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2016 Extra Class Study Guide: E9A - Basic antenna parameters

Posted: 26 Feb 2016 12:08 PM PST
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E9A Basic antenna parameters: radiation resistance, gain, beamwidth,
efficiency, beamwidth; effective radiated power, polarization

Antenna gain is one of the most misunderstood topics in amateur radio.
There are several reasons for this, including:

Antennas don’t really have gain in the same way that an amplifier has gain.
When you use a linear amplifier, you get more power out than you put in.
Since transmitting antennas are passive devices, there’s no way to get more
power out than you put in.
It’s not easy to measure antenna gain. There is no antenna gain meter that
you can simply hook up to an antenna to measure its gain.


So, what is meant by antenna gain? Antenna gain is the ratio of the
radiated signal strength of an antenna in the direction of maximum
radiation to that of a reference antenna. (E9A07) What this means is that
when you talk about antenna gain, you have to know what kind of antenna
you’re comparing it to.

When talking about antenna gain, antenna engineers often refer to the
“isotropic antenna.� An isotropic antenna is a theoretical antenna used as
a reference for antenna gain. (E9A01) An isotropic antenna is an antenna
that has no gain in any direction. (E9A02) That is to say it radiates the
power input to it equally well in all directions.

Let’s take a look at a practical example. I often say that the
1/2-wavelength dipole antenna is the most basic amateur radio antenna.
Well, the dipole actually has some gain over isotropic antenna. The reason
for this is that it is directional. The signal strength transmitted
broadside to the antenna will be greater than the signal strength
transmitted off the ends of the antenna.

The gain of a 1/2-wavelength dipole in free space have compared to an
isotropic antenna is 2.15 dB. Sometimes, you’ll see this value as 2.15 dBi,
where dBi denotes that an isotropic antenna is being used for this
comparison.

Since the isotropic antenna is a theoretical antenna, some think it’s
better to compare an antenna to a dipole antenna. An antenna will have a
gain 3.85 dB compared to a 1/2-wavelength dipole when it has 6 dB gain over
an isotropic antenna. (E9A12) You obtain this value by simply subtracting
2.15 dB from the 6 dB figu
Gain over a dipole = gain over an isotropic antenna 2.15 dB =

6 dBi 2.15 dBi = 3.85 dBd

Sometimes, the gain over a dipole is denoted as dBd.

Similarly, an antenna has a gain of 9.85 dB compared to a 1/2-wavelength
dipole when it has 12 dB gain over an isotropic antenna. (E9A13):
Gain over a dipole = gain over an isotropic antenna 2.15 dB =

12 dBi 2.15 dBi = 9.85 dBd

Antennas that are said to have gain are really focusing the energy that are
input to them. The higher the gain, the narrower the focus, or beamwidth.
The beamwidth of an antenna decreases as the gain is increased. (E9A06)

Effective radiated power

When you use an antenna that has gain, you are increasing the effectiveness
of the power input to it, at least in the direction the antenna is
pointing. The term that describes station output, taking into account all
gains and losses is effective radiated power. (E9A18) The effective
radiated power is not just the input power times the gain of the antenna.
You also have to take into account losses in other parts of the antenna
system.

This is especially true for VHF and UHF repeater systems, where losses in
the feedline, duplexer, and circulator can be significant. The power that
reaches the antenna may be substantially lower than the power output of the
transmitter.

For example, the effective radiated power relative to a dipole of a
repeater station with 150 watts transmitter power output, 2 dB feed line
loss, 2.2 dB duplexer loss, and 7 dBd antenna gain is 286 watts. (E9A15) To
calculate the answer, you have to first subtract the losses from the gain,
as expressed in dB to get the total gain of the system:
total system gain = 7 dB – 2 dB – 2.2 dB = 2.8 dB.

2.8 dB corresponds to a power ratio of approximately 1.905, so the
effective radiated power is the transmitter output power times the total
system gain:
effective radiated power = 150 W x 1.905 = 268 W.

Lets look at another example. The effective radiated power relative to a
dipole of a repeater station with 200 watts transmitter power output, 4 dB
feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd
antenna gain is 317 watts. (E9A16). In this case, the total gain of the
system is 10 dB – 4 dB – 3.2 dB – 0.8 dB, or 2.0 dB. 2.0 dB corresponds to
a power ratio of approximately 1.585, and the effective radiated power
equals 200 W x 1.585 = 317 W. In this system, high feedline and duplexer
losses are almost completely negating the benefit of using such a high gain
antenna.

Finally, the effective radiated power of a repeater station with 200 watts
transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB
circulator loss, and 7 dBi antenna gain is 252 watts. (E9A17) In this
example, the total gain of the system is 7 dB – 2 dB – 2.8 dB – 1.2 dB, or
1.0 dB. 1.0 dB corresponds to a power ratio of approximately 1.26, and the
effective radiated power equals 200 W x 1.26 = 252 W.

Feedpoint impedance, antenna efficiency, frequency range, beamwidth

Other antenna parameters are also important, of courese. One of the most
basic antenna parameters is the feedpoint impedance. Why would one need to
know the feed point impedance of an antenna? To match impedances in order
to minimize standing wave ratio on the transmission line. (E9A03) The
reason that it’s important to minimize the standing wave ratio, or SWR, is
that if you’re using coaxial cables, minimizing the SWR will also help you
minimize losses. If you minimize losses, you’ll radiate more signal.

Many factors may affect the feed point impedance of an antenna, including
antenna height, conductor length/diameter ratio and location of nearby
conductive objects. (E9A04) For example, we say that the feedpoint
impedance of a half-wavelength, dipole antenna is 72 Ω, but that’s only
really true if the antenna is in free space. When it’s closer to the ground
than a quarter wavelength, then the impedance will be different. That’s why
you have to tune the antenna when you install it.

Another antenna parameter that’s frequently discussed is radiation
resistance. The radiation resistance of an antenna is the value of a
resistance that would dissipate the same amount of power as that radiated
from an antenna. (E9A14) Radiation resistance plus ohmic resistance is
included in the total resistance of an antenna system. (E9A05)

If you know the radiation resistance and the ohmic resistance of an
antenna, you can calculate its efficiency. You calculate antenna efficiency
with the formula (radiation resistance / total resistance) x 100 percent.
(E9A09)

Vertical antennas are sometimes criticized as being inefficient antennas.
Soil conductivity is one factor that determines ground losses for a
ground-mounted vertical antenna operating in the 3-30 MHz range. (E9A11) If
soil conductivity is poor, ohmic resistance will be high. One way to
improve the efficiency of a ground-mounted quarter-wave vertical antenna is
to install a good radial system. (E9A10)

The frequency range over which an antenna satisfies a performance
requirement is called antenna bandwidth. (E9A08) Normally, the performance
requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this
parameter referred to as the 2:1 SWR bandwidth.



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