Antenna design

Updated May 15, 2008

Consider that antenna people aren't normal like the rest of us, they might not even be aware of the world wide web, and thus will never find this site. Many of them have been known to play the banjo in their spare time... it has to do with the simple life of designing linear, passive, one-port structures while the rest of us are beating our heads against the wall with large signal models of receiver/exciters, etc.

Here's a question that came our way about antennas... the space impedance is 377 Ohms, why don't antennas have this same impedance for best power transfer?

Here's how we see it (correct us if we're wrong). It seems like 377 ohms would make a great antenna, but this would be a very high system impedance to deal with, so the antenna itself is used as an impedance matching network. By transforming the impedance, virtually no power is lost and the "generator" is matched to the load. In the case of FM radios, often the antenna that comes in the box is a folded dipole that uses 300 ohm twin lead. But this is the exception to the rule!

Now, on to our growing antenna content... please send us more!

The RF Cafe has some good material on antenna patterns, check it out!

Phased array antennas (separate page)

Parabolic reflectors (separate page)

Horn antennas (separate page)

Microstrip patch antennas (separate page, new for September2007!)

Corner reflectors (separate page)

Freespace path loss Rule of Thumb (separate page)

Monopulse antennas (separate page)

Monopulse comparator networks (separate page)

Antenna gain rule of thumb

The picture below represents the most modern antenna design procedure, Kentucky windage!

Let's start our antenna page with some basic definitions:

Radiator
This is the basic element of an antenna (and is often called "the element".) An antenna can be made up of multiple radiators.

Boresight
The direction in which you are physically pointing the antenna, with the intention of maximum electromagnetic illumination. The word comes from the early military applications of microwaves, when radar was perfected to help shoot stuff down.

Boresight error (BSE)
The maximum radiation intensity is supposed to occur at boresight, but nothing works perfectly in the analog world, and often it is slightly skewed. The angle that the physical or optical boresight differs from the electromagnetic boresight is the boresight error. This is most important when your antenna is used in a tracking radar.

Range
The radial distance from an antenna to an object, particularly in radar. Along with azimuth and elevation, these form the spherical coordinate system that is used in antenna theory.

Azimuth
The angle from left to right from a reference point, from 0 to 360 degrees, or from -180 to +180 degrees. Linda Blair's head was spinning in azimuth in the original "Exorcist". The azimuth angle is typically given as Greek letter phi (). Often abbreviated AZ.

Elevation
The angle from horizontal, from -90 degrees (down) to +90 degrees (up). Typically denoted by Greek letter theta (). Often abbreviated "EL".

Reciprocity
Reciprocity means that an antenna performs exactly the same way in transmitting as it does in receiving a signal.

Antenna pattern

Isotropic radiator
A theoretical radiator that emits (or receives) electromagnetic radiation equally in all directions (both AZ and EL), with zero loss. There is no isotropic radiator in practice, which is proved by the "hairy ball theorem". No, we didn't make that up.

Omnidirectional radiator
An antenna that radiates equally in all azimuthal directions.

Dipole

Beamwidth

Gain
The maximum signal intensity of an antenna at a specified AZ/EL angle, typically at boresight, with respect to (usually) an isotropic radiator, expressed as dBi (decibels from isotropic). The narrower the beam width, the higher the gain. Gain is equal to directivity plus efficiency, when expressed as dB.

Here's a rule of thumb for antenna gain of a narrow-beam reflector antenna:

Antenna gain G=27000/(12)

Where 1 and 2 are the 3 dB (half-power) beamwidths in the principal planes, measured in degrees (not radians).

Directivity
The ratio of electromagnetic radiation of a real antenna at an AZ/EL angle (typically specified at boresight) to its radiation in all directions averaged over a sphere. Measured in the far field.

Efficiency
The efficiency of an antenna takes into account resistive losses, and is equal to the total radiated power divided by the radiated power of an ideal lossless antenna (or subtract them in decibels). Efficiency is not a function of AZ/EL angles.

Near field

Far field
Far from an antenna, the electromagnetic energy falls of as 1/R^2, proportional to the area of the ever-widening sphere.

Antenna ranges
Ranges are chambers that are used to study the behavior of antennas, which is a huge topic in itself. Any range vendors out there want to help us out?

Sidelobes
Unwanted gain response of an antenna, in a direction other than the main beam.

Peak sidelobe ratio
Ratio of the highest sidelobe to the central beam intensity of an antenna.

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