W5DXP's No-Tuner, All-HF-Band, Horizontal, Center-Fed Antenna
Cecil Moore, W5DXP.com, Rev. 1.3, Sept. 30, 2017
The No-Tuner, All-HF-Band, Horizontal, Center-Fed Antenna is our
old friend, the 80 meter halfwave dipole dressed up a bit. By varying the length of the
450 ohm ladder-line feeding the antenna, we can achieve an SWR of less than 2:1
on all frequencies on all HF bands with the exception of the lowest part of 80m. On 75m,
we are feeding the antenna with a half-wavelength of ladder-line. On 40m, we are feeding
it with 3/4 wavelength of ladder-line.
No antenna pruning required.
My transmission line really does tune my antenna system.
Special thanks to Walt Maxwell, W2DU (SK) and Jim Bromley, K7JEB.
The Ladder-Line Length Selector actually does tune the antenna system so no conventional
"antenna tuner" is needed - no coils and no capacitors. Switches or relays (remote control)
can be used for the switching function and should be sized according to the RF power levels
involved. W5DXP presently uses ten DPDT Knife switches attached to a piece of plexiglas
mounted in the hamshack window. For portable or backpacking use, the length selector
function can be performed simply by 1/2/4/8/16 foot pieces of ladder-line with mating
connectors on the end. The proper length of ladder-line is selected to cause resonance
in the antenna system.
Here's a table that explains it all. The transmission line always consists of a matching
section and from zero to six halfwavelengths of ladder-line. The impedance at the antenna
is shown along with the 450 ohm SWR and the impedance at the transmitter is shown along with
the 50 ohm SWR, i.e. the SWR seen by the transmitter. Please note that the data generated by
W5DXP occurred many years ago using EZNEC's generic 450 ohm lossless ladder-line which is an
approximation to the real world values. AC6LA has recently generated more accurate data using
AutoEZ with Wireman #554 ladder-line for a more exact feedline length, Z0, VF, and including
real-world losses. The following chart shows W5DXP's original lossless values and AC6LA's
real-world values. (Thanks, Dan, for the improved accuracy.)
Graphic Data Presentation Using Smith Chart (100k)
Here are the ten DPDT switches mounted on a piece of plexiglas that mounts in
W5DXP's hamshack window. It shows the ten DPDT switches with the one foot, two
feet, and four feet loops installed. The eight feet and 16 feet loops are not
installed yet in this picture. RF flow is right to left from banana socket set
to banana socket set. When installed in the hamshack window, the switches are
on the inside and the loops of ladder-line are on the outside.
Here's two ways to handle the shorter loops. On the left picture, the RF flow is
right to left into the banana sockets. The switches are shown in the shorted position, i.e.
the one foot loop is floating completely out of the circuit to avoid capacitive
effects. The bare copper wires in the center are the short. When the switches
are thrown into the other position, the one foot loop is inserted into the
circuit and the short is completely out of the circuit. The picture on the right is a way
to use a single DPDT knife switch to obtain one, two, or three feet of ladder-line depending on
the position of the switch.
This is a plot of all the current maximum points between the antenna and W5DXP's shack.
The transmission line is 90 feet long and the Ladder-Line Length Selector can add in an
additional zero to 31 feet for a total of 90 feet to 121 feet. 90 feet matches the antenna
on about 7.3 MHz and 121 feet matches the antenna on about 3.6 MHz. The matching points for
all the other HF bands lie between these two extremes. Note that if a fixed length of
ladder-line needs to be chosen for best results with this antenna, that length should
be around 100 ft. which should work with internal autotuners. Caution: Do not expect
a similar antenna erected in a different location to exactly match W5DXP's results. The
antenna environment has a large effect on the antenna characteristics so W5DXP's results are
only approximations when applied to other antenna locations and environments. Mounting
this antenna in an inverted-V configuration, for instance, is likely to change the
characteristics by an unexpected amount. "450" ohm ladder-line characteristic impedance
varies all the way down to 375 ohms for the #14 stranded configuration and velocity factor
varies among the different manufacturers and batches of ladder-line.
For those who don't have the space for a 130 foot antenna, here's a "Shorty" version
designed to work on all HF ham frequencies above 7 MHz. Like the bigger version, the 50
ohm SWRs predicted by EZNEC are below 2:1 for the bands of interest. This antenna will work
on 75 meters at reduced efficiency with a matching network or tuner.
Here is the physics that makes it all possible. Any 450 ohm SWR between 4.5:1 and 18:1
will result in a 50 ohm SWR of less than 2:1 IF the antenna system is fed at a current
maximum point. Moral: Make your center-fed HF antenna system at least a half-
wavelength long at your lowest operating frequency and feed it at a current maximum
point on the ladder-line.
Optimum Length For A Matching Section
This graph shows the optimum length for a matching section when
feeding a center-fed horizontal dipole with ladder-line through a 1:1 current-balun.
The bottom of the chart is normalized to wavelengths
so it works for most HF frequencies and most popular lengths of center-fed wire
dipoles. The left side of the chart indicates the optimum wavelength for a 450
ohm ladder-line matching section for connection to coax or connection to a multiple
of half-wavelengths of 450 ohm ladder-line.
Example: Assume a 102 ft dipole on 7.2 MHz.
102/(936/7.2) equals 0.785 wavelengths on 7.2 MHz. Reading the matching section length from the
graph yields 0.3 wavelength. A wavelength of 450 ohm ladder-line on 7.2 MHz is 886/7.2=
123 ft. 0.3 times 123 equals 36.9 ft for the 7.2 MHz matching section. Add 123/2 = 61.5 ft
if 36.9 ft is too short for a total of 98.4 ft.
What about feedline losses? AC6LA has been kind enough to generate a graph of the feedline losses
based on the suggested feedline lengths vs a fixed feedline length of 100 feet that uses an antenna
tuner to achieve a match to 50 ohms. As one can see, the feedline losses average around one dB and the tuner
losses are too small to matter.