Antenna Part 12. Antenna Parameters: How to Calculate Front-to-Back & Front-to-Rear Ratio.

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Front-to-back ratio, F/B is the ratio of the maximum power radiated by the antenna in the forward direction to the power radiated in the opposite direction. It measures the power difference between the front and the directly opposing side of the antenna. A high F/B also implies strong forward radiation and low radiation in the opposite direction.
Front-to-rear ratio, F/R is the ratio of the maximum power radiated by the antenna in the forward direction to the maximum power radiated in the backward direction. It indicates the antenna’s directional gain in the forward direction relative to its backward radiation. A high F/R signifies strong forward radiation and low backward radiation.
The front-to-back ratio (F/B ratio) of an antenna is a key performance metric that measures the antenna's ability to focus energy in its desired direction, while minimizing radiation or reception in the opposite direction (180 degrees from the front).
This ratio is defined as the ratio of the power radiated or received in the forward direction (main lobe) to the power radiated or received in the backward direction (opposite the main lobe). It is usually expressed in decibels (dB).
In a multiple-link path, microwave links operate on two frequencies, known as go and return (simultaneous transmit and receive). On the first hop, we will transmit on frequency f1 and simultaneously receive traffic from the far end on frequency f2. However, on the next hop down the chain, we will transmit on f2 and receive on f1. This setup means that two back-to-back antennas will both receive f1. Therefore, the antennas must be capable of blocking the signal received directly behind them to avoid interference. Correctly specifying the required front-to-back immunity will prevent this type of interference.
Although the antenna at the receiving end of a hop will collect some of the incident radiowave signal, much of the signal will bypass the receiving antenna and continue propagating through the atmosphere. If these signals were to intercept a different microwave link operating at a similar frequency, or another part of the same link, harmful interference could result. The beamwidth, together with knowledge of the transmitted powers, allows the link designer to calculate radiowave field strengths at any distance from the transmitter across an angular region, and thus assess the potential for interference.
Improve Front-to-Back Ratio with Shielding and/or Reflector Dish
Physical modifications to an antenna, such as adding shielding (on the sides and/or back) or a reflector (behind the antenna), can help improve the front-to-back ratio by minimizing backward radiation. Reflectors are typically parabolic dishes. Even a modest DIY effort to add inexpensive shielding or a reflector dish can boost performance. However, a professionally-made shield or reflector may significantly enhance performance.
Microwave antennas produce a highly directive beam, meaning they have a high level of responsiveness in one particular direction. Think of a torch beam that directs light within a narrow angular region. In antenna terminology, this region is called the main beam, and its direction coincides with the mechanical axis of symmetry of the reflector.
Beyond the main beam, the directivity decreases, forming several smaller side beams, or sidelobes, which are much less intense than the main beam (like dimmer areas with the torch) and reduce further in magnitude as the angle moves away from the axis.
The higher the front-to-back ratio, the more directionally efficient the antenna. A directional antenna always radiates some signal from the back of the antenna. A long-range directional antenna will have a front-to-back ratio of 15 dBi or higher. The high gain raises the signal above the signal noise, thereby increasing signal strength. Front-to-back ratio (34 dBi) for a directional antenna can be achieved by a large dish antenna.
The front-to-back ratio is important in situations where minimizing signal noise or interference is critical. A low front-to-back ratio indicates that the antenna is less directional, leading to poor reception from the front. However, a low front-to-back ratio can be advantageous when trying to pick up signals from behind or to the side of the antenna.
This key antenna parameter helps engineers design systems where the majority of the radiated power is directed forward, with minimal power loss to the rear. It is especially useful in directional RF systems to minimize interference from the reverse direction. A high front-to-back (F/B) ratio is ideal because it reduces the energy radiated behind the antenna, resulting in higher overall antenna gain.