The ZS6BKW antenna is a better match for our bands than the G5RV. The method of construction and tuning is essentially the same for both antennas but the dimensions are different.

The ZS6BKW is a “random length dipole” with ladder line acting as a 1:1 transformer. I prefer to use 450 ohm 18 AWG solid ladder line which is readily available on ebay in appropriate lengths (20m). The antenna is designed with the dipole arms electrically 1.35 wavelengths. The cut lengths must correspond to this (allowing for velocity factor). The ladder line should be 1/2 wavelength (corrected for velocity factor).

You will need

- 1 only 1:1 current balun
- 1 only 20m roll of 450 ohm ladder line (window line).
- 2 only lengths of wire for the dipole arms (about 14.8 m each to start). I use stainless steel woven wire for strength.
- 1 only HF antenna analyser- if you are intending building your own antennas, this unit will be essential.
- 1 patch lead with suitable connectors to go between your antenna analyser and your 1:1 current balun (usually PL259 connectors)
- 1 only 50 ohm resistor (not wire wound)
- soldering iron and solder
- Means to cut the stainless dipole arms.
- Side-cutters to cut the ladder line.

The first step is to get the ladder line to the correct length to act as a 1:1 transformer. This best done with an antenna analyser and a few bits and pieces. Cut your ladder line to 13.11 metres length (43ft). solder a 50 ohm resistive load across the two leads at one end of the ladder line. Do not use a wire wound resistor. Connect the other other end to your 1:1 current balun. I like to use solder-on ring terminals for this job.

In order to determine the velocity factor of the ladder line, so that you know what length to cut it. Calculate the half wavelength frequency and compare it to the measured frequency.

Measured frequency- Using your antenna analyser connected as described, determine the frequency at which the impedance is 50 ohms – usually around 12 MHz for typical ladder line of this type.

Calculated frequency- 1/2 wavelength in free space corresponds to 150/WL = 150/13.11 = 11.44 MHz.

Determine the Velocity Factor of the ladder line by comparing it to the free space calculated frequency. e.g. If you measured that 50 ohms occurs at 12.54 MHz with your analyser, then the VF = 11.44/12.54 = 0.91.

My measurements gave a VF of 0.891 for my ladder line. I then multiplied this velocity factor by 13.11 to arrive at the length to cut the ladder line. = 11.68m. Thus the ladder line is now the correct length to provide a 1:1 match with the antenna.

Now to the construction of the two dipole arms. You should at first cut these to a fraction longer than 14.46m (47 ft 5 inches). I add about 0.3 m to each end and cut mine to 14.76m. Making them a bit longer, to begin with, allows you to double back the ends to adjust length and to provide means to attach an egg insulator. I adjust the electrical length by doubling the uninsulated stainless steel wire back on itself and use stainless steel cable clamps to hold it. Use a suitable insulator piece such as perspex between the two arms. I connect the stainless steel wire to the ladder line using ring terminals and stainless steel bolts through the perspex.

Hoist the antenna to its final height and test SWR at 14.200 MHz. If the measured SWR is lower at a lower frequency, the antenna is too long.

Once you have it to the correct length, connect a suitable length of coax to the 1:1 current balun and to your radio and get yourself on air. The antenna is good for all 80m, 40m, 20m, 10m, 6m but not for 15 m.

VK4MDX

For a ZS6BKW the ladder line segment should be (around) 0.62 wavelengths, not 0.5 as above. (See Brian Austin’s original text).

Hi Robert, I recall that he actually specified a range of usable lengths depending on the impedance of the ladder line used. I recall a couple of graphs for L1 and L2 and feedline impedances. Nonethe less the original design with 400 ohm ladder line was as I recall, two lengths of wire at 14.2m fed with 11.1m of 400 ohm ladder line. 11.1 metres at the design band of 20m is approximately 0.5 wavelengths. The important point, mentioned in the blog above is that the ladder line should be a 1:1 transformer at this freq, so measuring with the antenna analyser as recommended above will give you the correct length.

Well no David, that’s not correct. Taking the 20m band as a reference the original design specified 1.35 for the dipole length and 0.62*Vf for the matching section. In his later paper Austin says “It turns out from the analysis that the optimum lengths of L1 and L2 are about 1.35 and 0.62 on 20m.” (not accounting for the velocity factor).

The graph that you refer to is in his later paper. The _shortest_ useful length of L2, which is used with the _longest_ length of L1 (approximately 29.5m) is shown as 12.8m, approximately 11.5m taking the Vf as 0.9 and this is specified with 300 ohm feeder, not 400. Austin notes though that using lengths at the extremes of the graph result in narrow bandwidths. I’m not sure where you get the 1:1 transformer notion from with variable L2 lengths.

But no matter, sorry for picking nits. In practice the impedance of ‘450 ohm’ line is usually nearer to 400 ohm (fortuitously) and Austin’s equations depend on the assumed height of the dipole anyway so as usual experimentation is needed to get the best.

73

Yes, that is why I used the antenna analyser to determine the correct length to achieve a 1:1. I suggest all constructors do the same if they can.

The 0.5 lambda (wavelength = WL) is just used to determine the velocity factor. As per the original BKW article(s), the BKW antenna requires a length of 0.62WL (for L2), which for 20m. band is 13.3m. To be clear, you want an electrical length of 13.3 m. This is in free space, so has to be shortened by the VF. To determine the velocity factor, the short or 50ohm method is used. This method gives you the frequency where L2 is 0.5 WL. So calculate the frequency in free space where 13.3m. is 0.5 WL and then measure at what frequency L2 is 0.5WL and then compare that with the theoretical value to get the VF. Then shorten the 13.3 by the velocity factor to get the correct physical length, which will then be 0.62WL at 20m.

From the center of my roof I can make the 2 arms of the antenne, but it it not so easy to have the open line spread out. Can I rol-up the 450 line and fasten it close to the balun transformer.

Hi Henk

No. You must not roll up the ladder line. What is the roof made out of? If metal, get the antenna away from the roof. If not metal just get the ladder line as vertical as you can.

I get the 1:1 notion from the description of the random length dipoles from W2DU’s Book Reflections. If you have not read it, you should. The ladder line acts as a 1:1 transformer at the frequency of design in these random length dipoles such as the G5RV and the ZS6BKW, that is fact. The feeder of different impedance will thus affect the optimum length, as will the velocity factor. Given the use of 400 ohm feed line in my case, and given the variabilities of velocity factor and given the design parameters of a random length dipole (of which this is one). The best way to achieve this is to use the antenna analyser to ensure the feed line is in fact a 1:1 transformer at the design frequency.