W5JH Projects

Magic Antenna Lengths

I have recently been considering the various lengths of dipole antenns for multi-band operation. L.B. Cebik (W4RNL) has written extensively on the 88 foot and 44 foot dipole antennas (see The "Ideal" Back-Up Antenna for 80-20 Meters). These lengths lend themselves well to multiple band usage when used with a low loss (as in open ladder line) feeder. The 88 foot length is popular for 80-20 meter operation and the 44 foot length is popular for 40-10 meter operation. Here is some stuff I have figured out.

Let's consider the 88 foot length first. The reason for the "magic" 88 foot length is two fold. Firstly (as the English say), it produces a broadside pattern for the 80-20 meter range. Secondly, the SWR and efficiency on 80 meters is still a "reasonable" figure. So how do we arrive at this length? To answer that question, lets consider the highest frequency - in this case 20 meters. A 88 foot dipole on the 20 meter band is a Double Extended Zepp. That means the antenna is 5/8 wave long on either side of the center insulator. The Double Extended Zepp is the longest dipole that will still produce a broadside pattern in its main lobe. You can prove this to yourself by modeling the antenna with your favorite antenna modeling software. On 20 meters, the 88 foot length dipole produces about 3 db gain over a similar height 1/2 wave dipole. On the 80 meter band, the same antenna has just a small amount less gain than a full size 1/2 wave length dipole at the same height. EZNEC reports the SWR on 80 meters at about 50:1 (using 450 ohm line). On 20 meters the SWR is considerably less - about 13:1. SWR is also somewhat dependent on the height above ground. These SWR values require a very low loss feedline. The recommended feedline is open ladder line used with your favorite parallel line tuner.

As mentioned above, the 44 foot dipole operates well over the 40-10 meter bands. On 10 meters, the 44 foot length is a Double Extended Zepp with gain. On 40 meters, the length is shorter than a 1/2 wave dipole but still has good efficiency. The SWR values are similar to those stated for the 88 foot model.

Looking closely at the two antennas, a pattern emerges. To make a generic dipole with a broadside pattern for a large frequency range, we can pick the highest frequency to be used and calculate a Double Extended Zepp. The antenna is then usable down to a frequency of that highest frequency divided by 4. Here are a couple of formulas:

      L = 1232 / Fh(Mhz)

      Fl(Mhz) = Fh(Mhz) / 4

      Where L = Length of dipole in feet, and Fl = Lowest usable frequency in Mhz.

Here is an example.

      Fh = 21 Mhz

      L = 1232 / 21 = 58.7 feet

      Fl = 21 / 4 = 5.25 Mhz

So, a 58.7 foot dipole would give a broadside pattern from 5.25 Mhz to 21 Mhz and at 21 Mhz have approximately 3 db gain over a 1/2 wave dipole. Why would we be interested in this antenna? Several reasons come to mind. If we ever get some band between 80 and 40 meters, this antenna might make sense! How about the case where the person doesn't have any interest in 10 meters and wants better performance or a lower SWR on 40 meters, then this length may work for them. Other band/frequency combinations also work out well. A 68 foot dipole (dang that's close to a 1/2 wave dipole for 40) works well on 40-17 meters producing 3 db gain on 17 meters. For SWLing, a 70 foot dipole would be a useful length.

Building The dipole

The dipole and feed line are built from two pieces of wire. To calculate the lengths of the wire, divide the dipole in half and add the length of the feed line. Cut two wires the same length. For a 88 foot dipole with a 65 foot feedline, the two wire lengths would be: (88/2)+65 = 109 feet. Each wire becomes 1/2 half of the antenna and also 1/2 of the feed line. Note: there are no breaks in the wire at any place. Therefore, there aren't any solder joints to go bad! Take a look at The FFD ANTENNA: A Field-Friendly Doublet, with Notes on Related Designs by Charlie Lofgren, W6JJZ.

Open Ladder Line and Tuners

Open ladder line is making a comeback as a great low loss feedline. Back many years ago, it was the ONLY type of feedline in use! When I say open ladder line I mean just that. I am not talking about the commercial 450 ohm "window line" being sold today as ladder line. Ladder line is usually homebrewed and will exhibit a much lower loss than the commercial window line especially when wet. See Balanced Transmission Lines in Current Amateur Practice, by Wes Stewart, N7WS. When making open ladder line, chose a large conductor. #14 or larger wire works best. The 168 strand PVC covered "flexweave" wire is excellent for homebrewed ladder line. The impedance choosen is not very critical but keep it above say 350 ohms and perhaps below 600 ohms. The modeling for the antennas discussed here was done using a 450 ohm feedline.

Very few companies are making parallel line tuners. The commercial companies (MFJ, etc.) produce single wire and coax fed line tuners that usually have 4:1 voltage baluns. Experience has shown that voltage baluns don't work well into complex impedances. If you plan to use a coax line type of tuner for parallel lines, make or buy a 1:1 or a 4:1 current balun. The 1:1 current balun is really just a high impedance choke and keeps the current constant on the parallel lines. Of course if you can find a good Johnson Matchbox at a reasonable price, go for it!


Without going into a great deal of detail about patterns, the Double Extended Zepp will produce approximately 3 db gain on the band it was designed for. On the other bands (the lower ones), it will produce less gain. At about 1/3 the design frequency, the antenna will have gain equal to a 1/2 wave dipole. This antenna is also useful for portable operation or as a low profile antenna for antenna unfriendly neighborhoods. As with any horizontal polarized antenna, this dipole needs to be put up as high as possible - preferably 1/2 wave length or higher at the lowest frequency to be used.


Copyright © 2001 Jerry W. Haigwood