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CHAPTER 2
VHF/ UHF RADIO PROPAGATION

Chapter 1 explains that HF, VHF, and UHF radio waves have different
propagation characteristics. VHF and UHF radio frequencies propa-
gate principally along LOS paths. On the other hand, HF waves, those
below 30 MHz, can also reflect off the ionosphere and then travel back to
earth. These sky waves, as they are called, give rise to one of the most
important attributes of HF radio and that is beyond the line of sight (BLOS) communication. Volume One, “HF Technology”, of this series “Radio
Communications in the Digital Age” provides a detailed description of HF
propagation. This chapter deals primarily with LOS propagation characteris-
tics of the VHF and UHF frequency bands.

While many HF propagation characteristics are associated with the ionos-
phere and wave reflections from it, the effects of local area topography
and conditions in the lower atmosphere mostly govern VHF and UHF propa-
gation. Similarly, ground wave propagation is a very important mode of HF
wave propagation, but at frequencies above 30 MHz, ground waves are
absorbed almost immediately and have a negligible beneficial impact.

Frequencies in the VHF and UHF bands usually penetrate the ionosphere and
speed out into space. That means that reflection off the ionosphere can not
be used to reliably extend communications range of these frequencies. For
the most part, the transmitting and receiving antennas must have a fairly
unobstructed path between them for communication to take place, hence
the term line- of- sight (LOS).

Height Matters for LOS Range

The visible horizon observed at approximately five feet above a flat surface of earth is less than 2.7 miles away (Figure 2- 1). This is approximately the maximum LOS radio range from a manpack radio on the back of a standing man to another manpack radio that is lying on the ground.

Figure 2- 1 shows that if the receiving radio were elevated to the back of a standing man, this maximum distance would be doubled. In this case the LOS distance would be 5.4 miles. But, if the second man was standing beyond this distance, say at 7 miles from the transmitting radio, the shadowing effects of the earth’s curvature would prevent the second man from receiving the radio wave. In this case, 7 miles is BLOS and is not within reach of VHF or UHF radios in these positions.

It is clear that the elevation of both the transmitting and receiving antennas is crucially important. For example if the receiving antenna were mounted on a 26- foot tower, the total LOS distance would be increased to 9 miles. Of course if the radiomen were both located on the tops of mountains, the LOS range might be as much as from 50 to hundred miles.
For ground- to- air UHF communications, the aircraft can be 100 miles away
or more and still maintain contact.

Transmit Power and Radio Range

For HF radio communications, transmit power is an important item. For very
long distances, particularly for both skywave and ground wave propagation,
every mile of distance attenuates (decreases) the signal. For most systems,
when doubling the distance, the radiated signal is divided by four!

Therefore, transmit power is often the limiting range factor. It is common to see 500- watt and 1- kW HF transmitters in vehicular or shipboard HF applications, and 10- kW or greater for HF fixed station broadcast sites.
VHF and UHF waves are also attenuated with every mile of distance. However,
for tactical manpack applications, it is most often the shadowing effects of irregular terrain, buildings, and other objects that limit the effective range and not transmit power.

Many manpack radios have two power settings: 2 Watts and 5 to 10- Watts.
The 2- Watt setting is often adequate and extends battery life when this
power level is selected. On the other hand, there are situations where
increased power is beneficial. In urban areas where high radio frequency
noise is prevalent, higher power increases the signal- to- noise (SNR) ratio and improves reception. Also, modern high data rate modulation wave-
forms require a high SNR to be effective.

UHF ground- to- air communications benefit from higher power because the
typical range is 100- miles or more. Lastly, although tactical manpack UHF SATCOM radios with only 18- Watts located in Europe can contact a satellite in an orbit 22,000 miles above the earth’s equator, communication is more reliable when higher power is used.

These higher power VHF and UHF radio sets are typically mounted in vehicles
or fixed stations with 50- watt power amplifiers to boost the power of the
manpack transceiver.

VHF and UHF Radio Reception Behind Ridges

For the most part, ridges and hills form shadows of VHF and UHF radio waves. However, there is an important exception when it comes to very sharp ridges or other kinds of abrupt barriers. This is caused by a phenomenon known as Diffraction (Figure 2- 2). When a VHF or UHF wave comes to a sharp edge, a portion of the wave bends around the edge and continues propagation as if a very low power radio was placed at the top of the ridge. It is important that the ridge be relatively sharp. A well- rounded hill or the curvature of the earth is not sufficient to cause this effect. This effect is important in a battle field situation where a soldier must seek shelter behind a ridge.

Reflections and Multipath Distortion

VHF and UHF waves can be reflected off of dense surfaces like rocks or
conductive earth, just like a beam of light can be reflected off a wall or a ceiling. Sometimes several paths exist between a transmitting and receiving antenna (Figure 2- 3). In this figure there is a direct LOS path between two radios, but there is also a reflected path from the bottom of a valley between them. It is clear that these two paths are of different length, and that the direct path is the shorter of the two. Since radio waves travel at a constant velocity, the direct path wave arrives at the receiver before the reflected path. This means that the same broadcast information reaches the receiver at two different times. The effect of this is much like echoes that one hears in an acoustically poor room. If the echoes are close enough to each other, it is hard to understand what is being said. In radio terminology, this is called multipath distortion.
Although it is annoying with voice communications, it is devastating to high data rate digital communication. A subsequent chapter will discuss some of the ingenious ways that have been devised to minimize the effects of this type of distortion. “Picket fencing” is a form of multipathing common to vehicular mounted radios. It is prevalent with VHF and UHF. The higher the frequency, the more pronounced the effect is. It is usually caused by interference or reflections of signals from man- made objects such as buildings, houses, and other structures. These objects cause constructive and destructive fields (or strengthened and weakened signals) so that when a vehicle travels through these fields, it receives
alternately stronger and weaker signals. There is usually a “swishing” sound in the receiver, as the signals rapidly grow weaker, then stronger, then weaker again.

The signal peaks and nulls are a function of wavelength. A 450- MHz signal
being received on board a vehicle travelling at 60 mph can “flutter” very
rapidly as the vehicle travels through the downtown area of a city. You can
experience the same phenomenon on the lower VHF bands, but the flutters
are not quite as rapid.

Sometimes this same effect is caused by signals of two stationary radios
reflecting off a moving aircraft above them. Multipath within a Building In tactical situations, manpack radios are frequently operated under cover in
buildings. VHF and UHF waves have trouble penetrating reinforced concrete
exterior walls, but they pass through windows and light interior wall partitions with comparative ease.

Figure 2- 4 shows a receiver in a room of a building with a transmitter located outside. In this case, there are three paths from the transmitter to the receiver, and none of them are direct.

Path 1 passes through the window nearest the receiver location, and is
diffracted around the sharp edge of the window frame to the receiver.
Likewise, path 2 just misses having a direct path to the receiver. It is
diffracted slightly by the window frame nearest the transmitter and then
passes through an interior wall on the way to the receiver. Path 3 goes
through a window and an interior wall before striking an outside wall of
the building and then reflecting back to the receiver.

Each of these paths has a different distance and, therefore, can cause
multipath distortion. Frequently just moving the receiver a few feet in some direction will avoid one or more of the available paths and the reception of the signal may be greatly improved.

VHF and UHF Wave Ducting

The suggested limits on LOS range are sometimes exceeded in practice.
One of the principal reasons for this is an effect called “ducting.” VHF
and UHF waves traveling through the atmosphere travel slightly slower
than they do in free space, and that is because the density of air slows
them down. The denser the air, the slower the wave speed through it.
Under normal conditions, the density of air is the greatest at the surface
of the earth and gradually reduces in density with altitude. Under fair, dry, and moderate weather conditions; the slight variations in air density have negligible effects on the path of radio waves passing through it.
Frequently there are abrupt changes in air density due to weather fronts
passing over an area or the heavy moisture burden of rain clouds. In such
cases VHF and UHF can bend or duct between air layers of different densities. Sometimes this ducting bends the radio waves downward so that the radio waves tend to follow the curvature of the earth. In such cases the LOS range is considerably greater than the optical LOS range. This type of wave propagation is impossible to predict; it is not practical to
plan on it for range improvement. However, when ducting conditions exit,
they generally do so for hours at a time.

SUMMARY

HF propagation can be LOS through ground waves or direct waves
and BLOS though the use of sky waves.

VHF and UHF frequencies cannot make use of skywave or ground
wave propagation and depend almost exclusively on the direct
wave. This restricts their use to LOS communications.

Radio wave propagation at VHF and UHF frequencies are primarily
affected by local area topography (hills and valleys) and atmospheric
conditions.

VHF and UHF range is usually limited by physical wave shadowing
of obstructions such as buildings and mountains.

Diffraction of VHF and UHF waves can bend them around sharp
edges such as window frames or sharp ridges.

Multipath distortion is caused by waves arriving at a receiver from
more than one path.

LOS range is greatly improved with increased height of either
(or both) transmit or receive antennas.

Ducting caused by certain weather conditions can sometimes
increase the range of VHF and UHF waves.