Page 400 - Complete Wireless Design

Communications System Design

Communications System Design 399

9.3.4 Link budget issues
Fresnel zone clearance must be considered when setting up a microwave link
across the earth. This is because optical line of sight between the transmitting
and the receiving antenna is only one factor in most microwave links, the oth-
er being the first Fresnel zone clearance.
The Fresnel zone is referred to as the radio line of sight, and will need more
clearance that the optical line-of-sight since a radio wave, when it passes near
an object (such as a building or a mountain), will become defracted or bent,
causing the radio wave to become degraded, even if the obstruction is many
feet below (or to the side) of the radio wave.
This is why we must confirm mathematically, and not optically, that we have
sufficient Fresnel zone clearance above any mountain or building to avoid
attenuation of the transmitted signal at the receiving station. The following
formula can be used in conjunction with Fig. 9.6 to verify this:

d d
2
1
h 72.1
f (d d
)
2
1
where h clearance between the top of any obstruction and the direct line
of sight that is required for zero attenuation to the transmitted
signal, feet
d distance between the transmitter and the obstruction, miles.
1
d distance between the obstruction and the receiver, miles.
2
f frequency of the transmitted signal, GHz
If the answer is required in meters:
d d
2
1
h 17.3
f (d d
)
1
2
where the terms are as defined above except that h is in meters and d and d
1 2
are in kilometers.
We could ignore the Fresnel zone clearance entirely but, depending on the
frequency and the geometry of the obstruction and whether the obstruction is
close to the optical-line-of-sight, the additional path losses could be anywhere
between 6 and 20 dB.
As we have discovered, the noise floor of a receiver system is a critical issue.
To calculate this, including the antenna’s NF as well as the receiver’s NF, we
can use the formula below. This formula works quite accurately for terrestrial
total receiver system NF calculations, but assumes a receiving antenna’s noise
temperature to be 290 K, which is a reliable antenna temperature estimate for
all antennas in an earthbound link environment. And since the formula cal-
culates the signal strength, in dBm, required at the input of the receiving
antenna for the receiver to output at a 0-dB SNR, we would have to confirm
that the transmitter on the other end of the link has the power to permit us,

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