Updated October 31,
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,
Dealing with 377
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
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
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
The RF Cafe has
some good material on antenna patterns, check
Now, on to our growing
antenna content... please send us more!
H-tree feed (new for October 2012!)
path loss Rule of Thumb
rule of thumb
The picture below represents
the most modern antenna design procedure, Kentucky
Let's start our antenna page
with some basic definitions:
This is the basic element of an antenna (and is often called "the
element".) An antenna can be made up of multiple radiators.
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.
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.
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 analysis.
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.
The angle from horizontal, from -90 degrees (down) to +90 degrees
(up). Typically denoted by Greek letter theta ().
Often abbreviated "EL".
Reciprocity means that an antenna performs exactly the same way
in transmitting as it does in receiving a signal.
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
"hairy ball theorem". No, we didn't make that up.
An antenna that radiates equally in all azimuthal directions.
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 area and wavelength:
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.
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.
Thus if you double the diameter
of an aperture, or double the frequency of operation, you will quadruple
the gain (increase by 6 dB).
Here's a rule of thumb for antenna
gain of a narrow-beam reflector antenna:
Antenna gain G=27000/(12)
are the 3 dB (half-power) beamwidths in the principal planes,
measured in degrees (not radians).
Far from an antenna, the electromagnetic energy falls of as 1/R^2,
proportional to the area of the ever-widening sphere.
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?
Unwanted gain response of an antenna, in a direction other than
the main beam.
Ratio of the highest sidelobe to the central beam intensity
of an antenna.
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