Monday, September 9, 2019

ZS6BKW Antenna

One of the antenna projects I wanted to get finished off this summer was get the 40m OCFD up in a flat-top configuration.

Got thinking about how I'd lack an antenna that covers 75m NVIS, thought about an 80m OCFD again. At 135ft long, the current situation with tree shapes, sizes and locations leave that easier said than done, and not for a lack of trying with out resorting to chainsaws. Having said that, getting one of two 80ft Fir trees closest to the house removed is on the list, so I may be-able put up longer wire antennas in the future.

What else is there that's under 100ft, at a minimum covers 40 and 20m with out the need for an ATU, and will work on 75m?

After considering a few ideas, I took a closer look at the ZS6BKW antenna which is an optimized variant of the well known G5RV. This antenna checks the boxes on these good points:
  • Light weight - no traps, coax etc hanging.
  • Low SWR on 40 and 20m, can run ~1kW with the AL-80B.
  • Respectable gain on 20m if you have the luck or luxury of orientating the broadside to favor somewhere of interest.
  • Will work on 75m with the remote ATU I already have.
  • Also covers 17, 12, part 10 and 6m with low SWR. 17m opens occasionally during the current solar minimum, and there is the Puget Sound 10m net on Sunday nights I like to take part in.

Spent a couple weeks reading everything I could find, and playing with models - one is included with MMANA-GAL, others can be found online.

Decided it was worth trying out. Initially I was going to order the parts from The Wireman and build it my self, but found a source of these pre-made cheaper and better built for $60 (normally $90) plus shipping from Amateur Radio Supplies => ZS6BKW G5RV 80, so went that route. 

Current setup:
  • Antenna apex at 50ft, ends in inverted V configuration.
  • K9YC inspired choke balun - 7 turns of RG-8X through two mix 31 clamp on ferrites.
  • Remote ATU between the choke and coax to the shack. 

How does it work?
  • Except for 15m, low SWR on six bands from 40 to 12m, parts of 10 and 6m.
  • Remote ATU dials it in on 75, 60, and 15m, brings the total to nine bands.
  • 30m is the odd one out, ATU fails to find a match in my case.

Nine bands where only three need an ATU, with 93 feet of wire in the air is OK by me. 30m might be an issue for some, but you can't always win 'em all :-) 

Links to further information:

What's next?
  1. Get the ends up higher - the test inverted V setup I have has the ends coming down too steep and into the trees, as expected resonance / low SWR favors the low ends of the bands.
  2. Maybe after the trees are "trimmed" it's possible I could move the antenna to where the vertical is with the existing radial field. This would allow me to operate it as a T (top loaded vertical) by bridging the two legs of the ladder line together at the base, and feeding it against ground resulting in an ok 80m DX transmit antenna in place of the vertical, eliminating one antenna from my setup.

What about low band DX? Not much progress there, and with winter approaching need to sort something out. Putting up the K9AY receive loop and building an 80/40m matching system for the 43ft vertical, possibly returning it to an inverted L is the "quick fix" solution.