The ZS6BKW antenna
6 bands (40 / 20 / 17 / 12 / 10 / 6m.) without a tuner or traps!
When OM Louis Varney G5RV designed his famous antenna in the 1950's, it performed excellently with the equipment of that day and age. With today's radio's, on the other hand, things are different. This led OM Brian Austin, ex ZS6BKW (now G0GSF) to redesign the G5RV, using modern antenna modeling software - something that was was not available when the G5RV was designed. He found that shortening the two legs of the G5RV dipole and lengthening the matching stub (the balanced feed line) resulted in an antenna that provides a good match to the 50 ohms impedance of today's transistorized radio's. This allows the antenna to be used on five bands without an antenna tuner! And as an added bonus, it was later discovered that the ZS6BKW antenna covers not five but six bands, as it does 6m, too, something that the original G5RV did not cover at all!
However, there is some bad news, too. On 80m and 15m the ZS6BKW design performs less well than the original G5RV, while on 30m it is just as bad. Also, the antenna's performance appears to be rather dependent on how well it is installed. If it is not high enough above ground or does not keep sufficiently clear of trees, buildings or conductive objects, performance can be affected rather seriously. However, in this respect it does not differ markedly from the original G5RV, which behaves similarly.
The horizontal part of the antenna (the dipole) can be made from electric fence wire, which is an excellent choice for this application. It has a better surface conduction and shows less corrosion than is the case with untreated steel wire, and can be put under mechanical strain without stretching over time like copper wire does. In the original G5RV design, the dipole length (L1) was 31.1m while the matching section (L2) was 10,37m of open line. In the optimized design by ZS6BKW the dipole itself is slightly shortened to a length (L1) of 28.4m, while the matching section (L2) is longer, and depends on the velocity factor of the feed line used. Different versions of this design circulate on the Internet, typically using 300 ohm tape (the type with a velocity factor of 0.85, and L2 being 11.1m) or using 450 ohm window line (with L2 being 12.2m). However, the best, cheapest and easiest way to make the matching section is to use ladder line, which can be easily constructed using Instant Ladder Line clip-on wire spreaders. In this case L2 is 13.08m.
While various literature claims that a 1:1 (or almost 1:1 match) without a tuner on various frequencies can be achieved, this seems more due to the lossy coax used by the ham doing the measurements than the antenna's performance! (Note that because lossy coax also dampens the reflected wave, coax loss will result in a better SWR measurement at the transmitter!) In OM Brian's own 1985 publication he merely speaks of "an acceptable match", better than 2:1 on five bands, which seems much more realistic. And that is fine - better than 2:1 is more than sufficient. A 1:1 match is not required, contrary to what many hams these days believe!
The table below lists a few SWR figures that compare theoretical (calculated) values found in literature, to the measured values of the author's own ZS6BKW antenna, as well as to the G5RV. (Only theoretical values for the latter, but past measurements done on actual G5RV's show that these match the actual performance fairly well.)
Band 80 40 30 20 17 15 12 10 6
ZS6BKW, literature 8.3 1.1 87 1.2 1.4 80 1.2 1.5 1.5
ZS6BKW, measured >10 1.7 >10 2 1.4 >10 2 2 1.7
G5RV 3.2 4.8 >10 2.5 >10 6.8 3.6 >10 N/A
(Note: these are the "best" SWR figures; they will of course vary while tuning across the band. The fact that the measured SWR values are higher than the ones in literature is probably due to "end effects".)
(Updated, 1 Jan. 2011) After having read this article, Brian Austin has responded to point out that his antenna design has been optimized for feed lines with an impedance of around 400 ohms. With this feedline impedance a better SWR will be obtained than when using a ladder line with an impedance in the 600 ohms range. So strictly speaking, a ZS6BKW antenna with a ladder line instead of window line is not a "real" ZS6BKW antenna.
While OM Brian is of course entirely correct in saying this, the fact remains that the above performance figures are still very usable on most frequencies, and an antenna built with ladder line that is on hand performs much better than one that requires window line that isn't! (End of update.)
Balun or no balun?
A lot of "religious debate" on whether or not to use a balun between the coax and the matching section has taken place over the years. Most of it is based upon OM Varney's own remarks on the subject, and experiments by other hams. However, Varney's statement that he did not notice any difference with or without a balun, and the conclusion that one is therefore better off without one, date from the 1960's and apply to the equipment of that era. Many hams, meanwhile, have experimented with 4:1 voltage baluns, which introduce a mismatch because the feed point impedance of the matching stub is much close to 50 ohms than to 200 ohms.
The fact of the matter is that any way you look at it, it is never a good idea to attach an unbalanced coaxial cable directly to a balanced load (which this antenna is). It introduces unbalanced currents in a balanced system, thus causing it to perform wildly different from what theory predicts, not to mention the fact that it causes shield currents in the coax, resulting in RF in the shack and TVI. A proper 1:1 current balun (i.e. a choke) between the coax and the balanced antenna feed point is therefore to be recommended.