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 (c. 1946) was designed by Louis Varney (G5RV, SK). The ZS6BKW (early 1980's) was derived from the G5RV by Brian Austin (G0GSF, formerly ZS6BKW). He developed computer programs that were used in conjunction with Smith charts to calculate the optimum dimensions for his variant of the antenna. Read G0GSF's own article about the antenna.
Construction of the ZS6BKW antenna is essentially the same as the G5RV except for the lengths of the radiators and matching section ladder line. The original G5RV "flat-top version" had a 102' (31.1 m) horizontal span with a 34' length of matching ladder line.
Note: The computations and antenna described in this article are based on the use of Wireman 5541 for the matching section (L2) and #14 AWG stranded copper wire with THHN coating for the radiators (L1).
Matching Section of ladder Line
The length of L2 (matching section) is important! According to the ZS6BKW design, the length of the matching section is electrically .62 wavelength (WL) for the 20 meter band. Since 14.2 MHz is near the midpoint of the 20 meter band, we will use it to calculate the initial value of WL that will be subsequently adjusted to compensate for the effects of velocity factor (VF).
L2 computed wavelength
WL in feet = 984 / f (MHz); 984 / 14.2 = 69.3’; 69.3’ X .62 = ~43’.
L2 electrical length
We need to adjust the calculated length of L2 (~43') to account for VF. If the VF of the ladder line is known, we can determine the electrical length of L2 by multiplying the calculated length by VF.
Example: If the VF of the ladder line is known to be .91, the electrical length of L2 would be approximately 39.1’ (43’ X .91 = ~39.1’).
If the VF of the ladder line is not known, it can be estimated by the following procedure. The alternative steps were suggested by KG0T.
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' piece of ladder line.
3. Connect a 50 ohm non-inductive resistive load across the leads at the feed point end (top) of the ladder 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.
KG0T alternative: Short the ladder line leads at the top end.
4. Hoist the ladder line vertically so the lower end is at least 1’ above ground and clear of nearby objects.
5. Connect an antenna analyzer to the lower end of the ladder line.
6. Use the analyzer to find the frequency where the ladder line impedance measures 50 +j0 ohms or as close as you can get to it. Record the frequency.
KG0T alternative: With the ladder line top end leads shorted, use an analyzer to find the frequency where the impedance measures 0 +j0 ohms or as close as you can get to it. Record the frequency.
7. Compute the 1/2 wavelength (WL) frequency for a 43’ (13.11 m) length of ladder 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 ladder line is the ratio of the computed frequency to the measured frequency. Divide the computed frequency by the measured frequency to estimate VF.
Example: If 12.54 MHz is the measured frequency for an impedance of 50 ohms (0 ohms KG0T alternative), the VF is ~.91 (11.44 MHz / 12.54 MHz).
Note: If you compute a VF of 1 or greater, something has gone wrong in the measurement process. A VF higher than .93 for ladder line is suspect based on published VFs of high quality ladder line. VFs of open-wire “feed line” can be in the range of .95 to .99.
9. Adjust the length of the ladder line (43’ X VF). With a VF of .91, the required length for L2 is ~ 39.1’ (43’ X .91 = 39.1’). Cut the ladder line to a length of 39.1'.
Tuning the Radiators
The L1 length of the ZS6BKW antenna is designed to be electrically 1.35 WL for the 20 meter band. The WL for the chosen frequency of 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) radiators each 47.5’ long. The extra length is to allow for adjustment during the tuning process. Generally, you can expect to have to shorten the radiators to around 46.75’ to compensate for the VF of THHN coated wire and other influencing factors such as height above ground, soil condition, and proximity to trees and structures.
2. Attach the matching section of ladder line to the antenna.
3. Hoist the antenna to its operating height.
4. The ladder 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 transmission line (TL). Be sure to include the balun and the coax TL. Measure the SWR at 14.2 MHz.
6. If the lowest SWR occurs at a frequency lower than 14.2 MHz, L1 is too long. If the lowest SWR occurs at a frequency higher than 14.2 MHz, L1 is too short.
TIP: If for some reason L1 is too short, add the same amount of wire to each end using "spade type" quick disconnect pairs and let the wire dangle vertically.
7. Incrementally adjust the length of L1 until the best SWR curve for the 20 meter band centered around 14.2 MHz is observed. Be sure to remove the same amount of wire from each side.
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. 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 ladder line was approximately 3’ above ground.
KI4PMI describing ZS6BKW construction details
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 good 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
- 440 ohm ladder line, 14 AWG, 19 strand copper-clad steel, designed for maximum legal power, VF .91. ↩