amateur radio training feeders & antennas v1.101 © essexham.co.uk
TRANSCRIPT
AMATEUR RADIO TRAINING
Feeders & Antennas
v1.101 © essexham.co.uk
FEEDERS & ANTENNAS
• What is a Feeder?• Antenna connectors• Types of antennas• Polarisation• Matching / SWR
Feeders
Coaxial Cable (coax) Ladder-line
• Unbalanced• Centre 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
A feeder connects the antenna to the transmitter
ConnectorsBNC PL259
Bayonet locking
We use 50Ω (not the 75Ω version which has a different size pin)
Screw-thread locking
Larger, more common
Antenna Types
¼ wave
Long WireDipoleYagi
5/8 wave
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)
• Four horizontal wires, called radials, form a ‘groundplane’, which act as a mirror for the radio signals
λ/4
Coax
Antennas: 5/8 Wave Ground Plane• Vertical antenna
• 5/8 of a wavelength long
• Better signals towards the horizon
• Coil at the base for coax ‘matching’
• Often used for VHF / UHF mobile
5/8 λ
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 x 2
6 x 4
9 x 8
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
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?
essexham.co.uk/train