This is part of an internal document that that gives an overview of the properties of antennas for non-engineers. We have divided the document into different posts where we discus each of the parameters: Frequency bands, gain and radiation pattern Polarisation Input Impedance and VSWR Port to port Isolation and Cross-polarisation Power Handling ability Antenna “Specmanship” Where applicable we have added some videos explaining the properties discussed.

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21 March 2016 AntennaParameters Part 1 - Frequency bands, gain and radiation pattern Page 1
ON THE PROPERTIES OF AN ANTENNA
This is part of an internal document that that gives an overview of the properties of
antennas for non-engineers.
We have divided the document into different posts where we discus each of the
parameters:
- Frequency bands, gain and radiation pattern
- Polarisation
- Input Impedance and VSWR
- Port to port Isolation and Cross-polarisation
- Power Handling ability
- Antenna “Specmanship”
Where applicable we have added some videos explaining the properties discussed. You
will find a link to a PDF below.
Introduction
An antenna is a device that converts energy from one form to another. When used in
transmit mode, currents in the coaxial cable (feeding the antenna) flow into the antenna
and the energy is converted to electromagnetic radiation which propagates into space.
When an antenna is used in receive mode, electromagnetic radiation interacts with the
antenna inducing currents into its components. These currents flow along the coaxial
cable connected to the antenna to a receiver.
In some ways the antenna is analogous to a speaker in a sound system. A speaker
converts electrical energy (from the wires powering the speaker) into sound energy which
we can detect using our ears. When operated in the opposite mode a microphone is
created. This device detects sound wave and converts them to electrical energy. An
antenna works with electromagnetic radiation and electric currents rather than sound and
electric currents.
An antenna is described by a number of attributes including frequency bands of
operation, gain, radiation pattern, polarisation, VSWR, input impedance, coupling, power
handling ability and so on.
This document describes each of these parameters.
1. Frequency bands of operation
An antenna is designed to work over a specific frequency band or over sub-bands within
a larger frequency range. Within the specified frequency bands antenna attributes such as
gain, pattern and VSWR should be fairly well controlled.
2. Gain and radiation pattern
The gain of an antenna represents the antenna’s maximum ability to focus
electromagnetic radiation in one particular direction (when transmitting) or its ability to
receive energy in that particular direction.
The radiation pattern of an antenna describes graphically how an antenna radiates or
receives EM radiation over a region in space. Both the gain and radiation pattern should
be considered when assessing the radiation characteristics of an antenna.

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Figure 1: A 3D radiation pattern (viewed from the front and back)
Figure 1 gives an example of a radiation pattern that shows how an antenna radiates its
energy over space (or receives energy from various directions). The scale on the RHS of
the plots gives a colour coded representation of the focussing ability of the antenna. The
direction of maximum focus is down the x-axis and one can see from the scale that the
gain (maximum focussing ability) is 8.5 dBi.
The radiation pattern presented in antenna brochures is usually one or more cuts taken
from the full radiation pattern. For example, the cuts that would be published for an
antenna whose full radiation pattern is shown in Figure 1 would be the x-y plane cut
(azimuth) and the x-z plane (elevation).
Figure 2: The azimuth (x-y plane) and elevation (x-z plane) 2D radiation patterns
The y-z plane would generally not be published as the antenna does not radiate much in
this plane. The y-z radiation pattern is shown in Figure 3 – note that the maximum
radiation in this plane is about -10dBi.

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Figure 3: The y-z plane pattern – normally not published
It was mentioned earlier that the gain and radiation pattern should be considered together
when assessing an antenna. To illustrate, Figure 4 shows the radiation pattern of antenna
whose gain is 8.8 dBi, but has very different radiation characteristics.
Figure 4: An omni-directional radiation pattern
The published patterns of such an antenna would include an elevation cut (any plane that
includes the z-axis) and the azimuth (x-y plane)

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Figure 5: The azimuth and elevation cuts for the omni-directional antenna
The gain of an antenna is always given with respect to another (well defined) antenna. In
other words, the gain of an antenna is always given in units that compare its maximum
radiation to the maximum radiation of another antenna (with both antennas having the
same input power). The units of gain used here are dBi, where the ‘i’ is used to specify
the reference antenna (in this case an isotropic source). So, if the gain is given as 8.5 dBi,
then this means that 8.5 dB more power is transmitted in the direction of maximum
radiation than that of an isotropic source. Another commonly used unit is dBd, where
the‘d’ stands for dipole. If the gain of an antenna is given as 5 dBd, then this means that 5
dB more power is transmitted in the direction of maximum radiation than is transmitted
by a dipole in its direction of maximum radiation.
An isotropic source is a source that transmits equally in all directions. Such a device does
not exist in reality, but it is very easy to visualise its radiation pattern – it’s just a sphere.
Figure 6: The radiation pattern of an isotropic source

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Note that the gain of the isotropic source is given with respect to another isotropic source
(i.e., it is compared to itself). That is why the gain of an isotropic source is 0 dBi - so,
0 dB more power is transmitted in the direction of maximum radiation (in this case all
directions) than that of an isotropic source.
We cannot directly see the radiation from antennas that work in the frequency bands
commonly used for cellular or Wi-Fi communications, so it can be difficult to visualise
what the antenna is doing. So let’s consider an antenna where we can see what is going
on.
2.1. Consider a lightbulb
In this section we are going to introduce an “isotropic” source that we can see and use it
to build an antenna. We will plot the radiation pattern of the antenna and compute its
gain. This exercise will reinforce the concept that the gain of the antenna is always given
with respect to a reference.
It was mentioned earlier that a true isotropic source does not exist in reality, but we can
get pretty close. Visualise a lightbulb with a very small battery attached – small enough
that it does not impede the radiation of light in any direction. This is our isotropic source
which radiates electromagnetic energy at about 100 THz (compared to 2.4 GHz for Wi-
Fi).
Let us enclose the lightbulb in an imaginary sphere (say 1m in radius). At each point on
the sphere, we can use a light meter to determine the intensity of the light radiated by the
bulb. Plotting this data in 3D we should end up with a pattern similar to that in Figure 6.
We can now create a reflector antenna using a shaped piece of metal and lightbulb as
shown in Figure 7.
Figure 7: A reflector antenna formed using the isotropic source
The same procedure as before can be used to measure the intensity of the light over the
surface of the imaginary sphere. Plotting the results in 3D, one might get something like
that shown in Figure 8.

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Figure 8: Radiation intensity of the lightbulb with reflector
Let us assume that the highest intensity of light emitted from this structure is 1000 times
that of the isotropic source.
If this was the case, then one would be able to make the statement that that 1000 times
more power is transmitted in the direction of maximum radiation than that of the isotropic
source (the lightbulb on its own).
The gain of an antenna is normally given in decibels. The following formula is used to
convert a linear power value to dB:
= 10 ×
30 = 10 × 1000
So, the gain of lightbulb antenna is 30dBi, where the ‘i’ indicates that an isotropic source
(the isolated lightbulb) was used as a reference.
The statement can now be made that the gain of the lightbulb antenna is 30 dBi, which
implies that 30 dB more power is transmitted in the direction of maximum radiation than
that of an isotropic source.
It should be noted that the input power to the bulb was not changed when introducing the
reflector. The 30 dBi gain was achieved simply by focussing the light emitted by the
lightbulb in a particular direction. The result of focussing the light means that less light is
radiated in other directions.
Put another way, if one added up the power of the light radiated in all directions by the
bulb antenna and did the same for the isotropic source (lightbulb on its own), then one
would come up with the same answer for both cases. This reinforces the fact that the
antenna only focussed the energy that was available from the lightbulb. That is all an
antenna can do.
Solwise - one of our distributors did a great video explaining gain.
https://youtu.be/wGE4tjATecY