Antennas
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Before we get into Antennas, it is important to have a clear understanding of some of
the terms associated with the subject of Antennas. These are-Standing Wave Ratio
(SWR) or Voltage SWR, Reflection coefficient and Return Loss.
Standing Wave Ratio or Voltage Standing Wave Ratio ( SWR or VSWR):
Standing Wave Ratio is a measure of how much energy is reflected back to the
source. A low SWR means less energy is reflected back and a high SWR means a lot
of energy is reflected back. Reflection occurs due to impedance mismatch. A
perfectly matched Antenna system will have an SWR =1 or 1:1 to indicate that it is a
Ratio. 1.5:1 is what industry aims for. There are several ways this Ratio is expressed.
First, let us define Reflection Co-efficient, 
 = Vr / Vi
(1)
Where Vr = reflected voltage
Vi = incident voltage, and
 = Reflection Co-efficient
It can be shown that  is also equal to = ZL – ZO / ZL + ZO
(2)
Where ZL = Load Impedance and ZO = Characteristic Impedance of the line
We can define the VSWR as follows:
VSWR = Vmax / Vmin, both expressed in r.m.s
Where Vmax = the maximum voltage on the line = Vi + Vr, and
Vmin = the Minimum voltage on the line = Vi - Vr
(3)
Using algebraic manipulations, it can be shown that
SWR = 1 +  / 1 - 
(4)
Substituting the value of  from Equation 2 into (4), we get
SWR = ZL /ZO
(5)
Also from Equation (4)
 = SWR – 1 / SWR + 1
Power Reflected = Pr = 2 Pi
Where Pi = incident power
(6)
(7)
Power absorbed by the load PL = Pi (1 - 2)
(8)
Power absorbed by the load in terms of SWR is given by the equation
PL = { 4SWR / (1 + SWR)2 }Pi
(9)
Hyder Khoja
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2/16/2016
Return Loss: is a measure of how much energy is returned or reflected back. Less the
energy is returned, better it is. Thus, larger the return loss better it is. Refer to the chart
and tables on P8.18, 8.19 C.S
Now do examples 8.2 and 8.3 P8.17, 8.18, 8.19 C.S and also examples 6.7,6.8 on P 186,
187 B.
Now do P87, 88, 89 90, 91 Course Pack and Chapter 5 C.S
Photocopy Antenna handouts (Pictures only Prthy)
When you want to convert from dBi to dBd,
Remember dBd = GdBi – 2.14
And
dBi = dBd +2.14
(10)
(11)
Also EIRP = Pt Gt, and
(12)
EIRP = ERP + 2.14
(13)
In this equation, EIRP is the Effective radiated Power with respect to an ISOTROPIC
RADIATOR and the ERP is the Effective Radiated Power of the Antenna at hand
Now do Example 8.4 P8.19 C.S
Also, Solve example 16-6, P260 E.W and 16-7 P262 E.W
Solve Example 8.1 P260 B, and 8.3, 8.4, 8.5, 8.6 P26-266 B, 8.7 P281and 8.10 P292,
8.11 P 301 B
Show Antennas on P304m 306, 308 B
Hyder Khoja
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2/16/2016