Problem with 2 way radio and siren

[Jay G.]

Ok heres the situation:
2003 Ford E450 Cutaway Chassis with Medtec custom box.

Radios:
Low band 46.10 mhz CDM1250
UHF CDM1250

Siren:
SVP - SA441MA - Magnum Siren.

Problem: When you tx on the LOW band radio it will key the siren no matter what position the siren selector switch is in

Any suggestions of what to check??

[spareparts]

Same sound from the siren each time or does it change?

Sounds like enough RF is getting into the siren to confuse it that one of the select lines is triggered. Look at the horn ring first. Disconnect it from the controller and try transmitting.

Martin

[jmr3865]

the antenna cable and the siren wires are too close together

[wx4cbh]

I concurr with the proximity factor of low band feedline and siren wiring. Years of installing low band Syntors in NCSHP cars tells me that the higher the SWR, the more likely the proximity of the control cabling with RF cabling will cause this type of problem, and on some low band antenna installations, you have to settle for 2:1 as the best that can be achieved with a particular antenna type, antenna location, mounting options, and vehicle profile combo.

Where the cables cross at a right angle or so is not where the problem will lie. We've occasionally had to run them on opposite sides of the vehicle in order to attenuate the parallel coupling that takes place. On remote mount sirens, grounding the shields on the siren control cables (if equipped) at both ends will deter the coupling but it still may be necessary to run them a good distance apart parallelwise in order to get some insurance on future problems. We've even had to sometimes run shielded audio and control wire where the original equipment does not provide it.

Also very important on all installations, but especially low band installations are the neat freaks who insist on neatly coiling excess cables and securing them in a position that creates a fairly good RF coil. This practice is almost guaranteed to cause untold RFI problems in accompanying equipment. If you've just gotta coil the cables, coil them in a large enough diameter coil that allows you to pull opposite sides of the coil in to the center and create a sorta flat bundle without compromising the minimum radius bend for the cable. This will help attenuate the RF coil effect.

[necsradio]

I had this problem with lowband and a pyramid repeater.
The radio would create an rf loop and stayed keyed.
I put a RF choke on both antenna leads and seemed to do the trick.

[kielhofer]

It depends on where the radio is.

I had a squad car that had the same problem, there was a Motorola mobile mounted either on top or bottom (can't remember which) of the siren controller. The radio had a mini-UHF connector and the shell and worked it's way loose so only the center pin was making contact. There was enough RF leaking out to get into the siren controler. After I tightened the RF connector the problem went away.

I have found out that sometimes the problems that make you bang your head against the wall have the easiest solutions.

Craig