Transcript Foundation: Feeders & Antennas

AMATEUR RADIO TRAINING
Feeders & Antennas
v1.2 © essexham.co.uk
FEEDERS & ANTENNAS
• What is a Feeder?
• Antenna connectors
• Types of antennas
• Polarisation
• Matching / SWR
Feeders
Feeder connects the antenna to the transmitter / receiver
Coaxial Cable (coax)
Ladder-line
• Unbalanced
• Inner conductor carries the signal
• Outer braided (screen) to keep the
signal within the cable
• Amateurs use 50Ω impedance - do
not confuse with 75Ω TV coax
• Balanced
• No screen
• Constant separation between
the two wires
Balun
When connecting an unbalanced feeder to a balanced
antenna, you need a BALUN (BALanced to Unbalanced)
Example:
A BALUN is required to connect unbalanced
coax to a balanced dipole antenna
Connectors
BNC
PL259
Bayonet locking
Screw-thread locking
We use 50Ω (not the 75Ω version
which has a different type of pin)
Larger, more common
Antennas
• Convert electrical signals into
radio waves (and vice-versa)
• Different sizes for different
bands
• In general, lower frequencies
need larger antennas
• Several different types used
by amateurs
Antennas: Dipole
• The dipole is a basic antenna
• Half a wavelength long, e.g: 10 metres long to work 14MHz
(the 20 metre band)
Half a wavelength λ/2
Antennas: ¼ Wave Ground Plane
• Vertical antenna
• Quarter of a wavelength long. e.g:
50cm long to work 144MHz (2
metre band)
λ/4
• Four horizontal wires, called
radials, form a ‘groundplane’,
which act as a mirror for the radio
signals
Coax
Antennas: 5/8 Wave Ground Plane
• Vertical antenna
• 5/8 of a wavelength long
5/8 λ
• Better signals towards the horizon
• Coil at the base for coax ‘matching’
• Often used for VHF / UHF mobile
Coax
Antennas: End-fed
Basic long-wire antenna
Unlikely to be correct length, so needs to be ‘matched’
More likely to cause interference (EMC) than other types
Antennas: Yagi
• Directional
• Focuses signal mainly in one direction
• Dipole with a reflector and several directors
Antenna Gain
• The Yagi antenna focuses signals in one direction, and has
“gain”
• Antennas have a measurement of gain in Decibels (dB)
Gain (dB)
Gain (times)
3
x2
6
x4
9
x8
10
x 10
ERP
Stands for “Effective Radiated Power”
The antenna gain multiplied by the transmitted power
ERP (watts) = gain x transmitted power
Example: 10 watts x 3dB = 20 watts
Polarisation
• Horizontal or vertical
• Receive and transmit antennas
should have the same polarisation
• Yagis can be vertically or
horizontally polarised
• Most VHF/UHF is vertical
• Much of HF operation is horizontal
Antenna Types
5/8 wave
Yagi
Dipole
¼ wave
Long Wire
Matching
• Each band requires a different length
antenna
• If the antenna is the correct length for
the band you want to work, there is a
“match”
• If not, you need to use an ATU
(Antenna Tuning Unit) – Commonly
used when working ‘multi-band’
• There is a practical on matching a
dipole.
SWR
• Use an SWR (Standing Wave Ratio) meter to measure the flow
of power back from the antenna. Ratio of 1:1 is ideal
• Example: SWR of 2:1 means that 10% of your power is
reflected back
• Poor SWR can damage the transmitter
Measure Forward &
Reflected power with an
SWR meter
A dummy load can be
helpful for station testing
Feeders & Antennas Summary
• ATU to match antenna to band
• Polarisation: Horizonal vs Vertical
• SWR: Standing Wave Ratio
• Gain:
3db = x2 ; 6dB = x4 ; 9dB = x8 ; 10dB = x10
• ERP (watts) : TX Power x Antenna Gain
• Antenna types: Dipole, ¼ wave, 5/8 wave, Yagi, End-fed
• Balun: To match BALanced antenna to UNbalanced feeder
• Connectors: BNC (bayonet) and PL259 (screw-thread)
• Feeder types (unbalanced coax ; balanced ladder line)
FEEDERS & ANTENNAS
Any questions?
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