Old XYL's Tales In Amateur Radio

#3: "A Full-Wave Dipole is Virtually Impossible to Match."

by Cecil Moore, www.W5DXP.com, Rev. 1.0, Aug. 8, 2017

History of the (Double) Zepp Antenna

Before WWII, open-wire balanced feedline was more popular than coax. Zeppelin airships were common and used a trailing antenna that came to be known as the Zepp antenna. It consisted of a half-wave end-fed wire fed by 1/4 wavelength of open-wire feedline. The 1/4WL of feedline transformed the high impedance of the half-wave end fed wire down to a reasonably low impedance acceptable to the vacuum tube finals of the day. The Zepp antenna is the grand-daddy of some of the end-fed antennas of today. Someone decided to add another 1/2WL of wire and connect it to the unconnected wire in the 1/4WL of feedline. The result was a full-wave dipole matched by 1/4WL of open-wire feedline and was known as a Double-Zepp, a very popular antenna at the time. So why does the full-wave Double-Zepp have a reputation for being difficult (if not impossible) to match in today's amateur radio circles?

Coax VS Open-Wire Feedline or Ladder-Line

Coax is the culprit. According to EZNEC, a 130 ft. 80m dipole has a feedpoint impedance of 5000 ohms on 40m where is it one full wavelength long. If one feeds the antenna with 50 ohm coax with no matching network, the SWR is 100:1 at the feedpoint. Thus the orgin of the myth that a full-wave dipole is a terrible antenna to be avoided at all costs. It is indeed a challenge to match coax to a very high impedance. Modern day end-fed 1/2WL antennas use an N:1 balun, where N:1 is a high value, e.g. 9:1, to try to transform the high impedance down to a level that will result in acceptable coax losses. But let's take a closer look at open-wire feedline's modern day equivalent, i.e. ladder-line. Let's compare the SWR for a 1/2WL dipole vs the SWR for a full-wave dipole using 450 ohm ladder-line. Assume the 1/2WL dipole has a feedpoint impedance of 50 ohms and the full-wave dipole has a feedpoint impedance of 5000 ohms. The SWR(450) for the 1/2WL dipole is 450/50=9:1. The SWR(450) for the full-wave dipole is 5000/450=11:1.

The SWR on 450 ohm ladder-line is approximately the same value for a 1/2WL dipole and a full-wave dipole!

Since the SWR is acceptable for ladder-line and approximately the same for both cases, the acceptable feedline losses will be approximately the same and usually lower than matched-line coax. So why do some hams assert that full-wave dipoles are to be avoided? It's a mental condition known as coax on the brain and was virtually unknown before WWII. Fortunately, there is an easy cure - just swallow a ladder-line pill and the condition will be cured. :)

Moral: There is absolutely no good reason to avoid a full-wave dipole, known as the Double-Zepp. Just feed it with an odd number of 1/4WL of ladder-line or open-wire line and enjoy about 2 dB of gain over a resonant 1/2WL dipole.

But please note that a feedline length of an odd number of 1/4WL will reverse the matching problem. It will cause a resonant 1/2WL dipole to have a very high impedance looking into the ladder-line. W5DXP feeds his 130 ft. dipole with 110 ft. (1/2WL) of ladder-line on 80m and 92 ft. (3/4WL) of ladder-line on 40m with no tuner required.[1]

[1] W5DXP's All-HF-Band No-Tuner Antenna