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Author Topic: too much RF?  (Read 352 times)
KA8SEP
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Posts: 38




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« on: November 07, 2005, 09:36:33 PM »

Hello,

While on 40M i got a comment that i have too much rf in my signal. it was sugesetted that i take the ground from my radio and run it back to the base of my trunk lip mounted hamstick.

wont this put rf into my rig?

Ted
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K0BG
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Posts: 9839


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« Reply #1 on: November 08, 2005, 05:36:39 AM »

Well, yes and no, as it depends on how you have installed and wired your antenna, radio, etc. Trunk lip mounts are second only to mag mounts as the worse way to mount an antenna. I'm sure you haven't done any bonding, or other noise abatement procedures. In other words, there is more to it than throwing the rig into the car and driving off.

There isn't enough room here to explain it all. I suggest you look over my web site, especially the wiring, bonding, and antenna articles. Once you have tackled those, and you still have the problem, send me an e-mail.

Alan, KØBG
www.k0bg.com
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WB2WIK
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Posts: 20542




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« Reply #2 on: November 08, 2005, 08:44:51 AM »

Alan's website has a lot of good info, don't be overwhelmed.

The over-the-air diagnostic "too much RF in your signal" is a nonsensical statement.  What someone meant to say is, "Sounds like you have RF feeding back into your mike and causing distortion."  That makes a lot more sense, and may or may not really be the problem.

I agree a trunk lip mount for an HF antenna is a good way to invite all sorts of problems.  Trunk lip mounts work pretty well for VHF-UHF antennas, but for HF (30 MHz and below) they introduce bad stuff including inadequate grounding and too-close proximity to your operating position in the vehicle, with the antenna literally looking at you through your rear window from very nearby.  Ideally, you'd want the antenna farther away and better grounded, or at least have substantial sheet metal in between you and your antenna.  Truly ideally, you'd want your mobile whip to be on a trailer fifty feet behind your vehicle.

Sometimes "RF feedback," if the only place it's getting into your rig is via the mike line (pretty common), can be solved by ferrite RF chokes installed over the mike cord.  This doesn't make the root cause of the problem go away, just patches up the problem so you can use your rig.

WB2WIK/6
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AA4PB
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Posts: 12667




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« Reply #3 on: November 08, 2005, 11:25:37 AM »

Connecting the radio and the antenna mount with a piece of wire is not likely to accomplish anything and may actually make things worse. That wire is not going to be a good ground to RF. Lets suppose that the wire happens to be 1/4 wavelength long. One end could be connected to the world's best RF ground and the other end would be high impedance - no RF ground at all at that frequency.

What you have to do is to provide good, low impedance connections (bonds) between the antanna mount and the chassis of the vehicle and between the various metal pieces of the vehicle. Take a look at Alan's site for some excellent information.

What you probably have is RF getting back into your microphone preamp circuits via the mike cable and causing distortion of your signal. The best solution is bonding of the antenna mount and vehicle parts.
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