Double-L Antenna For 80/160

Don Toman, K2KQ
toman@ibm.net

A popular misconception about vertical antennas for the low bands is that they must have elaborate ground systems. Here’s a vertical antenna for 80 and 160, fed with a single feed line that is simple, effective, and requires no ground system. You won’t beat the 4-squares, but you will hold your own against a grounded quarter wave with ridiculous amounts of copper in the ground.

Rather than get into the theory of why this antenna works, I will simply describe it here and let results speak for themselves. If there’s a demand, I’ll do a follow-up article on the relevant theory.

The antenna is a center-fed half-wave vertical with about 70 feet of vertical length with the remainder of the top and bottom of the antenna bent horizontal and parallel to each other. The antenna looks like a squared-off letter "C" fed in the middle of the vertical part.

Thus, the 160-meter antenna is a 270-foot dipole fed in the center with the bottom antenna wire bent parallel to the ground about 10 feet off the ground and the top at 80 feet off the ground. The horizontal parts are 100 feet long and parallel to each other.

The 80-meter antenna is a 130-foot dipole, fed in the center with 70 feet vertical and 30 feet horizontal 10 feet off the ground and 80 feet off the ground.

Think of it as an inverted L fed against an L. The two Ls are balanced with respect to each other and because the currents in the horizontal sections are out of phase, the antenna has a minimum of horizontal radiation.

The 80 meter and 160 meter antennas are separate, fed from a common coaxial feed line.

In my case, the 80-meter and 160-meter horizontal sections are about 30 degrees apart. The 160 horizontal wires run east while the 80 wires run east-northeast.

On both bands, the bulk of the current flows symmetrically in the center of the antenna, with the current peak about 45 feet off the ground at the feed point. On 80, the current loop peak is about 0.16 wavelengths above ground and on 160, the current loop is about 0.08 wavelength above ground.

The accompanying figure illustrates the antenna.

You can adjust the resonance of the antenna by adjusting the lengths of lower horizontal sections. The small asymmetry doesn’t bother anything. The center impedance of mine at resonance is very close to 50 ohms on 160 and close to 70 ohms on 80. The 160 antenna presents high impedance at 80 meters and the 80-meter antenna looks like a parallel capacitor across the 50-ohm 160 antenna. The 160 antenna covers 1800-1860 with under 2:1 SWR. I needed to take about 2 feet off the horizontal section to get mine resonant at 1830. If 80-meter current flows in the 160 antenna, it tends to flatten the current loop in the vertical section.

The 80 meter antenna is resonant at 3750 with the 130-foot length shown. The VSWR is under 2:1 over the DX part of the ‘phone band. It needs to have some length added to cover the CW portion. I haven’t tried to bring it to resonance in the CW band, but have chosen to feed it through a tuner.

I originally had this antenna hung from trees. This year I put up an 80-foot Rohn-25G with three sets of guys. I hung the wires from ropes attached to the tower so they are separated from it by about 3 feet. The coaxial feed line comes off perpendicular to the antenna and is then taped to the tower. The center conductor goes to the top and the shield goes to the lower part. Before the coax turns on to the tower, I’ve wrapped some 30 feet of it into a coil. I expected to see a lot of interaction, but the tower and guys seem well off resonance at the operating frequencies and I didn’t see any to worry about.

The first QSO on 80 was VK6LK, long path on SSB at sunset on September 12. I’ve worked a few ZSs, HF0POL, LU, and the usual Europeans. On 160, the first QSO was KP4SN on September 15. In the couple of weeks since hanging it from the tower, I’ve worked ZS6UT, TU2MA, TL5A, VK6VZ, VK6LK, VK3ZL and NL7Z and the usual horde of Europeans on 160 with no fuss. I run about 800 watts out from a Ten-Tec Centurion.

Last January, I had about 200,000 points with 750 QSOs in the CQ WW CW 160 contest using the predecessor hung from trees.

 

 

 

 
   


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