Back to Basics in Microwave Systems: Front-to-Back Ratio and Antenna Beamwidth

The lessons continue from the “Back to Basics in Microwave Systems” blog series. Derren Oliver explains front-to-back ratio and antenna beamwidth in this latest post. More information about this and other microwave antenna system topics is available in the Microwave Radio Antenna Link Fundamentals online course, which is available through the CommScope Infrastructure Academy.

front to back microwave backhaulIn the previous post of our Microwave “Back to Basics” blog series, Jim Syme spoke about radiation patterns and radiation pattern envelopes (RPE). He explained why they were so important in the planning and operation of microwave links and networks. In this post, I would like to discuss, in more detail, two of the most important characteristics of the RPE: front-to-back ratio (F/B) and antenna beamwidth.

The front-to-back ratio denotes the sensitivity of an antenna to radio waves in the region of 180 degrees plus or minus 40 degrees from the main beam direction - the area of space behind the antenna. It is defined in decibels (dB) relative to the peak of the main beam of the antenna. Details about the response immediately behind the antenna are important when evaluating the possibility of link “overshoot.” Let me explain:

In a multiple link path, microwave links will run on two frequencies known as go and return (simultaneous transmit and receive). Thus, on our first hop, we will transmit on frequency F1and simultaneously receive on the hop 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 layout means that two back-to-back antennas will both receive F1. The antennas therefore must be able to cut out the signal received immediately behind it if interference is to be avoided. Correctly specifying the required front-to-back immunity should avoid this type of interference.

Furthermore, microwave antennas produce a highly directive beam - 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 language, we call this region the main beam - the direction of which coincides with the mechanical axis of symmetry of the reflector.

Beyond the main beam area, the directivity reduces into a number of side beams, or sidelobes, which are much smaller than the main beam (less bright with the torch) and repeat with generally lesser magnitude with angle away from the axis.

The antenna, or half-power, beamwidth is the angular extent by which the power response has fallen to one half the level of the maximum level within the main beam. With the torch, it is the angle from the direction of maximum brightness, where the light is only half as bright.

Again, we use dBs to help us here. Half power coincides with a 3dB drop in power level, thus we often call this the 3dB beamwidth.

The size of the main beam is determined by the electrical diameter of the reflector. Think of the difference in a light beam between a hand held torch, and a large spotlight. Most of an antenna's radiated power is contained within the main beam region and therefore, knowledge of the beamwidth is useful to the link designer in assessing potential interference effects due to link overshoot.

Although the antenna at the receive end of a hop will be able to collect some of the incident radiowave signal, much of the signal will miss the receive antenna and continue propagating through the atmosphere. Were these signals to intercept a different microwave link operating at a similar frequency, or another part of the same link, harmful interference may result. The beamwidth figure, together with knowledge of the transmits powers involved, therefore allows the link designer to calculate radiowave field strengths at any distance from the transmitter, over an angular region, and thus make an assessment of interferencepotential.

The beamwidth of an antenna is also useful information for the antenna installer who has to accurately align the antenna in azimuth and elevation in order to receive or transmit the maximum possible signal strength. For example, knowing that the beamwidth is 0.5 degrees or 2 degrees will influence how accurately the process is performed.

Finally, remember that the tower and mount design gets progressively more complex (and expensive) as the beamwidth reduces. Wind induced tower twist and sway will cause the transmitted and received signal levels to vary unless due account is included in the mechanical design.

This blog post is adapted from content in the Microwave Radio Antenna Link Fundamentals online course, which is available through the CommScope Infrastructure Academy, and continues our look at the various performance characteristics of microwave antennas. Be sure to look for the next blog in our Microwave “Back to Basics” series where we’ll be looking at cross-polar discrimination.

Do you have any questions about front-to-back ratio or antenna beamwidth?