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VHF Interference

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burner50

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I'm having an interesting problem.


I have installed an Icom IC-208h in my wifes vehicle (1998 Dodge Durango). Power comes directly from the battery and thru the firewall using 10 gauge wire, properly fused, etc... The ground is under the hood and using the same grounding point as the ABS Computer (among other things) and was testing .4 ohm from this point to the negative side of the battery.

Antenna is a Mag Mount quarter wave VHF until I get a new NMO mount and the vehicle thaws enough to drill the roof. The antenna is placed on flat roof, forward of the luggage rack area on the roof. I dont have enough coax to relocate the antenna anywhere else on the roof.


When I key up on High Power (55 watts) VHF, it was knocking the FM radio out until I un-keyed. It is also knocking out the power to my cig lighter / Powerpoint and my GPS plugged into the outlets. (The LED in the cord goes out).


But when I key up on 50Watts UHF, (similar power drain) nothing like this happens.


I'm somewhat confused, and I understand that it could be the mag mount (hasnt happened to any other cars I've used this mag mount on), but I want to know for sure before I drill a hole in my wifes vehicle.


Anyone have any ideas?

EDIT:

I just realized that I havent hooked up a voltmeter while keying down to see if there is power fluctuations. I might try that tomorrow if I get time.


Anyone have any other ideas?
 

rescuecomm

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My brother's Dodge van's engine would cut out when keying an Icom 2AT with a 30 watt amp on a 1/4 wave mag mount. Changing the antenna to a 5/8 wave fixed it. Why? I don't know and would not have tried another antenna. You might try to respot the antenna or use medium power on the radio. It is RF getting back into the auto's wiring that is the problem. You could have some SWR on 2 meters and not on 440 that is causing feedback.

Bob
 

kb2vxa

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"The ground is under the hood and using the same grounding point as the ABS Computer (among other things) and was testing .4 ohm from this point to the negative side of the battery."

What "ground" exactly are you talking about? I have a sneaky feeling that going anywhere near a computer is just asking for trouble. Just like the B+ lead the B- lead should go to the battery and nowhere else, case ground should be accomplished by the radio mount itself being attached to a steel structural member or heavy copper braid strap to the closest point on the vehicle body.

Mag mounts are notorious for loose coupling to RF ground having no direct connection but relying on capacitive coupling to the planing surface. An SWR meter won't always tell the true story, you may have common mode currents present on the outer conductor of the coax radiating signal into the vehicle electronics and wiring. The only way you'll know what will happen with an NMO mount is to install one, there is no comparison whatsoever with a mag mount.
 

datainmotion

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My experience matches what Warren has stated. I don't think I'd ever share a ground bolt for any of a vehicle's computers with a transmitter. I'd also opt for an NMO mount rather than the mag-mount.
 

N0IU

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My radios get their positive and negative voltage directly from the battery with BOTH leads fused close to the battery. When I first installed an HF rig in my last Jeep Cherokee, I pulled the ground off the chassis and whenever I would key up on 20 meters and the cruise control was engaged, I would speed up! After I re-ran the power cables as described above, the problem went away.
 

n5ims

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"Just like the B+ lead the B- lead should go to the battery and nowhere else, case ground should be accomplished by the radio mount itself being attached to a steel structural member or heavy copper braid strap to the closest point on the vehicle body.
If you follow this advice, be absolutely certain that you place a fuse in both the B+ and B- leads. Failure to do so will cause your radio's power lead to be the entire car's common ground through your radio if there's any break or corrosion issues with the car's normal battery to ground lead.

See this thread for one story --> http://forums.radioreference.com/general-scanning-forum/14080-cb-scanner-antenna-proximity.html#post101112
 

burner50

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What "ground" exactly are you talking about?
It is a factory grounding point. A bolt attached to the metal body with a few wires terminated to it. As I said before it had .4 ohms to the negative side of the battery. I dont know what the "quotes" are all about on the word ground.

there is no comparison whatsoever with a mag mount.
ummmmm, I agree? The roof is covered in snow and ice, I have nowhere to melt it, nor anyplace warm to do the work. It was -9 today when I went to work and 2 when i got off work today.

scottaschultz said:
My radios get their positive and negative voltage directly from the battery with BOTH leads fused close to the battery. When I first installed an HF rig in my last Jeep Cherokee, I pulled the ground off the chassis and whenever I would key up on 20 meters and the cruise control was engaged, I would speed up! After I re-ran the power cables as described above, the problem went away.

What makes you think I dont have them both fused?



Anyway, I checked the voltage while keying down on high power and there was Zero voltage drop, so I'm doubting very much that it is a power problem, but I'm going to go move both leads to the battery and try a different antenna.


See here is the thing. If the truck is sensitive to RF, I'm not going to be able to use a radio in it. And I'm not drilling the hole until I'm sure I can actually use it.
 

code3cowboy

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VHF often kills the stereo in vehicles upon transmit. Every vehicle I have used a VHF radio in has had intermittent issues with the stereo being muted when transmitting. All of the guys I work with have seen the same phenomena, and none of us have ever seen any damage from this.
 

N0IU

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What makes you think I dont have them both fused?
Because you said:
Power comes directly from the battery and thru the firewall using 10 gauge wire, properly fused, etc... The ground is under the hood and using the same grounding point as the ABS Computer (among other things) and was testing .4 ohm from this point to the negative side of the battery.
You never actually said anything about the ground side being fused. I believe the point being made was to run both power leads directly to the battery as opposed to picking up the ground from the chassis.
 

burner50

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Well they are both fused as that is the way the wiring came from Icom.


Anyway, I re-located both the pos and neg to the battery, and same problem. Once I un-ran the coax and moved the mag mount further back on the vehicle, the problems seem to have been somewhat weakened. The FM Stereo still worked, and the GPS stayed "on".


I guess I'll move forward with the NMO, but placing it on the back of the vehicle instead of directly over the front seats (Shorter coax run that way)

Thanks for the assistance.
 

WatnNY

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I have always had the similar problem with VHF transmitting cutting out the stereo, but never UHF. Strange!

Mike
 

stevelton

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Ground is Negetive, not matter where you attach the black wire. So if the OP used the OEM power cable from Icom, and it has a fuse on the red wire and the black wire, and they are connected to the battery, then it IS properly grouned?! Sounds like there are a few on this thread that dont understand that.
As for the stereo, Hum, Math says, 108mHz and the 2 meter band are about 36mHz difference. 108 and the 440 band are about 332 mHz difference.
Sounds like normal front end overload.
What about trying a some kind of filter on the power lead on the back side of the radio, just in case RF is flowing down that.
Steven
 
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