Chelmsford Amateur Radio Society Intermediate Course 5 Antennas

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 Antennas & Feeders 1

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 Antennas & Feeders 2

Feeders • Feeder types: Coaxial, Twin Conductors Inner Conductor is shrouded by dielectric, with outer (braided) screen. For Radio 50 Coax is used (TV is 75 ) Two conductors kept at constant separation by insulation - no screen Balanced Feeder is available in 75 -300 Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9 Antennas & Feeders 3

Feeder Impedance A B B • Feeder Impedance is a form of AC Resistance • Impedance is based on the Ratio of A and B A Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9 Antennas & Feeders 4

Balanced/Unbalanced • 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 and unwanted radiation Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9 Antennas & Feeders 5

Decibels • Gains and Losses are expressed in d. B’s • 3 d. B is half steps and 6 d. B is quarter steps • 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 Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9 Antennas & Feeders 6

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 is both in Transmit and Receive modes. • 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 Antennas & Feeders 7

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 course value of 50 ohms. Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9 Antennas & Feeders 8

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 Antennas & Feeders 9

Electromagnetic Waves x Electric Field, E y Direction of Propagation z 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 Antennas & Feeders 10

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 Antennas & Feeders 11

Dipole Radiation Pattern for a Vertical Dipole: - Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G 3 PEM Slide Set 9 Antennas & Feeders 12

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 Antennas & Feeders 13