The ZS6BKW antenna is a better match for our bands than the G5RV. The method of construction and tuning is essentially the same for both antennas but the dimensions are different.
The ZS6BKW is a “random length dipole” with ladder line acting as a 1:1 transformer. I prefer to use 450 ohm 18 AWG solid ladder line which is readily available on ebay in appropriate lengths (20m). The antenna is designed with the dipole arms electrically 1.35 wavelengths. The cut lengths must correspond to this (allowing for velocity factor). The ladder line should be 1/2 wavelength (corrected for velocity factor).
You will need
- 1 only 1:1 current balun
- 1 only 20m roll of 450 ohm ladder line (window line).
- 2 only lengths of wire for the dipole arms (about 14.8 m each to start). I use stainless steel woven wire for strength.
- 1 only HF antenna analyser- if you are intending building your own antennas, this unit will be essential.
- 1 patch lead with suitable connectors to go between your antenna analyser and your 1:1 current balun (usually PL259 connectors)
- 1 only 50 ohm resistor (not wire wound)
- soldering iron and solder
- Means to cut the stainless dipole arms.
- Side-cutters to cut the ladder line.
The first step is to get the ladder line to the correct length to act as a 1:1 transformer. This best done with an antenna analyser and a few bits and pieces. Cut your ladder line to 13.11 metres length (43ft). solder a 50 ohm resistive load across the two leads at one end of the ladder line. Do not use a wire wound resistor. Connect the other other end to your 1:1 current balun. I like to use solder-on ring terminals for this job.
In order to determine the velocity factor of the ladder line, so that you know what length to cut it. Calculate the half wavelength frequency and compare it to the measured frequency.
Measured frequency- Using your antenna analyser connected as described, determine the frequency at which the impedance is 50 ohms – usually around 12 MHz for typical ladder line of this type.
Calculated frequency- 1/2 wavelength in free space corresponds to 150/WL = 150/13.11 = 11.44 MHz.
Determine the Velocity Factor of the ladder line by comparing it to the free space calculated frequency. e.g. If you measured that 50 ohms occurs at 12.54 MHz with your analyser, then the VF = 11.44/12.54 = 0.91.
My measurements gave a VF of 0.891 for my ladder line. I then multiplied this velocity factor by 13.11 to arrive at the length to cut the ladder line. = 11.68m. Thus the ladder line is now the correct length to provide a 1:1 match with the antenna.
Now to the construction of the two dipole arms. You should at first cut these to a fraction longer than 14.46m (47 ft 5 inches). I add about 0.3 m to each end and cut mine to 14.76m. Making them a bit longer, to begin with, allows you to double back the ends to adjust length and to provide means to attach an egg insulator. I adjust the electrical length by doubling the uninsulated stainless steel wire back on itself and use stainless steel cable clamps to hold it. Use a suitable insulator piece such as perspex between the two arms. I connect the stainless steel wire to the ladder line using ring terminals and stainless steel bolts through the perspex.
Hoist the antenna to its final height and test SWR at 14.200 MHz. If the measured SWR is lower at a lower frequency, the antenna is too long.
Once you have it to the correct length, connect a suitable length of coax to the 1:1 current balun and to your radio and get yourself on air. The antenna is good for all 80m, 40m, 20m, 10m, 6m but not for 15 m.
VK4MDX
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