My experiments with verticals go on. I recently built a half-square antenna and found it both simple to get up and running as well as efficient. In fact given a choice I would go for a half-square most of the time, it produces very nice signals and is really simple to build.
The HSQ has a history of some 50+ years. Originally invented in 1948 by W6BCX it apparently managed to avoid its rightful fame until it was extended into what is now known as a bob-tail. If you know what a bob-tail looks like the HSQ is a two-legged version of it. While the bob-tail has many virtues (or so I am told — I still have to find that out myself) it has a more complicated feeding system and more overall size than the HSQ. The HSQ has lower gain than the bob-tail, but apart from that they are fairly similar. Note specially the low radiation angle in the diagram below:
The half-square is a vertically polarized bidirectional antenna. Compared to a horisontally polarized dipole its gain is low as it will not get the extra circa 5 dB due to ground reflexion. But compared to a normal quarter-wave-stick its has some gain and a favourable directional pattern. A “waist” of some -15 dB and specially the low radiation angle makes it a useful DX-antenna.
Building it is simplicity itself. Traditional dimensions are two quarter-wave vertical lengths, interconnected by a half-wave-length of phasing wire. My simulations show that the “waist” is most pronounced if the lengths are adjusted somewhat:
The HSQ is very easy get up and running. The lengths in the figure differs somewhat from the traditional dimensions (which would be 0.25 and 0.5 respectively). Simulations show that the dimensions in the figure give a more marked “waist” in its radiation diagram. This could be beneficial under some circumstances eg. suppressing QRM from the sides. The lengths are expressed in wave-lengths, with no adjustment made for wave velocity in the cable used.
The half-square is fed with 50 ohm coax at the upper right corner in the picture. The braid of the coax goes to the first vertical leg and the center conductor to the horisontal phasing line. If the “waist” is required the SWR will be about 1:1.5, otherwise trim the vertical legs until a SWR of 1:1 is found. The radiation pattern will have a broader “waist” but gain and performance will be the same. Note that the pronounced “waist” can be used as a QRM suppressor if the antenna is properly oriented.
I built and tested a HSQ for 14.05 MHz in the winter of 2005. Its performance was exceptional compared to the dipole I had to compare with. While the dipole was not ideally mounted, and as such gives a skewed comparison, I had a fair number of QRP contacts on it but the signals were always stronger on the HSQ than the dipole. And in some instances the distant stations were only copyable on the HSQ, and completely covered in noise on the dipole.
So all in all: This is a simple antenna to build with excellent performance.