The ZS6BKW Multiband HF Antenna
This is the antenna for you guys who want to get on HF effectively, and haven’t too much space or cash to throw around. Actually, it’s a design from ZS6BKW (aka G0GSF), similar to the G5RV, but it actually resonates on five bands, (well 6, actually) and doesn’t rely on a tuner (ATU) to make it work. The design appeared in TT (RadCom) Jan & Feb 1993, but is also in Pat Hawker’s “Antenna Topics” (publ. RSGB 2002) It’s only 90 ft long (27.51 metres), with a 40 ft (12.2 m) downlead.
ZS6BKW
Horizontal or inverted V layout
So, it’s a cousin to the G5RV (which only resonates on 14 & 24 MHz), but better as it needs no ATU on 40, 20, 17, 12, 10 and 6 metres.
When Telford DARS were doing the 50MHz Trophy contest down at Bridgnorth, I took the necessary bits along to try out this antenna. For simplicity, I set it up as an inverted-vee configuration. The reason was simple – you only need one support to hold it up, not two. I also had the club’s MFJ Antenna Analyser with me so that I could see what was happening. I recorded the data – see below.
Incidentally, all centre fed antennas can be supported by just one mast, with the ends left to droop down. The ‘rule of thumb’ is that the angle at the apex should never be less than 90 deg, otherwise cancellation between the two halves occurs. Furthermore, as it is the current peaks along an antenna that do most of the radiation, having the centre at the highest point is a positive advantage, rather than supported at each end with a big droop at the centre (current point nearest to ground). This is another reason for not being too fussy about the ends of a centre-fed antenna being lower, or bent around. It will have minimal effect on radiation efficiency. The only thing is never have the ends dropping right down to ground level – because the ground will seriously de-tune the antenna and it will not work – believe me, I’ve tried it. Just a yard or so off the deck makes all the difference. Simply have end insulators (or plastic strips etc), then wire or twine to the tying-off points. This effectively raises the ends of the antenna sufficiently clear of the ground. So, the ‘BKW can be horizontal (two supports) or inverted-vee layout (single support), as shown. Incidentally, the same applies to a simple dipole.
The antenna wire can be solid copper, stranded, insulated or not. A lot of rubbish is printed about the merits or otherwise of different sorts of wire. It’s largely hogwash. Wire is wire at these frequencies. Wet string? well that’s a different matter.
In the original design, 300 ohm twin was used, but I prefer the 450 ohm stuff. It’s much stronger and losses, especially in wet weather, are lower when impedances are high down the line. Back in 1985, 450 twin wasn’t readily available, there was only 75 and 300 twin, or the option of making your own open-wire feeders (which actually are the best of all – around 600 ohm, but these do tend to twist or get caught in trees etc! Yes – bitter experience and soldered joints here too!)
Finally, if you want to use it on other HF bands (3.5, 10, 21 MHz), an ATU (just like at the bottom of your ‘5RV!) will do the business, but preferably at the bottom of the 450 ohm feeder with a balanced output, not after a length of 50 ohm coax, if you’ve had to use it to reach your rig. Of course for 1.8 MHz (160m), you could short out the feeder twin, and feed it like a Marconi antenna, with a suitable ATU. Not very clever, however.
Here are the MFJ figures I recorded on the test antenna:-
Best in-band frequency: SWR “R” at feedpoint Notes
3.38 MHz (80m) 7:1 20 tunes easily with ATU
7.00 MHz (40 m) 1:1 40 puurrfect
10.1 MHz (30 m) high high needs atu
14.06 MHz (20 m) 1:1 40 wonderful
17.85 MHz (17 m) 1:1 50 below 1.3:1 in 18MHz band
21.00 MHz (15m) high high needs atu
24.69 MHz (12 m) 2:1 100 OK, even without an ATU
28.62 MHz (10 m) 1.3:1 60 No sweat!
50.27 MHz (6 m) 1.3:1 60 A surprise: 6m. too!
Just to show the “proof in the pudding”, I used it on 7 and 14 MHz, and got excellent reports, as one would expect with a half-decent antenna! Didn’t have time to use it on all bands, but I leave that to you (to tell everyone how good it is).
Martyn Vincent G3UKV
Kevin VK2FUO advises to put a 1 to 1 current balun on the ZS6BKW antenna 450 ohm feeder line where it joins the 50 ohm coax to the radio.
VK2FUO acknowledges author Martyn G3UKV
Hi in the diagram it’s 93.60 ft long and the feeder is 39.1 ft long. But in your article it’s 90ft long and the feeder is 40 ft. ???? Can you please clarify
In the above table of SWR and resistance is the “R” the resistance where the flat top reaches the 450 ohm line?
Thanks and 73,
Pete
WB2UAQ
Yes, please. How long for each?
Wayne. Please read carefully. The 90 and 40 feet he is referring to the design if the antenna around the time when it was first introduced (2002). Use the dimensions in the diagram.
Can the matching section make a horizontal dogleg for part of the 39.1 feet?
the 1:1 balun sure makes a big difference in how this antenna works!
i built a larger version 204 ft. long with 64 ft. of ladder line.works great on 160 mtrs at 65 ft in the air as a flat top.
I just got this antenna on the air today, using the dimensions described above, or very close to them. I promptly worked two Russians and a Slovenia station with 55 an 57 reports. And, I have an S7 noise level. One end of the antenna is about 35 feet high, the center about 25 feet, and the other end, maybe 20 feet high. So far I’m really happy with this antenna.
On 40 and 20, (my main interest) the lowest SWR occurs at the bottom of the band (7.0 and 14.0 mhz). I would like to move the SWR low points nearer the center of the respective band. Obviously, I can trim the flat top portions. But would raising the antenna and/or adjusting the 450 ohm matching section accomplish the same thing?
I have the same question as Phil. Watching for replies.
Yes, you can easily trim the ladder line to adjust the resonant points higher in the band. You are fairly close to the ground if your center is just 25 ft high. You are getting some ground induced proximity loading which electrically lengthens the antenna and makes it resonate low. My suggestion is to trim the ladder line shorter, in small increments. Try to aim for your favorite band (note that a small adjustment on 7 mhz will have a larger impact on subsequent higher frequencies, eg, 14, 18, 28 etc.
people cant follow simple directions!
If using a clamp-on ferrite for the 1:1 balun, just moving the 1:1 balun would be easier than cutting the ladder line.
My question, and referring to Dave’s dogleg question, does the ladder line become a vertical radiator? Ideally you would want all the radiation off the dipole sections.
There will be essentially no radiation by the open wire feed line if a “good” 1:1 balun or choke (i.e. high impedance) is inserted at the open wire to coax connection. A balun or choke also helps on receive by reducing noise pickup on the open wire feed line.
The dogleg will require that the choke impedance be larger due to the unbalance introduced by the dogleg. Twisting the open wire line helps the wire balance because it reduces the unbalance caused by the ground and nearby objects.
I’ve been doing a lot of reading about this antenna on other sites. My question is about the length of the 50ohm line. I’ve seen everything from 70ft to 50ft to as short as possible. What did you have the best results with. When finished it will be flat topped at around 40ft.
For the ZS6BKW antenna, the coax feedline length is not critical. unlike the G5RV that relies on the coax for “losses” to reduce SWR on the non-resonant bands (all bands except 14 and 24 mhz),
the ZS6BKW presents a near 50-ohm R at the junction of the coax to the ladder line.
So, the coax can be any length that gets you to the shack. Shorter the better, of course, to minimize losses.
Also remember there is a 1:1 current choke balun at the coax-to-ladder line feedpoint so the coax is not radiating – only carrying RF to the antenna.
I built a 92 foot ZS6BKW with 40 feet open feeder-(14 ga THHN at 4 inch spacing)- did not have 450 ohm window available. Apex is 40 feet-ends are about 15 feet at approx 120 deg spread. Tried to attach feeder to balanced manual antenna tuner and didn’t tune well at all. Simply soldered 50 ohm coax to feeder line and resonates well on 40,20,17,12. Not 10,15 or 30 at all. Does my wide feedline change things too much-do i need 450? do i need more elevation on the ends? I see i need to add a current balun as well.
Thanks much