By Ed Humphries – N5RCK
The March 1991 issue of CQ Amateur Radio contained yet another discussion of multiband wire antennas. In his column ”Radio FUNdamentals”, Bill Orr, W6SAI writes about the original W9CXX multibander with its’ complex copper tubing matching section.
He then goes on to discuss the popular G5RV developed by R.Varney, which is widely built and commercially available. Orr points out the deficiencies of the G5RV: when built in the original design it delivers reasonable SWR on the 7, 14, and 24 MHz bands, but into a 75 ohm coax feedline that is awkward to load up on modern transceivers; when built with 50 ohm coax the SWR is poor on all bands, but it performs reasonably well when used with a ”transmatch” antenna tuner.
The column skips over an intermediate antenna design discussed in the March 1986 issue of Ham Radio. Bill’s column back then pointed out that W5ANB first proved you could successfully modify the G5RV, load it with 50 ohm coax and run without any antenna tuner. But the best design (so far HI) he discusses in both articles is the one by ZS6BKV. Brian Austin used computer modeling to help him design a 5 band tuner-less antenna. Orr’s CQ column reprints the design using only the dimensions for a 300 ohm matching section (I presume TV flat lead qualifies).
In his original column Orr also presented the figures for using 400 (handmade open-wire leads) or 450 ohm (ladder-line) as the matching section. Since 450 ohm ladder-line is somewhat stronger than the commonly available 300 ohm TV lead-in, I’m here giving both sets of figures so you can make your own choice.
< 90' 3" for 450 ohm matching section or 92' 2" for 300 ohm > o-----------------------------oo-----------------------------o || The ZS6BKV Antenna || || || 40' for 450 ohm || || 36' 9" for 300 ohm || ||
At the end of the matching section Orr recommends a 1:1 balun; others would say that several loops of coax at the feedpoint will do as well to help keep rf off the feedline. The feedline to the transceiver is common 50 ohm coax; RG 58/U is fine for hf for most runs. This antenna should give low SWR on 7, 14, 18, and 24 MHz bands. At 28 MHz the SWR is really only good from 28.5 to 29.0. Tests showed the best SWR curves when the antenna was erected at about 42 feet above ground. When run as an inverted-V (90 degree) the resonant frequency came down 80 kHz for 14 MHz and 125 kHz for 24 and 28 MHz. The March ’86 article printed SWR curves, and the March ’91 article printed field patterns for all 5 covered bands.
* Origin: Ham Echo Moderator / HDN Coordinator (214) 226-1181 (1:124/7009)