Most Common source of Intermod

[motomeso]

Just a general question for the users here,

I had an intermod problem related to a paging system system here in Ontario,Canada on 149.770mhz that I was able to filter out with a Sinclair cavity filter.

Anybody else have intermod issues? If so what was the source or sources and what did you do to correct the problem?

 

[N_Jay]

Just a general question for the users here,

I had an intermod problem related to a paging system system here in Ontario,Canada on 149.770mhz that I was able to filter out with a Sinclair cavity filter.

Anybody else have intermod issues? If so what was the source or sources and what did you do to correct the problem?

Are you sure it was intermod, and not other overload conditions.

The solution to true intermod is to find the source (nonlinear element) and either remove it or reduce the level of signal exciting it.

 

[brandon]

I'm using 2 PAR filters. One is their generic one for the 152 MHz pager band. The other is a custom filter for 159.360 which seems to be used by CDF as the input to 151.385. I must be close to their transmitter because it came in all over the 150-174 MHz band before I got the filter. Even with the filter their signal is still full quieting but at least doesn't cause any more intermod problems.

 

[motomeso]

No it was intermod, it was mixing with a few other sources and causing havoc here a fair bit. Taking one frequency out of the mix seemed to calm things down a fair bit. I do I have an overload issue with a full duplex VHF phone system near by but the paging system was causing the most problems here. When somebody uses the phone system I can pretty much hear it on every VHF frequency programmed in the scanner. Lucky for me it is only used a few times a day and I don't think it is worth another filter personally.

 

[Bucko]

Like ve3nsv I have a pager interference problem too. The pager's tower I can see from my window and is on 152.48. It has been in business for several years now and just gave up on trying to stop the problem. I have tried notch filters which help but I ended up losing gain where I needed it. Of course some scanners are worse than others about getting tore up and it does not matter whether it is a Shack scanner or Uniden.

 

[kb2vxa]

Hi NSV and all,

To get your general question out of the way with a general answer, there are far too many to even begin to discuss here, the possibilities are endless. Third order is the most common and most difficult to deal with unless the frequencies are known and you know how to do the complex equasions to deal with it. This alone leaves it a hit or miss proposition for those less than a degree or Amateur Extra license. If you think that's tricky, the source may be outside the radio altogether, a rectifying contact between two metal surfaces somewhere in the neighborhood. Ug, sure gets sticky! Maybe someday I'll tell you about how a bad ground injected audio from a broadcast station into a repeater last summer and how it was discovered and fixed. WOR on 2M? Yeah, it happened.

OK, so we get lucky once in a while like you did, you found the frequency of the strongest signal and notched it out. The best solution is a proper receiver which has a good front end but that is usually a pricy one and then too you have to be knowlegable of more than the published specs. Nope, they never include the dynamic range figures critical to the "equasion", you probably will never find out unless you have acess to a testing lab and some awfully expensive equipment and of course know how to use it, that leaves out the casual user. Back to the hack and whack method, that sometimes means hacking together a coaxial stub which often works surprisingly well but again you have to know the frequency and the equasion to plug it into to get the right length. Cheap and dirty? Sure is but often is quite enough before we have to get out the big guns, bandpass, bandstop and notch filters.

Continued next week, 1001 uses for bits of scrap coax or how I towed a '56 Chevy with RG-8U.

 

[woody_46]

Continued next week, 1001 uses for bits of scrap coax or how I towed a '56 Chevy with RG-8U.

I'm looking forward to the towing story..


My problem is overload from strong FM transmitters. The one station is so strong I can hear it as background music when stopped on some freqs. That's why I've given up on an oudoor antenna with my trunking scanners. I also have a strong pager nearby that causes intermod. I agree with the above poster about radio design being a major part of the problem. Thank goodness for attenuators.

I actually posted a question about notch filters in the Eastern Canada forum, no response there. Here's the filter I'm looking at, any comments on it?.. http://www.durhamradio.com/s/product.php?productid=44839&cat=1825&page=1 I realize it's a compromise as opposed to a filter on a narrow freq range. As posted above, finding the exact offending freqs, and how many can be difficult.

 

[rjschave]

I'm using 2 PAR filters. One is their generic one for the 152 MHz pager band. The other is a custom filter for 159.360 which seems to be used by CDF as the input to 151.385. I must be close to their transmitter because it came in all over the 150-174 MHz band before I got the filter. Even with the filter their signal is still full quieting but at least doesn't cause any more intermod problems.

Can someone please explain full quieting to me?

Thanks.

 

[woody_46]

Can someone please explain full quieting to me?

Thanks.

I looked it up to get a good desrciption for you.

A phenomenon on FM transmissions where the incoming signal is sufficient to engage the receiver limiters - thus eliminating the noise due to amplitude fluctuations.

Basically it means a strong signal free from distortion.

 

[Bill2k]

Full quieting is used to describe how a transmission station sounds. In other words, the station comes in real strong with absolutely no static in the background during transmissions.

Bill

 

[rjschave]

Thanks for the info.

 

[Al42]

Guys, let's use proper terms here.

Intermod is 2 or more signals mixing, creating a signal on a third frequency, which you're picking up. You won't get intermod if you live next door to a 600 watt transmitter - you'll get primary overload.

Sometimes it's difficult to tell the difference but with intermod you usually hear two voices at the same time, or you may hear paging tones and a voice - and at least one of the the voices starts and/or stops in mid-sentence most of the time.

If you're just hearing "splatter" (what you'd hear listening to 95.5 FM if you're parked outside the 95.7 transmitter), it's not intermod, it's overload.

Why care?

Overload can only be improved by eliminating the signal in your scanner, which may be impossible. Intermod can sometimes be eliminated much more easily - you have to find the source, which may be something as simple as a corroded screw on a rain gutter. (Replace the screw, or just turn it 1/8 turn so it makes better contact, and you might eliminate the intermod completely.)

 

[woody_46]

Thanks Al, I have both when using an outdoor antenna. Intermod can sometimes also occur when the frontend of the receiver is overloaded (saturation of the RF amplifier passing unwanted signals to the mixer stage) . That's why this is so maddening, and difficult to pinpoint at times. It is not an exact science.

 

[Tweekerbob]

Here is some additional info re: IMD and also an introduction to different methods in reducing it, courtesy of ARRL.

 

[woody_46]

Here is some additional info re: IMD and also an introduction to different methods in reducing it, courtesy of ARRL.

Thanks, nice article!

Now lets add another problem to the equation>desensing.lol. It's another condition from overloading. People need to keep this in mind when connecting a scanner to an outdoor antenna. I've mistaken poor performance to a bad antenna, or coax, when in fact the scanner was the culprit. I now check my antennas with a known good receiver (has a good frontend & S-meter). That way I will know a new scanner I'm trying may be overloading with the outdoor antenna.

 

[N_Jay]

Thanks Al, I have both when using an outdoor antenna. Intermod can sometimes also occur when the frontend of the receiver is overloaded (saturation of the RF amplifier passing unwanted signals to the mixer stage) . That's why this is so maddening, and difficult to pinpoint at times. It is not an exact science.

What is realy maddening is that fact that it IS an exact science. :lol: :lol: :lol:

 

[woody_46]

What is realy maddening is that fact that it IS an exact science. :lol: :lol: :lol:

Okay, guess we need you to explain it to us, what do I need to to get all of my scanners working properly with an outdoor antenna.

 

[mbstone99]

149.77

Just a general question for the users here,

I had an intermod problem related to a paging system system here in Ontario,Canada on 149.770mhz that I was able to filter out with a Sinclair cavity filter.

Anybody else have intermod issues? If so what was the source or sources and what did you do to correct the problem?

Actually 149.77 is you didn't know is pretty much a province wide frequency for pagers used by Bell Mobility.

Matt

 

[Tweekerbob]

Well, N_Jay is right, it is an exact science. But it is a very complicated science too, IMD that is.

If overload/desense is your true problem then there are a few simple techniques to help with that:

1. Use antennas cut/designed only for the bands of interest to you. Using a "wideband" antenna will bring in all kinds of "other" signals, most are of no interest and if they are strong, will have the potential to cause overload. An antenna tuned to a specific band will receive all frequencies, but it will adequately (in most cases) attenuate all signals not associated with the antenna's tuned frequency or odd harmonics of the tuned frequency. (see that article for further info on harmonics).

2. If the offending signals are relatively close in proximity to your frequencies of interest (like pagers or FM broadcasts, for example) consider coaxial stubs or better yet, purchase the appropriate notch filters to provide a decent null for those frequencies.

3. If the two above methods fail concurrently, then go outside and look up to make sure you are not living under a radio tower; you have a strong signal indeed. You will then probably have to dial in some attenuation via an onboard ATT in your radio or purchase an external one. 10dB of attn. should probably do the trick, and not affect your ability to hear moderately strong signals, but if necessary try 15 and 20 dB.

4. Move to the country (away from radio towers) .

There are other methods, but these are the cheapest and easyist for the hobbyist to accomplish and will in most case eliminate about 90-100% of the problem. Barring moving to the country, you can also purchase surplus commercial gear (single banded) and use a single band antenna.

 

[N_Jay]

Okay, guess we need you to explain it to us, what do I need to to get all of my scanners working properly with an outdoor antenna.

You need to know what it is you are trying to receive, and know what it is that is causing the interference.

From that you need to figure out what is the mode of the interference. IM, Overload, Desense, Image, etc.

Then you need to take the appropriate steps to correct the situation.

Most of the time you need to reduce the level of the interfering signal sufficiently. Other times you need to correct the source of the interference (such as with external IM).
Sometimes there is nothing you can do, such as an overloading adjacent frequency.
Sometimes it is the receivers fault (such as an interfering image).

Sorry of this sounds like I am not answering your question, but it is like asking "Why won't my car start?", there are 100's of reasons and very few of them are mysterious.