Construction Project – QRP 4:1 BALUN

At First use of a ZS6BKW antenna 1 February 2018, I mounted the ZS6BKW antenna on a 10 metre (32.8 feet) telescopic pole. For SOTA purposes the ZS6BKW antenna (enhanced G5RV) is quite versatile allowing operation on 80m through to 6m with the exception of 30 and 15m. With all the benefits invested this antenna, there is one obvious downside, the 11.1 metre (36.4 feet) 300 twin lead. Assuming your objective is to keep the feedline elevated and not resting on the ground, you need a tall telescopic pole.

So what’s this post about? I have modified the 11 metre 300 ohm twin lead and cut it to an arbitrary length of 8.2 metres (27 feet) (no science or magic formulas), just 8.2 metres which allows me to operate the radio about 1 metre away from the base of a 7m (23 feet) pole. Of note the support pole is now the stock standard 7 metre squid pole purchased from Haverford.

So what happens to the twin lead feedpoint impedance, it can’t be the same? Well a 28.4 metre ZS6BKW when combined with a 11.1 metre length of 300 ohm twin lead has impedance of approximately 50 to 100 ohms on 40, 20, 17 and 10m bands. Cutting the feedline to 8.2 meters changes the impedance to the point the antenna VSWR is now >10 on most but not all HF bands. Now the ZS6BKW is a 28.4 metre (93 feet) doublet, long live the doublet πŸ™‚

How do you work with a VSWR of >10? Simple, terminate the modified 300 ohm twin lead with a 4:1 BALUN or 6:1 BALUN (balanced to unbalance) impedance transformer as close to the radio as is possible, but wait with all good solutions there is a catch. You now require some form of impedance transformation circuit, commonly known as an Antenna Tuning Unit (ATU) to deal with the remaining 50 ohm mismatch, a manual or automatic T-match or L-match will work. Sorry no free lunch with this antenna.

I have a hombrew T-match plus an IC-703 with an inbuilt auto ATU. How do I construct a homebrew QRP 4:1 BALUN? See pics below – the male BNC panel mounted plug connects directly to the ATU or transceiver antenna socket. Secure the 300 ohm twin lead copper wire ends to the BALUN M4 screw posts and clamp between two washers. Tighten the wing nuts for a firm hold.

Don’t forget the upside to this type of antenna is the instant band changes. You don’t need to lower the pole and change links as you would with a multiband linked half-wave dipole.

Materials:

  • Section of 3mm (1/8th inch) kitchen cutting board
  • FT114-61 or FT140-61 toroid
  • Enamelled copper wire 20 AWG
  • 2 x 20 mm M4 screws, washers and wing nuts
  • 3 solder tabs
  • Panel mount male BNC plug
  • Short length of RG316
  • 2 x cable ties
Homebrew 4:1 BALUN
Homebrew 4:1 BALUN
30 MHz wide VSWR scan of the BALUN into a 200 ohm resistor
Rear view
Circuit diagram

My Icom IC-703 internal ATU is capable of matching the impedances shown below. The 4:1 BALUN is connected directly to the IC-703 antenna socket. Unlike the ZS6BKW, I can confidently transmit on the 30m and 15m bands. For SOTA purposes, I use 3.625, 7.090, 10.130, 14.310, 18.140 and 28.490 MHz +/- QRM. I rarely use the 15 and 12m bands.

Impedance and VSWR of the 28.4m doublet combined with a 8.2 metre 300 ohm twin lead and 4:1 BALUN. Looking into the 4:1 BALUN BNC plug.

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