...VHF/UHF Propagation

- Up the bands and out of range

In the past few years there has been a growing movement from VHF frequencies (50-300 MHz) to UHF (300-900 MHz). Many, however, have found the move fraught with difficulties. ...

When planning a communications system the usual practice is to look at radiation patterns. However, patterns developed by antenna manufactures will not necessarily mirror what can be achieved in real conditions, especially in the case of mobile networks using high gain antennas for extended range. The reason for this lies in the differences in propagation of the direct wave.

Waves at UHF frequencies are shorter than those at VHF frequencies with the result that the range decreases progressively as the frequency gets higher, giving less radio coverage. Added to this, the intensity of a wave varies inversely with the distance from a source, so that, if a field strength is 100 millivolts per metre at 1 mile, it will only be 50 millivolts per metre at 2 miles, and so on. Optimum range is approximately 80km at VHF under the very best conditions (ie over salt water with no obstructions, ideal mounting, minimal losses, etc.). In practice such a range would regularly be achievable with higher gain antennas.

As with HF communications, the capacity of an antenna to transmit and receive signals will vary considerably. A transmit antenna will have all the power of the transmitter (less system losses) to radiate the signal. However, quality of reception will be subject to many environmental factors, not just the power of the transmitting antenna and where it is mounted.

Both the VHF and UHF direct wave are affected by multipath propagation created by reflection and scattering of the signal when it encounters obstacles like buildings, trees and the like. Signals affected by multipath effects arrive at the receiver later than those that are unobstructed and from different paths, resulting at best case in fading, at worst case in total disruption of the signal.

UHF signals are also far more subject to polarisation reversal due to topography than VHF signals with the result that unless the receive antenna can pick up both vertically and horizontally polarised signals, the received signal will suffer from flutter and fading.

Mobile networks suffer most of all. Often there is no direct line-of-sight path to the base station so that the received signal is made entirely of multiple reflections and scattered waves. The amount of fading can vary from around 20dB or less to more than 30dB, so that a vehicle moving at 50km/h can experience several fades per second.

Because the direct wave is longer at VHF frequencies, it has the ability to bend around obstacles through a process known as diffraction - to fill in around corners to some extent. However, as the frequency increases this ability diminishes progressively, and the size of the shadow zone where no signals reach becomes larger. As a result, a VHF network in a built-up area or a hilly terrain is often much more successful than one at UHF frequencies. .

Moving vehicles are also affected by Doppler Shift, positive as the vehicle moves towards the signal and negative as it moves away from the signal. The amount of shift can be as much as 53 Hz in a vehicles moving at 60km/h and communicating on 900 MHz.

Moreover, both mobile and hand held transceivers experience a change in the phase relationship between the various components of the received signal as the receiver moves about.

The rule of thumb for erecting VHF and UHF antennas is the higher the better - within reason of course as long cable runs will result in more losses than the gains you are aiming for from additional height.

Erecting antennas approximately 12 metre (40ft) above buildings will ensure that signals are not affected by associated electrical interference and other obstacles. If you require lightning protection, the antenna tip will need to be about 2 metres below the tip of the lightning rod.

It is possible to determine theoretical range, as distance to the radio horizon increases with additional antenna height (See table). Calculating similarly for both the transmitting and the receiving antenna and adding the results will give the maximum possible range between two antennas. So, your range is dependent on the distance to the radio horizon of the other antenna.

Where maximum possible range is important, it is necessary to ensure that the correct coaxial cable is used to minimise transmission line losses. As cables vary according to specifications and deteriorate over time, it is also essential to bear this in mind. In the marine environment a cable with tinned inner and outer conductors, such the RG59 cable as used by Moonraker, will survive longer.

As a general rule, RG58 should not be used for outdoor runs of more than about 5-6 metres (16-20ft). RG213 is suitable for outdoors runs up to about 30 metres (100ft). For longer runs it is necessary to use Heliax type cables which have much lower losses but are not flexible.

Using the indicative attenuation charts can give you a good idea of what the losses would be with the different types of cable. For every 3dB down, half the radiated power is lost. Thus a 100m (350ft) run transmitting between 148 and 174 MHz using RG213 will give a 9.1dB loss (3.5 x 2.6). Using a 50w transmitter, only 6.25w would be radiated (-25w + -12.5w + -6.25w). However, using Heliax 1/2 in., the loss would be reduced to 2.8dB (3.5 x 0.8 and around 25w would be radiated.
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Obviously, the question of antenna height for vehicles is a special case. The best position for mounting is centre roof. Mounting in other positions will not give as good results. Glass mounted antennas also have the disadvantage of presenting an obstruction to the view of the driver and can be affected by window heating elements, wiper blades, and radiation from the vehicle's electrical equipment from which it may not be effectively screened if it is not mounted far enough away.

As mentioned above, when transmitting a signal, assuming the antenna is well designed and installed, almost all of the transmitter power is available for radiation, regardless of antenna height. Thus, a 48cm (19 in) half wave antenna transmitting on 300 MHz will have the same radiation efficiency as a 40.8m (134ft) half wave antenna transmitting on 3.5 MHz.

In receive mode, however, the quality of reception is dependent on the physical length of the antenna, as the antenna can only extract the signal from the section of the wave front that is about 1/4 wave from the conductor. This area is around 43m (142ft) in diameter at 3.5 MHz, but at 300 MHz it is only around 49cm (1.6ft) in diameter.

Because energy is evenly distributed throughout a wave front, the UHF antenna only receives a small portion of the energy from a given field strength compared to the HF antenna. In fact, the amount varies directly with the square of the wavelength, so that the HF antenna receives about 7000 times more energy.

So, the UHF antenna can only extract a fraction of the energy from the wave front that the much longer HF antenna can. This is without the multipath reception complications experienced at UHF frequencies.

Comparing two antennas with the same design characteristics, one for 432 MHz, the other for 144 MHz, the UHF antenna can only intercept one third of the energy in receiving as the VHF antenna. For a UHF antenna to be able to receive as well as a VHF antenna, both antennas need to be the same physical length and with similar radiation characteristics. In this case it would need around three times as many elements. This means that it is a good idea to utilise antennas of a higher gain for the higher frequencies.

While the aperture or capture area of the array of an antenna decreases as the frequency is increased, it is possible to increase the receive efficiency (and compensate for the increased radiation path loss due to the shorter wavelength). This is achieved by stacking elements to form collinear arrays, thus increasing the physical height of the antenna.
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Taking a theoretical 1/4 wave antenna of 0dB, a 5/8 wave collinear, (5/8 wave of operating frequency), will have a gain of + 3dB on the 1/4 wave antenna. Similarly a 1/2 wave will gives + 2dB gain and a 7/8 wave + 4dB gain compared to the 0dB 1/4 wave antenna.

Generally speaking, a 1/4 wave antenna will give you a beamwidth of approximately 10% of its operating frequency. However the longer the collinear, the lower the angle of radiation with a resultant reduction in beamwidth. A 5/8 wave collinear will have a beamwidth of approximately 8 per cent of its operating frequency.

Because larger gains are required as the frequency is increased and less energy is achieved with each additional element, this means that for vertical collinears there is an upper limit of around 11.15dBi (9dBd) due to mechanical reasons with a practical upper limit of around 7dBi (4.85dBd).

Also the beamwidth narrows increasingly with each additional element. Lower angle radiation is increased at the expense of the higher angles to create a longer beam. So, above these limitations, the bandwidth becomes too narrow with the result that more than 4 elements are rarely used.

Reduced beamwidth is a significant factor, especially for mobile networks, where slight changes in the orientation of a vehicle or vessel may mean that the signal will not be able to get out or come into the receive path.

Because the wavelength at UHF is less than 1 metre, the signal is also subject to variations in the orientation or position of the antenna. These can reduce the signal level by several tens of decibels. For example an omnidirectional antenna of 8dBi will have a typical half power point of under 30 degrees, which on a vehicle will have a significant impact on communications efficiency.

Reflecting on the nature of VHF and UHF propagation, it is easy to see where all the problems have arisen, especially for mobile networks and areas with hilly or mountainous topography. Not all areas are suitable for UHF networks without comprehensive repeater systems, as used by the cellular networks. So practical issues need to be examined when making the all important choice.

While many are swayed by the increased capacity for data transmission at UHF frequencies, this becomes irrelevant if the system does not perform up to expectations. If you live in a hilly region and/or are planning a network, mobile or otherwise, it makes good sense to look before you leap.