Transmitting Interferes With Scanners

Hello all, I do not know too much about radio installs so please bear with me if this is an obvious problem.

A group of myself and others work together in a photography business, and we recently decided to take our operations efficiency up a notch by renting a radio system. We are utilizing Kenwood NX800 mobiles at this time (total of 3 for now) on an NXDN system provided by Mobile Relay Associates here in So Cal. Two of us had our radios installed professionally by a business that does installs, and mine was installed myself. We all are experiencing one common issue though. Whenever we transmit, our radios are cutting out our scanners or creating horrible interference noise, but is only specifically on frequencies in the range of 450-512. This inhibits us from hearing things while we are talking which could be a serious issue on a long transmission. For my setup specifically, I am using a Uniden BCD436HP connected with a magnet mount antenna, a PRO106 with just a standard antenna, and a Motorola MTS2000. My NX800 is hooked up to a magnet mount antenna, and the other two NX800s are running off drilled antennas. I highly doubt it is the way they were installed because all three of them are having the same problem.

Any one have ideas?

cheap front ends on your scanners

Yeah, it happens on motorolas, as well...

It's called de-sense. As toasty said, you are overloading the front end of your scanner. It's a temporary problem limited to only when you transmit on your radio. The only way you can stop it is getting your receive antenna far enough away from the transmitting antenna, which is about 20 wavelengths in the horizontal plane. On a car, impossible to do.

another option is a notch filter to put on the scanner.
all depends how much time and/or money you want to spend on the problem.

When you transmit on 450-ish UHF, you totally wipe out the rest of the UHF band and probably whack the 150-ish VHF and 800 bands as well. Common situation, and the type or brand of radio has nothing to do with it.

As a photographer you understand the balance between background and foreground or subject lighting. Here is a non-radio example: If I walk a block away on a dark night and signal you in Morse code with a red flashlight you will see the flashing. But if we do the same thing at 5PM with the sun behind me the sunlight will be too bright for you to see my flashing light well enough to decode my signal. My red light is still reaching your eyes, but the sun is too strong to make it out unless my flashlight is very, very bright.

The same applies to receivers in your car. Your scanners are able to hear distant radios (the flashlight) just fine with no strong signals coming from the car, but when you transmit the off-frequency RF energy from your radio (full sunlight) will desense (or overload) your scanners to the point they can no longer see other signals (the flashlight).

It's sort of of like using a f-stop five-brazillion lens to take a picture of the sun at 5PM, and expect to see details on the ground. The problem is that your scanners are fixed at f1.1.

RF Filtering is also similar. If the sun were to turn cyan and I was using a red flashlight you might be able to filter out the top of the spectrum and improve things, but will still lose a lot of the red spectrum. Filtering only magenta (461.075 MHz) from all other reddish and yellowish colors (450-512) also kills close colors in the spectrum (460.500). [Forgive me if I got my colors wrong - its been a while, but you get the idea.]

Does this help?

Edit: Bandpass and notch filter combinations can make it work, but are expensive and must be designed to pass a wide range but block one very specific frequency to the scanners, and do the reverse on your radio. Tuning filters is an art and not cheap either.

If hearing the scanner is essential while you are transmitting, about the only solution I can think of would be to go to something like Zello push to talk on your smart phone, That would put your transmitters up in the GHz range with less than 30mW typically. Using a higher powered (even 5 W ) mobile transmitters will pretty much wipe out any scanner within 20 feet of the transmitter antenna.

Bruce

You could reduce the transmit power, it will help minimize the desense.

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krokus said:

You could reduce the transmit power, it will help minimize the desense.

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Even at 5w he will wipe out the scanner.




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Setup a scanner in a fixed location and stream the audio over the Internet to a mobile device in each of your vehicles.

...what about just dealing with desense like EVERYONE else with a mobile radio setup?

IF YOU'RE TALKING YOU AIN'T LISTENING!!!

Thank you all for the responses. I'll have to reevaluate our situation and go with the best solution.

However, if we just deal with it and not listen while we talk, can that still cause damage to my scanner? Reason I ask is because we have been just dealing with it for the last few days or so, but the other night I noticed that my 436 was not receiving anything in the UHF band with the exception of LA County Fire and Sheriff (Very strong signals), but the PRO106 was still hearing the ones it normally could. I went out again last night, and it was the same problem but not has bad. There were only a couple or a few departments that weren't coming through, but did come through on the PRO106. When I changed locations, the ones not heard did come through, but that's only because I got closer to them.

Did I read somewhere that desense can lead to damage on the front end?

It takes a ridiculous amount of power to damage a front end when the transmitting radio is not directly coupled to the receiver.

Anyone who says "a cop was transmitting in the lane next to me and my radio blew out" is either a liar or didn't realize their radio was on its way out the door already.

Some of us hams have 110w 2m radios in our cars with a scanner antenna on the same ground plane 21" away from the other antenna. No problems. None. Desense? Yes. Tragic failures? No.

You will not have permanent damage.

Please ignore the crudeness of the following picture...

Credit: "Managing Interstation Interference" by George Cutsogeorge

I always get the 800 SZ trunked radio desensing my BCD996XT. I have the PCTEL MLVP 800Mhz on the edge of the trunk and the scanner antennam, a PCTEL MLB4700, granted it is very close (abut a foot away) I dont fear the damage to the scanner as the transmitter is only 5 watts. I am soon to put a Motorola 800MHz antenna kit towards the rear of my roof and put the low band radio back in. Also I cant use the other side of my trunk because it has another MLPV (430?) for the UHF spectra sitting in the center console, and the center of the roof is currently taken by a V/U dual bander for my amateur radio. You guys think that antenna on the roof would have less desense than on the trunk? I have a 2003 Nissan Sentra if you need to look up the car.

Keep your transmit time down when listening to a scanner. Or when you are TX have the person that is RX listen to the scanners for you and so on.

You could put a diode limiter in line with the scanner antenna to reduce the chance of damage when you transmit, but otherwise it won't be practical (read cost effective) to filter the scanner so that it doesn't overload when you transmit.

Put another way, when you're transmitting, you won't be receiving on the scanner. End of discussion.

I've said it before and I'll say it again. When you're talking you aren't listening anyways. It's almost a moot point.

krokus said:

You could reduce the transmit power, it will help minimize the desense.

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I keep my FT-7900 at 5 watts. Although my BCD536 is not yet installed, I don’t anticipate a problem. Previously, I used a dual-band handheld (FT-51R) and handheld scanner (Uniden BC3000XLT), both side by side in the car, with both antennas on the car’s trunk. Since 5 watts was the most I could go with that setup, I never had a problem with the scanner when I was transmitting. When I kicked in the Mirage 40 watt amp? Yep. Sure did.

Pretty common that transmitting on a radio will wipe out a scanner in the same vehicle. Especially if you have the antennas mounted fairly close to each other. Does the same to me on my Subaru Legacy, but I have one antenna mounted on my trunk and the other up front. I'm only transmitting 20 watts on VHF, but it smokes my scanner everytime I key up. So as others have said it; if you're talking, you ain't listening.