Portable 28 MHz (10 Meter) 2 Element Beam Using 102″ Steel Whips by KL7JR/VE8RST/VY1RST
“I built an initial POC version with bamboo and wire and it works ok centered on 28 Mhz and the radio tunes it ok using the antenna tuner. The final version is very much the same way (elements do sag) and we had a lot of fun on top of a 15 mast on a high hill in Peterborough. Gain and Directivity is useful and we managed to work some caribean stations. Cost will be $25 x4 for the whips plus the dipole brackets and a mast so figure on $200. VE3IPS”
John KL7JR has a lot of CB antennas for ham or 11m use that he has played with
The old standby 102 inch steel CB whip has been around for a long time and used on the amateur bands by many hams over the years. This design will incorporate four of the steel whips and allow us to concentrate our signal to a specific area and give us 3 to 4 dBd to boot (*)! It is direct feed with 50 ohm coax. It will also be easy to assemble and fairly transportable at a cost of about $100 depending on how you connect the whips to the boom or use new or used materials. Here’s a method I chose which will make it a snap to assemble and install in cold weather, field day, etc.
A 102 inch steel whip is a quarter-wave on the CB band (27 MHz). Using the N3DNO antenna calculator we arrive at the following dimensions for our desired frequency of 28.450 MHz (+-): driven element is 8 ft and 4 inches long each (there’s 2 for a total of 16 feet 8 inches), the reflector is 17 ft and 7.3 inches long with the spacing at 6 ft and 7 inches from the driven element. Note that the reflector is one continuous piece electrically. (Not insulated from the boom.)
Now, keeping that in mind, and trying to maintain the 8 ft and 6 inch length of the CB whips (I didn’t want to cut the whips down as I may want to reuse them some day for other projects) we find these dimensions (trial and error using the above antenna calculator): for 27.8 MHz the driven element is 8 ft and 6 inches long (the actual length of each whip+-) on each side, and the reflector is 18 feet long (**) with a spacing of 6 ft and 9 inches. The difference in the desired frequency of 28.450 MHz vs. the “not to cut the whips” frequency of 27.8 MHz is only .650 MHz which should be easily handled by any antenna tuner.
To mount the whips to the boom, you can u-bolt a short piece of “L” channel (+- 6 inches long with the predrilled holes) to the boom and then mount the antennas to that using insulated connectors for the driven elements and non-insulated connectors for the reflector.
(Or if you are like me and want something a bit more attractive, you can use a pair of
commercially made mounts (MFJ mini dipole mount #347 or others like in the picture above).
The 102 inch steel whips are available from DX Engineering (#DXE-WP-102) or through Radio Shack (?). This design may also work on 12 meters as well. This antenna is in the experimental stages, but the above dimensions should be good starting points, and you may chose to cut your whips down to the design dimensions above.
Keep in mind that whether or not you use the commercial mounts on the reflector, or your own mechanical design, that you will have to make certain both whips are connected together forming one continious electrical connection. Other wise it will not work! If you have trouble tuning for lowest swr or just don’t want to bother, then use your tuner. A 1:1 balun at the feed point is also suggested.
73 de Yukon John, KL7JR
** You can add 6 inches of wire to each of the 102 inch whips to get the 18 ft length, or not. Experiment either way.