On Thu,3/31/2016 6:06 AM, km4...@gmail.com wrote:
When you operate mobile, do you use some sort of magnetic mount to support the whip? Also does the metal of the vehicle take the place of a counter poise?
I'm not Wayne, but I've studied this issue. Yes, the vehicle chassis needs to serve as the counterpoise for a mobile antenna, but I have yet to see a mag mount that does that effectively. VHF/UHF mag mounts are designed to do that by means of capacitance between the mount and the roof, but nearly all that I have seen have no contact between the coax shield and the enclosure of the mount!
At HF, I see no practical way for any mag mount I've seen to have anywhere near enough capacitance to a roof to work as a counterpoise at HF.
If you want to work mobile, you need to make a solid connection to the frame, AND it needs to be a part of the frame that is not insulated from the rest of the frame by PAINT. That isn't easy in most modern vehicles.
Two examples. With a Volvo S80 I owned about 15 years ago, I used a license plate mount for Hamsticks. The license plate holder was insulated from the trunk roof, and the trunk roof was insulated from the rest of the body by the hinges, so I had to bond around both. That worked pretty well, but I suspect there were still pieces of the body that were insulated by paint.
Second example. My current vehicle, a 2006 Toyuota Sequoia (big SUV) that I bought in Nov to move to CA from IL. It was winter in Chicago, so K9IKZ let me bring it into the loading dock of his biz, and we poked around to try to find good contact with the body. Lots of paint in the way -- I found bolts a few inches from each other with no continuity between them. I eventually mounted the antenna socket to the roof rack, and found a nearby bolt that did get to the body.
That worked pretty well as an antenna, but the vehicle has really bad susceptibility to HF RF -- at 100W on 20M, the main computer that runs the vehicle goes into "limp home mode." I've never bothered to try to fix it -- I was in the process of moving when I learned that (on an isolated stretch of I-80 in the NV desert), so didn't have time to chase it down, and because it was RF on the body that was exciting vehicle wiring, I figured that it would have been pretty difficult to fix. :) And my only interest in HF mobile is for long trips without the XYL, which I no longer take after I finished moving. I've heard that other big SUVs are far better in this regard.
73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
