The G5RV
Louis Varney, G5RV, designed his antenna after the Second World War and published it in the RSGB Bulletin in 1958. The top is a 3/2-wave dipole on 20m. It's 31.1 m (102 ft) long, and a 10.36 m (34 ft) open-wire matching section drops from the centre to transform the impedance on 20m so that it can be fed with coax. On other bands, the matching section and the coax together present an impedance that a tuner can usually handle, but the SWR on the coax is often high. Varney himself intended it to be used with a tuner on every band except 20m.
The half-size G5RV has a 15.5 m (51 ft) top and a 5.18 m (17 ft) matching section. It covers 40m to 10m with a tuner, and 80m only with a lot of help from the tuner.
The ZS6BKW
Brian Austin, ZS6BKW, used computer modelling in the 1980s to re-optimise the G5RV's dimensions. His version has a slightly shorter top, about 28.4 m (93 ft), and a longer matching section of 450 Ω line with an electrical length of 13.6 m. It gives an SWR under about 2:1 on 40m, 20m, 17m, 12m and 10m without a tuner, and usually on 6m too, which makes it a popular upgrade from the G5RV.
Why the velocity factor matters
The matching section is a transmission line, so its electrical length is what counts. Waves travel more slowly in ladder line and twin feeder than in free space, so the physical length is shorter. Window line varies between brands, from about 0.88 to 0.95, and wet weather lowers it further. If your SWR minimum lands just outside a band, adjusting the matching section length by a few centimetres is often more effective than trimming the top.
Installing it
Hang the matching section straight down from the centre for as far as possible, and keep it away from metal. Fit a 1:1 current choke where the matching section meets the coax, since the antenna's feed is unbalanced on several bands and the coax braid will radiate otherwise. Keep the coax from the choke to the radio as short as practical, because high SWR on the coax adds loss. The coax loss calculator shows how much.
Questions
How long is a G5RV?
The full-size G5RV is 31.1 m (102 ft) across the top, with a 10.36 m (34 ft) open-wire matching section. With 450 Ω window line, cut the matching section to about 9.7 m (32 ft) to allow for its velocity factor.
What are the ZS6BKW dimensions?
About 28.4 m (93 ft) across the top, with a 450 Ω matching section 13.6 m long electrically. With window line of velocity factor 0.91, that's 12.4 m (40.6 ft).
Does a G5RV need a tuner?
Yes, on most bands. It's designed to match well on 20m, and needs a tuner elsewhere. The ZS6BKW matches on more bands without one.
G5RV or ZS6BKW?
The ZS6BKW gives a low SWR on more bands (40, 20, 17, 12 and 10m) without a tuner. The G5RV is a little longer and needs a tuner, but some find it slightly better on 80m.