Main Difference – Omni vs. Directional Antenna
Antennas are devices that transmit or receive electromagnetic waves. Depending on how they transmit and receive electromagnetic waves, antennas can be classified into directional and omni (omnidirectional) antennas. The main difference between omni and directional antenna is that directional antennas only transmit and receive signals in one direction, whereas Omni antennas transmit and receive electromagnetic waves in all directions along a particular plane.
What is a Directional Antenna
A directional antenna is an antenna that transmits or receives signals most strongly in one direction. In order to transmit/receive a signal from a directional antenna, the transmitter and the receiver need to be pointed towards each other. When the directions are aligned, the receiver receives the best possible signal.
Parabolic “dish” antennas are examples of directional antennas. The parabolic shape helps to reflect and concentrate the signal towards the receiver. These antennas are often used to receive satellite television. Since the antenna is directional, it needs to be pointed in the direction of the satellite.
Satellite TV antennas on a building.
Other types of directional antennas include Yagi Uda antennas and log-periodic antennas, shown below:
A Yagi Uda antenna (left) and a log-periodic antenna (right).
The diagram below shows a radiation pattern for a simple directional antenna:
Radiation pattern for a type of directional antenna
Note that in this case, the signal is transmitted most strongly in the z direction.
What is an Omni Antenna
An omni antenna transmits/receives signals in such a way that, along one plane, the signals are sent out in all directions with equal strength while the strength of signal varies as you move in any plane perpendicular to that plane. A simple omnidirectional radiation pattern is shown below:
The radiation pattern for a simple omni antenna (shown as grey).
In this diagram, if you move along the plane shown in blue, the strength of the signal does not change if you maintain a constant distance between yourself and the transmitter. However, if you move along the plane shown in red, whilst maintaining the same distance between yourself and the transmitter, the strength of the signal you receive would change. In fact, the signal would disappear when you are directly above or below the transmitter.
Omni antennas are used when signals need to be transmitted in many directions. For instance, mobile phone signals and FM radio signals are transmitted using omni antennas, because they are not meant to be received by a receiver at a fixed location.
A whip antenna on a car that receives radio signals is an example of an omni antenna.
Difference Between Omni and Directional Antenna
Direction of Transmitted/Received Signals
Directional antennas transmit or receive signals most strongly along one direction.
Omni antennas transmit or receive signals in all directions along a plane.
Antenna Gain
Directional antennas produce a larger signal gain compared to Omni antennas, if they both have the same efficiency and receive signals with the same power.
Applications
Directional antennas are used to transmit electromagnetic waves to fixed receivers (e.g. satellite televisions in homes, signals from spacecrafts to their control centers, etc.).
Omni antennas are used to transmit electromagnetic waves to receivers whose locations may not be fixed (e.g. mobile phone communications, FM radio).
Image Courtesy
“Reception” by Jerzy Kociatkiewicz (Own work) [CC BY-SA 2.0], via flickr
“Yagi-Uda antenna array” by HEA Engineering Subject Centre (Own work) [CC BY 2.0], via flickr (Modified)
“LPDA (Log.-periodic) Antenna, Frequency 400-4000 MHz for Amateurradio, EMC” by BAZ Antennas (BAZ Spezialantennen, Germany) [CC BY-SA 3.0], via Wikimedia Commons (Modified)
“Example antenna radiation patterns…” by James Wlodarczyk (Own work) [CC BY-SA 3.0], via Wikimedia Commons (Modified)
“Car radio stock antenna, which has been extended and displayed with the car shown in portrait view orientation for context…” by Zuzu (Own work) [CC BY-SA 3.0], via Wikimedia Commons
