Cecil Moore, W5DXP, Rev. 2.1, 03/17/18
The ZS6BKW antenna evolved from an optimization of the G5RV antenna that, in general, sacrifices 75m operation to gain 17m and 10m operation. Where the G5RV is basically a four HF band antenna (80m, 40m, 20m, and 12m) the ZS6BKW is a five HF band antenna (40m, 20m, 17m, 12m, and 10m).
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.
The ZS6BKW is not a perfect antenna. The SWR on 80m is too high and the 40 ft ladder line matching section is a little too long on 17m and a little too short on 10m putting resonance outside of the privileges of the technician license. The standard ZS6BKW is mismatched on 60m, 30m, and 15m and requires the installation of a parallel capacitor across the ladder line to achieve resonance on those bands. Here's what the standard ZS6BKW SWR graph looks like before modifications.
With a few modifications, the following measured SWR values were obtained from the modified ZS6BKW.
There's a really neat method of using a single capacitor to match a mismatched ladder line fed dipole on a single band. We can choose a location on the 450 ohm ladder line where a single shunt capacitance of the proper value will twist whatever SWR exists at that point to an SWR equal to 450/50=9:1. Then at the next current maximum point on the ladder line, the impedance will be a perfect 50 ohms using only one low loss capacitor with no tuner required.
Given the fixed length of 40 feet of ladder line (TheWireman.com #553) there exists a point on the ladder line where a single shunt (parallel) capacitor will resonate the ZS6BKW on these three bands each of which has a unique point on the feedline and a unique value of capacitance that will resonate the system. The trick is to locate that specific point and determine the value of the capacitor to be attached while keeping the total length of ladder line at 40 feet. Fortunately, AutoEZ  makes the task fairly easy. Note that only one capacitor can be attached at any one time and must be disconnected for any band other than the associated band. This is easily done with a remote-controlled relay. Here are the results of W5DXP's efforts to resonate a ZS6BKW on 60m, 30m, and 15m using AutoEZ. Length L1 is the length of ladder line measured from the ZS6BKW feedpoint to the point on the ladder line where the shunt capacitor is to be installed. Length L2 is the rest of the 40 feet of ladder line terminated at the choke-balun output terminals.
Band L1 Cap L2
60m 21.5 ft 128 pf 8.5 ft
30m 29.9 ft 104 pf 10.1 ft
15m 35.5 ft 44.5 pf 4.5 ft
Here is the above data presented on 3 Smith Charts plus SWR graphs:
One low-loss shunt capacitor, properly connected to the ZS6BKW ladder line, is all it takes to achieve resonance on 60m, 30m, or 15m. Again, any shunt capacitor attached to the ladder line results in single band resonance and must be disconnected for operation on any other band.
Following are the measured impedances looking into the 1:1 choke balun for W5DXP's standard ZS6BKW on 80m. If we install a series capacitor to neutralize the inductive reactance, we will obtain a lower SWR. Here are the numbers vs frequency.
SWR, series cap, New SWR
3.50 MHz, 16.7+j0 ohms, 3.0:1, NA (Resonance), 3.0:1
3.60 MHz, 19.8+j24.5 ohms, 3.2:1, -j24.5 = 1804 pF, 2.5:1
3.70 MHz, 24.0+j50.3 ohms, 4.4:1, -j34 = 855 pF, 2.1:1
3.80 MHz, 30.0+j81.4 ohms, 6.5:1, -j57 = 515 pF, 1.7:1
3.90 MHz, 39.4+j120 ohms, 9.3:1, -j82 = 340 pF, 1.3:1
4.00 MHz, 55.0+j171 ohms, 12.6:1, -j108 = 233 pF, 1.1:1
Reasonable SWRs were obtained centered around 3.845 MHz using a 500 pF series doorknob capacitor - illustrated in the following block diagram.
Since the reactance of the 500 pF capacitor decreases with frequency, it can be left in the circuit on all of the higher frequency bands. For instance, it has almost no effect on 10m where the reactance is only -j11 ohms. Here are the SWR curves for 80m for W5DXP's standard ZS6BKW and with various values of series capacitors in the circuit.
For a lower SWR, the following circuit resulted in an SWR of 1.2:1 on 3.83 MHz.
Note that W5DXP uses knife switches for SW1 and SW2 which gives independent control over the state of the two capacitors. The parallel capacitor is switched in for 80m operation and switched out for all the higher frequency bands. If the parallel capacitor alone is switched in the circuit, the frequency of lowest SWR is around 3.62 MHz, good for CW and data.
Switching a capacitor in and out of a circuit located at the coax/twinlead junction on an antenna like the ZS6BKW is a challenge that depends on the particular installation. Worst case it can be done with two SPST relays. Is the parallel capacitor worth the effort? Following is an SWR graph with and without the parallel capacitor. The series capacitor only configuration results in an SWR less than 3:1 for 3.72-3.87 MHz. With both caps switched in, the SWR is less than 3:1 for 3.77-3.9 MHz with a near-perfect match on 3.83 MHz. That's about 72% coverage of 80m depending on which capacitors are switched in or out.
If one wants to vary the length of the ladder line from 39.5' to 40.5' to 41.5', here's a way to do it. In series with the transmission line, it allows us to select zero, one foot, or two feet of ladder-line. Of course, this feature could be automated using two DPDT relays.
The selectable 1'/2'/3' loops above using the DPDT knife switch allows us to cover about 200 kHz of the 80m band with an SWR bandwidth of 3:1.
Hopefully, something in this article will be helpful to hams wishing to adjust their ZS6BKW's resonant frequency or use their ZS6BKW on 80m while reducing coax and tuner losses. Unfortunately, without doing something at the antenna feedpoint, there is no way to reduce losses in the ladder-line matching section which on 80m, can be in the ballpark of 2 dB. Open-wire feedline would probably reduce those feedline losses compared to the windowed ladder-line.
 AutoEZ is available at www.ac6la.com including a free demo version. (W5DXP is just a satisfied customer with no monetary connection.)