The Dipole Antenna
by N4UJW

A dipole antenna is the simplest, usually the least expensive and most popular type of radio antenna used in ham radio and radio communications and has been around longer than you can remember.

Before we get into what a dipole antenna is, we should understand a couple of important scientific facts about antennas in general.

The Half Wave Dipole is a Reference Antenna!

The dipole antenna is THE "reference" antenna that is used for db gain numbers you may see when an antenna company or individual advertises a gain figure for his super duper whiz bang antenna.

What is "gain" of an antenna and how is it referenced and used in the use of various antennas? Let's try to understand it.

Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. A high-gain antenna will preferentially radiate in a particular direction. Specifically, the antenna gain, or power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna.

So if an antenna has a published "gain" of say, 20 dBd, that "20" number is referenced to a half wave dipole antenna. (The d in the dBd represents a dipole). If the "gain" number is stated as 20 dBi, (the i in dBi represents isotrophic), then in theory, it has LESS gain than an antenna referenced to a dipole. The "i" (isotropic) is added to show that the gain numbers exist as if the antenna was measured for gain in "free space".

"Free space" is like saying that the antenna exists in an environment that has nothing around it that will add or take away from the performance of the antenna. And remember that it is radiating equally in ALL directions. In other words.....outer space and it only exists in a vacuum!

In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is then given in dBd (decibels over dipole):

The half wave dipole antenna consists of a conductive wire or rod that is half the length of the maximum wavelength the antenna is designed to operate at end to end. This wire or rod is split in the middle, and the two sections are separated by an insulator at the center.

Each wire or rod is connected electrically usually to a 50 ohm coaxial cable at the ends of the center insulator closest to the middle of the antenna. This gives 2 identical lengths of the conductor on each side of the center insulator. 

Radio frequency voltages at the design frequency are applied to dipole antennas at the center, between the two conductors. They are used alone as antennas, and as the driven elements in other types of antennas that are sometimes slightly modified from an exact half wavelength.

The word Dipole means "two poles."

The dipole antenna is made of a wire broken in the center and where broken, each half of the wire connects to an insulator that divides the wire in two equal sections as stated above. Two wires from the voltage source, which is the transmitter, are connected across the insulator. On one side of the dipole, the current in the form of moving electrons flows first from the voltage source (the transmitter) toward one end of the dipole. At the end, it reflects back down the coax toward the voltage source. The same thing occurs on the other half of the wire on the other half cycle of alternating current.

An antenna that is the right length for the current to reach the far end of the wire just as the polarity changes is said to be resonant. Because electricity travels at 95% the speed of light in a wire, the number of times the polarity changes in one second (frequency) determines how long the wire has to be in order to be resonant. 

A resonant coax-fed dipole antenna will have a low SWR and will radiate efficiently on the band for which it is resonant, but it will not work well on all bands. For example, if the tuning range of your tuner has a sufficient range, you will be able to load up any antenna with it, but it will not necessarily radiate a signal efficiently. It may have high tuner and feed-line losses.

Most dipoles consist of two pieces of wire of equal lengths with one of the two ends connected together through an insulator. The far ends of the wires are also connected to insulators. The two conductors of a feed-line are separated and connected across the gap at the center insulator. The antenna is held up by rope that connects the insulated ends of the antenna to two supports. It is a "balanced" antenna, because equal currents flow on both halves of the antenna. Coax is an unbalanced feed-line. The possible effect of using an unbalanced feed-line on a balanced antenna like a dipole usually creates radiation on the feed line which we do not usually want.  The dipole that is stretched between two high supports is called a flattop dipole, distinguishing it from other configurations that are described later in this article.

The simplest antenna system of all is the half-wave resonant dipole fed with coax and no tuner or matching device. The only reason for using a half-wave resonant dipole antenna is to eliminate the need for a matching device such as a tuner. The feed-point impedance will be near 50 ohms at ordinary heights and they can be fed directly with 50-ohm coax from the output of todays modern radios which are designed for a 50 ohm "load" which is what we want.

So you may ask, why don't we use the dipole as a full wavelength antenna? The simple answer is that it would be very difficult to match to the transmitter because of the very high impedance which would be much higher than the 50 ohms that the transmitter requires. In the half wave dipole, the two halves of a dipole are fed 180 degrees out of phase, meaning when one side is fed positively, the other side is fed negatively. That is why a feed-line has two conductors. Of course, the sides swap polarity on each half cycle.

Some types of Dipole antennas.

Figure 1. Flat Top Dipole

In Figure 1. above in flat top configuration, the 2 halves of the dipole with center insulator are supported on each end by an insulator with the feed line coax supported by the center insulator and attached to each half of the dipole. The coax then leads to the the transmitter.

2. Inverted-V Dipole (see figure 2 below)

Another configuration for the half wave resonant dipole is one having one support in the center and the ends stretched down toward the ground. The single support can be a tree, mast, or tower. The ends of a dipole have high RF voltages on them, and need to be at least 10 feet above ground for safety. This antenna is called an "inverted-V," because the shape of the dipole looks like a "V" turned upside down. Most dipoles illustrated in this book can be put up in the inverted-V configuration. This configuration works well because the current is concentrated on the middle two-thirds of the antenna at the apex. The current in an antenna is what is responsible for the radiation. The ends of the antenna have very little current in them and it does not matter if the ends are close to the ground. The middle of the antenna is up high where the radiation is taking place and that is the place you want the radiation to be. An inverted-V has an advantage that the horizontal space required for it is less than what is needed for a flattop dipole. The angle between the wires on an inverted-V needs to be greater than 90 degrees. The gain of an inverted -V is 0.2 dBd and it has a radiation pattern nearly omni-directional. Since it is easy to construct and works so well, the inverted-V is the most commonly used dipole. An explanation of the decibel will come later.

Figure 2. The Inverted-V Dipole

3. Dipole Shape Variations

The wire of a dipole does not have to be run in a straight line. A dipole does not have to be perfectly horizontal. Thats the way it is usually depicted in books and magazines, but you can bend the legs of the antenna up, down or sideways.

Figure 4. Two Dipole Shape Variations

There are many more shapes and methods
of mounting a dipole type antenna other than shown on this page.

If you make either wire one-half wavelength long and carefully prune it to resonance, you can use it without a tuner on and near its resonant frequency. Both antennas have the current part at the top where most of the radiation takes place. The vertical parts of these antennas radiate a weak vertically polarized wave. The only reason these dipoles are contorted this way is to make them full-sized and to fit in the available space. Other shapes are possible, and you can be creative at your location.

4. Calculating the Length of a Half-Wave Resonant Dipole

The approximate length in feet of a half-wave resonant dipole is found by dividing 468 by the frequency in MHz. 468 / Frequency = total dipole length in feet. You may (should) remember that formula from your study to pass your exam.

Some examples of using the standard dipole formula:

468 / 3.5 MHz = 133.7 feet total

468 / 7.0 MHz = 66.85 feet total

The actual physical length of a dipole antenna will be determined by several factors. Using larger diameter wire will make the dipole resonate lower in frequency. Therefore, to make it resonant at the higher desired frequency, it must be shortened. Raising a dipole higher above ground will make it resonate higher in frequency. An insulated wire will make the dipole resonate lower in frequency than a bare wire.

Using the above formula, cut the antenna a little longer than the calculations say. If the SWR is best at a lower frequency than you desire, the antenna will have to be made shorter by pulling the excess wire through the end insulators, folding the ends of the extra wire back on itself. Then wrap the ends of the overlapped wire on itself so it won't come loose. This causes the excess wire to "short" itself to the rest of the antenna. If you are using insulated wire, you will need to cut off the excess wire. The reverse is true if the antenna resonates too high in frequency. The extra wire can be let out to make it resonate on a lower frequency. This is why you originally cut the wire a little longer. 

Additional good reading about Dipole antennas:

Your First HF Dipole by K4DPK

 BUILDING and Tuning A DIPOLE THE EASIER WAY! 

Editor note: 

Portions of this article were edited and used from....  

Understanding Antennas For The Non-Technical Ham
A Book By Jim Abercrombie, N4JA (SK)