ACMA Frequency Allocations

In Australia the regulation, allocation and use of Radio Spectrum is controlled by the Australian Communications and Media Authority (ACMA).

To find out who has been allocated specific frequencies visit the ACMA Frequency Allocation Database

Australian radiofrequency spectrum allocations chart - 496k - [ pdf ]

Antenna Polarisation

Antenna polarisation is a very important consideration when choosing and installing an antenna.

Most systems use either vertical, horizontal or circular polarisation. Knowing the difference between polarisations and how to maximise their benefit is very important to users.

Polarisation
An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The electric field plane determines the polarisation or orientation of the radio wave. In general, most antennas radiate either linear or circular polarisation.

A linear polarised antenna radiates wholly in one plane containing the direction of propagation. Where a circular polarised antenna, the plane of polarisation rotates in a circle making one complete revolution during one period of the wave. If the rotation is clockwise looking in the direction of propagation, the sense is called right-hand-circular (RHC). If the rotation is counter clockwise, the sense is called left-hand-circular (LHC).

An antenna is said to be vertically polarised (linear) when its electric field is perpendicular to the Earth's surface. An example of a vertical antenna is a broadcast tower for AM radio or the "whip" antenna on an auto-mobile. Horizontally polarised (linear) antennas have their electric field parallel to the Earth's surface. Television transmissions use horizontal polarisation.

Circular polarised wave radiates energy in both the horizontal and vertical planes and all planes in between. The difference, if any, between the maximum and the minimum peaks as the antenna is rotated through all angles, is called the axial ratio or elliptically and is usually specified in decibels (dB).

If the axial ratio is near 0 dB, the antenna is said to be circular polarised, when using a Helix Antenna. If the axial ratio is greater than 1-2 dB, the polarisation is often referred to as elliptical, when using a crossed Yagi.

Important Considerations
Polarisation is an important design consideration, as each antenna in a system should be properly aligned for maximum signal strength between stations. When choosing an antenna, it is an important consideration as to whether the polarisation is linear or elliptical. If the polarisation is linear, is it vertical or horizontal? If circular, is it RHC or LHC?

This is becomes a greater concern in Wireless LAN devices as line-of-sight (LOS) paths are required due to the low power levels involved, consequently the polarisation of the antennas at both ends of the path must use the same polarisation.

In a linearly polarised system, a misalignment of polarisation of 45 degrees will degrade the signal up to 3 dB and if misaligned and 90 degrees the attenuation can be more than 20 dB.

Likewise, in a circular polarised system, both antennas must have the same sense. If not, an additional loss of 20 dB or more will be incurred. Also note that linearly polarised antennas will work with circularly polarised antennas and vice versa. However, there will be up to a 3 dB loss in signal strength. In weak signal situations, this loss of signal will mean a great deal.

Cross polarisation is another consideration. It happens when unwanted radiation is present from a polarisation, which is different from the polarisation in which the antenna was intended to radiate. For example, a vertical antenna may radiate some horizontal polarisation and vice versa. However, this is seldom a problem unless there is noise or strong signals are nearby.

Typical Applications
Vertical polarisation is most commonly used when it is desired to radiate a radio signal in all directions over a short to medium range.

Horizontal polarisation is used over longer distances to reduce interference by vertically polarised equipment radiating other radio noise, which is often predominantly vertically polarised.

Nevertheless both horizontal and vertical polarisation may be deployed over long distance if a reflector is deployed to focus the energy being emitted.

So consequently the decision is using the polarisation, which offers the best rejection of local unwanted signal.

Circular polarisation is most often used in satellite communications. This is particularly desired since the polarisation of a linear polarised radio wave may be rotated as the signal passes through any anomalies (such as Faraday rotation) in the ionosphere.

Furthermore, due to the position of the Earth with respect to the satellite, geometric differences may vary especially if the satellite appears to move with respect to the fixed Earth bound station. Circular polarisation will keep the signal constant regardless of these anomalies.

These Antennas make very good point-to-point long run connections due to a combination of linear noise rejection and high gain. The two most common a crossed yagi or helix

When setting up an exclusive communications link, it may be wise to first test the link with vertical and then horizontal polarisation to see which yields the best performance (if any).

If there are any reflections in the area, especially from structures or towers, one polarisation may outperform the other. Further, if there are other RF signals in an area, using a polarisation in the opposite predominant high level signals will give some isolation as discussed earlier.

On another note, when radio waves strike a smooth reflective surface, they may incur a 180 degree phase shift, a phenomenon known as specular or mirror image reflection. The reflected signal may then destructively or constructively affect the direct LOS signal.

Circular polarisation has been used to an advantage in these situations since the reflected wave would have a different sense than the direct wave and block the fading from these reflections.

Diversity Antennas
Even if the polarisations are matched, other factors may affect the strength of the signal. The most common are long and short-term fading. Long term fading results from changes in the weather (such as barometric pressure or precipitation). Short term fading is often referred to as "multi-path" fading since it results from reflected signals interfering with the LOS signal.

Some of these fading phenomenon can be decreased by the use of diversity reception. This type of system usually employs dual antennas with some kind of "voting" system to choose the busiest signal. This is commonly used in many 802.11 wireless network equipment.

However in theory for the best results when using external antennas they should be at least 20 wavelengths apart, so that the signals are no longer correlated, particularly in medium and long-distance situations.

Cherenkov Light

Cherenkov light appears when a particle travels through matter faster than light can. This effect is the optical equivalent of a sonic boom, which occurs, for example, when a jet travels faster than the speed of sound.

But how can a particle go faster than light without violating the laws of physics? The speed of light in a vacuum is the ultimate speed limit: 300,000,000 meters per second. It's thought that nothing can travel faster.

However, light slows down when it goes through water, glass, and other transparent materials—in some cases by more than 25 percent. Hence a particle can slip through material faster than light does, while at the same time staying below the speed of light in a vacuum.

When this happens, a particle emits bluish Cherenkov light, which spreads out behind it in a hollow cone that is shaped like the cone of a sonic boom. This light gives the water surrounding a nuclear reactor core its distinctive blue glow.

Compton Scattering

Compton Scattering, is where an interaction between charged electrons in mater and high energy photons result in the electron being given part of the energy, causing a recoil effect of another high energy photon.

Folded Dipoles

The drawing below shows the essential elements of a folded dipole which consists of two parallel elements having a constant spacing S with each element having a certain diameter, d1 and d2.

The ends of the parallel elements are connected to form a continuous loop and the feed-point is at the centre of the element having the diameter d1.

Consequently, in this calculation we use the value of single-wire dipole, the feed-point impedance will be transformed upward by the ratio R according the equation seen below.

To make things a little easier here is a Folded Dipole Calculator...

In free space the impedance, of a resonant 1/2 wave dipole antenna with a centre feed is approximately 72 Ohms. Hence, a folded dipole using equal diameters for both elements will have a ratio of 4 and therefore an impedance of about 288 Ohms.

Please note: This is only a guide for experimentation, when looking around the Internet you'll find many variations to this calculation due of a number of reasons ranging from rounding down/up or ways to match the impedance of the antenna with the feed line.

Half Wave Dipole

The current distribution along a half-wave dipole is roughly sinusoidal. It falls to zero at the end and is at a maximum in the middle. Conversely the voltage is low at the middle and rises to a maximum at the ends. It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum. This provides a low impedance feed point which is convenient to handle.

To calculate the element length of a half-wave dipole the formula is Wavelength = Speed of Light / Frequency is used. However as electricity travels slower through copper than space, it must be reduced by the velocity factor Vf by about 0.951 before converting it into a 1/2 wavelength.

Here is a simple Java calculator.

Ohm's Law

Ohm's law is one of the most useful electrical calculations you can ever know. The law states basically that the current through a conductor between two points is directly proportional to the potential difference (voltage drop) across the two points, and inversely proportional to the resistance between them.

The mathematical equation is I = E / R

Where I = Current in amps, E = Voltage in volts and R = Resistance in ohms

It is just a matter of transposing this equation to calculate the value of the other two known values.

I = E / R

E = I x R

R = E / I

Resistor Colour Code

Types of Radiation

Alpha Radiation
Alpha radiation is heavy, very short-range and is an ejected nuclei . Characteristics of alpha radiation are:

• Does not penetrate human skin or clothing and is not an external hazard
• Can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds.
• Thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation.
• Cannot be detected through layers of water, dust, paper, or other material.
• Travels only a short distance (a few cm) in air.


Examples of some alpha emitters: americium 241, radium, radon, uranium, thorium.

Beta Radiation
Beta radiation is a light, short-range and is an ejected electron. Characteristics of beta radiation are:

• May travel a few metres in air and is moderately penetrating.
• Beta radiation can penetrate human skin and prolonged exposure may cause skin injury.
• Beta-emitting can be very harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds.
• Can be detected with a thin-window GM probe, some beta emitters are very low-energy and are difficult to detect
• Clothing provides moderate protection against beta radiation.

Examples of some pure beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.

Gamma and X Radiation
Gamma radiation and x rays are highly penetrating electromagnetic radiation. Characteristics of this radiation are:

• Able to travel several metres in air and many centre metres in human tissue and are sometimes called "penetrating" radiation
• Sealed radioactive sources and machines that emit this radiation are an external hazard to humans.
• These electromagnetic radiations differ to light and radio waves only in the amount of energy they have.
• Dense materials are needed for shielding, clothing provides little shielding but can prevent contamination of the skin by gamma-emitting materials.
• Detected by GM probe and survey meters with a Scintillators such as sodium iodide.
• Gamma radiation and/or characteristic x rays often accompany the emission of alpha and beta radiation during radioactive decay

Neutron Radiation
Neutrons may be emitted during either spontaneous or induced nuclear fission, nuclear fusion processes, very high energy reactions such as in the accelerator-based neutron sources and in cosmic ray interactions.

• Able to travel several metres through most materials
• Sources and machines that emit this radiation are extremely hazard to humans.
• Sometimes considered a more severe hazard due to neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters
• Dense materials are needed for shielding
• Most commonly detected by with a Scintillators and other specialised equipment.
• Neutrons also degrade materials; intense bombardment with neutrons creates dislocations in the materials, leading to embrittlement of metals and other materials.