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eHam Forums => Mobile Ham => Topic started by: KA3QOT on August 09, 2008, 11:51:43 AM



Title: Grounding When Portable from Car
Post by: KA3QOT on August 09, 2008, 11:51:43 AM
I often work portable from my car. My setup includes a Kenwood TS-570S(G) powered by an AGM sealed battery right next to it. I place both on the backseat of my car. My antennas include various hamsticks which are placed in a magmount on the roof of my car. I've worked reasonably well in this configuration.

Today, I tried to operate on 40m. Though I had a close-to-flat SWR, I received some nasty shocks when transmitting. This happened three times or so before I gave up. Occasionally, I've gotten tiny shocks but these were strong enough to make marks on my skin.

I am not using a ground, which I'm guessing may be the issue. How can I ground when operating? I usually operate from a park nearby and don't want to permanently install the radio in my car.

73,

Dennis KA3QOT


Title: Grounding When Portable from Car
Post by: K0BG on August 09, 2008, 12:10:35 PM
The reason you're getting "bit" is because there are common mode currents flowing over the coax. This is a direct result of an in adequate ground plane under the antenna. Remember, what ever current flows in the antenna, must also flow in its ground plane.

The fact you are using mag mounts adds to the problem, as the coax shield is not connected to the body at a point directly under the antenna. Adding ground straps won't help the situation much.

You can use split beads to isolate the common mode currents, and turn the leakage into heating the ferrite. Or, mount the antenna directly on the vehicle.

Alan, KØBG
www.k0bg.com


Title: Grounding When Portable from Car
Post by: KA3QOT on August 09, 2008, 12:49:54 PM
Thanks, Alan, for the great advice.

Regarding split beads, should I go with a 43 or 77 mix? Also which diameter size and should I place them at the back of the radio? Anyplace you can recommend that sells them?

73,

Dennis


Title: Grounding When Portable from Car
Post by: K0BG on August 09, 2008, 02:19:21 PM
You need to use Mix 31, and it needs to be on the coax very near the antenna. Just remember; if the choke isn't large enough, it will get hot enough to destroy the ferrite. So, use the 3/4 inch ID ones, and get as many turns as you can. If you can't get at least 10, use two beads.

Also, a choke just masks the real problem, and that's inadequate mounting.

Alan, KØBG
www.k0bg.com