Homebrew Travel Antenna
This is a low cost homebrew two band travel antenna for 20 and 40 meters. It is based on the B&W Travel Antenna concept with a telescoping whip and a loading coil.
A piece of 1/8 inch thick aluminum bar stock is formed into a window bracket which is fastened to the window sill with a C-clamp. The end of the bracket is bent down at a 45 degree angle to permit the antenna to lean out and away from the building. A short length of RG-174 coax cable with alligator clips on one end is the feed line. A 33 or a 16 foot foot length of wire with another alligator clip is used as the counterpoise.
The telescoping whip, from Radio Shack, is six feet long when extended.
The loading coil is wound on a 5 3/4 inch length of closet pole wood 1 1/4 inches in diameter. There are 46 turns of #18 stranded wire used on the coil. I placed taps at 14 turns and also 44 turns from the whip (top) end of the coil. On 20 meters, the coax feed line is clipped to the coil so that 14 turns are used for loading. For 40 meters, the clip is used at either 44 or the 46 turn tap.
Above the loading coil. I glued in a 13 inch piece of 1/4 inch dowel rod to increase the radiator length to seven feet. To enable the telescoping whip to quickly plug into the loading coil assembly, I used fishing pole ferrules (from Bass Pro). These are the metal parts that permit long fishing poles to be made in two or more sections for travel and slipped together when used.
On the bottom of the loading coil, a 1/4 inch stud cut from a 2 inch bolt is tapped into the wood coil form. A wing nut is used to secure the coil to the aluminum window bracket.
You may notice the round button object left of the C-clamp in the photo. This is a steel washer that has been super-glued to the aluminum bracket to permit an optional magnetic mount 2 meter whip to be placed on the bracket to extend it out of the window for improved 2 meter operation.
If you do not have a window sill to fasten the C-clamp, then the window may be lowered down onto the bracket. If this option is used, I tie a length of nylon cord from the antenna to some stationary object in the room to reduce the chance of it falling to the ground.
The coil tap positions were found experimentally. I used a sewing needle clipped to the coax cable "hot" alligator clip and pierced the coil wire insulation to make contact with the wire to test various tap locations. I moved the needle position one turn at a time while measuring reflected power. Once the best tap position was found, the insulation was stripped and a solder lug or a wire was soldered to the coil turn.
Results using this antenna are similar to a mobile antenna's performance. While never as good as full size dipole, it does work and QRP contacts can be made while traveling.
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Updated November 29, 2003