The length of a commercial base-loaded mobile whip is nearly a full electrical 1/4-wave because the coil is there primarily to provide a convenient way to adjust impedance and run at DC ground, and leaving as much whip length as possible helps performance. If you build your own, and saving space is critical, a base-loaded antenna could be designed to be as short as needed. Whether it will disappoint later is a another matter.
There have been guys who've cut down the 48" helical fiberglass CB whips to work on 6 meters with fair results. Distributing the loading along the length can sometimes work out better than putting the entire load in the base. The name brand used to be "Wondershaft," but I suspect these days you'd have to look for something from Valor, maybe even shop at a truckstop. I cut one for 10m FM by carefully prying off the protective rubber tip cap, and removing a little at a time off the top with a hacksaw. The windings on the fiberglass CB whips get very close spaced right at the top, so it took only about 1/4" off the tip, IIRC, to move it 2.5 MHz, all the way up to 29.6 MHz. You might actually get all the way to 54 MHz before getting down to 1m in length, and performance may suffer if you're into the wide-spaced windings at that point. Fortunately, it would be a fairly forgiving style of antenna to cut-and-try for a homebrewer, since you could change things up before applying whatever protective coating goes on last. The CB types use an ordinary 3/8"-x-24 threaded mount and are series-fed, but they're prone to static noise. If I was trying to put one on a tower, I think I'd try to leave the whip electrically short, and add at least a small coil at the base to allow shunt feeding to be at DC ground. Depending on construction, that might also allow some last-minute adjustment in impedance to match the environment, before the coil was sealed up. But then, if you're building a base coil anyway, maybe a 1m stick of aluminum tube for the vertical and a couple more turns in the base loading coil would be the better answer for durability. 73, Paul, AE4KR ----- Original Message ----- From: Chuck Kelsey To: Repeater-Builder@yahoogroups.com Sent: Wednesday, February 03, 2010 9:11 AM Subject: Re: [Repeater-Builder] Re: Space-saving 6m repeater antenna? If the length is that restricted - about 40 inches - you are probably out of luck. The ground plane is about half the length of a folded dipole, so it is actually substantially shorter. .... A base-loaded mobile antenna is not all that much shorter than a 1/4-wave. The one on my car is overall 50". A 1/4-wave antenna would be 54"...
