“Secret” antenna

It may be called “secret” on two reasons. First, despite the number of publications in the Internet it is not very popular among our hams as if it was “hidden” for them. The second, when it is suspended between the trees in the yard it does not catch the eye of your neighbor who usually is irritated with something beyond his everyday scope. My neighbors are sure that my Yagi “intercepts” all the TV signals intended for them in the whole adjacent area J .

This wire antenna was mentioned as “Half-square” in “Amateur Adventure”, small local magazine, and the publication author KK7C does not pretend to be an “inventor” of the configuration. The major purpose of the article was to offer ready-to-use antenna kit so no exact dimensions were given and the method of connecting the feedline was not clear.

The antenna (Fig.1)

looks like “pi” character with two legs being of l /4 and the horizontal portion of l /2. Thus it presents two vertical radiators fed out of phase. The significant feature of this antenna is that its radiation resistance is close to 50 Ohms. It is fed in the corner by %) Ohm coaxial cable with the central conductor connected to the horizontal part and the braid – to a leg.

Before making this Halfsquare for 80m band I have suspended a small replica for 24,9 MHz and because I have a sloppier for this band to compare with. No significant difference was observed while listening to the NCDXF beacons but when UA9OC gave me the signal from the direction perpendicular to the antenna vertical plane any doubt disappeared: the Halfsquare gave its 4 dB plus against the sloppier. Then 40m antenna was suspended and finally the 80m. It turned to be not easy to lift it to l /4 (20m) above the ground to the top of a tree: I made a halberd with the bowstring of 1mm steel wire and the arrow from aluminum 6mm pipe. The 0.3mm fishing line was attached to the arrow and when this line was thrown over the upper brunch another line of 1.2mm was attached to the thin line and pulled over the tree. Then the antenna wire was pulled to its position. One tree turned to be not enough high so the 3m end one leg of the antenna was bent and suspended 2 meters above the ground.

Some trimming was necessary to tune the antenna to the desired frequency. It is necessary to keep in mind that the leg near the feed point is trimmed for the length (+/-) D L = (D F/300 000)/4, m, where D F is necessary frequency shift. The second (remote) leg is trimmed 3 times more.

May be it is not the best antenna for low bands, at least it loses to GP, but it does not need any radials! At least it is much better than Inverted “V” with the apex about 18m high. I think it can be successfuly used in a field condition.