Chelmsford Amateur Radio Society Intermediate Course 5 Antennas

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Chelmsford Amateur Radio Society Intermediate Course (5) Antennas and Feeders Chelmsford Amateur Radio Society

Chelmsford Amateur Radio Society Intermediate Course (5) Antennas and Feeders Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 1

Basic System 50 Ohms Output Transmitter Standing Wave Ratio Meter Antenna Matching Unit Antenna

Basic System 50 Ohms Output Transmitter Standing Wave Ratio Meter Antenna Matching Unit Antenna Feeder Receiver Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 2

Feeders • Feeder types: Coaxial, Twin Conductors Coax: Inner Conductor is shrouded by dielectric,

Feeders • Feeder types: Coaxial, Twin Conductors Coax: Inner Conductor is shrouded by dielectric, with outer (braided) screen. For Amateur Radio 50 Coax is used (whereas Video/TV uses 75 ) Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Twin Feeder: Two conductors kept at constant separation by insulation - no screen Balanced Feeder is available in 75 -600 Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 3

Feeder Impedance A B • All Feeders have a Characteristic Impedance, Z 0 (eg

Feeder Impedance A B • All Feeders have a Characteristic Impedance, Z 0 (eg 50 or 75 Ohm Coax, 300 Ohm twin feeder etc) B A Chelmsford Amateur Radio Society Intermediate Licence Course • Z 0 is based on the Ratio of A and B (and the nature of the spacing dielectric) • If correctly terminated by a resistive load then Z 0 determines the ratio of Vrms / Irms in the feeder Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 4

Balanced/Unbalanced/Match • Coax is unbalanced - Inner has signal, Outer is at ground •

Balanced/Unbalanced/Match • Coax is unbalanced - Inner has signal, Outer is at ground • Twin Feeder is balanced - conductors have equal and opposite voltages/currents/fields • Mounting Twin Feeder near to conducting objects will cause an imbalance in the conductors, a change in its Z 0 and unwanted radiation (or loss of immunity from external interference) • Similarly, severe bends or crushing coax changes its Z 0 and causes internal mismatches • Any mismatch within a feeder, or its termination, will result in its input impedance no longer being its characteristic impedance Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 5

Decibels • Gains and Losses may be expressed in d. B’s • 3 d.

Decibels • Gains and Losses may be expressed in d. B’s • 3 d. B is half steps, 6 d. B is quarter steps, 10 d. B is tenth • You will need to remember this table for exam: 3 d. B x 2 or a half 6 d. B x 4 or a quarter 9 d. B x 8 or an eighth 10 d. B x 10 or a tenth Example: 3 d. B Gain doubles power, whilst 3 d. B Loss halves it Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 6

Feeder Losses • ALL feeders have loss - the longer the feeder the greater

Feeder Losses • ALL feeders have loss - the longer the feeder the greater the loss. Twin Feeder has a lower loss than Coaxial cable • This loss affects both the Transmit and Receive paths • For some standard cables the loss is: Per 100 m RG 58 RG 213 10 MHz 4. 8 d. B 2. 0 d. B 30 MHz 8. 2 d. B 3. 2 d. B 144 MHz 21 d. B 8. 6 d. B Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 7

Antenna’s • All Antennas have a feed point impedance. • This is determined by

Antenna’s • All Antennas have a feed point impedance. • This is determined by the dimensions which will relate to the wavelength of the applied signal and the height of antenna • Dipoles are a half wave length long and are a resistive match at only one frequency. • If you replace the antenna by a resistor of the same value as the feed point impedance the transmitter will not be able to tell the difference • Dipoles in theory are 73 ohms but in practice approx 65 ohms so close enough to the nominal course value of 50 ohms Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 8

Voltage Standing Wave Ratio • If the feed point impedance is incorrect then it

Voltage Standing Wave Ratio • If the feed point impedance is incorrect then it will not match the impedance of the feeder and some energy will be reflected back down the feeder. • When this reflected energy is returned to the Transmitter it is again reflected back to the antenna and is radiated. • The combined energy is known as the forward and reflected power and gives rise to the Standing Waves on the feeder. I 2 V 0 1/4 WAVELENGTH Chelmsford Amateur Radio Society Intermediate Licence Course 1/4 WAVELENGTH Carl Thomson G 3 PEM 1/4 WAVELENGTH Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 9

VSWR & Impedance • If we have a mismatched antenna, energy is reflected back

VSWR & Impedance • If we have a mismatched antenna, energy is reflected back to the Transmitter where it is again reflected back to the antenna and is eventually radiated - Energy is not lost • The combined forward and reflected power and gives rise to the Standing Waves on the feeder. • The reflected signal will change the input impedance of the feeder so that it is no longer the characteristic impedance and the feeder no longer presents the correct impedance to the transmitter. • Antenna Matching Units transform this effective impedance back to nominal so that the radio operates correctly - even though the antenna remains imperfect Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 10

Electromagnetic Waves x Electric Field, E y z Direction of Propagation Magnetic Field, H

Electromagnetic Waves x Electric Field, E y z Direction of Propagation Magnetic Field, H • Electromagnetic radiation comprises both an Electric and a Magnetic Field. • The two fields are at right-angles to each other and the direction of propagation is at right-angles to both fields. • The Plane of the Electric Field defines the Polarisation of the wave. Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 11

Polarisation • Polarisation is the plane of the antennas radiating electric field. • Common

Polarisation • Polarisation is the plane of the antennas radiating electric field. • Common polarisations are Horizontal and Vertical. • Transmitter and receiving antenna polarisation need to match for optimum signal strength, especially at VHF/UHF • Verticals ( /4, 5 /8) give vertical polarisation. • Yagi and Dipoles antenna’s may be either horizontal or vertical depending on their mounting. Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 12

Dipole Radiation Pattern for a Vertical Dipole: - Chelmsford Amateur Radio Society Intermediate Licence

Dipole Radiation Pattern for a Vertical Dipole: - Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 13

Yagis Direction of Radiation Boom Feeder Reflector Driven Element Radiation Pattern Directors Unwanted Sidelobes

Yagis Direction of Radiation Boom Feeder Reflector Driven Element Radiation Pattern Directors Unwanted Sidelobes Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9: v 1. 2, 31 -May-2009 Antennas & Feeders 14