Lightweight ZS6BKW doublet for portable operations

A recurring topic of discussion amongst amateur radio operators who activate at portable field locations is “Which antenna do you use?” or something similar.

Each person has their own preference.

I started my SOTA operations using a home built Link Dipole. It was heavy, as it was built with materials on hand. I subsequently built a lightweight version, as described elsewhere on this blog. Both versions of the antenna suffered some mechanical failures due to the repeated deployment causing fatigue fractures in the wire.

Many operators make other antenna choices: end fed half wave, random length wire, small transmitting loop (a.k.a. magnetic loop), vertical antenna and the list goes on…..

One key consideration when making antenna choices for portable operation must be antenna efficiency. As portable antennas will often be operating at relatively low heights, ground losses will dominate antenna performance. Adding additional losses due to input transformers or physically small antennas can easily affect the success of an activation, especially when operating QRP.

After yet another fatigue fracture induced fault in a heavier link dipole during a Park activation, I decided to build a new antenna which would be stored in the back of the vehicle. I decided to build a doublet and thus remove the need to raise and lower the antenna to open or close links, a required activity for every band change with a link dipole. I considered several options, but decided on the ZS6BKW antenna. One downside of such a doublet is that one needs an antenna matching unit (or ATU if you prefer) on some bands, but this was not an issue for me as I had such units at hand. For SOTA activations, my KX2 has the ATU option installed.

The ZS6BKW is a based on the legendary G5RV doublet. Brian Austin G0GSF (ex ZS6BKW) wrote the article, first published in RadioZS (June 1985) and also in Sprat (#130, Spring 2007). There are many articles on the internet, but perhaps the most useful reference can be found here. This article includes the article “The ZS6BKW Antenna – from the horse’s mouth” plus some additional information.

My first ZS6BKW has built using uncoated 49 strand “600 lb” stainless steel wire, purchased at a local hunting & fishing store. It is 1.8 mm diameter. One attraction was that it was unlikely to break during normal operation in the field! I used 450 ohm ladder line for the feed line section. The antenna worked well, so when my lightweight SOTA antenna failed again, I considered options for its replacement.

I determined my antenna dimensions using Figure 2 of the Austin article. The figure allows one to choose a doublet length and the appropriate feedline length based on the feedline impedance. 300 ohm feedline suits longer doublet length – a possibly less desirable characteristic for portable use.

For SOTA operation, something lighter was desirable. I spent a little time over a long period looking to source some 300 ohm “TV ribbon”. Not a lot of time each time that I thought to go searching for it, but these thoughts came infrequently! I eventually found some in a hardware store in Traralgon when wondering around waiting for the vehicle to be fitted with new tyres. There were two rolls on the cable rack – one with about half of the cable gone, plus a roll which was nearly full. The latter was covered with a thick layer of dust. The price tag said 30 cents per metre. When an assistant arrived, I asked for a price for the whole roll, hoping for a discount. After several minutes, she returned and advised that the price was incorrect and should be more than twice the price estimated ($30 for 100 m) based on the displayed price. She did advise that I could have the whole roll for $30. I accepted.

The next task was to consider the wire for the horizontal span. I ruled out the lightweight DX-Wire used on the lightweight link dipole: it is very light and reasonably strong, but it is prone to fatigue fractures if you manage to induce a kink in the wire. I settled on some 1 mm stainless steel wire rope purchased at the local large green-clad hardware warehouse. A roll of 50 m cost less than $15. The wire is made up of 19 strands of 316 Marine Grade stainless steel wire, with 17 kg working load and breaking load of 85 kg. Larger sizes are also available, but the small size fitted my needs. The 50 m roll would make 1.5 antennas.

I also purchased some 1 mm ferrules to form small loops at the ends of the wire.

I made a centre insulator/joiner from some 1.6 mm fibreglass pcb material, without any copper cladding, cut from a scrapped board. Once I had cut the required shape, I made several holes: 2 x 1.6 mm holes each side of centre to feed in the inner ends of the horizontal wires, a larger central hole at the top as a tie-on point, some holes joined to form 2 slots to feed in the feedline, a hole to anchor a terminal block and some holes to take cable ties to fix the feed to the centre insulator.

The doublet centre assembly.

The prototype doublet centre. Each side of the doublet comes in through two (2) holes and then bends into either side of the terminal block. You could apply some conductive compound to the ends of the wire if desired. Similarly, the feedline enters via two slots cut in the insulator and then into the terminal block. A cable tie anchors the feedline to the pcb below the first slot to further reduce stain on the feedline.

I assembled the antenna using small loops at the ends and directly tying off some builder’s line to the loops, thus saving a small amount of mass from end insulators. Prior to forming the loops, I had slid some heat shrink tubing over the wire so that I could later cover any sharp ends on the wire. I erected the antenna in the backyard using a 10 m squid pole, with the antenna in a typical inverted V configuration. I then checked the impedance with an antenna analyser and soon had the system resonant at 14.23 MHz.

I next fashioned a small scrap of single-sided pcb material to form a termination for the rig end of the feedline and to join to some RG316 coax. I added some holes to use cable ties to fix the feedline and the coax to the board and soldered the lines to the hand-cut tracks. It was a little crude, but works for me.

As a precaution, I fitted a choke balun on the coax near the feedline junction.

A small pcb fashioned by hand to facilitate the junction between the coax and the 300 ohm feeder. The coax is fed through a ferrite to form a choke. A Velcro tie facilitates winding up and storing the antenna assembly.

The entire antenna and winders comes in at 460 g.

The antenna assembly ready for the backpack. Each half of the doublet is wound on a “Wonder Wabbit Winder”.

I have the option of raising the antenna centre with a squid pole or by throwing a line over a tree branch. The antenna seems to work well. Band changing is now a breeze: change bands, dial up the required frequency and hit the TUNE button. Ready to go! The KX2 ATU usually manages to find a match. To date, I have made contacts on all bands from 80 m through to 15 m. Propagation conditions being what they are at/near the bottom of the solar cycle, I have not yet used the antenna on on 12 or 10 m.

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