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Antennas
A good antenna works A bad antenna is a waste of time & money Antenna systems can be very inexpensive and simple They can also be very, very expensive

Antenna Considerations
–The space available for an antenna –The proximity to neighbours –The operating frequencies you will use –The output power –Money

Antenna Types
High Frequency 1.6 - 30 Mhz + 50 Mhz 160 - 6 metres Anantenna’s size/length depends on the frequency It’s functionality largely depends on the height above ground, as well as the polarity and it’s configuration

Some Math
Velocity of propagation 300,000,000 m/sec For 1 wavelength, above 30 MHz
Frequency (f) = 300 / wavelength Wavelength (λ) = 300 / frequency
Frequency measured in megahertz Wavelength measured in meters

Above 30 MHz, λ = 300/fmetres or 984/f feet For a half wave λ = 150/f metres or 492/f feet Below 30 MHz λ = 286/f metres or 936/f feet (including the velocity factor 0f 0.95) For a half wave λ = 143/f metres or 468/f feet

The length of a half wave dipole for 3.65 MHz
The length of a half wave dipole for 3.65 MHz L = 143/f = 143/3.65 = 39.18 metres The higher the frequency the shorter the antenna The lower the frequencythe longer the antenna

Types of Antennas
Simple wire – Dipole – Folded dipole – Trap dipole – Offset or Windom antenna – Phased dipoles – Vertical or horizontal (both) – Beverage wave antenna

Types of Antennas
–Metal –Vertical –Yagi –Trap Yagi –Phased arrays –Loops –Vertical or Horizontal –Horns for super ultra high frequencies –Mobile antennas

Antenna Polarization
Vertical orhorizontal Electrical vs Magnetic radiation (Diagram) Vertical waves travel @ 90◦ to the earths surface Horizontal waves travel parallel to the earth’s surface Usually wire antennas are horizontal but an inverted ‘V’ dipole has a vertical component Yagi type antennas can be either vertical or horizontal Circular antennas can be both Usually, horizontally polarized antennas hear less noiseIsotropic Antenna
The isotropic antenna is a hypothetical point source. It does not exist in reality but is considered as an important starting point considering different antennas from the theoretical to the practical The pattern is a Cardioid - a donut shape or a sphere
Dipole Radiation Pattern

Polarization - Practical
Antennas radiating a vertical polarization are best received by an antenna oflike polarization Cross polarization reduces reception by as much as 30 db Bouncing DX signals probably have both polarizations Designing antenna polarization usually depends on the frequency being used - at 70 cm in th eUHF band the elements are very short so either polarization is possible. Usually vertical is used as repeaters are vertically polarized.

Resonance
Antenna length is dependanton frequency The lower the frequency the longer the antenna elements Examples 80 metres 40 10 6 2

3.750 Mhz 7.055 28.5 52 145

124 ft 66 16.4 9 3.2

Isotropic Source
Polarization by Element Orientation Page 48 / 9

An isotropic antenna is a: hypothetical point source What is the antenna radiation pattern for an isotropic radiator? A sphere Polarization of an antenna is determined by:the electric field What does horizontal wave polarization mean? The electric lines of force of a radio wave are parallel to the earth's surface What does vertical wave polarization mean? The electric lines of force of a radio wave are perpendicular to the earth's surface

Polarization by Element Orientation

Con’t

What electromagnetic wave polarization does a Yagi antenna have when itselements are parallel to the earth's surface? Horizontal What electromagnetic wave polarization does a half-wavelength antenna have when it is perpendicular to the earth's surface? Vertical VHF signals from a mobile station using a vertical whip antenna will normally be best received using a: vertical ground-plane antenna A dipole antenna will emit a vertically polarized wave if it is: Parallel with...