Antennas
5f.1 Understand the concept of an antenna polar diagram.
Identify the polar diagrams for the half–wave dipole and Yagi antennas.
Identify the directions of maximum and minimum radiation.
Antennas do not usually transmit equal amounts of power in all
directions. A polar diagram shows where the power is directed.
A dipole antenna has a polar
diagram as shown opposite. At about half a wavelength high there
are two main lobes which transmit most of the power, least power is
radiated off the ends of the dipole. It
is usually quite difficult to get an antennas for the lower bands a
half wave high and so the polar diagram will be distorted.
A 3 element Yagi antenna has a
driven element, a reflector which is longer than the driven element and
a director which is shorter than the driven element.
This antenna will produce a
major lobe where most of the power will be transmitted and a back lobe
and side lobes which radiate much less power.
5f.2 Recall that the gain of an antenna is measured in dB, and
understand how to calculate the Effective Radiated Power (ERP) for a
known RF power and antenna gain (in multiples of 3 dB and 10dB).
The
gain of an antenna is measured in dBs. The power from a transmitter is
usually given as dBW, where 1 Watt is equivalent to zero dBW
The table opposite shows you some values for Watts v dBW. The only ones you have to learn are
3dBW = doubling the power
10dBW = 10 times the power
ERP stand for Effective Radiated Power. On some bands the power limit is given
as ERP. To calculate the ERP we need to know the RF power delivered to
the antenna and the gain of the antenna.
Example 1:
We have a transmitter that delivers 100W to the antenna. The antenna has a gain of 6dB. What is the ERP in dBWs and in Watts?
Example 2
A VHF hand-held has a power out of 4 Watts and a Yagi antenna with a gain of 12dB. What is the ERP?
4 Watts =6dBW
So, total ERP = 6+12 = 18dBW
In terms of Watts, a 12 dB gain = multiplying the power by 4 and then 4 again
4x4x4=64 Watts
Watts
dBW
0.1
-1.0
1
0.0
2
3.01
3
4.77
4
6.02
5
6.98
10
10.0
15
11.76
20
13.01
30
14.77
40
16.02
50
16.98
100
20.00
200
23.01
500
26.98
1000
30.00
Example 1
Example 2
5f.3 Recall that a three-element Yagi has a half-wave driven element, a
reflector that is slightly longer than the driven element and a
director that is slightly shorter than the driven element.
Recall that Yagi antennas may have more than one director. 3-element Yagi
In a 3 element Yagi the driven element cut to the required frequency
The reflector is slightly longer and the director is slightly shorter.
The gain can be increased by increasing the number of directors. Each director added is slightly shorter than the previous one.
5f.4 Recall that electromagnetic radiation comprises both an electrical field and a magnetic field.
Recall that the two fields are at right-angles to each other and that
the direction of propagation is at right-angles to both fields.
Recall that it is the plane of polarisation of the electric field that defines polarisation of the wave. Polarisation
Radio waves are part of the electromagnetic spectrum.This includes
x-rays, light rays and infra-red. This means they have both an electric
field and a magnetic field. These are at right angles to each other.
The direction of propagation is at right angles to both fields. Amateurs say that an antenna is either horizontally polarised or
vertically polarised. So, what are they talking about? The answer is
that the direction of the electric field determines the
polarisation of an antenna
5f.5 Recall that VHF and UHF signals will normally be received most
effectively when the transmitter and the receiver have the same antenna
polarisation and that this is less important at HF because the
polarisation may change during ionospheric reflection.
At VHF and UHF using a a vertically
polarised antenna used to receive a horizontally polarised signal can lead
to 20dB loss in signal. The same loss is encountered with a horizontally polarised transmitting antenna and a vertically polarised receiving antenna.
At HF the polarisation is less important, as the passage to the
ionosphere and back results in the original polarisation changing in an
unpredictable way.
Dummy loads
5g.1 Understand the use of a dummy load and its construction.
A dummy load is a large 50 Ohm
resistor that can take the place of an antenna. The output from a
transmitter is connected to the dummy load. The RF energy is turned to
heat and there is little radiation of radio waves. This is a useful
method of testing transmitters without radiating a signal that could be
a nuisance. There are a number of points to bear in mind when building
a dummy load:
The resistor has to have no inductance or capacitance. This
means that wire round resistors should not be used as these are
inductors. Only carbon resistors.
Although it is possible to obtain high power resistors they
are not very common. It is easier to build up a 50 Ohm resistor from a
number of low power resistors connected in parallel. For example if 20x
1k Ohm resistors are connected in parallel the total resistance is
equal to 50 Ohms. If each resistor is rated at 2Watts the total power
handling ability is 2 X 20 = 40 Watts.
The resistors have to be surrounded by something that is a good conductor of heat. Oil and silver sand are frequently used.
The resistor needs to be built into a metal enclosure to
reduce RF radiation and to ensure that heat can be transferred to the
air. The metal also supports an RF socket.
