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CHAPTER 3
ELEMENTS IN A VHF/ UHF RADIO SYSTEM

Now that you have an overview of how radio waves propagate, let’s take a look at how they are generated. The primary components in a VHF or UHF radio system fall into three groups: transmitters, receivers, and antennas. In most modern radio tactical sets, the transmitter and receiver are contained in a single unit called a transceiver.

This chapter presents an overview of these radio system elements.

The Anatomy of a Multiband VHF/ UHF Transceiver

In times past, a tactical transceiver was restricted to a single band. That
is to say that a separate radio was required for HF, VHF, and UHF service.
With the increased requirement for greater troop mobility there is
enormous pressure to compress all of these separate radios into a single
multiband radio. Thanks to electronics miniaturization; multiband,
multimode radio systems are a reality.

Currently, transceivers capable of VHF, UHF, and Tactical Satellite (TACSAT) service are common. A combined HF/ VHF/ UHF and TACSAT transceiver is the latest innovation in this area. The simplest transceiver must generate a modulated signal to the antenna and to receive a signal from an antenna, demodulate it, and feed the information to a headset, computer, or some other human or machine interface. A multiband transceiver must perform these functions for each of its frequency bands (Figure 3- 1).

Most functions of the multiband transceiver are common to all frequency
bands; however, the electronic means to accomplish these functions
differ depending upon the operating frequency band. Thus those functions that are associated with VHF transmit and receive frequencies must be grouped separately from those that perform that function for the UHF band. That is why most of the RF portions of the transceiver must be duplicated for each band, as shown in Figure 3- 1.

Transmit Path Begins with the Digital Signal Processor

The transmitted voice or data information is applied to a common block
in a multiband transceiver called the Digital Signal Processor (DSP). The
DSP is actually a powerful but miniature computer that turns the input
information into a digital form that is manipulated within the computer.
The functions performed by the DSP include audio bandwidth filtering,
voice digitization, encryption, and modulation. The output of the DSP is
actually a Low Frequency (LF) modulated carrier that is an exact replica of
what is to be transmitted, except for its frequency. This signal is referred to as being at an Intermediate Frequency (IF).

UHF Frequency Up- Conversion, and Frequency Synthesizer

If a UHF frequency is selected, the IF signal at the output of the DSP is
applied to the UHF up- converter circuits. Another block of circuits, called a frequency synthesizer creates the various signals that are required by the up- converter to create the desired UHF output frequency.

Power Amplifier and Transmit Filters

The up- converted signal is then applied to a wideband power amplifier
which covers the entire transmit band selected. In this case it is the UHF
band and the amplifier that handles signals from 90 to 512 MHz. The signal power output of this amplifier is typically operator selected from 1 to 10 watts.

Following the power amplifier is a group of switched low pass filters that
“clean up” its output. These remove noise, spurious signals, and harmonics
generated by other transmitter circuits including frequency harmonics
generated by the power amplifier. This process reduces interference with
adjacent communications channels.

UHF Antenna Port

The output of the UHF low pass filters is applied through a Transmit/
Receive (TX/ RX) switch (shown in Figure 3- 1 in the TX position) to the
UHF antenna port of the transceiver. UHF antennas have a 50- ohm
input impedance.

Receive Path Begins with Switched Bandpass Filters

A receive UHF signal is applied by the antenna to the antenna port, and
then through the TX/ RX switch to a group of switched bandpass filters.
The purpose of these filters is to remove signals above and below the
desired signal.

RF Amplifiers and Down- Converter

The filtered input signals are applied to several radio frequency amplifier
stages (shown as one block in Figure 3- 1). The typical input signal has a
signal strength in the micro- watt range (one millionth of a watt). The RF
amplifiers boost this signal to the milli- watt range for further processing.

The next step in this process is to down- convert the signal to the LF IF
frequency used by the DSP block. Again, this is accomplished by the
down- converter in conjunction with signals from the synthesizer. In
modern radios, this process is performed in several separate amplification
and down conversion steps. It is shown in Figure 3- 1 as occurring in
just one step for simplicity.

DSP Demodulation and Decryption

The final steps in the receive process are performed by the DSP. Here
the IF signals from the down- converter is demodulated and decrypted to
form the base band signals (audio or data) that are used by the operator.
VHF Band Portion of the Transceiver

The VHF transmit and receive functions are similar to those of the UHF
band except that they are performed by the VHF portions of the radio.
However there is one additional function required in the VHF band and
that is antenna matching.

VHF Whip Antenna Matching

The whip antenna frequently used with a VHF manpack radio does not present a 50- ohm impedance to the radio over the 30 to 90 MHz band. In order to maximize the power radiated from this type of antenna, a series of switched matching circuits are used in the transmit path following the switched low pass filters. The correct matching network is selected automatically by the frequency selector switch on the transceiver front panel.

50- Watt Multiband Transceiver Group

It is common for radios used in vehicles and in fixed stations to require
higher power than the tactical manpack transceiver can deliver on its own.
In these applications, the manpack transceiver is attached to a mounting
base that includes power amplifiers and some additional antenna ports
(Figure 3- 2).

Power Amplifiers

A multiband vehicular adapter is likely to have two or more power amplifiers that are tailored to the frequency ports of the manpack
transceiver. Figure 3- 2 shows a vehicular adapter with both VHF and UHF
transceiver ports. Each of these ports is associated with a power amplifier
that is capable of producing 50 watts of output power.

Each of these amplifiers has a receive bypass path which is selected by the
transceiver keyline. In the key- down transmit condition, the bypass is open and the signal is applied to the power amplifiers. However, in the receive condition, the amplifiers are bypassed so that the signal from the antenna ports can pass back to the receiver circuits in the manpack transceiver.

VHF Low, VHF High, UHF, and TACSAT Antenna Ports

Most multiband transceivers have two antenna ports, one for VHF and
the other for UHF. In vehicular and fixed station installations it is common to have antennas that are larger and more efficient than those used with a manpack alone. It is therefore, convenient to have four antenna ports.

The first port is used for low band VHF over the 30 to 89.999 MHz range.
But the UHF path is spread between three separate antenna ports, as
shown in Figure 3- 2.

The output of the UHF amplifier is applied to a diplexer, which splits the UHF port into two frequency ranges, 90 to 224.999 MHz and 225 to 512 MHz.
Each of these frequency outputs is applied to a corresponding antenna port.
The 225 to 512 MHz path is further divided by a relay switch into a UHF
path and a TACSAT path. This is because the TACSAT path requires a
collocation filter, a Low Noise Receive Amplifier (LNA), and a separate
antenna port. This additional filtering and amplification on the TACSAT
path is useful because of the typically low level of received signal from the tactical satellite in orbit 22,300 miles above the equator. The collocation filter is there to remove radio noise generated by vehicle ignition, motors, and other transmitters that would otherwise obscure the faint signals from the satellite.

The Antenna Group

The antenna is one of the most critical elements in a radio circuit. Here,
we will look at typical antenna types and their applications.

Antenna Characteristics and Parameters

Some of the most commonly used terms describe antennas are impedance, gain, radiation pattern, take- offto angle, and polarization. Every antenna has an input impedance that represents the load to be applied to the transmitter. This impedance depends upon many factors such as antenna design, frequency of operation, and location of the antenna with respect to surrounding objects.

The basic challenge in radio communications is finding ways to get the
most power possible, where and when you need it, to generate and
transmit signals. Most transmitters are designed to provide maximum
output power and efficiency into a 50- ohm load. (Ohm is a unit of
measurement of resistance.) Some antennas, such as log periodic
antennas, can provide a 50- ohm load to the transmitter over a wide
range of frequencies. These antennas can generally be connected
directly to the transmitter. Other antennas, such as dipoles, whips, and
long- wire antennas, have impedances that vary widely with frequency
and the surrounding environment.

HF applications use an antenna tuner or coupler. This device is inserted
between the transmitter and antenna to modify the characteristics of
the load presented to the transmitter so that maximum power may be
transferred from the transmitter to the antenna.

For most VHF and UHF applications, the antennas have built- in broadband-
matching units so separate antenna coupler units are generally not required.

Antenna Gain and Radiation Pattern

The gain of an antenna is a measure of its directivity — its ability to focus the energy it radiates in a particular direction. The gain may be determined by comparing the level of signal received from it against the level that would be received from an isotropic antenna, which radiates equally in all directions. Gain can be expressed in dBi; the higher this number, the greater the directivity of the antenna. Transmitting antenna gain directly affects transmitter power requirements. If, for example, an omnidirectional antenna were replaced by a directional antenna with a gain of 10 dBi, a 100- watt transmitter would produce the same effective radiated power as a 1- kW transmitter and omnidirectional antenna.

In addition to gain, radio users must understand the radiation pattern of
an antenna for optimal signal transmission. Radiation pattern is deter-
mined by an antenna’s design and is strongly influenced by its location
with respect to the ground. It may also be affected by its proximity to
nearby objects such as buildings and trees. In most antennas, the pattern
is not uniform, but is characterized by lobes (areas of strong radiation) and nulls (areas of weak radiation). These patterns are generally represented graphically in terms of plots in the vertical and horizontal planes (Figure 3- 3), which show antenna gain as a function of elevation angle (vertical pattern) and azimuth angle (horizontal plot). The radiation patterns are frequency dependent, so plots at different frequencies are required to fully characterize the radiation pattern of an antenna.

In determining communications range, it is important to factor in the
take- off angle, which is the angle between the main lobe of an antenna
pattern and the horizontal plane of the transmitting antenna. For VHF
and UHF applications, low take- off angles are generally used for LOS
communications; high take- off angles are used for ground- to- air, close
air support.

The orientation of an antenna with respect to the ground determines its
polarization. Most VHF and UHF whips and center fed monopole antennas
are vertically polarized.

A vertically polarized antenna produces low take- off angles. The main
drawback of vertical whip antennas is their sensitivity to ground
conductivity and locally generated noise. Center fed monopoles avoid the
sensitivity to ground conductivity and are preferred for vehicular mounts.
Horizontally polarized antennas, such as a 1/ 2- wave dipole, have high
elevation angles. This type of antenna is particularly useful when the
transmitter is near a forest or jungle. This allows the radiation to get
above the trees rather than having them absorbed. Diffraction at the
treetops tends to bend the radiation down so that it follows the treetops.
For best results, the transmitting and receiving antennas should have
the same polarization.

VHF Antennas

There are a countless variety of antennas used in VHF communication.
We’ll focus here on some of the more common types. The vertical whip antenna is frequently used since it is omnidirectional and has low take- off angles. It is vertically polarized. A typical vertical whip
radiation pattern is shown in Figure 3- 4. A reflector, consisting of a second vertical whip, can add directivity to the radiation pattern of a whip.

Another useful type of antenna is the center fed 1/ 4 wave dipole, which
is basically two lengths of wire fed at the center (Figure 3- 5). This is a
horizontally polarized antenna and is frequently used for vehicular and
fixed station applications.

The radiation pattern can change dramatically as a function of its distance
above the ground. Figure 3- 6 shows the vertical radiation pattern of a
horizontal dipole for several values of its height (in terms of transmitting wavelength) above the ground.

An inverted vee (sometimes called a “drooping dipole”) produces a
combination of horizontal and vertical radiation with omnidirectional
coverage. See Figure 3- 7. For fixed station use on high elevations (high hills or mountain tops) a log periodic directional antenna can be used for very long LOS communications of 100 miles or more. See Figure 3.8

UHF and SATCOM Antennas

For most UHF manpack applications, the transceiver is mounted with a
short, stubby antenna that resembles a hot dog in shape. This antenna
is used for relatively short LOS distances and its virtue is its small size.

For vehicular or shelter mounted applications, an effective general- purpose UHF antenna is a center- fed dipole (Figure 3- 9a). This antenna looks like a thick whip antenna. It is constructed within a fiberglass tube and consists of a dipole mounted vertically within the tube along with its feed point. Its significant virtue is that it is relatively independent of the ground quality. It has a low take off angle, and it is vertically polarized. The center-fed dipole antenna has a pattern similar to the whip pattern shown in Figure 3- 4.

There are center- fed dipoles designed for VHF frequencies as well. The UHF Tactical Satellite (TACSAT) antenna has a unique inverted umbrella shape (Figure 3- 9b). It produces a directed beam that must be pointed directly at the satellite in order to be effective.

For fixed station use, an elevated whip or center- fed dipole greatly
increases the LOS range (Figure 3- 9c). This antenna assembly consists of
a mast and a vertical whip or dipole mounted above ground plane rods.
Again, this antenna structure can be used for both VHF and UHF applica-
tions with the proper selection of antenna and ground plane rod lengths.
Another popular UHF antenna used for fixed station use is the Biconical
Antenna shown in Figure 3- 9d. An antenna of this type has been designed
to cover the 100 to 400 MHz range. Its broadband capability makes it an
excellent choice for wide band Transmission Security (TRANSEC) modes such
as frequency hopping. Refer to Chapter 7 for a discussion of TRANSEC.
The Biconical Antenna is usually mounted on a mast similar to the one
shown in Figure 3- 9c.

SUMMARY

A radio system consists of a transceiver and an antenna group.

The transceiver provides both transmitting and receiving functions.

The transmit function consists of modulation, carrier generation
frequency translation, and power amplification.

The receive function consists of RF signal filtering, amplification,
frequency down conversion, and demodulation.

Antenna selection is critical to successful VHF, UHF, and TACSAT
communications. Antenna types include vertical whips, center- fed
dipoles, biconical antennas, directional log periodic arrays, and
umbrella TACSAT antennas.

An antenna coupler matches the impedance of the antenna to that
of the transmitter, transferring maximum power to the antenna.

The gain of an antenna is a measure of its directivity — its ability
to focus the energy it radiates in a particular direction.

Antenna radiation patterns are characterized by nulls (areas of
weak radiation) and lobes (areas of strong radiation).