Origins
(Austin 1987) described a multiband HF antenna that is very popular with hams some thirty years later.
In his article, Austin explained the characteristic of a single wire multiband antenna with a series section matching transformer. The geometry is quite similar to the G5RV with hybrid open wire and coax feed, but Austin pursued lengths of the dipole legs, and matching section length and Zo to optimise VSWR(50).
The design was never an ‘all band’ antenna, but rather a multiband antenna with low feed point VSWR(50) on several bands. Austin tabulated the frequency relationship of the optimised bands for the case of a 400Ω matching section, and they were in the ratio of 1:1.97:2.52:3.47:4.04. If the first frequency was chosen to be 7.2MHz, the other centre frequencies would be 14.2, 18.1, 25.0 and 29.1MHz.
NEC model
To give insight into behaviour of the ZS6BKW I have built and NEC-4.2 model of a ZS6BKW with dipole 28.5m (L1) of 2mm dia copper wire at height of 10m above ‘average’ ground (σ=0.005 εr=13), and 13.44m (L2) electrical length of 400Ω lossless transmission line. L2 was tweaked to optimise alignment of the VSWR(50) response with the ham bands. The model assumes no feedline common mode current.
Above is the VSWR(50) response of the model from 3-30MHz. Minimum VSWR near the nominated five bands is quite low. Note that VSWR(50) at 80m is quite poor. Continue reading The ZS6BKW five band antenna – discussion of an NEC model