Ham Radio Dipole
|This Page Covers
1. Antenna System
2. Dipole Formula
3. Resonant Length
4. Transmission Line
5. Dipole Hardware
It is called a half-wave antenna because its length corresponds to an electrical half wave at the frequency for which it is cut.
The classic dipole is also called a balanced antenna because it is "fed" at its exact center.
The half-wave ham radio dipole is the most common antenna used by amateur radio operators.
Here is a simplified illustration of the classic dipole antenna.
The characteristic impedance of a half wave dipole is around 73 ohms.
However, if the horizontal dipole is between 0.1 and 0.2 wavelength above ground, its impedance will be somewhat lower and closer to 50 ohms ... the characteristic impedance of commonly used coaxial transmission line such as RG-8, RG-8X and RG-58.
As a bonus, the impedance will remain low at odd harmonic multiples of the basic frequency! Yes, you can operate a 40 meter (7 MHz) dipole on 15 meters (21 MHz) too. A two-for-one antenna!
Keep in mind that the antenna is only part of a coherent system! An antenna system is composed of...
The formula to calculate the (approximate) overall physical length of a dipole is:
Length (meters) = 142.58 / frequency (MHz)
The length of wire given by the formula is cut in half to make both sides of the dipole.
The above formula gives an approximate result because the length at the desired resonant frequency is affected by its operating environment conditions, such as...
Most dipoles will require a little "pruning" to resonate at the desired frequency. I recommend that you...
Cut your dipole wire some 2-3% longer than the length given by the formula.
Make a note of the length obtained in step 1.
Raise the dipole to its operating height.
Measure the SWR at several frequencies within the intended frequency band. (Use only a few watts and pick a quiet time on the band to make your tests).
Note the frequency (F_min) at which minimum SWR is obtained.
Multiply (F_min) by the antenna length recorded in step 2.
Divide the result of the above multiplication by the desired frequency of operation, to obtain the final length.
Trim both ends of the dipole down to the final length obtained in step 7.
You can feed your dipole with coax, as already mentioned above.
However, coaxial cable is unbalanced! When feeding a balanced load, such as a dipole, with an unbalanced transmission line, the antenna will induce RF currents on the outer shield of the coax.
These unwanted RF currents spell nothing but trouble, not the least of which are RFI and stray RF in the shack to play havoc with your computer or other delicate electronic devices!
Fortunately, you can prevent these unwanted RF currents from traveling back on the outer shield of your coaxial transmission line ... with a choke.
To make a choke, all you need to do is wind a portion of the coaxial transmission line to form a coil.
A choke balun made of coax is most effective when a single layer is close-wound on a form, such as 4in. plastic drain pipe or 6in. "schedule 40" PVC pipe.
The table below lists values for each HF amateur band. Form size and number of turns are optimized for each band.
|Single Band RF Chokes (most effective)|
I recommend one choke near the antenna feed point, and another one about a quarter wavelength down the line, before the coax enters your radio shack.
If you intend to use your dipole on its odd harmonics (more than one band) then use the following table to construct your RF chokes.
|Multi-Band RF Chokes|
|Freq. Range (MHz)||RG-8, RG-58, RG-59, RG-8X, RG-213|
|3.05 m. (10 ft.), 7 turns
5.49 m. (18 ft.), 9-10 turns
2.44 m. (8 ft.), 6-7 turns
Many kinds of wire, insulators and rope can be pressed into service for a temporary installation.
But for a permanent and safe all-weather installation, here is what I recommend.
I use either #14 stranded (7x22) hard-drawn copper wire, for spans less than 45 meters (150 ft.) between stable supports.
or the very strong VariFlexTM #13, 19 strand, copper-clad steel wire when the antenna needs to be strung between trees.
You can use a ceramic "dogbone" as center insulator, but you will have to wrap your coax around it, tie it securely, then split the center conductor and braid to connect to each side of your ham radio dipole ... and seal to prevent water from seeping in the coax! Not exactly the best.
I prefer using a center insulator with a SO-239 ... much easier to seal.
Of course, you could use a 1:1 balun at the dipole feed point. It will serve as center insulator and coax connector. The balun will improve the radiation pattern somewhat ... if your dipole is at least 1/4 wavelength above ground.
Finally, I use #5 Delta CIN ceramic end insulators for their resistance and long leakage path.
I have had a dipole strung between two large trees for years with (3/16") Mil Spec Dacron® rope. It is still up there! This rope is very strong and abrasion resistant.
Take your ham radio dipole down at least once a year (twice or more if you have had severe weather) to check for damage (frayed ropes, damaged coaxial shield or connection, etc).