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Author Topic: RF on HF- moble  (Read 382 times)
KC8ORP
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Posts: 11




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« on: September 08, 2007, 05:48:56 AM »

I cant get rid of the RF when on HF in my moble, grounded everything, getting it back through the radio speakers, FERRITE where on power leads Huh or coax??.. the radio is the Icom 706MK2G remote mounted
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AA4PB
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Posts: 13032




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« Reply #1 on: September 08, 2007, 07:39:27 AM »

The ferrites need to go on the car radio's speaker and power leads - near the radio. Most modern car radios have power to the amplifier even when the radio is turned off. A few even have a separately located amplifier that is powered all the time.
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KE3WD
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Posts: 5689




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« Reply #2 on: September 08, 2007, 09:50:42 AM »

Before applying any "fix" at all, what you really need to do is some things to isolate where the interference is coming from first.  

*With key on and engine off, is the interference still there? If so, suspect interference from some types of electric fuel pumps, or maybe the car's comp control system, the latter being a very rare case anymore.  

*Does interference only happen when engine is running?

*Is this interference pulse interference, such as you might expect to hear if the radio is picking up spark plug pulses, changing speed from idle to faster pulses when you touch the accelerator?  Suspect ignition noises.  If diesel, suspect injector control noise.  

*Is the interference a whine that changes pitch higher with engine acceleration?  Suspect Alternator noise.  

*What bands is the interferenc worse or better?  


Finding the source of the noise first can let you deal with it from the noise source rather than at the radio.  Always better to do it that way.  


KE3WD
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KE3WD
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« Reply #3 on: September 08, 2007, 09:53:25 AM »

BTW - Are both the postive and negative power leads going straight to the vehicle battery?

This is one of those things that, if the system works okay without doing that, no harm or foul probably, but if there is noise and you haven't done the direct hookup, as per ARRL Handbook guidelines, using a fuse on both lines as close to the battery as possible, it should be your first line of attack.  

The battery also can act like a huge filter, you see.  


KE3WD
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AA4PB
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Posts: 13032




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« Reply #4 on: September 08, 2007, 11:11:28 AM »

WD: Your advise is sound if he were getting RFI into his ham rig. As I read his post, the ham rig is causing noise in his car radio speakers.

Perhaps he needs to clarify which way it is.
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N1WOM
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Posts: 60




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« Reply #5 on: September 08, 2007, 12:51:50 PM »

KC8ORP wrote: "I cant get rid of the RF when on HF in my moble, grounded everything, getting it back through the radio speakers, FERRITE where on power leads Huh or coax??.. the radio is the Icom 706MK2G remote mounted"

Key phrase: "getting it back through the radio speakers"

Follow AA4PB's advice.
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K0BG
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Posts: 9901


WWW

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« Reply #6 on: September 08, 2007, 01:06:11 PM »

Here is the crux of the problem. Because most late model vehicles are "prewired" for such things as Navi units and On-Star, the power amplifiers are powered on anytime the ignition is on, whether or not the radio is actually turned on. In fact, most GM units are powered even when the key is off!

In most cases (not all), when the radio is off, the low level preamp circuitry contains numerous transistor junctions. These rectify the induced Rf, and what you end up with is obvious.

Sometime, applying beads to the interconnect wiring and the amplifier will fix the problem. However, in some non-premium sound system cases, the radio and the power amp are on the same chassis, and it is impossible to filter the interconnection.

You can also get rectification in the final amp transistors, and if enough signal is present, you'll hear that too.

To tell which is which, pull the fuses for the radio. Remember, there may be more than one fuse (radio and its associated amp). If you still have the problem, it is rectification in the finals. If the problem goes away, the fix should be obvious.

You didn't say what your vehicle was (is), but if it is a GM, there is a tech bulletin about this very issue. Although it covers On-Star equipped vehicles, the fix is the same.

Alan, KØBG
www.k0bg.com
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AA4PB
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Posts: 13032




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« Reply #7 on: September 08, 2007, 01:49:10 PM »

Alan's advise is good but I would submit that even when the amp and the radio are on the same chassis, the RF most likely gets into the assembly via long external wires. Filtering the speaker leads and power leads should help to keep RF out of the unit. It is not necessary to actually get inside to filter the interconnections between the amp section and the radio section.
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KE3WD
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Posts: 5689




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« Reply #8 on: September 08, 2007, 08:44:37 PM »

Didn't catch that bit about the speakers, my bad.  


Quick and dirty method, try placing about 15pF disk capacitor across each speaker voice coil.  


KE3WD, ashamed for missing that.  
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