A Safety Multi-Band Mobile Antenna
By Robert Wilson, AL7KK

fan_a.gif (1150 bytes)bout five years ago I was looking for a fully automatic, all band, mobile antenna for my four-wheel drive car here in Alaska, USA. In addition it needed to be easy to tune, and it should not hit other cars, trees, or people as I passed.

After 50 years as a ham and antenna researcher I simply applied the best available existing technology in a slightly different way for my solution. Trap resonators for ham bands are available at most amateur radio stores. The technology is well developed and reliable. However, using a number of resonators together has never advanced past the stage of placing them in all directions on the top of a pole. The result is a dangerous, unwieldy, and totally unpractical piece of hardware. This does not have to be, so I put them all in a line to minimize the frontal areas and reduce the danger to passing cars, trees and people. It works very well!

I called it a "FAN-TENNA" because it does have some resemblance to a fan. The drawing in Figure 1 and accompanying Parts List shows all the required parts and layout. You buy the standard resonators and attach them to the mast that you make with a soldering torch, tube cutter, and drill. All the material can be obtained from a quality hardware store.

Figure 1

fan_fig1.gif (9739 bytes)

Parts List
1. 2"copper end caps, (2 each)
2. 3/8x24 bolts, 2.5"long (2 each)
3. 3/8" split lock washers (9 each)
4. 3/8" fender washers, hardened if available (6 each)
5. 3/16"x20" aluminum strap, (6061-T6 if available)
6. 2"x48" rigid copper tubing (1 each)
7. Resonators for 80-10 meter bands (6 total)
8. 3/8" nut

Please note that it is necessary to put a small hole (about 1 mm diameter) somewhere near the middle of the tube. This will prevent problems with expanding hot air as you solder the final cap on the mast. Later on the hole can be filled with a self-taping screw, glue, or covered with a nameplate. Also tighten the nuts and bolts very firmly to prevent the mast from turning at some later time.

When put on a vehicle it is necessary to use a small diameter rope, or cord, attached to the auto rain gutter both fore and aft to prevent mast slippage. I have not yet found a mast base strong enough to hold all the resonators without this extra brace.

Tuning was easy with my ICOM 706 set to read SWR. I also tried a SWR meter and got the same results. A little adjustment of the length of each resonator reduced the SWR on each band to a reasonable level. There was very little tuning interaction of the resonators when distributed as shown in the drawing. On one of these antennas I also added a 2 to 1 autotransformer at the antenna base to improve the SWR, but it is not an absolute necessity.

Results were as good as any single band resonator type antenna, but I never had to get out and change resonators. The antenna is flat and in line with the car and thus will not hit adjacent cars or people, hook trees in an irreversible way, and if you really want the fan can be mounted horizontal for driving into a garage (but tuning is more difficult). In addition I am now able to pass cars without seeing a look of total terror in the other drivers eyes. Rather I get a look of "What in the world was that???" -30-

Robert Wilson is a Senior Member of IEEE, an engineer and physicist. In the past he has been employed by the FAA, Voice of America, Martin-Marietta, Comsat, RCA, DoD, University of Colorado, University of Wyoming, and the State University of Iowa. His work has carried him to 21 countries. In addition he has been a Merchant Marine Radio Officer, and is a long time radio amateur with the call AL7KK. He now lives on a lake in Alaska where he operates the Arctic Antenna Co.

~ October 2001 antenneX Online Issue #54 ~

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