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 priviledges of the technician license. The standard ZS6BKW is mismatched on 30m and 15m and requires a major change in the length of the ladder-line to achieve system resonance on those bands. Here's what the standard ZS6BKW SWR's looked like before modifications.
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.
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.
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.