ZS6BKW Modifications


How to Transform Your ZS6BKW Into an All-HF-Band Antenna

by Cecil Moore, W5DXP, Rev. 3.0, 09/01/18

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 center-fed dipole fed with ~30 ft of 300 ohm twinlead plus coax the rest of the way to the shack.

The ZS6BKW is a 92 ft center-fed dipole fed with 40' of 450 ohm ladder 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 to minimize coax feedline radiation and RF-in-the-shack.

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 a relay-switched parallel capacitor (or open stub) 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.

The author's goal was a multi-band HF antenna that didn't require an external tuner. Implementing all of the improvements necessary to achieve the above SWR curve is probably more than the average ham wants to undertake. So these improvements will be taken one at a time from the easiest to the most time-consuming. First, let's look at lowering the SWR on 80m using a single series capacitor that can be left in the circuit permanently. The following graph shows the effect of a single series capacitor from no capacitor to a 250pf capacitor.

Assuming a choice of 500pf which would result in an SWR of 2:1 on 3.85 MHz, here is how to install the capacitor on the coax side of the 1:1 choke-balun.

The author chose to install a 750 pf capacitor which gives an SWR of 2.5:1 at 3.77 MHz. Since the reactance of a capacitor decreases with frequency, this capacitor can be a permanent installation because it will have little effect on the higher frequency bands. Of course, if the choke-balun is outdoors, the capacitor will need to be weatherproof.

Now we will turn our attention to the next easy-to-fix problem and that is the resonant frequency on 10m which is about 28.7 MHz, 2-3 kHz higher than the tech frequencies of 28.3-28.5 MHz. What can we do to bring the resonant frequency down to 28.4 MHz? If we mount a 10 inch parallel open stub of 300 ohm twinlead about 5 feet up from the choke-balun, we will shift the resonant frequency to 28.4 MHz with an SWR of about 1.8:1 at that point. Here's a picture of the author's 10 inch stub. Because the parallel reactance is so large on the lower frequency bands, the stub can be made permanent without appreciably affecting conditions on any other band.

The following modification looks like it would have little effect on the ZS6BKW. In fact, if one wants to avoid an antenna tuner, it has a large effect especially on the higher frequency bands. It can be implemented locally with a DPDT knife switch or remotely with a DPDT relay. The following device will add zero or one or two feet to the ZS6BKW matching section.

Here's what that will accomplish:
The length of the matching section naturally has more effect as the frequency is increased. The longer length results in a lower frequency of resonance and the shorter length results in resonance at a higher frequency. Note that the shorter length eliminates the need for a tuner on 17m. The above graph is an actual scan of the author's ZS6BKW using an AIM-4170D antenna analyzer. The resonant frequency on 10m has been shifted to 28.5 MHz and high SWR on 80m has been improved. Here is an expanded graphic of what the plus or minus one foot of matching section can accomplish on 80m

Now we come to the modifications that most hams will probably think are not worth the effort. However, in the spirit of completeness for an all-HF-band antenna, the author has designed and tested each of these modifications on his ZS6BKW and they work as advertised. The 80m SWR can be further improved to perfect (at one frequency) by the addition of a parallel capacitor on the coax at the 1:1 choke-balun terminals. Unfortunately, that capacitor must be removed for operation on the higher frequency bands and thus requires remote control of a relay (or manually lowering the antenna each time 80m operation is desired.) The following graph shows what happens with the various combinations of a series capacitor and a parallel capacitor at the balun input.

And here is how to do the switching with knife switches when one has access to the balun position as does the author. Most hams would have to use a relay (or two) for this function.

We have three bands left on which the ZS6BKW is known to be a very poor performer, 60m, 30m, and 15m. How can we get the ZS6BKW to perform well on those bands? The answer is simple - the implementation is a little difficult. We need to switch in a parallel capacitor at a point on the 450 ohm matching section that will result in a low SWR at the balun. One SPST relay is all that is needed for each band. The author had surplus DPDT relays in his junkbox and that is what he used below:
If voltage is supplied to the relay, the capacitor is switched in. If no voltage, the capacitor is switched out. Thus if we have three relays for 60m, 30m, and 15m, only one of them will be activated at a time. Here's where to hang the relays and the capacitor values that are required.
For those of you who can read a Smith chart, here they are for the 60m, 30m, and 15m bands along with the SWR graphs for those bands.
If all of the modifications described above are implemented, the modified ZS6BKW literally becomes an All-HF-Band Antenna.