by Martin E. Meserve
Electrically Shortened Center-Fed Dipole
The drawing below is a diagram of a lumped-constant loaded dipole antenna that is
intended to fit in available space, rather than requiring a full 1/2 wavelength, at a specified
frequency. As long as the physical length is longer than 0.2 wavelengths, it will work well at
any height between 30 and 90 ft. The antenna can be fed directly with 50 Ohm coaxial line or,
as the drawing shows, through a 1:1 Balun. This drawing shows up several time on this page,
however the other instances will have different annotations.
This electrically shortened dipole is similar in appearance to a trap dipole except the
"traps" consist of coils only, causing the antenna to operate not unlike two mobile inductively
loaded whips, mounted horizontally base-to-base.
The performance of any antenna depends upon many factors, not the least of which is its design.
This program will help you arrive at a mathematically precise design, but the overall performance
of any antenna also depends upon construction integrity, location, height, orientation, terrain,
surrounding objects, etc. Some antenna adjustment and pruning to compensate for these variable
conditions is nearly always required.
The inductance required is based on the antennas total width (A), the distance from the center of the
antenna to the loading coil (B), the wire diameter (D) of the radiating elements, and the required
operating frequency (f). The equation for determining the inductance required is listed below.
This web page eliminates the need for you to try and enter this messy equation in your calculator/computer.
Note that the dimension C is the distance from the input of the Loading Coil to the end insulator.
Reduce your available space by 6" to 12" on each side to allow for securing and insulating the dipole
from it's supports. he exact distance required depends on your mounting method.
There is a minimum space requirement, which is displayed early in the program. Shorter lengths will work
but will be less efficient. For maximum efficiency the antenna should be as long as possible, consistent with available space.
The Initial Parameters section asks for some basic information,
like the intended operating frequency, the available horizontal space, and the wire size. It then
lists the minimum space you should use and the space required for a full size dipole. If the available
space is not big enough, you may wish to choose an alternative antenna. You can get creative and combine
the Inverted Vee antenna with loaded radiators to fit in just about any space.
The ratio of the antenna length to the wire size does have some effect on the overall length of the antenna, but
the difference is small and you will need to do some pruning anyway. So just use a wire size that is
close to what you may be using.
The Possible Antenna Configurations section presents you with 9
variations for the antenna dimensions. They are listed in order from least to most efficient.
For maximum efficiency select the combination using the largest inductor that is practical for your
The Custom Dimensions section lets you define other lengths for
Dimension B. The program will then calculate the necesary inductor value and Dimension C for these lengths.
The Possible Antenna Configurations section and the
Custom Dimensions section have buttons that will bring up a program
for designing the inductor of your choice. In the Possible Antenna Configurations
section choose the antenna configuration you with to use and press the Design Coil button.
In the Custom Dimensions section, just press the Design Coil button.
The Custom Coil link brings up a
program for designing the inductor you have decided on.
Once you have the inductor value the physical characteristics of the loading coils can be developed using the
Coil Design program (available through this link or
from the Navigation Bar), or, Barker & Williamson pre-wound air core inductors
are available in a variety of diameters, turns per inch, and lengths.
Barker & Williamson air core inductors are pre-wound coils with plastic supports to maintain
their shape and structure. It is purchased in stock lengths and then trimmed down to the size required.
This is a good source of very stable and accurate coils. Use the link in the Navigation Bar
to display a catalog listing of available coil stock.
For an acceptable Q choose a coil whose length does not exceed twice its diameter.
Another antenna intended for restricted space, using a slightly different approach for the loading coil, is
the Shorty 40. The Shorty 40
is intended for use on 40 Meters, but it will fit in the space required for a 20 Meter dipole.
In the area on the right of the drawing, specify the Center Frequency, in MHz,
for the band you want the antenna to operate in, the Length Available
for Dimension "A" and the Wire Size.
A Full Size dipole for x would require
x. For maximum efficency, the Shortened Dipole should be no shorter than
x. This is the
minimum length for maximum efficiency. Shorter antennas will also work, but at reduced efficiency.
The table below lists 10 variations for the placement of a loading coil. They are listed, in order,
from least to most efficient. Using the buttons next to the antenna numbers select the configuration
you would like to use. Then use the
Coil Design program, or, Barker & Williamson pre-wound air core inductors
are available in a variety of diameters, turns per inch, and lengths.
For maximum efficiency select the combination with the highest coil Q that is practicable for your application.
For an acceptable Q, choose a coil whose length does not exceed twice its diameter (L/D Ratio <= 2:1).
The loading coils should be as far from the center feed point as possible, bearing in mind that as the
distance for the feed point increases, coil size increases and the self-resonant frequency decreases
until it reaches the operating frequency, at which point coil Q decays to zero, and the antenna efficienty approaches zero.
If one of the selections does not meet your requirements, the Custom Dimensions
section will allow you to define other lengths for Dimension B and Dimension C. The program will
calculate the necessary inductor value for these lengths.
The items listed below are the dimensions for Antenna Number
x, which is to be used on x.
The items listed on the right are the details for the loading coil.
Note: If you see negative dimensions or NAN
for the inductor value, you probably entered a dimension for B that is too big.
For those who are interested, this is the equation for calculating the inductance required, when you
are working with a dipole that is less than 1/2 wavelength. It is listed in the
ARRL Antenna Handbook, but it does
not have a lot of explanation associated. It's pretty messy, but I use it in several
of my web pages, in a much simplified form. I simply grouped it into one Multiplier (M1), four
Numerator (N1-N4) and two Denominator (D1-D2) equations. In the first equation:
While the equation requires these specific dimensions, the calculator allows for other dimensions to
be used (e.g. Feet or Meters). The necessary conversion is done in the calculator program.
The output of the equation is Reactance, in Ω. This can be converted to Inductance with:
L (µH) = XL/(2·PI·f). Or, the Multiplier (M1), used in the original equation,
could be modified to
M1 = 106/(68·PI2·f2)
so that the output would be directly in µH.