1 Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G3PEM Slide Set 9 Antennas Feeders Chelmsford Amateur Radio Society Intermediate

1Chelmsford Amateur Radio SocietyIntermediate Licence Course

Carl Thomson G3PEM Slide Set 9Antennas & Feeders

Chelmsford Amateur Radio Society Chelmsford Amateur Radio Society

Intermediate CourseIntermediate Course(5) Antennas and Feeders(5) Antennas and Feeders



System

Transmitter

Receiver

Antenna Matching

Unit

Standing WaveRatio Meter

50 OhmsOutput

FeederAntenna



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



Feeder Impedance

• Feeder Impedance is a form of AC Resistance

• Impedance is based on the Ratio of A and B

B

A

A

B



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



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 x2 or a half

6dB x4 or a quarter

9dB x8 or an eighth

10dB x10 or a tenth



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 RG213

10 MHz 4.8 dB 2.0 dB

30 MHz 8.2 dB 3.2 dB

144 MHz 21 dB 8.6 dB



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.



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 V

1/4 WAVELENGTH 1/4 WAVELENGTH 1/4 WAVELENGTH 1/4 WAVELENGTH

0

2



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.

z

x

y

Electric Field, E

Magnetic Field, H

Direction of Propagation



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.



Dipole Radiation Pattern

Radiation Pattern for a Vertical Dipole:-



Yagis

Driven ElementReflector

Boom

Directors

Feeder

Direction of Radiation

Unwanted Sidelobes

Radiation Pattern

Documents

1 Chelmsford Amateur Radio Society Intermediate Licence Course Carl Thomson G3PEM Slide Set 9 Antennas Feeders Chelmsford Amateur Radio Society Intermediate