The G5RV is a 102 ft dipole fed with ~30 ft of parallel line plus coax the rest of the way to the shack. The ZS6BKW is a 92 ft dipole fed with 40' of parallel line plus coax the rest of the way to the shack. Both antennas need a 1:1 choke-balun at the BALanced twinlead to UNbalanced coax junction.
W5DXP has a rotatable dipole for 20m-10m operation but 40m is his favorite band. The ZS6BKW works as well on 40m as a 1/2WL 40m dipole so up it went and worked as advertised. Now what could be done to the standard ZS6BKW to obtain a reasonable SWR, i.e. below 2:1, on 80m?
W5DXP took a look at the feedpoint impedance looking into the ladder-line on 80m and it looked a lot like the feedpoint impedance of an 80m mobile antenna with a resonant resistance value of around 20 ohms accompanied by a 2.5:1 SWR. Hmmmm, a shunt coil will twist that 20 ohms to 50 ohms on a 75m mobile antenna while raising the resonant frequency. A shunt capacitor will twist 20 ohms to 50 ohms while lowering the resonant frequency. A quick analysis indicated that lowering the resonant frequency on 80m was the preferred shift since it was already in the upper portion of the band.
A little experimenting indicated that a series capacitor (or two) would be better than a shunt capacitor. W5DXP has a bunch of 500 pF doorknob capacitors rated at 20 kv and 30 amps so the following two electrically equivalent configurations were tried.
IC-756PRO----coax----500 pF cap--1:1 choke====40' Ladder-Line====92' Dipole
It worked very well on 80m and by varying the length of the ladder-line feeding the ZS6BKW from 39 ft to 43.5 ft, the following SWR results were obtained on 80m. If the standard 40 ft of ladder-line is used on the ZS6BKW, the resonant frequency on 80m was about 3.865 MHz.
W5DXP was working on a scheme for switching the capacitor in and out of the antenna system (depending on the band being used) when the following thought occurred: Since the reactance (-j84 on 80m, -j11 on 10m) and therefore the effect, of the capacitor decreases with increasing frequency, what happens if the capacitor is left in the antenna circuit full time? The answer to that question resulted in some very interesting measured results for the single 500 pF capacitor configuration.
On 80m, I liked the results of extending the matching section to 41.5 ft so that is the length chosen for the final design. The resonant frequency is 3.825 MHz and the 50 ohm SWR is 1.7:1 at that frequency. The 3:1 SWR bandwidth is 3.745-3.915 MHz although the autotuner in W5DXP's IC-756PRO will tune down to 3.6 MHz.
On 40m, the resonant frequency is 7.185 MHz with a 50 ohm SWR of 1:1. That is an improvement over the original ZS6BKW design. The SWR at the band edges is below 2.8:1.
On 20m, the resonant frequency is 14.1 MHz with a 50 ohm SWR of 1.2:1. The 3:1 SWR bandwidth is 14.0-14.33 MHz.
On 17m, the resonant frequency is outside the band and the 50 ohm SWR is 2.8:1 inside the band - acceptable to most autotuners.
on 12m, the resonant frequency is outside the band and the 50 ohm SWR is 2.1:1 inside the band - acceptable to most autotuners.
On 10m, the 41.5 ft of ladder-line results in the resonant frequency being 28.615 MHz with a 50 ohm SWR of 1.3:1, i.e. no tuner required. The 3:1 SWR bandwidth is 28.17-29.06 MHz. That is a definite improvement over the original ZS6BKW where the 10m resonant frequency is above 29 MHz.
Here are the actual measured SWR curves for 80m, 40m, 20m, and 10m for the 92'/41.5' ZS6BKW. The blue curves are the SWR values for the single 500 pF capacitor configuration. The green curves are the SWR values for the dual 1000 pF capacitor configuration.
If the 1:1 choke-balun is in an enclosure, the doorknob capacitor can be installed in the same box. Increasing the 40 ft matching section to 41.5 ft is recommended to center the resonant frequencies in the middle of the bands. W5DXP's dipole length of 92 ft of insulated wire is like 93-94 feet of bare wire.
The simple act of adding the capacitor (and increasing the length of the ladder-line to 41.5 ft) not only put most of the 80m band within range of his IC-756PRO's autotuner, but it also put most of the popular frequencies on the original five HF bands within range of the autotuner. W5DXP is particularly happy with the SWR of 1.5:1 on 28.5 MHz where a tuner is not required.
The single capacitor (Method 1) and the dual capacitor (Method 2) are electrically equivalent and the differences in the real world are relitively minor as can be seen from the SWR graphs. The advantage that Method 2 has over Method 1 is that Method 2 puts less stress on the balun and on the capacitors since the voltage across each of the two capacitors is 1/2 of the voltage across the single capacitor. It may also be easier to install two capacitors in series with the ladder-line than to install one capacitor in series with the coax cable. In actual operation, it is difficult to tell the difference between the two methods.
W5DXP's exact results will not be achieved at different locations and different heights above ground but hopefully, the results will be similar enough to be useful.
The capacitor mod does not change the very high SWR on the ladder-line on 80m, e.g. maybe 40:1, so the losses in the ladder-line are not negligible. W5DXP estimates that losses may be in the ballpark of 1.5 dB or about 1/4 of a standard S-unit. Anyone considering using a ZS6BKW on 80m, with or without the capacitor mod, should consider the losses involved. On the other hand, if this mod allows one to easily match the ZS6BKW on 80m (or even operate without a tuner) where matching was previously difficult to impossible, it is probably worth tolerating the losses. The capacitor mod also decreases coax losses on 80m by lowering the SWR on the coax and decreases tuner losses on 80m by raising the impedance seen by the tuner.