Rob's web

A flexible coax antenna

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - A flexible coax antenna

Fig 1
Fig. 1. Portable Coax Antenna made from common coax cable.

A novel but effective flexible coaxial antenna and feed line can be made from a length of coax cable and an insulator. It can be rolled up in a small space for use as an emergency-portable antenna or it can be hung permanently from a fixed support. Feed-line and radiating portions are all one piece so that there are no joints or soldered connections except at the transmitter end of the line where the plug of your choice is installed. Although a compromise antenna, it is simple to construct and is an easy solution to the portable antenna problem.

Select your coax cable and cut a length equal to about one-quarter wavelength at the desired frequency plus the length required for the feed-line. Any of the popular coax cables such as RG-58/U, RG59/U or RG-8/U may be used. At one end of the cable, slice down and pull off the outer insulated cover (being careful not to damage the shield underneath) for a distance of one-quarter wavelength plus about 10 inches. Push back the exposed shielding to loosen it up, then fold it back over and down the feed-line portion of the cable. The shield will form the bottom section of the coaxial antenna. The shield will not reach down the cable for the entire length it was cut - thus the reason for cutting it 10 inches longer than necessary. Pull the shield down as tight as possible, measure a quarter wavelength at the desired frequency less 5 per cent, and trim off the excess shield. A few extra turns of tape should be applied to keep the shield from creeping up the cable. Now measure out on the insulated unshielded center conductor one-quarter wavelength less 5 per cent. Remove all of the insulation beyond this point. Attach a supporting insulator to the top of the antenna using the length of stripped wire. Now wrap the entire length of exposed shield with plastic tape, making it firm at both ends to prevent slippage.

The coaxial antenna can be rolled up and tucked away in the car. Merely connect it to the transmitter and attach the insulator end to a tree when using the mobile rig at a fixed location. A metal hook formed at the insulator will facilitate "hanging" the antenna.

Melvin H. Dunbrack, W1BHD.