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Antenna Design

Click here to go to our page on antenna measurements (new for November 2016)

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 modeling, frequency translation, transient analysis, stability factor, channel temperatures, noise figure, etc.

Antenna Design

Dealing with 377 Ohms

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!

This further explanation came from Glenn:

The reason antenna impedances are not 377 ohms is that somebody in RF land decided that 50 ohms was a good number for their stuff. The antenna is the transformer that tries to match the free space 377 ohms (set by physics) to the ridiculously low 50 ohms (set by whimsy).

Actually, the fifty ohm standard was a compromise between high power handling (maximum at 30 ohms) and low loss (minimum at 77 ohms) for air coax which is used to connect high-power broadcast transmitters to antennas on top of towers. Read more about the coax compromise here.

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

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

H-tree feed (new for October 2012!)

What's that antenna?

AESAs

PESAs

Circular polarized feed

Rectennas

Vivaldi antenna

Phased array antennas

Parabolic reflectors

Horn antennas

Microstrip patch antennas

Corner reflectors

Freespace path loss Rule of Thumb

Monopulse antennas

Monopulse comparator networks

Antenna gain rule of thumb

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

Antenna Design

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, it forms the spherical coordinate system that is used in antenna analysis.

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 (Antenna Design). Often abbreviated AZ.

 

   AzElorientation                Antenna Design     

  

Elevation
The angle the horizontal (xy plane), from -90 degrees (down) to +90 degrees (up). Related to the spherical coordinate (Antenna Design),which is measured from the verical (z-axis). 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

An antenna pattern, or radiation pattern, is a 2D (or 3D contour) plot which shows the angular variation in an antenna parameter such as  the relative field strength in the far-field. The pattern is usually presented in polar coordinates and with a dB scale. 

AntennaPattern

An example of 2D antenna pattern in an azimuthal cut

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
A common antenna type which, in its simplest form, consists of a straight wire cut in the middle so that each half may be connected to one of the two conductors of a transmission line. 

Beamwidth

The width of the main lobe (or main beam) spanning a -3dB difference in gain. In the above example, the beamwidth of the antenna pattern is about 60 degree.

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.

Directivity of an isotropic radiator which radiates equal intensity in all directions, is 0 dB. Directional antennas are measured against the isotropic radiator, because they favor one direction, their directivity is positive when expressed in dB. Directivity can be calculated as a function of the "effective area (aperture)" and wavelength:

Antenna Design

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.

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 times efficiency, or directivity plus efficiency when expressed as dB.

Antenna Design

Thus if you double the diameter of an aperture, or double the frequency of operation, you will quadruple the gain (increase by 6 dB).

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

Antenna gain G=27000/(Antenna Design1Antenna Design2)

Where Antenna Design1 and Antenna Design2 are the 3 dB (half-power) beamwidths in the principal planes, measured in degrees (not radians).

Near field
The region close to an antenna where the electromagnetic fields do not follow a simple 1/R relationship with the range R.

Far field
The region far from an antenna where the electromagnetic power densiity (power per unit area) falls off 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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