Dipole Antenna: What is it? (And How Are They Designed)

Contents

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Key learnings:

Dipole Antenna Defined: A dipole antenna is a type of RF antenna consisting of two identical conductive elements, operating effectively at half the wavelength of its frequency.
Basic Design: The antenna’s simple structure—two rods connected in the middle—makes it a fundamental model for many other types of antennas.
Radiation Patter: Dipole antennas typically emit signals in a pattern that is strongest perpendicular to the antenna, ideal for broad area coverage.
Variations: Different types of dipole antennas, such as the folded dipole and the fan dipole, are designed for specific uses, enhancing signal strength and impedance.
Applications: Widely used in telecommunications, dipole antennas serve crucial roles in transmitting and receiving radio frequencies in numerous devices.

What is a Dipole Antenna?

A dipole antenna, also called a doublet or dipole aerial, is two equal conductive rods or wires fed at the centre, the usual first model of a linear antenna.

For the common half-wave dipole the total length is about $= \frac{\lambda}{2}$ in free space. Each arm is then about a quarter wavelength, not a half wavelength by itself.

The conductor is split at the centre. An insulator holds the two arms apart.

Those two arms meet a feeder, often a coaxial cable, at the centre. A half-wave dipole is usually centre-fed because that is where current is high and voltage is low.

Basic Dipole Antenna With Center Feed Point — Basic Dipole Antenna With Centre Feed Point

An RF voltage source is applied between the two arms. That voltage drives a current in the wires, and the wires radiate an electromagnetic wave.

On a resonant half-wave dipole, current is highest and voltage is lowest at the centre. Current is lowest and voltage is highest at the open ends.

The basic doughnut pattern is shown below. Maximum radiation is broadside, perpendicular to the wire.

A radiation pattern is a plot of how the antenna sends energy into different directions in space.

Radiation Pattern of a Basic Dipole Antenna

The dipole is a transducer: it turns RF current into a radiated wave when transmitting, and a wave into RF current when receiving.

Dipole Antenna Design

The same dipole form is used on HF, VHF and UHF (ultra-high frequency) bands of the radio frequency spectrum. The physical length scales with wavelength.

Worked example: a 1 MHz half-wave dipole.

Selection of Length of the Dipole Antenna

Free-space wavelength falls as frequency rises:

$\begin{align*} \lambda \propto \frac{1}{f} \end{align*}$

(1) $\begin{equation*} \lambda = \frac{c}{f} \end{equation*}$

where c is the speed of light, $3*10^8 \,\, m/s$

f is frequency in hertz

$\lambda$ is wavelength in metres

Thus,

$\begin{align*} \begin{split} \lambda = \frac{3*10^8}{f(Hz)} = \frac{300000000}{f(Hz)} = \frac{300000}{f(KHz)} \end{split} \end{align*}$

(2) $\begin{equation*} \lambda = \frac{300}{f(MHz)} \,\, meters \end{equation*}$

A free-space half-wavelength is then

$\begin{align*} \begin{split} \frac{\lambda}{2} = \frac{300}{2*f(MHz)} =\frac{150}{f(MHz)} \,\, meters \\ \frac{\lambda}{2}=\frac{150*3.28}{f(MHz)} =\frac{492}{f(MHz)} \,\, feet \end{split} \end{align*}$

(3) $\begin{equation*} \frac{\lambda}{2}=\frac{150*39.37}{f(MHz)}=\frac{5905}{f(MHz)} \,\, Inches \end{equation*}$

So at 1 MHz a free-space half-wave is 150 metres, 492 feet or 5905 inches, the total length of both arms together.

A real wire looks a little electrically longer than its physical length (end effect). A common first cut is about 5% shorter than free-space λ/2 so the antenna resonates near the wanted frequency.

That is why builders often start at about 95% of a free-space half-wave, then trim.

The practical $\frac{\lambda}{2}$ length is the free-space figure times a factor K:

$\begin{align*} \begin{split} \frac{\lambda}{2}=\frac{492*K}{f(MHz)} = \frac{492*0.95}{f(MHz)} \end{split} \end{align*}$

(4) $\begin{equation*} \lambda = \frac{468}{f(MHz)} \,\, feet \end{equation*}$

K depends on wire thickness and frequency. The stored 0.95 / 468-feet rule is a common HF starting point, often quoted up to about 30 MHz. Confirm on an analyser.

Selection of the Feed Impedance or Radiation Resistance

Feed impedance is voltage over current at the feed point. A half-wave dipole is usually fed at the centre, where voltage is low and current is high.

For maximum energy transfer, the feed impedance should match the feeder. A mismatch raises SWR and reflected power. It does not by itself set radiation efficiency.

A thin half-wave dipole in free space has a radiation resistance near 73 Ω. Over real ground, and with finite wire, 60 Ω to 70 Ω is a common measured range. Length and height change that number.

75 Ω coaxial cable is a close match to that ~73 Ω feed. 50 Ω coax is also used; the mismatch is then modest. Coax is unbalanced, so a balun is still the usual interface.

300 Ω and 600 Ω open-wire line is a better fit to folded dipoles, whose feed impedance is several times 73 Ω.

Use Balanced Feeder or Balun

A dipole is a balanced antenna, so a balanced feeder is the natural match. On a parallel-wire line the two conductors carry equal and opposite currents, so their far-field cancels and little power is radiated by the line. Spacing is often about 0.01 wavelength. If the feeder is coax, use a balun.

Coaxial Cable

Coaxial cable is still the most common run from the radio to the antenna.

In coax the inner and outer currents are equal and opposite, so the fields stay inside the cable shield.

Nearby objects therefore couple less to the coax run. The cable is still unbalanced, so use a balun at the dipole.

Strain Insulator

A strain insulator is an electrical insulator that takes the pull of a hanging wire.

It sits between two wire lengths so they do not connect electrically. Dipole ends and overhead lines use that part.

A 1 MHz half-wave layout is shown below.

Types of Dipole Antennas

The half-wave dipole is the usual starting type. Other common forms follow.

Folded Dipole Antenna

A folded dipole is one half-wave element folded into a long thin loop, not two separate antennas in an array.

One conductor is continuous. The other is split at the centre and fed by a balanced transmission line. The ends join, so the two sides share the same end voltage and, for equal wire size, similar current.

The pattern is the same shape as a single dipole: bidirectional, maximum broadside. The feed impedance is higher.

Two-Wire Folded Dipole Antenna

A two-wire folded dipole is that single folded loop made from two parallel conductors.

If both wires have the same radius, they share current equally. For feed current I, each path carries about I/2. Same power at half the feed current means about four times the impedance. The figure is below.

A common equal-wire formula is

$\begin{align*} Z = n^2 * 73 \end{align*}$

where n is the number of $\frac{\lambda}{2}$ parallel conductors

For two equal wires that is

$\begin{align*} \begin{split} Z = R_r = n^2*73 \\ = 2^2 * 73 \\ = 4 * 73 \\ Z = 292 \,\, \Omega \end{split} \end{align*}$

That ~292 Ω is close to 300 Ω twin-lead, so a matching unit is often skipped.

Three Wire Folded Dipole Antenna (Folded Tripole)

Three parallel conductors of half-wave length make a three-wire folded dipole, or folded tripole.

Equal radii share the current three ways. Feed current I then puts about I/3 in each conductor.

Same power at one-third the feed current raises the impedance about nine times. The figure is below.

For three equal wires that is

$\begin{align*} \begin{split} Z = R_r = n^2*73 \\ = 3^2 * 73 \\ = 9 * 73 \\ Z = 657 \,\, \Omega \end{split} \end{align*}$

657 Ω is near 600 Ω open-wire line, so a matcher is often skipped.

Folding is mainly an impedance step-up. Unequal wire sizes shift the current share and therefore the feed impedance. It does not give a free choice of any impedance.

Advantages of a Folded Dipole Antenna

Why people use a folded dipole:

The higher feed impedance is easier to match to 300 Ω line.
It is a little broader-band than a thin single wire, which helps FM and TV.
Gain and pattern stay close to a single dipole. The usual reason it appears in the Yagi-Uda antenna is the higher feed impedance, not extra gain.

FM Dipole Antenna

An FM dipole is a half-wave dipole cut for the VHF FM broadcast band. The figure below is a typical indoor/outdoor wire. Polarization must match the station: many FM broadcasts are mixed or circular, not only vertical.

Maximum radiation is still broadside to the wire. A vertical dipole is vertically polarized and is nearly omnidirectional in the horizontal plane. A horizontal dipole is horizontally polarized and has the usual figure-8. The stored figure is below.

Radiation Pattern of A FM Dipole Antenna — Radiation Pattern of a FM Dipole Antenna

The usual FM broadcast band is 88 MHz to 108 MHz.

Fan Dipole Antenna

A fan dipole is several dipoles on one feeder, spread like a fan.

It is also called a parallel dipole.

Each wire is cut for one band and all meet at one feed. On a given band the resonant pair presents a low impedance. The off-band wires present a higher impedance, so most of the power takes the resonant pair. Isolation is not perfect.

The figure is a 4-band fan: 80 m, 40 m, 20 m and 10 m in parallel on one feeder.

On 80 m the 80 m pair is the low-impedance path. The 40 m, 20 m and 10 m wires take less current.

On 40 m the 40 m pair takes most of the current. Kirchhoff’s Current Law still sends some current into the other wires, so they radiate a little.

Half-Wave Dipole Antenna

The half-wave dipole is the usual type. Its total length is about one half-wavelength ( $\frac{\lambda}{2}$ ) at the operating frequency.

It is also called a Hertz antenna, as distinct from a Marconi (quarter-wave over ground) antenna.

It is two quarter-wave arms with the feed at the centre. The two ends sit at the same high RF voltage relative to the centre.

Current along the wire is approximately sinusoidal, a standing wave. The layout and the V/I sketch are below.

Current and Voltage Distribution In A Half Wave Dipole Antenna — Current and Voltage Distribution in a Half-Wave Dipole Antenna

Maximum radiation is broadside to the wire, as in the figure below.

Radiation Pattern of a Half Wave Dipole Antenna In Free Space — Radiation Pattern of a Half-Wave Dipole Antenna

The same half-wave idea can be scaled from LF to microwave. One physical antenna does not cover 3 kHz to 300 GHz. At 3 kHz a half-wave is about 50 km.

Short Dipole Antenna

A short dipole is much shorter than a half-wave i.e.,( $<\frac{\lambda}{2}$ ). The next paragraph gives the usual textbook range.

Radiation resistance falls as the wire gets shorter, and the feed looks capacitive. Current along a short dipole is approximately triangular.

Many texts take $\frac{\lambda}{50}$ to $\frac{\lambda}{10}$ . i.e., $\frac{\lambda}{50} < l < \frac{\lambda}{10}$ . Current is again approximately triangular. The figure is below.

Current Distribution in a Short Dipole Antenna

In the plane perpendicular to the wire the pattern is a circle. The 3D shape is still a doughnut, a little fatter than a half-wave dipole. The comparison is below.

Radiation Pattern of a Short Dipole Antenna

Radiation Pattern of a Short Dipole Antenna And Half Wave Dipole Antenna — Radiation Pattern of a Short Dipole Antenna in Comparison With the Half-Wave Dipole Antenna

A short dipole is used when a full half-wave will not fit. Efficiency is then lower unless a matching network and low-loss loading are used.

What is a Dipole Antenna Used for (Applications)?

Where dipoles are used:

Dipoles are used throughout radio and telecommunications.
The same dipole can transmit or receive. In two-way radio it often does both.
Half-wave dipoles appear on radio and television receivers.
A folded dipole is a common Yagi-Uda driven element for terrestrial TV on Z₀ = 300 Ω twin-lead, because the fold raises the feed impedance toward that line.
Folded dipoles are also used on FM and TV broadcast antennas where a wider match is wanted.
VHF and UHF dipoles and dipole-based arrays are used in land mobile, public safety and industrial radio.
An FM dipole is a common receive antenna for 88 MHz to 108 MHz.
A parabolic dish is a different antenna. Some dish feeds use a dipole or a dipole-like probe at the focus.