Doublet Antenna
 

This is my favorite multi-band dipole. It is often confused and called a variety of names, the most common is probably "Zepp," or “center-fed Zepp.” Other names include the “tuned doublet” and “dipole with tuned feeders.” This is essentially a dipole cut for the lowest frequency of interest and fed with 450 ohm ladder line. An antenna tuner with a built in balun matches the feed-line impedance to the 50-Ω coax to your radio.

                

Unfortunately, this antenna is widely misunderstood. Let’s clarify a few points—perhaps we’ll even debunk a few myths in the process:

1. Ladder line is used because of its inherent low loss at HF. It does not radiate (assuming proper balance), nor does
     high SWR cause it to radiate.

2. High SWR on a transmission line merely increases line losses relative to those of the matched line. It does not degrade
    the antenna's performance.

3. Any power that is not dissipated in the feed line, tuner or transmitter is radiated by the antenna.

4. It is a multi-band antenna and will transmit on whatever frequency it is cut for, and above.

5. You can get gain from this antenna on the higher frequencies. (multiple wavelenghts)

The Doublet can be remarkably efficient, despite the relatively high SWR along its feed line. The secret is the ladder line’s low loss.

Do not install a balun between the ladder line and antenna; they are both balanced already!

The antenna length should be about a half wavelength at the lowest frequency of interest for best results.

Typical lengths for an antenna constructed with stranded, jacketed copper wire: (wire gauge is not critical for this formula)

160m overall length - 254' or 127' per side.
80m overall length - 134' or 67' per side.
40m overall length - 67' or 33.5' per side.

There is no magic feed-line length; however, a slight adjustment may be required for your particular installation. Remember, the feed-line impedance varies along a mismatched line. If your tuner has difficulty on some bands or you experience “RF in the shack,” add or subtract 1/8 wavelength (for the troublesome band) of feed line. You may need to repeat this if other bands are difficult to tune, but a little perseverance will fix them all.

Advantages of the Doublet include all-band coverage, minimal weight and low cost. Twisting the ladder line helps stabilize it in windy environments. I generally twist it 180° for every two feet or so.

Disadvantages are the expense of a tuner and the need to retune when changing frequency or bands. Ladder line lacks the convenience of coax. (You must keep it away from metallic objects by at least twice its width—about two inches for one-inch-spaced 450-Ω line.)  

 
 
 
Copyright © 1995 - 2012, AI4JI
All Rights Reserved