Geometric Principle

• The principle of the parabola is to take a given point--the feed horn--from which the radio beam originates and draw lines from that point to a curving surface--the parabolic reflector--in such a manner that any line from the feed point to any point in the reflector will bounce off the reflector in parallel with all other lines that originate at the feed point and bounce off the reflector at any other point. The larger the parabola the more parallel lines will develop and the tighter the beam. Geometrically, an infinitely large parabola would produce a perfect beam width of zero degrees. Since this is impossible, we must settle roughly for a one-degree beam width with a 10-foot diameter with a gain of roughly 43dB.
  
  

The Feed Horn

• The two main design concerns of the feed horn are that it does not block the transmitted signal as it bounces off the reflector, and that it distributes the signal evenly to the reflector without sending any part outside the parabola's diameter. This would also reduce the front-to-back ratio. The feed horn must also be geometrically positioned so that all the beams from it bounce off the reflector as close to parallel with each other as possible. It must also be strong enough to resist the wind or not be susceptible to weather damage.
  
  

Back and Side Radiation

• The amount of signal that actually leaves the antenna in the main beam, in ratio to the signal that splashes out the sides or through the back, is called the front-to-back ratio. This ratio is important for FCC licensing as the "back" signal has the potential to interfere with other radio signals. In areas with a lot of microwave traffic it may be necessary to use a more expensive high performance antenna that will have a lesser front-to-back ratio to obtain a license.
  
  

Standard Vs. High Performance Antennas

• Standard antennas are the cheapest and easiest to install. High performance antennas have higher gain and better front-to-back ratios but are more expensive. They are also heavier and have higher wind loads and therefore will require stronger tower and support mechanisms. They can come with covers that protect them from birds and debris and can also be heated to prevent icing up in colder climates. Both standard and high performance antennas can be equipped with pressurized feed horns to prevent water accumulation.
  
  

The Microwave Path

• The microwave path needs to be designed so that the receive signal is high enough above the threshold of the radio to provide a sufficient fade margin to withstand signal attenuation caused by atmospheric conditions. The path signal receive level is the sum of the transmit power, minus the loss of the transmission line, plus the gain of the near antenna, minus the loss of the path, plus the gain of the distant antenna, minus the loss of the distant transmission line. The threshold of the radio is a design characteristic of the radio. A typical face margin is usually no less than 30dB.