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Front to Back Ratio: Directivity of Antenna to Reduce Interference

Front to Back Ratio Applies to Directional Antennas Only

Front to Back Ratio Defined and Explained

  • The measure of directivity of a directional antenna:  It is dependent on the directivity of the antenna with the measurements taken on a 180 degree axis and measured in decibels (dB).
  • A ratio of the magnitude of directional radiation in an antenna, directed in a single direction, divided by the radiation wasted in or by the antenna.
  • A measure of the antenna gain, transmitted or received signal strength at the front and the back of a directional antenna expressed as a ratio. 
  • It can be alternatively expressed as the ratio between the maximum forward and backward antenna gains. 
  • Higher Front to Back Ratio is the key to overcome signal noise (interference) and thereby increase the range and performance of an antenna.

Improve Front-to-Back Ratio with Shielding and/or Reflector Dish

Physical modifications to the antenna such as shielding (on the sides and/or or back) and reflectors (in back of the antenna) can be used to increase the front to back ratio by minimizing backward radiation.  Reflectors are typically parabolic dishes.  Normally you will see some increase in performance even from a modest effort to add inexpensive shielding or a reflector dish:  A DIY effort will increase performance.  A professionally-made shield or reflector dish may increase performance very significantly.

When evaluating an antenna, Front to Back Ratio doesn’t necessarily fit with antenna gain (dBi). This is because an antenna can have poor front-to-rear ratio but have great gain. The overall performance will depend on the antenna you are using. In case of a directional antenna, both Front to Rear Ratio and antenna gain should be high for it to work efficiently and have a strong signal.

How Front-to-Back Ratio is Calculated

Front-to-back ration can be calculated from the radiation patterns of directional antennas which are plotted on a polar plot. These antenna co-polar radiation patterns are created by rotating an actively transmitting antenna within an anechoic environment and measuring its gain at intervals. On the plot, 0 degrees corresponds to the front position and 180 degrees relates to the back of the antenna. Radiation strength is plotted on a logarithmic scale in decibels. Lobes of energy are plotted giving a distinctive visual appearance from which the front to back ratio can be discerned.

Why is front-to-back ratio important?

The higher the ratio, the more directionally efficient the antenna is because a directional antenna always radiates some signal out of the back of the antenna. A great, long-range directional antenna will have a Front to Back Ratio (gain) of 15dBi or higher: The high gain of the antenna gain raises the signal above the "signal noise" that exists in almost every environment - thereby increasing signal strength.  The best front to back ratio of a directional antenna is a large dish antenna such as 34dBi.

Front to Back Ratio, which is expressed in dB, is important in situations where signal noise / interference needs to be minimized. A low Front to Back Ratio means the less directional an antenna is hence poor reception from the front. A low Front to Rear Ratio can only be good when one is trying to pick up the signal from behind or the side of an antenna.

This key antenna parameter enables engineers to establish setups where the majority of the source power is radiated in a forward direction with minimum power loss at the rear. It can be used in directional RF systems to ensure that interference in the reverse direction is kept to an absolute minimum. A high front to back or F/B ratio is optimal as there is less energy radiated behind the antenna with a corresponding higher antenna gain.

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