Feeders and Antennas
Coax
Cable
Coaxial cable, or coax is a type of cable that has an inner
conductor surrounded by a tubular insulating layer,
surrounded by a tubular conducting shield. Coaxial cable
differs from other shielded cable used for carrying lowerfrequency signals, such as audio signals in that the
dimensions of the cable are controlled to give a precise,
constant conductor spacing, which is needed for it to
function efficiently as a radio frequency transmission
line.
A. Outer plastic sheath
B. Woven copper shield
C. Inner dielectric insulator
D. Copper core
From Wikipedia
Twin
Feed
Twin-lead cable is a two conductor used as a
transmission line to carry radio frequency (RF)
signals. It is constructed of two multi stranded
copper or copper clad steel wires, held a precise
distance apart by a plastic ribbon. The uniform
spacing of the wires is the key to the cable's
function as a parallel transmission line; any
abrupt changes in spacing would reflect radio
frequency power back toward the source.
Coax is an unbalanced feeder - the inner carries the voltage and the
outer is the earth.
Coax is used by many stations as the outer acts as a screen
Twinn feeder is considered to be a balanced feeder. It has equal and
opposite voltages/currents/fields.
If you wish to connect an unbalanced feed to a balanced antenna
(coax feed a dipole) you need an interface. This interface is known as
a BALUN (BALanced – UNbalance.
The BALUN prevents RF currents flowing on the
braid. If this occurred the screening properties our
lost.
Different types of RF connectors:
PL259
BNC
When using a PL259 , BNC plugs and sockets it is essential
that the inner conductor and outer braid are connected
correctly. Poor connections can lead to equipment mal
functioning and can also lead to unwanted expense having to get equipment repaired.
It is essential that the screening is continuous from start to
finish.
PL259,s can be used when developing a HF or VHF station. Most amateur
stations will use these plugs somewhere. They have a screw thread to secure
the connection .
BNC connectors have a bayonet connection and mainly used in circuits that
are running low power. There are two types a 50 and 75 Ohm.
The difference – different inner pin sizes
Antennas
An antenna (or aerial) is an electrical device which converts electric power into radio waves, and
vice versa. It is usually used with a radio transmitter or radio receiver. In transmission a radio
transmitter supplies an oscillating radio frequency electric current to the antenna's terminals, and
the antenna radiates the energy from the current as electromagnet waves (radio waves). In
reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce
a tiny voltage at its terminals, that is applied to a receiver to be amplified. Antennas are essential
components of all equipment that uses radio.
From Wikipedia
Some ant are designed to be directional and therefore give maximum gain in a
set direction – beams. Many wire ant will be fixed and it will soon be
established which geographical areas the ant favours.
Recall that these are polarised according to the orientation of the antenna, e.g.
a horizontally oriented antenna will radiate horizontally polarised waves
Review following antenna
Dipole
Quarter wave ground plane
Five-eights ground plane
Yagi
End Fed wire
Dipole:
The most common form of dipole is two straight rods or wires oriented
end to end on the same axis, with the feed line connected to the two
adjacent ends
HF - Would need to be fed via
a BALUM
Quarter wave ground plane
Radials represent ground plane and are a quarter wave long
Five-eights ground plane
The 5/8 wave has a slight gain over the 1/4 wave antenna shown above.
Like the 1/4 wave, the 5/8 wave is also used vertically and gives an omni-directional radiation
pattern but the thing to note is that the 1/4 wave has no coil, whereas the 5/8 wave requires the coil
at the base of the antenna for impedance matching.
Yagi
The Yagi is a directional antenna. It is said to have gain because it focuses the radio waves into a,
generally, single direction and is therefore not wasting power radiated in directions where it is not
required. The Yagi can be used vertically or horizontally. The diagram shows the antenna in the
vertical position. The design of a Yagi antenna is, in its' most basic form, a row of dipoles, held in
place by a boom. Spacing between the elements (Reflector, Driven and as many Directors as
required) is fairly critical so the whole antenna is mounted on a boom. The Reflector and Directors
may be electrically connected or insulated from the Boom but the Driven element must be insulated
from the boom. Length of the elements is also a critical factor in design.
End Fed Wire
The end fed wire is simply a random length of wire attached to the centre of a coax feeder or, more
usually, linked directly onto the rear of a suitable ATU that can take single wire. This is a poor
antenna (a compromise) as it is not tuned to any particular frequency and thus generally preforms
badly relative to a dipole.
ERP - Effective radiated Power
Most manufacturers inform you of the gain of their antenna by using the scientific notation dB which
stands for decibels. Whilst this may appear more complex you will be meeting it again in the
Intermediate and Advanced courses.
Gain in dB
Gain multiple factor
3 dB
x2
6 dB
x4
9 dB
x8
10 dB
x 10
Power leaves your transceiver and travels up to the antenna. If you are using an antenna which has
what is called "GAIN" then effectively you will be getting more out of the antenna than you are
putting in. This is only possible because of the antenna construction.
So what is this EFFECTIVE power. As the power is being radiated we called it EFFECTIVE RADIATED
POWER (erp) and this is given by this formula :ERP = power fed to antenna from the rig x antenna gain
using linear units and no allowance for feeder loss.
So if you have a transceiver which has power output of 10 watts and the antenna has a gain of 10
the ERP = 10 watts (output) x 10 (gain) = 100 Watts EFFECTIVE RADIATED POWER
This means that if the licence conditions state that the maximum ERP is 10 Watts, and the radio in
use only gives output 1 watt of RF Power, then an aerial with a 10 times gain will produce the
highest legal power for that frequency. Also, if for the same frequency, the radio in use has a
maximum RF Power output of 5 watts, and the aerial in use has a gain of 2 times, then the ERP will
be 10 Watts.
Polarisation of Antennas
Polarisation of antennas are normally Horizontal or vertical. For the receiving
station to get maximum signal strength it needs to have the same polarisation
as the transmitting station.
Vertical antenna give vertical polarisation – mobiles with mobile 5/8 mag
mounts
Yagis and dipoles may be set up to be either polarisation – depends on
how they are erected.
In some complex situations the above rules may not apply.
Antenna Matching and SWR
Antennas preform at their best when they are designed for a particular
frequency and used on that frequency. However the challenge come when
the antenna is to be used on more than one band. It is essential that the
SWR (Standing Wave ratio) is kept as low as possible. This measurement of
the antenna system to the normal impedance of the radio. If the SWR is high
the output power will be reflected back and could damage the transceiver
Some HF antennas will cover two bands but normal practice is to use ATU
(Aerial Tuning Unit). Many ATU’s also have a SWR meter which enable you to
monitor any transmission and take appropriate action should the SWR
change.
Dummy Loads permit radio tests without radiating a signal
Revision
PL259
BNC
Revision
Twin Feed
Revision
What is the above ??????
Yagi antenna – shown vertically polarised
Revision
Is coax cable a balanced or unbalanced feeder ???
Coax is an unbalanced feeder - the inner
carries the voltage and the outer is the earth
Revision
What does this symbol represent
Antenna/Aerial