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5/1/2015
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
properties, particularly the radiation pattern.

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
needs adding 6 dB to the output readings
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
additional element to the counterpoise
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

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
adapting circuit to match the 50 Ω load

Saturation may occur, even unnoticed

FET input is prone to be destroyed by electrostatic discharges

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

Various methods to mitigate these sources have been defined but none seems to be the ideal
solution

Standards
Sta
da ds d
diverge
e ge in indicating
d cat g how
o to use tthe
e rod
od a
antenna
te a
Blue line: measurement with
optical link evidences the
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/$31.00@2013 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
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Uncertainty improvement
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