If you use a mag-mount for an HF whip, be sure to use one with three magnets, not one. I tried a Hustler 17-m whip with a one-magnet mount, and it tipped over at freeway speeds.
I agree that the capacitance of a mag-mount is insufficient. The coax braid may actually be serving as the better part of the counterpoise. I gave up on the mag-mount and now have a bracket on the spare tire mount. Works great. On our KX3 web page you'll find a link to an application note about mobile installations. There are many subtleties. 73, Wayne N6KR On Mar 31, 2016, at 10:33 AM, Jim Brown <[email protected]> wrote: > On Thu,3/31/2016 6:06 AM, [email protected] 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:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
