Antennas, Polarisation and Effective Radiated Power (e.r.p.) (5b1, 5b2, 5c1, 5c2, 5c3)

Antennas

 

When transmitting, the antenna converts electrical signals in the feeder into radio waves in the air.

 

When receiving, the antenna converts radio waves in the air into electrical signals in the feeder.

 

The sizes of h.f. and v.h.f. antennas are different because they are related to wavelength (λ) of the radio signal.  They operate on the same principles. 

 

You need to be aware of 5 of the most popular types of antenna used in amateur radio.

Dipole

 

Dipole (horizontal)

 

 

A Dipole can be used horizontally as shown above.  It radiates to the right-angles (into and out of the screen), but not off the ends of the wires.

 

 

Dipole (vertical)

 

·          When a dipole is used vertically, it radiates in all directions but not off the ends of the wires (not up into the sky and down into the ground).

 

 

 

 

 

 

 

λ/4 (¼-wavelength) ground plane

 

 

·          Note the λ/4 ground plane has elements λ/4 (¼-wavelength)

 

·          It is fed with coaxial feeder

 

·          The λ/4 ground plane is always used vertically and has omni-directional (all round) coverage.

 

 

 

 

 

 

 

 

 

 

 

5/8 λ (⅝ wavelength)

·          It has slightly better gain that the the λ/4 ground plane.

 

·          It is fed with coaxial feeder

 

·          The 5/8 λ is always used vertically and has omni-directional (all round) coverage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Yagi

 

·          The Yagi antenna is directional.  The one illustrated here is pointing to the right.

·          The Yagi antenna also has gain because it directs power in the wanted direction (like a car headlamp) rather sending it in all directions.

 

·          The Yagi can be used either be used horizontally or vertically.  The one illustrated in the diagram here is vertically polarised.

 

 

 

 

 

 

 

 

 

 

The Long Wire

The other type of antenna is the long wire. 

 

This is exactly what it says – it is a long piece of wire at least 1 λ (1 wavelength) long.

 

Note the need for a wire going from the transmitter to earth.  This is usually a copper stake in the ground.

 

 

 

 

 

 

 

 

Polarisation

 

·          Polarisation is the plane of the antennas radiating electric field.

 

·          Common polarisations are Horizontal and Vertical.

 

·          Transmitter and receiving antenna polarisations at each end of the radio contact need to match for optimum signal strength.

 

·          Verticals (λ/4, 5/8 λ) give vertical polarisation.

 

·          Yagis and Dipoles may be either horizontal or vertical polarisation depending on their mounting.

 

Here are some real yagis in use:-

 

ç This 5-element Yagi is horizontally polarised and is pointing to the left.

 

 

 

 

This 5-element Yagi is vertically polarised and is pointing to the right.  (The box underneath is an electrical rotator) è

 

 

 

 

 

 

Effective Radiated Power (e.r.p.)

 

Where an antenna has some gain, it’s effective radiated power is calculated as the power of the transmitter x the gain of the antenna.

 

For example a transmitter producing 10Watts connected with feeder to a Yagi Antenna with 4 times gain will have an effective radiated power of 10 x 4 = 40 Watts e.r.p.

 

(You may see reference to dBs in manufacturer’s catalogues – you don’t need to know about dBs for the Foundation Licence)

 

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