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
Shack of VK2KMI 
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.
Very well presented article. May I suggest to those of you that obtain your ladder line look for stranded in favor of solid core that can fracture over time. It’s really good to see people still constructing.
Thanks Mark. Yes stranded is better although it still eventually breaks if stressed, it takes longer
Actually stranded is worse, if it is copper coated steel. On 3.5 MHz, the skin depth is higher than the actual copper layer.
https://owenduffy.net/transmissionline/300/davis.htm
I’ve never used steel wire for feedlines. Always copper and usually not stranded but, stranded copper does not seem to break as easily.
I use Stainlesssteel 3.5mm² with loads of substrings for my antenna’s. It’s incredibly strong, durable and works the same as copper, no difference at all. Also, it’s a lot cheaper. And I know that our coastguard is using it too as copper kept breaking because of the strong winds. The only thing is, you need to use a bit thicker then copper. As such I use 3.5mm²
We’re talking about feedline. My antenna is stainless. The feedline is solid copper ladderline.
Ok, but I would not use solid copper for a feeder, I use the same stainless or stranded copper as it’s not as easy to break as solid. However, I use openline (windowed) as it’s better to keep the space equal. Also the stranded copper can handle more power.
As I mentioned, I agree that stranded is better because it is less prone to break, but not immune. I used the term ladder line but it is commercial windowed line and the stranded stuff is difficult to find in this country. As for power handling, for short distances it’s unlikely to make much difference. The propagation on a ordinary stranded copper wire is usually slower than solid at RF and the loss is higher.
I do not agree that solid copper is faster. As the surface of stranded is a multiple size of solid it will have a lot less resistance and more surface to transfer the signal. At our physics lessons in school is was even proven that stranded needs far less mm² then solid to transfer the same signal. Also, when you get lower in the bands it will use less surface. I do not see where solid is better other then it’s cheaper to produce. I do not know about your country, but here in Belgium is also harder to find, I know of only 1 shop that carries it.
https://www.hfkits.nl/product/450-ohm-lintlijn-cq553-flexibele-geleiders/
You don’t have to agree to be wrong. https://www.flukenetworks.com/blog/cabling-chronicles/considerations-choosing-stranded-vs-solid-cable
https://harborenergysolutions.com/which-is-better-stranded-or-solid-wire-and-why/
https://forum.allaboutcircuits.com/threads/use-of-stranded-wire-versus-solid-wire-for-rf-purposes.129273/
HF signals use the surface of the wire, it doesn’t use the inside. As such it’s simple, the more surface you have the better the signal flows. Stranded has more surface then solid. The higher the frequency the more important the surface becomes, or even above it. That is why dialectic and wire coating becomes an issue pretty quickly in coax as well with 300 Ohm TV-lintline. Your last link shows how fast the surface is growing, the first post.
I don’t need a physics lesson. I know the physics. You need to read the links I provided. You are wrong about stranded v solid feedlines. Stranded wire is lossier and the velocity factor is lower PERIOD.
It realy doesn’t matter one bit, as the antenna will pervorm the same, stranded or solid. The big issue with these antennes is the right choise of coax to feed the window-line and the use of a good common-mode-chocke. The radiators and transformer-line have no impact if the correct lengths are taken. The coax is a different matter, some use rg-58 or 213 and that is a big error, as well as using a terriod 1:1 balun. I use ferrite-clamps (3m of the stuff) and Aircell7 (30m) and it works very good upto 1.5KW. I have made these antenna’s out of copper, litze and stainless-steel….made no difference. As the losses are not there to be found.
Again, were not talking about antennas and we’re not feeding with coax. We are using the wire as feedline (window line, ladder line etc).
“Skin effects are less pronounced on multi-stranded conductors” https://lectromec.com/skin-effect/
“The degree to which frequency affects the effective resistance of a solid wire conductor is impacted by the gauge of that wire. As a rule, large-gauge wires exhibit a more pronounced skin effect (change in resistance from DC) than small-gauge wires at any given frequency”
The short story is that the contribution of skin effect on aprox 1 to 2mm solid or stranded wire is tiny, but stranded results in higher losses at HF and the velocity factor is lower.
The G5RV and the ZS6BKW were designed to be fed via open wire not coax. As I say in my many posts regarding these antennas, a common mode choke is essential if connecting coax to the open wire feeder(window line or ladder line). I cover this in one of the posts in this blog. Due to the high impedance of the feedpoint at some non resonant frequencies, the return current will flow on the outside of the coax shield (due to skin effect). Essentially making the outside of the coax a third leg of the antenna and bringing RF into the shack.
I strongly recommend reading Walter Maxwells Feedlines for a detailed explainer of these antennas and the need for a choke. http://www.w3pga.org/Antenna%20Books/Reflections%20III.pdf
Correction. The book is called Reflections III
Walter his books are my bible, have read them all. However, you are writing about the ZS6BKW. The window-line is just a transformer, it’s not the feed-line as in transmission-line. The transmission-line is coax. Because if the window-line was a feed-line, it could have any length, but that isn’t the case. As for common-mode-currents, that happens at any SWR/impedance, as it’s a problem of coax because of the ‘3th-wire’ it has. Another thing, the ZS6BKW doesn’t function on 80m, it’s radiation is practically nothing. It’s only different then the G5RV as they changed the SWR to lower levels. However the radiators are too short for 80m to work and the difference to a real G5RV is enormous. Also, I did say I use a common-mode-choke, but not a 1:1 balun (is a choke too) as it’s has problems with QRO and high SWR/impedance differences. Ferrite-clamps do not have this problem and don’t have losses. If you have read Maxwell his books, you would have noticed that attenuation of coax is the only thing that matters and SWR has little to no impact on losses, as long as the coax is low-loss and has high-velocity. Something many overlook (or don’t want to know) like the ‘designers’ of the ZS6BKW did, they only focused on SWR and their computer-program. It is not an improved G5RV, it’s a mangled G5RV with worse result on 80m.