I have written about the ZS6BKW some time ago in September 2012. I suggest anyone reading this post, obtain a copy of Walter Maxwell’s excellent book Reflections III. In it he explains the misconceptions about a G5RV and provides excellent information. The book is available here http://store.cq-amateur-radio.com/Detail.bok?no=183
Let me summarize some of the key learnings about a G5RV antenna.
1. The G5RV is a random length dipole just like any other random length dipole.
2. Varney (G5RV) designed the antenna to have multiple lobe radiation pattern on 20m.
3. Varney intended the antenna to be fed with open line feeder from transmitter to antenna.
4. The G5RV if built correctly will have an SWR of 1.7:1 at the resonant frequency on the 20m band. SWR will be very high on other bands.
5. A transmatch (tuner) is required.
6. The open wire feeder is cut to a specific length so as to act as a 1:1 impedance match ie. the impedance of the antenna will be seen by the transceiver.
7. If feeding with coax to the open wire feeder, a 1:1 current balun is required at the junction of open wire and coax. ie. where you go from BALanced to UNbalanced. Without this the coax will act as a third component of the antenna and will radiate, reducing efficiency, increasing receive noise and causing interference.
8. The ZS6BKW variant provides a better match on most modern bands and is what I now use. http://www.nc4fb.org/wordpress/zs6bkw-multi-band-antenna/