# Resolving Uncertainty in CISPR , MIL-STD

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Rod antenna in general (1)
Resolving Uncertainty in CISPR , MIL-STD Emissions Testing
by Reducing Limitations of Active Rod Antennas
Roberto Grego

A rod antenna is a particular case of
dipole antenna, where one monopole is
“hidden”.

First example of (transmitting) rod
antenna: the Marconi’s

The ground acts as a reflector,
like a mirror creates the appearance
of someone behind the glass.

The optimal rod length is of ¼ λ (x 0,95)
where λ = wavelength
EMC Sales & Marketing Manager
at Narda Safety Test Solutions S.r.l. - Italy
Rod antenna in general (2)

Rod antenna in general (3)
At ¼ λ current and voltage have
sinusoidal distribution along the rod
 Ideally, the ground plane should be infinite, forming the missing
half of the dipole.
 With a perfectly conductive, infinite ground plane the radiation
pattern would be identical to that of a a dipole, with its maximum
radiation in the horizontal direction, perpendicular to the antenna.
 Practical ground planes (counterpoises) may vary in size and
conformation according to the application, affecting the antenna

For λ < 1/8 current and voltage tends to
linear distribution along the rod
Rod antenna in EMC (2)
Rod antenna in EMC (1)
 In EMC the rod antenna is used as receiving antenna of radiated
emissions
 Standards define the operative parameters:
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Rod length (100 – 104 cm)
Counterpoise size (60x60 cm)
Frequency range
Field strength range (limits & detectors)
Grounding
Distance from EUT
Calibration method
Standard
Frequency range, MHz
Limits range, dBµV/m
CISPR/IEC/EN
0.15 ÷ 30
20 ÷ 86
MIL-STD
0.01 ÷ 30
24 ÷ 90
RTCA DO160
0.15 ÷ 25
35 ÷ 60
…..
 The rod length becomes very small in reference to the wavelength:
 30 MHz:
 10 kHz:
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λ = 10 m
λ = 30 km
The rod becomes like a probe in near field
Self-capacitance of the rod is dominant
The electric length is half of physical length, i.e. 0.5 m
The assumption of linearity up to 1/8 λ is matched
With unit = dBµV/m, the linearization to 1 m
Equivalent circuit:
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Drawbacks of Rod antenna in EMC (1)
Rod antenna in EMC (3)
 Theoretical Capacitance of 1 m length Ø 8 mm Rod Antenna above
an ideally infinite Ground Plane, according to CISPR 16-1-4
 Intrinsic drawbacks:
 Effect of size-limited counterpoise
 Impedance of the bonding to ground (if present)
 Impedance of the RF cable
 Capacitive effects of the environment (chamber, EUT )
 Antenna geometry variations due to the coaxial cable shielding acting as an
Drawbacks of Rod antenna in EMC (2)
Drawbacks of Rod antenna in EMC (3)
= RESONANCE

Z = cable shield impedance due to its intrinsic capacitance and inductance, plus parasitic
capacitances

Rod capacitance changes as a function of Z

Result is a voltage divider circuit, variable with frequency, cabling, grounding and
environment characteristics
Drawbacks of Rod antenna in EMC (4)
Drawbacks of Rod antenna in EMC (5)
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Simulation of the effect of simple structural changes to the rod self-capacitance
Simulation of the effect of simple structural changes to the rod self-capacitance
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Drawbacks of Rod antenna in EMC (6)
Drawbacks of Rod antenna in EMC (7)
Common countermeasures to reduce resonances
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“Shorten” the cable image by counterpoise grounding
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Increase the impedance to ground by ferrite beads around the cable
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Connect the counterpoise to the setup grounding, or leave it floating
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The low capacitance (10 ÷ 15 pF) makes the rod an high-impedance source

An active hi-Z (FET) preamplifier is required, followed by an impedance
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Saturation may occur, even unnoticed
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FET input is prone to be destroyed by electrostatic discharges
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Dynamic range and sensitivity are determined by the preamplifier

Measuring broadband, impulsive noise may be challenging
Actual overall uncertainty may vary respect to calculations
Experiences of alternative methods (1)
Experiences of alternative methods (2)
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Eliminating the cable effect by replacing it with an optical fiber
The sum of all the facts and circumstances as described is that:

Emission measurements with the rod antenna may be affected by unpredictable sources of
uncertainty
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Various methods to mitigate these sources have been defined but none seems to be the ideal
solution
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Standards
Sta
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Blue line: measurement with
effectiveness in reducing
uncertainty.
Alternative methods have been experienced:

Eliminating the cable effect by replacing it with an optical fiber

Embedding a full receiving unit into the rod antenna
Reference:
Experiences of alternative methods (4)
Experiences of alternative methods (3)
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Replace the cable with an optical fiber to avoid its effect
AND
Improve the performances by embedding a receiving unit
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Harry W. Gaul (2013) – “Electromagnetic Modeling and Measurements of the 104 cm Rod and Biconical
Antenna for Radiated Emissions Testing Below 30 MHz” – 978-1-47990409-9/13/[email protected] IEEE

Calibration by the Antenna Substitution Method
Auto-ranging input attenuator
Frequency preselection
Digital F/O link free of thermal drifts, saturation, analog noise
Possibility of auto-calibration
Pulse response according to CISPR 16-1-1
Reference: A. Gandolfo, R. Azaro, D. Festa “Innovative
Field Receiver based on a New Type of Active Rod
Antenna”, IEEE EMC&SI Symposium – Santa Clara, CA –
March 2015
Reference: A. Gandolfo, R. Azaro, D. Festa “Innovative
Field Receiver based on a New Type of Active Rod
Antenna”, IEEE EMC&SI Symposium – Santa Clara, CA –
March 2015
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Conclusion
Intrinsic
drawbacks
Cable+
grounding
effects
Preamp+
calibration
Thanks for attending!
Uncertainty improvement
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November 12, 2015
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