Course: BEng (Hons) in Electrical and Electronic Engineering

1. EEEE 6490345 RF AND MICROWAVE ELECTRONICS
Terrestrial Microwave Links
FACULTY OF ENGINEERING AND COMPUTER TECHNOLOGY
BENG (HONS) IN ELECTRICALAND ELECTRONIC ENGINEERING
Ravandran Muttiah BEng (Hons) MSc MIET

2. Terrestrial Microwave Link
Many wideband point-to-point radio communications links employ
microwave carriers in the 1 to 20 GHz frequency range.
Antennas are located on high ground to avoid large buildings or hills and
repeaters are used every 40 to 50 km to compensate for path loss.
1

3. 2
The following points can be made about these links:
(1) Microwave energy does not follow the curvature of the earth in the
way that Medium Wave (MW) and Long Wave (LW) transmissions
do. Microwave transmissions are thus restricted to Line-Of-Sight
(LOS).
(2) Microwave transmissions are particularly well suited to point-to-
point communications using narrow beam, high gain and antennas.
(3) At about 1 GHz circuit design techniques change from using lumped
to distributed elements. Above 20 GHz it becomes expensive to
generate big amounts of microwave power.
The bands in current use are near 2 GHz, 4 GHz, 6 GHz, 11 GHz and 18
GHz.
On a 6 GHz link with a hop distance of 40 km the free space path loss is
140 dB. With transmit and receive antenna gains of 40 dB, however, the
transmission loss reduces to 60 dB.

4. 3
Microwave links were widely installed during the 1960s for analogue
Frequency Division Multiplexing (FDM) telephony. In these systems
each allocated frequency band is subdivided into 30 MHz radio channels.
Figure 1 shows how the 500 MHz band is divided into 16 separate
channels on 29.65 MHz centre spacings. Adjacent radio channels use
orthogonal antenna polarisations, Horizontal (H) and Vertical (V), to
reduce crosstalk.
The 500 MHz bandwidth microwave link can accommodate traffic
simultaneously in all sixteen 30 MHz channels.
Analogue Systems

5. 4
Figure 1: Splitting of a microwave frequency allocation into radio channels
250 MHz 250 MHz
252 MHz
248.9 MHz 248.9 MHz
29.65 MHz 44.5 MHz
3.1 MHz
𝑓1 𝑓3 𝑓5 𝑓7 𝑓2
′
𝑓4
′ 𝑓6
′ 𝑓8
′
𝑓2 𝑓4 𝑓6 𝑓8 𝑓1
′
𝑓3
′
𝑓5
′ 𝑓7
′
Transmit (receive)
Low block
Receive (transmit)
High block
H (or V)
V (or H)

6. 5
Superheterodyne Receiver
In the superhetorodyne receiver a microwave channel filter, centred on the
appropriate radio channel, extracts the 30 MHz channel from the
transmissions. The signal is mixed down to an Intermediate Frequency
(IF) for additional filtering and amplification.
Figure 2: Extraction of a single radio channel in a microwave repeater
Antenna
Circulator
Other Channels
Channel Filter
Isolator Mixer
Preamp
Local
Oscillator
IF Amplifier

7. 6
Digital microwave Public Switching Telephone Network (PSTN) links
operate at 140 Mbit/s on a carrier frequency of 11 GHz using Quadrature
Phase Shift keying (QPSK) modulation.
The 64 Quadrature Amplitude Modulation (QAM) system offers practical
spectral efficiency of 4 to 5 bit/s/Hz implying that the 30 MHz channel can
support a 140 Mbit/s of multiplexed telephone traffic signal.
Digital Systems

8. 7
Figure 3 is a schematic of a digital regenerative repeater, assuming
Differential Phase Shift Keying (DPSK) modulation, for a single 30 MHz
channel.
Here the circulator and channel filter access the part of the microwave
spectrum where the signal is located.
Digital systems have a major advantage over analogue systems as they
operate at a much lower Carrier-to-Noise Ratio (CNR).

9. 8
Figure 3: Digital DPSK regenerative repeater for a single 30 MHz radio channel
DPSK
Demodulator
Data
Regenerator
Clock
Timing
Recovery
Drive
Interface
Phase
Modulator
AGC
Preamp
IF Amplifier
Local
Oscillator
𝑓1
RF Source
𝑓1
′
CirculatorCirculator
Baseband
Interface