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UHF Radio Feedback

Submitted: Monday, Sep 03, 2007 at 21:19

alcokev

Article Overview - UHF Radio
If you are considering a trip into Australia’s vast outback, then a UHF Radio is an important communications device to have. View Full Article...
hi there i have a problem with my uhf, it works fine with the engine off, but when the engine is on there is interference with the output there is a buzzing sound behind my voice at the receivers end. what could be causing this. its on a 1995 pajero, its earthed and is connected to the 12v ignition wire that runs to the radio for power.
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ThreadID: 49292 Replies: 5
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AnswerID: 260186   Submitted: Monday, Sep 03, 2007 at 22:09

Eric Experience replied:

alcokev.
Try running the radio directly from the battery. Eric.
Reply 1 of 5
AnswerID: 260191   Submitted: Monday, Sep 03, 2007 at 22:33

RFLundgren (WA) replied:

You dont say what motor is in your Paj. If it is a pertol motor you will need to get a niose supressor to insert to stop the interference. I am assuming a pertol as when I had my Pajero which was the 2.8 TD I did not have any issues at all in regards to noise from the radio. I had at the time both UHF and CB.

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Cheers

Richard

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Reply 2 of 5
AnswerID: 260216   Submitted: Monday, Sep 03, 2007 at 23:48

LGen1 replied:

If you have it earthed to the battery it may be the problem,
Earth it to the body of the vehicle.
Reply 3 of 5
FollowupID: 521666   Submitted: Tuesday, Sep 04, 2007 at 11:17

Ingtar posted:

What? Earthing to the body of the vehicle introduces more points where noise could be induced. Direct to the battery is the best option, and the battery will work as something of a buffer.

Another option is to look what your power/aerial cables are run next to... if they run too close to other electrical items they may cop interference even if it is not feeding through the power circuit.

Other than that, noise suppressors as others have mentioned.
FollowUp 1 of 4
FollowupID: 521758   Submitted: Tuesday, Sep 04, 2007 at 20:08

LGen1 posted:

Irrespective of what Ingar posted, try it.
I had the same problem and a radio tech. removed the earth from the battery and earthed it to the body of the vehicle, nothing else was done, and the problem was eliminated.
FollowUp 2 of 4
FollowupID: 521789   Submitted: Tuesday, Sep 04, 2007 at 22:34

Ingtar posted:

Sorry LGen, I just like to understand why things do what they do... I have always been taught to go direct to the battery. Only things I can think of is that possibly the vehicle acts as one big shield, or moving the earth simply cleaned up a dodgy connection.

Sometimes too many things are taken to a battery and that causes problems because they can't all get a clean connection.
FollowUp 3 of 4
FollowupID: 523589   Submitted: Friday, Sep 14, 2007 at 18:00

Mike Harding posted:

This is one of those odd times when you may both be correct :)

High frequency radio signals are complex and obstreperous animals and 477MHz is a fairly high frequency. In any particular vehicle installation it may be that by changing something as simple as a cable length or the location of the ground point - changes which wouldn't matter in a low frequency or DC situation - you significantly alter the electrical properties at UHF of the whole installation.

RF engineering in these sorts of situations is something of a black art and it's usually worth trying things which one may not think would make a difference. At 477MHz a piece of wire which your multimeter would register as zero ohms may have significance impedance (resistance).

Mike Harding
FollowUp 4 of 4
AnswerID: 260217   Submitted: Monday, Sep 03, 2007 at 23:52

Olcoolone replied:

One other thing you might like to try is get some coax cable and connect the positive from the battery to the center core at one end and at the other end connect the radio via a fuse, the outer braid should be connected earth at both ends.

The idea is to sheild the positive wire from noise......

The other thing is to take the belt of the alternator and start the car and try using your radio, if the noise when transmitting has gone a away you know it is charging system noise and you can address it. DON'T RUN THE CAR FOR MORE THE 2 to 3 MINUTES AND DO IT WHEN THE CAR IS COLD.

If the noise is still there you will have to try and find it, many things like fuel pumps, ecu's, distributers and coils are a common sorce of RF.

It might even be voltage induced noise or a problem with the radio. Most noise when transmitting is caused from dirty power, you don't see it much in mobiles but more when people use cheap unfiltered 240v power supplys

Check the voltage at the radio when transmitting with the engine on and off. (when checking the off one turn you head lights on for 30 seconds and then off, then test the power at the radio.

You could even try running the radio from a external power source (power supply,another vehicle) and see what happens.

Regards Richard
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Reply 4 of 5
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AnswerID: 260236   Submitted: Tuesday, Sep 04, 2007 at 07:35

Grungle replied:

Try fitting a noise supressor. Very cheap to buy and is a common fix for the problem described (although some vehicles like Fords are extremely hard to get rid of this noise).

Found a link to a site that goes through the process of trying to eliminate the noise from a 4wd that has a whine present on his CB ->http://www.stu-offroad.com/cb/cb_install-4.htm

Good luck!

Regards
David

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Reply 5 of 5
FollowupID: 523561   Submitted: Friday, Sep 14, 2007 at 16:22

alcokev posted:

all is sorted, hooked it upto 12v always on wire to the radio instead of the ignition switch one. cheers for the help everyone, was away in bendethera last weekend and it was pretty wet. but the pajero didnt stop. kevin
FollowUp 1 of 1

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