Short Dipole Antenna : Design, Working, Radiation Pattern, Effectiveness & Its Applications

A dipole antenna is a simple & also most frequently used type of antenna within wireless networking devices, radio & telecommunications. As the name suggests, this type of antenna mainly includes two identical conductive elements like rods or metal wires. A common example of a dipole is the TV antenna found on broadcast TV sets. The signal range of this antenna is vertically 75 degrees & horizontally it is 360 degrees. There are different kinds of dipole antennas available like a half wave, multi half wave, folded, and short dipole antenna. So this article discusses one of the types of dipole antenna like a short dipole antenna.

What is Short Dipole Antenna?

A type of dipole antenna that is formed with two conductors where the total length of this antenna is much shorter than a half wavelength is known as a short dipole antenna. This type of antenna includes two co-linear conductors that are located back-to-back, although with a little gap among them for the feeder. This antenna’s one end is open-circuited whereas the other end is fed through an AC source. The frequency range of this antenna ranges from 3 KHz – 30 MHz. This antenna is most frequently utilized in low-frequency receivers and also where space is a problem otherwise where a non-resonant antenna is required. The object of this antenna supports only polarized fields. The short dipole antenna diagram is shown below.

Short Dipole Antenna Design

This antenna design mainly depends on different factors like the required frequency, antenna size & the transmission line impedance being utilized. For designing this antenna, you have to compute the element’s length & the spacing among them. The space between the elements is very significant & is decided by the preferred impedance of the antenna. The element’s length can be decided through the operation frequency. For this antenna, the length of each element is equivalent to half the radio wave’s wavelength being broadcasted.

The construction of this antenna is very simple with a wire which is shorter than the wavelength. At one end of this antenna, a voltage source is connected whereas at another end, a dipole shape is made by terminating lines.

This antenna design with an ‘L’ length is shown above. The wire of antenna leads should be below one-tenth (1/ 10) of the wavelength (λ) which is given below.

L < λ/10

Where
‘L’ is the wire length for the short dipole.
‘λ’ is the wavelength.

Working

The short dipole antenna working is, once an alternating current supplies throughout the antenna then it creates an electromagnetic field which emits outside from the antenna.

The antenna polarization can be defined as the way the electromagnetic fields are generated by the antenna as energy emits away from it. So these directional fields decide the direction where the energy is received or goes away from an antenna. The short dipole antenna polarization is linear.

Radiation Resistance

The radiation resistance is one of the main antenna parameters. This is mainly necessary to decide the overall feed impedance & thus the necessary matching. The radiation resistance of this antenna can be calculated as

Rr = 20(πL/λ)2

Where:

‘Rr’ is the radiation resistance within ohms
‘L’ is the antenna element length.
‘λ’ is the wavelength.

For this antenna, the input feed impedance includes various elements like capacitance, series inductance, Ohmic resistance & radiation resistance. So, all these elements need to be vectorially merged to get the feed impedance.

This antenna is shorter as compared to a half-wavelength and it works under the normal half-wavelength resonant frequency. Thus, the major reactive element will be the capacitive reactance. So the impedance of the antenna is difficult and it includes the resistive component & capacitive reactance. Instead of using a coaxial feeder, a balanced feeder using is simple between the antenna as well as any impedance matching circuit.

Directivity of Short Dipole Antenna

The directivity of the antenna can be defined as the measure of the radiation pattern concentration in a particular direction which is expressed in decibels. If the directivity is higher, then the concentration of the beam radiated through an antenna is more. An antenna that equally radiates within all directions then it would be omnidirectional, so the directivity will be 1 (0 dB).

A high directivity of antenna is not better always, for instance, mobile devices need omnidirectional antennas so they need a low or no directivity antenna. The antennas with High-directivity are utilized in permanent installations like wireless backhaul, satellite television, etc. because the information needs to send & receive in a particular direction over long distances.

The short dipole antenna directivity mainly depends on the field’s component only. So it can be measured to be 1.5 (1.76 dB), which is extremely low for physical (or) non-theoretical antennas.

Radiation Pattern

The radiation pattern is the graphical representation of the energy radiated through an antenna. The radiation pattern describes how the energy from the antenna radiates out into space. The short dipole antenna’s radiation pattern is within the omnidirectional pattern that is shown below.

The short dipole & the infinitesimal dipole antenna radiation pattern is the same as a half-wave dipole. So, if the dipole is in a vertical shape, then the radiation pattern will be in circular shape. The shape of the radiation pattern is a “figure of eight” once viewed within the two-dimensional pattern.

How to Make a Short Dipole Antenna?

To make a simple short dipole antenna, you will need the following materials:

• Coaxial cable (such as RG-58 or RG-6).
• Wire cutter/stripper.
• Soldering iron and solder.
• Connector suitable for your application (e.g., BNC, SMA, or F-type connector).
• Insulating material (e.g., PVC pipe or wooden dowel).
• Mounting bracket or mast (optional).

Here’s a step-by-step guide to creating a short dipole antenna:

Determine the Desired Frequency: Identify the frequency for which you want to design the dipole antenna. The length of the dipole elements will be based on the wavelength of the desired frequency.
Calculate the Element Lengths: Use the formula L = λ/2, where L is the length of each dipole element, and λ (lambda) is the wavelength. Divide the wavelength by two to get the length of each element.

Strip the Coaxial Cable: Strip the coaxial cable’s outer insulation, exposing the inner conductor and shielding. Be careful not to damage the inner conductor during this process.

Measure and Cut the Coaxial Cable: Measure and cut the coaxial cable to the desired length for each dipole element. Make sure to cut the cable at the center, ensuring equal lengths for each element.

Strip the Inner Conductor and Shielding: Strip approximately 1-2 cm (0.5-1 inch) of insulation from the inner conductor at each end of the cut coaxial cable. Strip the shielding to expose the braid.

Solder the Connector: If required, solder the connector to the exposed inner conductor and shielding at each end of the coaxial cable. Ensure proper soldering and strain relief to maintain a sturdy connection.

Mount the Dipole Elements: Attach the dipole elements to an insulating material, such as a PVC pipe or wooden dowel. Make sure the elements are parallel and spaced apart by a distance equal to the width of the insulating material.

Mount the Antenna: Install a suitable mounting bracket or mast to secure the dipole antenna in the desired location. Ensure that the antenna is positioned away from any obstructions that may affect its performance.

Connect the Antenna: Connect the antenna to the appropriate RF equipment using the connector you soldered to the coaxial cable.

Test and Fine-tune: Test the dipole antenna by connecting it to the RF equipment and checking for signal reception or transmission. If necessary, fine-tune the antenna length or positioning for optimal performance.

The dimensions of the dipole elements will depend on the desired frequency, so ensure you calculate the correct lengths based on the wavelength. Also, note that while a simple dipole antenna can be effective for certain applications, more advanced designs or commercial antennas may provide better performance in specific scenarios.

Effectiveness of a short Dipole antenna

The effectiveness of a short dipole antenna can be evaluated using various parameters. Here are some common measures to calculate and assess the performance of a short dipole antenna:

Radiation Pattern:  The radiation pattern can be measured experimentally using specialized equipment or simulated using antenna modeling software. A well-designed short dipole antenna should exhibit a symmetric radiation pattern with uniform coverage in the desired direction.

Gain: Antenna gain measures the concentration of radiated power in a specific direction compared to an ideal isotropic radiator. It is typically expressed in decibels (dBi) or relative gain. The gain of a short dipole antenna is generally lower compared to other antenna types, such as yagi or patch antennas. The gain can be calculated using theoretical formulas or measured experimentally.

Directivity: Directivity represents the ability of an antenna to concentrate radiated power in a specific direction. It is a measure of the antenna’s ability to focus energy in a particular direction, excluding losses. Directivity is closely related to the antenna’s radiation pattern. The directivity of a short dipole antenna can be calculated using formulas based on the antenna dimensions and structure.

Efficiency: Efficiency indicates the ability of the antenna to convert input power into radiated power. It considers losses within the antenna system, such as resistive losses, dielectric losses, and mismatch losses. The efficiency of a short dipole antenna can be calculated as the ratio of radiated power to the total input power. Typically, shorter dipole antennas have lower efficiency compared to longer antennas.

Impedance Matching: Impedance matching is crucial for optimizing the transfer of power between the antenna and the transmission line or RF circuitry. Poor impedance matching can lead to significant signal reflections and loss of signal strength. The impedance of a short dipole antenna can be calculated based on its dimensions and compared to the impedance of the feeding transmission line or system.

Bandwidth: The bandwidth of an antenna represents the range of frequencies over which it operates effectively. It indicates the ability of the antenna to receive or transmit signals across a range of frequencies without significant degradation in performance. The bandwidth of a short dipole antenna is related to its physical dimensions and resonant properties.

Standing Wave Ratio (SWR): SWR measures the impedance match between the antenna and the transmission line. It quantifies the amount of power reflected back from the antenna due to impedance mismatch. Lower SWR values indicate better impedance matching and reduced power losses. SWR can be measured using an SWR meter or calculated based on impedance measurements.

These parameters provide a comprehensive evaluation of the effectiveness and performance of a short dipole antenna. However, the specific calculation methods may vary based on the antenna design, frequency of operation, and measurement equipment available. For precise calculations, specialized antenna analysis software or equipment may be required.

Advantages & Disadvantages

The advantages of a short dipole antenna include the following.

• Its construction is simple.
• These are available in small sizes.
• It has high power efficiency.
• It is a very simple wire antenna.
• It offers the benefit of getting balanced signals
• The size of the dipole gap has no impact almost on the antenna’s performance.
• These are omnidirectional once transmitting & receiving signals.

The disadvantages of short dipole antennae include the following.

• It has less radiation.
• It has high resistive losses.
• These antennas don’t receive signals from far away
• Power dissipation is high.
• These are not efficient.
• It has less radiation.
• It has less S/N ratio.

Applications

The applications of short dipole antennae include the following.

• These antennas are used in narrow-band applications.
• These are used mainly for tuner circuits.
• These antennas are used in power transmission, wireless communications, or surveillance at wavelengths that range from micrometers to hundreds of KMs.
• This is used instead of the full half-wave dipole antenna within specific applications wherever the full half-wave dipole antenna is very large.
• This antenna is used mostly in low-frequency receivers.

Thus, this is an overview of a short dipole antenna, design, working, advantages, disadvantages, and its applications. This is a kind of antenna that is very shorter than a half-wavelength. This antenna is applicable wherever space is limited like in wireless communication systems & portable radios. Here is a question for you, what is folded dipole antenna?