Some Useful ZS6BKW Modifications

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. 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.

Following are the measured impedances looking into the 1:1 choke balun for a 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 capacitor, New SWR
3.60 MHz, 14+j49 ohms, 7.1:1, -j49 = 902 pF, 3.6:1
3.70 MHz, 22+j80 ohms, 8.4:1, -j80 = 538 pF, 2.3:1
3.80 MHz, 34+j116 ohms, 10:1, -j116 = 361 pF, 1.5:1
3.87 MHz, 50+j147 ohms, 10.5:1, -j147 = 280 pF, 1:1
3.90 MHz, 59+j162 ohms, 10.8:1, -j162 = 252 pF, 1.2:1

One thing is obvious. It is much easier to obtain a low SWR centered somewhere between 3.75 and 4.0 MHz than it is to obtain a low SWR below 3.75 MHz. Reasonable SWRs were obtained centered around 3.815 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 is the SWR curve for 80m with the 500 pF capacitor in the circuit.

For a lower resonant frequency, the following circuit resulted in an SWR of 1.4:1 on 3.735 MHz.

The parallel capacitor is switched in for 80m operation and switched out for all the higher frequency bands. 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 a remote relay. Is the parallel capacitor worth the results? Following is an SWR graph with and without the parallel capacitor. The green curve is the SWR response with only the series 700 pF capacitor in the circuit. The blue curve is what happens when the 800 pF parallel capacitor is added to the circuit. The parallel capacitor is preferred in the circuit for 80m operation without an antenna tuner but note that the parallel capacitor should be switched out below 3.68 MHz which reduces the SWR.

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.