5a.1 Recall the correct cable
to use for RF signals and that coaxial cable is most widely used
because of its screening properties. There are two basic types of cable used to connect the antenna to the RX/TX:
Coaxial
cable consists of a central wire surrounded by a non-conducting
material which in turn is surrounded by a copper sheath. Finally the
outside is covered in PVC type materials. Coax is very popular because
the outer copper sheath which is earthed at the TX/RX helps to screen
out interference.
Open
wire (twin) feeder consists of two parallel wires spaced between 1cm
and 15cm apart. This is cheaper and there is less loss of signal.
5a.2 Recall that the plugs
and sockets for RF should be of the correct type and that the braid of
coaxial cable must be correctly connected to minimise RF signals
getting into or out of the cable. Identify BNC and PL259 plugs as shown in Table 2.
It is important that the correct plugs
are used on the end of coaxial RF cables. These should match the
impedence of the coax and the coax sheath (braid) should be connected
correctly to prevent RF radiation from the plug. Two common plugs used by amateurs are:
BNC plugs - stands
for Bayonet Neill Concelman. This is commonly found connecting TV
equipment, oscilloscope probe connections and as an antenna connector.
It has the advantage of providing a quick attachment with a simple push
and twist action. Beware that there are 50 Ohm and 75 Ohm varieties. For
radio use make sure it is 50 Ohm.
PL259 plugs - this is the most common antenna connector found on amateur radio equipment.
A good quality PL259 will work up to a GHz. Different types are
available with different sized sleeves for different diameters of coax
cable. They should have a PTFE dialectric surrounding the centre pin.
Some dialectrics will melt if too much heat is applied. The hardest
part of fitting coax is soldering the braid to the plug.
Types of antenna
5b.1 Recall that the purpose
of an antenna is to convert electrical signals into radio waves (and
vice-versa) and that these are polarised according to the orientation
of the antenna, e.g. a horizontally oriented antenna will radiate
horizontally polarised waves.
On transmit an antenna is designed to
convert electrical signals from the feeder to electromagnetic waves
which pass through the air and through a vacuum.
On receive an antenna is designed to convert electromagnetic waves in the air to electrical signals in the feeder.
The electromagnetic waves passing
throug the air or vacuum are polarised. This means that they are
horizontal or vertical. This is easier to understand when you look at a
VHF or UHF antenna. If the elements are upright, then the signal
radiated is vertically polarised. If the elements are horizontal then
the signals radiated are horizontally polarised. At HF the polarisation may change as the signal is reflected by the ionosphere.
5b.2 Identify the half-wave dipole, λ/4 ground plane, Yagi, end-fed wire and 5/8λ antennas. Understand that the sizes of
HF and VHF antennas are different because they are related to
wavelength, though they operate on the same basic principles. Understand that the λ/2 dipole has a physical length approximately equal to a half wavelength of the correct signal.
For the Foundation exam you need to be able to recognise the antenna types shown opposite. The length of the antenna is related
to the wavelength the antenna is designed for, So, a half wave antenna
will be approximately half of the wavelength it is designed for. A λ/2 antenna for 2m would be approximately 1/2 a metre long.
A λ/2 for 4m would be 2 metres long
A λ/2 for 160m would be approximately 80 metres long!
NB λ/2 means a half wave. λ is the abbreviation for wavelength. A λ/4 is a quarter wave.
Two common vertically polarised antennas are the λ/4 groundplane.
The vertical part is λ/4 long and is connected to the inner of the
coax. There are 4 radials which are at 90 degrees to the radial. These
are also λ/4 long. The radials are connected to the braid of the coax.
The 5/8 vertical
consists of a vertical part that is 5/8 of a wavelength long. To make
this match 50 Ohm coax, an inductor (coil) is connected to the bottom.
The centre of the coax is connected to the end of the coil, or to get
an even better match is tapped along the coil at a point that produces
a 50 Ohm match. The braid of the coax is connected to 4 radials which
are λ/4 length. 5/8λ antennas give more gain at ground level that a 1/4λ vertical.
A Yagi antenna consists of a driven element to which the coax is connected, a reflector behind this which is slightly longer than a λ/2 and one or more directors in front of the driven element that are cut to be slightly less than λ/2. A Yagi provides gain in the direction of the directors. End fed wire antennas
consist of a length of wire strung out to a tree or pole. The end of
the wire is connected to an AMU (Antenna Matching Unit) and the chasis
of the AMU is connected to earth or a counterpoise wire. The resonant
frequency of the antenna will depend on its length. These antennas are
common on the HF bands, 132 feet is a popular length.
