Some Useful ZS6BKW Modifications

Rev. 1.4, 11/22/17

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 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.

With a few modifications, the following measured SWR's were obtained from the modified ZS6BKW.

Two old telescoping mobile AM antennas were installed side-by-side in the ladder line so the length of the ladder line can be varied by 28 inches. Making the ladder line length adjustable is all it takes to resonate the ZS6BKW on the desired frequencies in each band. Here's what it looks like.

If one wants to vary the length of the ladder line from inside the shack 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 one foot, two feet, or three feet of ladder-line.

What about 60m? That band is just about worst case for a ZS6BKW. The 90 ft length of the dipole is close to 1/2 wavelength and the 40 ft tuned feeder is close to 1/4 wavelength yielding a sky-high impedance at the coax/twinlead junction. If the tuned feeder is lengthened to 1/2 wavelength, i.e. ~78 ft, then the antenna system should work well on 60m but W5DXP has not tested such a configuration.

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.

Frequency,   impedance,         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.

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.