Intermediate Course (5) Antennas and Feeders

Transcription

1 Intermediate Course (5) Antennas and Feeders 1

2 System Transmitter 50 Ohms Output Standing Wave Ratio Meter Antenna Matching Unit Feeder Antenna Receiver 2

3 Feeders Feeder types: Coaxial, Twin Conductors + - Inner Conductor is shrouded by dielectric, with outer (braided) screen. For Radio 50W Coax is used (TV is 75W) Two conductors kept at constant separation by insulation - no screen Balanced Feeder is available in W 3

4 Feeder Impedance A B B A Feeder Impedance is a form of AC Resistance Impedance is based on the Ratio of A and B 4

5 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 5

6 Decibels Gains and Losses are expressed in db s 3 db is half steps and 6dB is quarter steps You will need to remember this table for exam: 3dB 6dB 9dB 10dB x2 or a half x4 or a quarter x8 or an eighth x10 or a tenth 6

7 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 100m RG58 RG MHz 4.8 db 2.0 db 30 MHz 8.2 db 3.2 db 144 MHz 21 db 8.6 db 7

8 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. 8

9 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. 2 I V 0 1/4 WAVELENGTH 1/4 WAVELENGTH 1/4 WAVELENGTH 1/4 WAVELENGTH 9

10 Electromagnetic Waves x Electric Field, E y Magnetic Field, H z Direction of Propagation 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. 10

11 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 (l/4, 5l/8) give vertical polarisation. Yagi and Dipoles antenna s may be either horizontal or vertical depending on their mounting. 11

12 Dipole Radiation Pattern Radiation Pattern for a Vertical Dipole:- 12

13 Yagis Direction of Radiation Feeder Boom Radiation Pattern Reflector Driven Element Directors Unwanted Sidelobes 13