as front-end overload which decreases the sensitivity of your scanner.
BB

 

[popnokick]

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Instead of a spectrum analyzer consider using a USB SDR ( Software Defined Radio) like the sub $20 NooElec R820T and software to view your adjacent channel activity. Lots of info in the SDR forum here on RR.

 

[737mech]

FM trap

The cheap way to find out is try the radioshack fm trap. Another way is to ask others in your area what is working for them, or what they have learned about your area. One note about the PAR filters, they work great.
You could ask Dale at PARelectronics for help. He is very good at answering emails quickly.

 

[LIScanner101]

I think I'll give the RS FM trap a shot,

 

[popnokick]

The RS FM trap works... IF your problem is desense caused by nearby FM broadcast signal(s). It's inexpensive to try but if FM broadcast is not the cause of your desense, then it won't help. If you use an SDR USB dongle and one of the software programs like HDSDR or SDR#, you'll be able to see immediately in the spectrum waterfall if there are any extremely strong signals operating nearby.
Do you have CloseCall on your scanner? You could enable that and see what happens. If the signal is strong enough to cause desense, it is likely to be picked up by CloseCall.

 

[zz0468]

I’ve seen posts where people have said their scanner has gone into desense...

...They don’t always say HOW they know the source...

I often wonder the same thing. Based on the vague descriptions in their posts, I can only guess that there's a lot of the story they leave out when they bring their tales of woe to the forums.

In SOME cases, the source can be obvious... Hearing music in the spattering sidebands of a nearby FM station, putting a notch filter in is a good first guess.

But what a lot of people don't get is, what you actually hear as interference may not actually be the cause. Example... there is a multi-carrier cell site in a church near my house. It radiates multiple signals in the 870-896 and 1900 MHz ranges. But when I try to use a scanner to listen anywhere in the 440-470 range, I hear a VHF telemetry signal who's quarter kilowatt transmitter is 20 miles away all over the UHF band. At first glance, one would blame the telemetry station, but it turns out it's the cell site crunching the front end of the scanner. And I don't even hear the cellular stuff at all. A 550 MHz low pass filter in front of the scanner settles the problem right down.

In my case, I would love to improve my overall scanning experience but I honestly don’t even KNOW if I have any offending signals...

Does your receiver have any symptoms at all? Or could you possibly be worrying for nothing?

How does one even go about testing to see if there is any desense?

Desense is a particular problem where a strong signal source reduces the effective sensitivity of a receiver listening in on a different frequency. It doesn't necessarily come with visible symptoms.

The only way to test for "desense" is with a signal generator, and an isolated tee inserted into the antenna line of the suspect receiver.

Maybe it's intermod you're worried about.

One easy test for non-technical scanner listeners is to insert a 10 or 20 db attenuator pad in front of the receiver. If it settles down and actually sounds better with weaker signals being allowed into the receiver, then you have an intermod (and/or possibly a desense) problem.

What if the signals I’m trying to pick up are just too weak to pick up, and no filter in the world would make any difference?

The find something closer to listen to. Remember, some systems are specifically designed NOT to cover outside their area, and trying to hear it from 50 miles away will be an exercise in frustration and futility. The typical remote transmitter site has a LOT of directional antennas on the towers, meaning there is a lot of radiation NOT going in certain directions. Without fully understanding the system you're trying to listen to, it all boils down to luck as to whether there is enough signal in your direction to bother with. There frequently is not.

I would really appreciate any help in understanding if I even have a desense problem to begin with!

I would venture to say that, if you're not actually aware of a problem, then go on your merry way and enjoy what you hear. If adding a higher or larger antenna suddenly introduces strong and odd signals where there were none before, then you need to consider a filter, once you figure out what filter you need.

Without a spectrum analyzer, it's guess work and luck.

There are a lot of good ideas being offered in the thread... But I would start at defining whether or not you even have a problem to begin with. When you do, there's really little doubt that something's not right. It's just not sounding like you're there yet.

I think I'll give the RS FM trap a shot,

For what? You haven't actually even mentioned that you have a problem.

 

[SCPD]

The RS FM trap works... IF your problem is desense caused by nearby FM broadcast signal(s). It's inexpensive to try but if FM broadcast is not the cause of your desense, then it won't help. If you use an SDR USB dongle and one of the software programs like HDSDR or SDR#, you'll be able to see immediately in the spectrum waterfall if there are any extremely strong signals operating nearby.
Do you have CloseCall on your scanner? You could enable that and see what happens. If the signal is strong enough to cause desense, it is likely to be picked up by CloseCall.

Close call with global att on just picked up 96.3 in band 2 on bct15x... full strength.... but with att off it skips by 96.3?
uhf and 800 whiped out they put this station in about 2 or 3 years ago... think the fm trap will bring back uhf and 800?

I have the par vhfsym152ht tuned to 152.48 pager and par vhfsym158ht tuned to 157.74 and pagers that maxed the signal out in att mode knocked 2 pager frequencies in vhf out completely
Dale worked with me and tuned the notches perfectly...


Oh yes another question about my filters...Didn't want to bother Dale (again)..
should I connect these end to end or should I leave 1/4 wave of coax between each notch filter for best recovery. and should the 152.48 MHz pager filter be closest to the scanner or 157.74mhz pager filter
or does it not matter orientation of the frequency when in series

 

[737mech]

You can cascade par filters any way you like. The only time you need coax between is if you are using the same exact filter, then you need it.

 

[prcguy]

As mentioned there should be no interaction cascading different frequency filters but when cascading identical filters to multiply the benefits of a notch or improve skirts on a band pass, the cable lengths are very critical and it would be nearly impossible to get it right without sweeping the filters and cables with a network analyzer.

I've assembled and aligned countless filters and duplexers with new cables and the end length rarely comes out to a calculated length due to pickup loops and other components in the filters adding to the electrical length.
prcguy

You can cascade par filters any way you like. The only time you need coax between is if you are using the same exact filter, then you need it.