Bobtails and Half Squares|
These antennas offer big gun DX performance on the low bands,
yet they are surprisingly simple to build. Try one- you will like it!
For many years, the conventional wisdom has been that bobtails and half squares
are mainly low band DX antennas and are not the best choice for close-in
work. When built in the customary manner (close to the ground, voltage fed),
they are excellent low angle radiators.
Although there are times when DX signals arrive at
higher angles, and these exceptions actually happen
quite often, lower propagation angles are usually the best
for DX work.
W6BCX wrote in Ham Radio Magazine, March 1983, page 28:
"For distances beyond 2500 miles, angles of signal departure below about 15
degrees are almost always the most effective. This is true regardless of
propagation path, whether it's one acute geometric bend near midpoint or a
chordal or ducting mode... and... regardless of ionospheric tilt. Although it's most
noticeable on 10 and 15 meters, it still applies to long haul 40 and 75/80 meter
QST Magazine for January 2001, page 57 confirms this in
a set of 3 charts showing optimum elevation angles to Europe, the Far East, and
South America from different parts of the US. All the chart entries are 15
degrees or less, some as low as 3 degrees on 10 meters. The lone
exception is the northeast US to Europe path on 80 meters, where the
angle is 20 degrees.
The established norm for low band arrays has been operation close to the ground, voltage fed using a small counterpoise for a ground return. When built this way, they produce a good, low angle pattern with a deep overhead null. The pattern is shaped like a figure 8 broadside to the array, and it also has fairly deep side nulls. This means a lot less noise is received from some directions. Overall noise background can be much lower than with a single vertical, especially when noise sources fall into one of the nulls. This is very important on the low bands because lightning static travels farther at these frequencies, making the noise problem much worse than at 14MHz and higher.
W6BCX discouraged the use of direct coax corner feed because this limits the antenna to a single band and corrupts the clean pattern of a voltage fed antenna (due to coupling between the antenna and feedline.) On the low bands, your 'skip zone' from which other stations cannot be heard shrinks substantially from the larger diameter circle that is seen on the high bands. This means that you can hear signals from close-in stations within a few hundred miles coming in at very high angles. (Skip zones are actually ring shaped- a circle of local stations can always be heard via ground wave) If the antenna has poor high angle discrimination, close-in stations can be crashingly loud. These are your competition in a pileup while you are trying to hear some faint, far away DX station. A deep and wide overhead null is therefore a very good thing for low band DXing. W6BCX said that using corner feed would produce more undesirable high angle, horizontally polarized, end fire response.
As height is increased, the antenna develops additional lobes at higher angles. On the low bands, close-in stations can be considerably louder compared with 'on the deck' performance. This behavior can be clearly seen in a set of elevation plots for a 15 meter half square antenna in CQ Magazine, July 2000, page 60. Comparing patterns at three different heights (.41/.60/.77 wavelength horizontal wire) is quite revealing. Also included is a plot of a reference dipole at about 1/2 wavelength showing its much poorer overhead null and much higher angle lobes. One not so obvious detail is the effect of having the tails of this antenna a mere .17 wavelength (8 feet) off the ground. A closer look at the lowest height plot shows that the antenna is already beginning to grow secondary lobes at about 65 degrees, although the main response is still much stronger at 15 degrees. The situation worsens rapidly as the antenna is raised further, with the low angle lobes shrinking and the high angle lobes taking over. Another not so obvious detail is that these arrays do perform best slightly off the ground, providing maximum gain and slightly lower lobe angles while still suppressing the growth of higher angle lobes. On 80 meters, for example, antenna modeling says that about 15 foot tail height (.05 wavelength) is best over average ground. These are small differences, however- those who build antennas with the tails barely off the ground may not be able to hear any difference if they raise them higher. It very much depends upon the local ground quality and, to some extent, antenna dimensional details.
Moving up to the high bands brings considerably more choice in DX antennas- verticals are no longer quite such a necessity. Horizontally polarized antennas perform much better here than on the low bands, where they are 'cloud warmers', radiating most transmitter power at uselessly high angles. On 20 meters, a half wave is only 35 feet, which is an easily obtainable height for many hams. Reasonably good performance can be expected from antennas at this height, although there are certainly good reasons why so many towers for horizontal antennas are much higher! For operators unable to go higher, vertical antennas are sometimes a better choice, providing significantly lower angle radiation and at the same time eliminating the need for expensive tower installations. One disadvantage- arrays of verticals, which provide gain, directivity, and reduced noise pickup, are usually built in a fixed positions, not rotatable like tower-mounted yagis and related types. Hams who have good to excellent local ground quality may get better results with verticals; those with poor ground conditions may be disappointed with them. It is necessary to experiment with different antennas to find out what works best at your location.
Bobtails and half squares have been disparaged for use on the high bands, yet lots of hams keep trying them and getting good results. I once tried an elevated 20 meter bobtail myself, as an emergency antenna, and can report that it put a potent signal into northern Europe. Mine was voltage fed using an LC network and counterpoise on the middle tail, at about 17 feet tail height (1/4 wavelength). The support ropes angled down from about 40 foot high tree branches, and the feedpoint was on the roof peak of my house. The antenna orientation was broadside N-S (horizontal wire E-W). The height chosen was probably a very good compromise, improving on DX performance the same antenna would have at ground level. It got the job done, allowing me to work a rare DXpedition when my tower-mounted yagi was broken. Moxon, G6XN, is a big fan of vertical arrays for the high bands, as evidenced by the extensive coverage given them in his book "HF Antennas For All Locations".
Many ham DXers do not have the option of putting up a tower, yet they do have room for wires. They may have tried dipoles or other horizontally-polarized antennas, or ground-mounted verticals on the high bands and come away disappointed. A bobtail or half square may be just what is needed to put some excitement back in their DXing. For those interested in working mostly DX, a conventional voltage-fed array would be well worth trying, but if ragchewing with closer-in stations is also important, a new approach to the bobtail developed by NJ0F (now K0BHP) may be of interest. His 17 meter design offers a lot of performance from a single antenna. He increases height considerably, enough to encourage more high angle energy, and directly corner feeds at one end with coax. While corner-fed half squares are nothing new, corner-fed bobtails are not often seen on the bands. His modeling of the antenna shows that the major lobes split in two, producing a cloverleaf pattern rather than the usual figure eight. This antenna still has nulls off the ends, and adds two roughly broadside nulls as well. Within the four lobes, it provides significant DX gain and directivity. For closer-in work, it is more nearly omnidirectional. Overall, it delivers a lot of good, clean fun, yet is easy to build, utilizing all-wire construction and direct coax feed. NJ0F reports that the antenna seems to be a band opener, and it retains low angle DX lobes. NJ0F's 17m Bobtail Antenna Design
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