Are you a fan of the G5RV antenna? Then you may be interested in the ZS6BKW antenna. The ZS6BKW antenna is an optimized variant of the venerable old G5RV. The G5RV antenna (circa 1946) was designed by Louis Varney (G5RV, SK) and the ZS6BKW (early 1980's) was derived from the G5RV by Dr. Brian Austin (G0GSF, formerly ZS6BKW). Dr. Austin used extensive computer modeling and experimentation to determine the optimum dimensions for his version of the antenna. Construction of the ZS6BKW antenna is essentially the same as the G5RV except for the lengths of the radiators and matching section window line. The original G5RV "flat-top version" had a 102' (31.1 m) horizontal span with a 34' length of matching ladderline.
Note: The computations and antenna described in this article are based on the use of 14 gauge 450 ohm window line for the matching section (L2) and #14 AWG THNN wire for the radiators (L1).
Matching Section of Window Line
The correct length of matching window line for the antenna is important! According to Dr. Austin’s design, the length of the matching section is .62 wavelength at 14.2 MHz or approximately 43’ in length. Wavelength in feet = 984 / f (MHz); 984 / 14.2 = 69.3’; 69.3’ X .62 = 43’.
We should adjust the calculated length of matching window line (43') to account for velocity of propagation (VF). If we know the velocity factor (VF) of the window line, we can determine the required length of matching line by multiplying the calculated length by the velocity factor.
Example: If the VF of the window line is known to be .91, the length of the matching window line would be approximately 39.1’ (43’ X .91 = 39.1’).
If we do not know the VF of the window line, we can estimate it using the following procedure.
1. The wavelength at a frequency of 14.2 MHZ is 69.3’ (984/14.2 = 69.3’). Multiply 69.3' by .62 to get 43’.
2. Cut a 43’ length of window line.
3. Connect a 50 ohm resistive load across the leads at the high end of the window line. Do not use a wire wound resistor for the load. Wire wound resistors are notorious for exhibiting inductive properties because of their coil construction. Introducing inductive properties (reactance, etc.) could cause erroneous analyzer readings.
4. Hoist the window line vertically at least 1’ above ground and clear of nearby objects.
5. Connect an antenna analyzer to the lower end of the window line through a 1:1 current balun.
6. Use the analyzer to find the frequency where the window line impedance measures 50 ohms. Record the frequency.
7. Compute the 1/2 wavelength (WL) frequency for a 43’ (13.11 m) length of window line. Remember WL in meters = 300 X f (MHz). By rearranging the WL formula, we can compute the frequency. The 1/2 WL frequency is 150 / WL; (150 / 13.11) or approximately 11.44 MHz.
8. The approximate VF of the window line is the ratio of the computed frequency to the measured frequency. If 12.54 MHz was the measured frequency for an impedance of 50 ohms, the approximate velocity factor is .91 (11.44 MHz / 12.54 MHz).
Note: A valid VF cannot be greater than 1. If you compute a VF greater than 1, something has gone wrong in the measurement process. A VF of higher than .91 for window line is suspect based on the published VFs of high quality window line. VFs of open-wire “feed line” can be in the range of .95 to .99.
9. Adjust the length of the window line (43’ X VF). With a VF of .91, the length of matching window line for the antenna would be approximately 39.1’ (43’ X .91 = 39.1’).
Tuning the Radiators
The dipole length of the ZS6BKW antenna is designed to be electrically 1.35 wavelengths (WL) long at 14.2 MHz. The WL for 14.2 MHz is 69.3’ (984 / 14.2 = 69.3’). Then 1.35 WL is 93.5’ (1.35 X 69.3’ = 93.6’).
1. Cut (2) dipole radiators 47.5’ long allowing extra length for adjustment purposes. Generally, you can expect to have to shorten the radiators to slightly under 46.75’ to compensate for the velocity factor of THNN coated wire and other environmental factors.
2. Attach the matching window line to the antenna.
3. Hoist the antenna to its operating height.
4. The matching window line should hang vertically from the antenna at least 1’ above ground and clear of nearby objects.
5. Connect an antenna analyzer to the the source end of the coax TL and measure the SWR at 14.2 MHz.
6. If the measured frequency with the lowest SWR is lower than 14.2 MHz the antenna is too long. If the measured frequency with the lowest SWR is higher than 14.2 MHz the antenna is too short.
7. Adjust the lengths of the antenna radiators until the minimum SWR is measured at 14.2 MHz. Be sure to adjust each radiator the same amount.
Note: The resonant frequency of a dipole antenna is dependent upon its height above ground as well as other environmental factors.
KI4PMI and I constructed the ZS6KBW antenna described above and analyzed it extensively using an AIM-4170C. The SWR curves generated by the analyzer are displayed below. Measurements were made with and without the 1:1 current balun in place. No discernible differences were noted.
Note: 25’ of RG-8X completed the TL arrangement and the antenna was hoisted to a height where the lower end of the matching window line was approximately 3’ above ground.
KI4PMI describing ZS6BKW construction details
2 Meter Band
6 Meter Band
10 Meter Band
12 Meter Band
17 Meter Band
20 Meter Band
40 Meter Band
80 Meter Band
The EZNEC models for the antenna are available in a zipped file at the link below.
With the exception of the 6, 15, and 80 meter bands, the analyzer displayed excellent SWR curves and bandwidths. The antenna was deemed unusable on the 15 meter band due to a very high SWR curve. However, we were able to tune both the 6 and 80 meter bands using an LDG AT-100PRO autotuner. In spite of poor band conditions and an inopportune time of day, we made contacts on 80 meters, 40 meters, 20 meters, and 17 meters. The 17 meter contact was located in Italy.
The ZS6BKW is a very nice wire antenna and well worth the effort to build.
KI4PMI & NC4FB