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

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

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

3.  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) Twin Feeder:Two conductors kept at constant separation by insulation - no screen Balanced Feeder is available in 75-600 

4. A B B A Feeder Impedance • All Feeders have a Characteristic Impedance, Z0 (eg 50 or 75 Ohm Coax, 300 Ohm twin feeder etc) • Z0 is based on the Ratio of A and B (and the nature of the spacing dielectric) • If correctly terminated by a resistive load then Z0 determines the ratio of Vrms / Irms in the feeder

5. 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 Z0 and unwanted radiation (or loss of immunity from external interference) • Similarly, severe bends or crushing coax changes its Z0 and causes internal mismatches • Any mismatch within a feeder, or its termination, will result in its input impedance no longer being its characteristic impedance

6. Decibels • Gains and Losses may be expressed in dB’s • 3 dB is half steps, 6dB is quarter steps, 10dB is tenth • You will need to remember this table for exam: 3dB x2 or a half 6dB x4 or a quarter 9dB x8 or an eighth 10dB x10 or a tenth Example: 3dB Gain doubles power, whilst 3dB Loss halves it

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 affects both the Transmit and Receive paths • For some standard cables the loss is: Per 100mRG58RG213 10 MHz 4.8 dB 2.0 dB 30 MHz 8.2 dB 3.2 dB 144 MHz 21 dB 8.6 dB

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 nominal course value of 50 ohms

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

10. 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

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

12. 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.

13. Dipole Radiation Pattern Radiation Pattern for a Vertical Dipole:-

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