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.
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 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