Antenna Standards in the
Frequency Range 30 MHz to 18 GHz
Beat Mühlemann, Frédéric Pythoud
Swiss Federal Office of Metrology and Accreditation, Bern-Wabern, Switzerland
Agenda
Š 1. Introduction
Š 2. Definitions and principles
Š 3. Experimental methods
Š 4. Summary
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1.1 Motivation and objectives
Š Wide variety of methods to define the antenna factor, and thus
to define the electric field:
y Dipole as reference up to 1 GHz (The antenna factor can be computed
numerically)
y Standard Gain horn (The gain can be computed numerically)
y Ridged horn (simulation techniques are required to determine the
antenna factor)
y Measurements with 3 antenna method
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1.2 Drawbacks
Š Dipole as reference up to 1 GHz :
y calibration of the baluns has to be performed in addition to the simulation
Š Standard Gain horn:
y Formulas accuracy not sufficient
y connector quality not taken into account
Š Ridged horn
y Formulas accuracy not sufficient
y connector quality not taken into account
Š Measurements with 3 antenna method
y High accuracy needed
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2.1 Definitions and traceability
Š The antenna factor AF has the
dimension (1/m). This quantity is
derived from the meter.
Š The antenna factor of a
receive antenna is the
ratio of E/U, where U
is the voltage measured
at a conventionally accepted
value of 50 Ω.
Electromagnetic
wave
Antenna
Voltage U
E (V / m )
AF (1 / m ) =
U (V )
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2.2 Principles
Š The insertion loss is given as:
U1 c ⋅ d ⋅ Z L
AF1 ⋅ AF2
=
U2
f ⋅η
V
1
V
2
d
Š This equation is true under the following assumptions:
y U1 is the forward voltage (ref. 50 Ω) measured at the input port of antenna 1
y U2 is the voltage (ref. 50 Ω) measured at the input port of antenna 2
y Free field propagation (no field reflections)
y Far field
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2.3 In other words
Š Antenna factor is well defined (up
to 18 GHz) by the following
experiment (using 3 antenna
methods)
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3.1 Experimental Methods
U1 is the forward voltage (ref. 50 Ω)
measured at input port of antenna 1
Measure S21 with a calibrated VNA
(full 2 port)
U2 is the voltage (ref. 50 Ω)
measured at input port of antenna 2
Measure S21 with a calibrated VNA
(full 2 port)
Free field propagation (no field
reflections)
METAS averaging procedure
Far field
Either large distances or take into
account in uncertainty evaluation.
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3.2 METAS approach
Site attenuation
direct
indirect, floor
indirect,ceiling
+ S 21
+ S 21
S 21 = S 21
indirect , frontwall
indirect ,backwall
+ S 21
+ S 21
Principle: move both antennas in order to:
•
Keep the distance constant
•
Modify the reflected path length of a half wavelength:
indirect
indirect
( position2 ) = − S 21
( position1 )
S 21
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3.3 Motion details
Š Longitudinal motion:
λ
dl =
4
Š Vertical motion
λ
2
dh =
1 + (D / H )
4
Š Side motion
dw =
H
Position 2
Position 1
D
λ
2
1 + (D / W )
8
Š (see EMC Zurich 2005)
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3.4 Comment on Gain and Antenna Factor
Š Isotropic Gain is defined as:
wmax
gi =
wiso
same . radiated . power
4πwmax
=
Wtotalradiated
Š Antenna factor and Gain definitions are very different and refer
to totally different quantities (E-Field + Voltage / Radiated
Power Density).
Š In the case where the Antenna impedance is 50 Ohms (Real
only without any imaginary value), then:
AF =
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9.73
λ g
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4. Summary
Š Antenna Factor is very important since it provides the
reference for E-Field (under the assumption that the Volt is
properly defined)
Š There is no need for primary standard in the range 30 MHz to
18 GHz. The antenna factor is derived from the meter.
Š Experimental determination of the antennas are improving
and thus, there is no need of complex simulations to
determine the antenna factor of geometrically defined
structures.
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Thank you
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