Radio Propagation

We would never guarantee coverage without carrying out a range test, but normally, communications between two licensed handportable radios will reach several hundred yards, or even a mile or so in open areas. Place them on a hill, or introduce a talk-through repeater, and this could rise to thirty or more miles.

Radio signals travel in line-of-sight but, like light, can penetrate some things and bounce (reflect) off others. Small buildings (without much metal) and trees provide little obstacle but hills (unless you're on the top) and tunnels can present real problems.

Height, aerial type, transmit power and obstacles are the main issues. Weather, despite folklore, has little effect on reception.

Never believe the range quoted by unlicensed radio suppliers. The usual claim of up to 10 km is grossly exaggerated.

 

When someone is thinking about a radio system, the one thing they always want to know is 'how far can these radios work?'.  Well, to use an old adage - 'How long is a piece of string?'.  There are so many variables that, when I get asked the question, and I do very often, I don't exactly try to duck the question. But with any radio system, the only definitive way is to try it.

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Very often, for a given system in a given situation, I could say with confidence that they will, or will not, do the job.  If I was asked if a pair of professional radio handportables would be able to communicate across a small, flat, field, where both radio users could see each other at all times, I would say 'yes, they'll work'.  Put one of them in a building, behind a hill, or put them both in a town centre, then I’d have to put more thought, and possibly tests, into it.

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First a little science lesson:  The information (speech or data) you want to be conveyed as a message between the two radios is encompassed in, and sent by, a radio signal varying according to the message.  Radio signals and light are basically the same things – electromagnetic field (EMF) conveyed by photons.  The only difference between light and radio (and gamma rays, ultraviolet and infrared light) is the frequency of the waves – they occupy different parts of the electromagnetic spectrum.  And it is this frequency (sometimes measured as a wavelength, λ) that gives the EMF its properties. 

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The radio spectrum is the part of the electromagnetic spectrum useful for radios and is divided into ‘bands’, each band consisting of adjacent frequencies which have similar properties.  Our UHF (ultra high frequency) walkie talkie sends and receives signals at around 450Mhz (1 Mhz = 1 million cycles per second).  The UHF band is between 300Mhz to 3,000Mhz (or 3 GigaHertz).  Red light, by comparison, is in the light, or visible, spectrum at a frequency around 450,000 Ghz.  Light is more commonly measured in wavelengths ( λ).  To change frequency (f) to wavelength (λ), you need to divide by the speed of light (c).  Hence to find the wavelength of red light:  λ=f/c, λ=450,000Gh/(3x108)   λ=666nm (nanometres, 10 -12 metres).

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The eyes receive the light waves that the brain circuitry decodes into levels or colours and brightness, the antenna receives the radio waves that the radio circuitry decodes into useful speech or data. 

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As well as needing a different receptor (eyes, camera, radio aerial, microwave dish) the different bands propagate (travel) differently.  Light, as we know, will not travel through most solid materials, glass being an obvious exception.  It also travels through water fairly well.  Radio waves, however, can pass through windows and walls, but cannot pass through water very well at all.  It also does not much pass through metal. Metal-clad buildings present a real problem.

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Both will travel ‘line of sight’ – in other words in straight lines (unless passing from one medium to another) at the speed of light (186,000 miles [300,000,000 metres] per second).  When a signal cannot pass through a material, it bounces off it or is absorbed by it.  This is how we see objects.  We see grass because frequencies that we see as green light bounces off it to our eyes, and it absorbs other colours.  Hence in a built-up area, radios may communicate when the users cannot see each other (are not in ‘line of sight’).  The signal could be travelling through walls or bouncing off them to the other radio. Valleys, rivers and coastlines, being low, are notoriously difficult places to cover.

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You can see from this that calculating accurate reception distances is impossible.  However, there are several other aspects, other than frequency, that can affect a radio’s range:

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• The transmitted power (to the antenna)

• The frequency of the signal

• The ability of the antenna system to transmit this power well and in the direction we require

• The distance to the receiving radio

• The objects between the two radios - hills, buildings, trees etc (therefore the height of the aerials)

• The quality of the receiving radio’s antenna system

• The sensitivity of the receiver

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A big factor in transmission range is the height of the transmitting and receiving antennae.  The higher the combined heights of the antennae, the better they will overcome 5 above, including, in systems with very good range, the curvature of the earth.   

                                     

An antenna, if it were a metal ball, (hypothetically assuming no connecting wires), would send the signal in all directions (up, down, and to all sides).   Usually the up and down signal is wasted, as our two users would be to the horizontal of each other.  Most antenna, therefore, are lengths of metal, sending the signal out from the long side and little up or down (omni-directional).  Most of the signal, again, is wasted as we are transmitting in all horizontal directions, 360 deg, whereas the other user is only in one direction.  However, his position can change, or other users may want to be contacted, so an omni-directional antenna is required.

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Where the position of the two aerials are always fixed, i.e. a TV receiver (which usually only receives the signal from one transmitter site), then a directional antenna is used, which sends the signal to, or receives the signal from, mostly in one horizontal direction.  This is usually in the form of a Yagi antenna, as on the roofs of our houses.  The TV transmitting station, however, needs to transmit in all directions to reach all homes in its area.  So it uses an omni-directional antenna, usually on a tall mast on high ground.

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We say that a directional antenna has ‘gain’, or amplification.  The antennas described so far do not increase the transmit power, which remains at, say, 4 watts for a handportable radio. The gain is ‘effective’ gain, which concentrates the power available in the direction we want.  If we concentrate the 4 watts in one direction, we can make the signal simulate 8 watts of power for that direction.  Effective power gain is measured in decibels (dbs).  3 db gain is doubling the effective power.

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We have covered most things that affect the range of radio communication, and when designing a system, range is one of the prime concerns.

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A simple back-to-back portable system is what many schools and factories may use. Consisting of just several handportable radios, this would typically be the cheapest system and the range may be perfectly acceptable for small areas of a few acres. For larger areas you may find that radios near the centre of the premises, or those upstairs in a building, would be able to reach all other users but those at the perimeter cannot reach others at the opposite side. Should this be unacceptable then a central base station would be considered.

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If most of the messages passed among users were to and from a controller, or supervisor, in an office, then the range of the system could be greatly enhanced by installing a base station with its own antenna, often fixed to the roof or a high wall. This ensures that the controller has the height advantage to reach the parts other users cannot reach. In this way, range can be increased from a few hundred metres to a few kilometres (centred around the base). This is all very well for the controller, but if the portable users wanted to converse with each other, they are no better off. They would need a repeater.

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Many base stations are also configured as repeaters.  They re-transmit whatever signal they receive from the users back out to the other users. All conversations are directed through the repeater base station in this way and the users get the benefit of the high base aerial even when talking to each other. Users outside of the, now fixed, coverage area couldn't converse with each other on the repeater channel even if they were standing next to each other. To get even greater range, some base stations (usually shared systems), are located off-site on some remote high point (tall building, mountain etc). These could give ranges of tens of kilometres and more. With our repeater in Durham, users can converse from, say, Cramlington to Spennymoor, a distance of 50km as the crow flies. Several sites can be linked together to give an even greater range, as with the cellular networks. Some 'repeaters' are even located in satellites, giving a massive, if expensive, range for Sat Phones.

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So while predicting the range of a radio system can be tricky, a system can always be designed to cater for the user's requirements regarding coverage. A good radio supplier will offer a system with due regard to the users' needs and terrain limitations, with full knowledge of available equipment.

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Alan Davies

Managing Director

Cygnal Ltd