can you protect a scanner from RF

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Kfred

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Any one know how to keep from destroying a scanner when using a 100 watt high band mobil radio? How far apart would the antenna's have to be on the roof of a truck? I have seen references to using back to back diodes on the antenna but do not know if this would work or how much power they could take without being destroyed and leaving the scanner unprotected. Kfred.
 

Cowthief

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Keying relay.

Hello.

The normal solution is a keying relay, something that disconnects the scanner or other receiver from high RF levels.
There are keying relays that work on RF level alone, so except for the power connection and of course the RF connection there is little to do.
Most are now solid state, PIN diodes, just like an RF TR switch.
2 diodes back to back will work also but will add RF noise to your signal.
So, the best solution may be to simply herd into the junkbox.
The RF switch a nintendo uses works very well as a low power keying relay.
Power is derived from an RF preamp that drives a diode bridge.
So, RF level exceeds a given level and the preamp goes into saturation, driving the diode bridge that drives the PIN diodes.
This is what is inside just about every modern military radio made, EMP protection.
On military radios one must reset the radio once the event is over, if anyone is still alive to do this.
The one I got was military surplus that was intended for a multi-unit installation of the AN/VRC-12.
After that, with a firm understanding of how this works, I have been able to run several radios in vehicles with no trouble whatsoever.
And, if you are worried that 2 diodes are going to fail you need to reexamine your proposed install.
Even if you can keep the RF out of the coax, the RF levels are too high on the other antenna(s).
Remember, an antenna is intended to be away from other objects, including other antennas.
An antenna that close can easily affect the SWR of the transmitter.
Normally, a VHF 100 watt transmitter antenna mounted on the trunk lid will give no trouble to another antenna mounted on the roof, both distance and vertical separation come into play.
 

SAR923

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If it's any help, I had an Explorer with a 50 watt UHF radio feeding an A/S broadband antenna that was mounted on the forward part of the roof, a 100 watt VHF radio feeding an A/S 5/8 wave mounted in the center of the roof, and a Uniden 785 that was hooked up to a Larsen 150/400/800 antenna toward the back of the roof. With the exception of some ear bending feedback if I keyed up on VHF and the sanner was stopped on another VHF frequency, I never had any damage to the scanner. A keying relay will stop the feedback problem but I doubt you'll need it to protect the scanner.
 

Kfred

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Early, TX
Thank you

Thank's for the rapid reply, this is the kind of information I was looking for, Kfred
 

RodStrong

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I've ran a combo of high power and mid power VHFs, along with low to mid power UHF mobiles in various vehicles I've driven over the years, and they've never screwed up the mobile scanners I've used. Like the other poster, I get feedback occasionally, due to various conditions, but I've never knowingly hurt my scanner.

Good luck.
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
The military radios that incorporate a cut out relay for RF or EMP overload do not require resetting, they simply switch the antenna path back to the receiver when the offending RF level drops below a safe level.
prcguy
Hello.

The normal solution is a keying relay, something that disconnects the scanner or other receiver from high RF levels.
There are keying relays that work on RF level alone, so except for the power connection and of course the RF connection there is little to do.
Most are now solid state, PIN diodes, just like an RF TR switch.
2 diodes back to back will work also but will add RF noise to your signal.
So, the best solution may be to simply herd into the junkbox.
The RF switch a nintendo uses works very well as a low power keying relay.
Power is derived from an RF preamp that drives a diode bridge.
So, RF level exceeds a given level and the preamp goes into saturation, driving the diode bridge that drives the PIN diodes.
This is what is inside just about every modern military radio made, EMP protection.
On military radios one must reset the radio once the event is over, if anyone is still alive to do this.
The one I got was military surplus that was intended for a multi-unit installation of the AN/VRC-12.
After that, with a firm understanding of how this works, I have been able to run several radios in vehicles with no trouble whatsoever.
And, if you are worried that 2 diodes are going to fail you need to reexamine your proposed install.
Even if you can keep the RF out of the coax, the RF levels are too high on the other antenna(s).
Remember, an antenna is intended to be away from other objects, including other antennas.
An antenna that close can easily affect the SWR of the transmitter.
Normally, a VHF 100 watt transmitter antenna mounted on the trunk lid will give no trouble to another antenna mounted on the roof, both distance and vertical separation come into play.
 

kb2vxa

Completely Banned for the Greater Good
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For what it's worth the simplest way to avoid background chatter, feedback screeches and the like is turn off whatever noise source you may be listening to before picking up the microphone. It's only common courtesy, I wish certain annoying operators had some consideration for the listener, it's like trying to hold a conversation in a boiler factory.

Anyway like the others I have never had a problem having taken antenna separation into account, vertical in particular. That makes me wonder how the porcupine whackers get away with it but that's just more fodder for Hamsexy I suppose. (;->)
 
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