EMISSION CONTROL (EMCON)
When EMCON is imposed, RF emissions must not exceed -110 dBm/meter
^{2} at one nautical mile.
It is best if systems meet EMCON when in either the Standby or Receive mode versus just the Standby mode (or OFF).
If one assumes antenna gain equals line loss, then emissions measured at the port of a system must not exceed -34
dBm (i.e. the stated requirement at one nautical mile is converted to a measurement at the antenna of a point
source - see Figure 1). If antenna gain is greater than line loss (i.e. gain 6 dB, line loss 3 dB), then the -34
dBm value would be lowered by the difference and would be -37 dBm for the example. The opposite would be true if
antenna gain is less.
Figure 1. EMCON Field Intensity / Power Density Measurements
To compute the strength of emissions at the antenna port in Figure 1, we use the power density equation
(see Section 4-2)
[1] or rearranging P
_{t}G
_{t}
= PD (4
πR
^{2}) [2]
Given that P
_{D} = -110 dBm/m
^{2}
= (10)
^{-11} mW/m
^{2} , and R = 1 NM = 1852 meters.
P
_{t}G
_{t}
= P
_{D}(4
πR
^{2}) = (10 mW/m
^{2})(4
π)(1852m)
^{2}
= 4.31(10)
^{-4} mW = -33.65 = -34 dBm at the RF system antenna as given.
or, the equation can be
rewritten in Log form and each term multiplied by 10:
10log P
_{t} + 10log G
_{t} = 10log P
_{D}
+ 10log (4
πR
^{2}) [3]
Since the m
^{2}
terms on the right side of equation [3] cancel, then:
10log P
_{t} + 10log G
_{t} = -110 dBm
+ 76.35 dB = -33.65 dBm = -34 dBm as given in Figure 1.
If MIL-STD-461B/C RE02 (or MIL-STD-461D RE-102)
measurements (see Figure 2) are made onseam/connector leakage of a system, emissions below 70 dBμV/meter which are
measured at one meter will meet the EMCON requirement. Note that the airframe provides attenuation so portions of
systems mounted inside an aircraft that measure 90 dBμV/meter will still meet EMCON if the airframe provides 20 dB
of shielding (note that the requirement at one nm is converted to what would be measured at one meter from a point
source).
The narrowband emission limit shown in Figure 2 for RE02/RE102 primarily reflect special concern
for local oscillator leakage during EMCON as opposed to switching transients which would apply more to the
broadband limit.
Figure 2. MIL-STD-461 Narrowband Radiated Emissions Limits
Note that in MIL-STD-461D, the narrowband radiated emissions limits were retitled RE-102 from the previous
RE-02 and the upper frequency limit was raised from 10 GHz to 18 GHz. The majority of this section will continue
to reference RE02 since most systems in use today were built to MIL-STD-461B/C.
For the other calculation
involving leakage (to obtain 70 dBμV/m) we again start with:
and use the previous
fact that: 10log (P
_{t}G
_{t}) = -33.6 dBm = 4.37x10
^{-4} mW (see Section 2-4).
The
measurement is at one meter so R
^{2} = 1 m
^{2}we have:
= .348x10
^{-4} mW/m
^{2} = -44.6 dBm/m
^{2} = P
_{D} @ 1 meter
Using the field
intensity and power density relations (see Section 4-1)
Changing to
microvolts (1V = 10
^{6} μV) and converting to logs we have:
20 log (E) = 20 log (10
^{6} x
36.2x10
^{-4}) = 20 log (.362x10
^{4}) = 71.18 dBμV/m = 70 dBμV/m as given in Figure 1.
Some words of CautionA common error is to only use the one-way free space loss
coefficient
α_{1} directly from Figure 6, Section 4-3 to calculate what the
output power would be to achieve the EMCON limits at 1 NM. This is incorrect since the last term on the right of
equation [3] (10 Log(4
πR
^{2})) is simply the Log of the surface area of a
sphere - it is
NOT the one-way free space loss factor
α_{1}.
You cannot interchange power (watts or dBW) with power density (watts/m
^{2} or dBW/m
^{2}).
The equation uses power density (P
_{D}), NOT received power (P
_{r}). It is independent of RF and
therefore varies only with range. If the source is a transmitter and/or antenna, then the power-gain product (or
EIRP) is easily measured and it's readily apparent if 10log (P
_{t}G
_{t}) is less than -34 dBm. If
the output of the measurement system is connected to a power meter in place of the system transmission line and
antenna, the -34 dBm value must be adjusted. The measurement on the power meter (dBm) minus line loss (dB) plus
antenna gain (dB) must not be higher than -34 dBm.
However, many sources of radiation are through leakage,
or are otherwise inaccessible to direct measurement and PD must be measured with an antenna and a receiver. The
measurements must be made at some RF(s), and received signal strength is a function of the antenna used therefore
measurements must be scaled with an appropriate correction factor to obtain correct power density.
RE-02 Measurements
When RE-02 measurements are made, several different antennas are chosen dependent upon the frequency range
under consideration. The voltage measured at the output terminals of an antenna is not the actual field intensity
due to actual antenna gain, aperture characteristics, and loading effects. To account for this difference, the
antenna factor is defined as:
AF = E/V [4]
where E = Unknown electric field to be determined in V/m ( or μV/m)
V = Voltage measured at the
output terminals of the measuring antenna
For an antenna loaded by a 50 Ω line (receiver), the theoretical
antenna factor is developed as follows:
From Section 4-3 we see that A
_{e} = G
_{r}λ
^{2} /4
π, and
from Section 4-1, E
^{2} = 377 P
_{D} therefore we have:
Reducing this to decibel form we have:
20 log AF = 20logE - 20logV =
with λ in meters and Gain
numeric ratio (not dB) This equation is plotted in Figure 3.
Since all of the equations in
this section were developed using far field antenna theory, use only the indicated region.
Figure 3. Antenna Factor vs Frequency for Indicated Antenna Gain
In practice the electric field is measured by attaching a field intensity meter or spectrum analyzer with
a narrow bandpass preselector filter to the measuring antenna, recording the actual reading in volts and applying
the antenna factor.
20log E = 20log V + 20log AF [7]
Each of the antennas used for EMI measurements normally has a calibration sheet for both gain and antenna
factor over the frequency range that the antenna is expected to be used. Typical values are presented in Table 1.
Table 1. Typical Antenna Factor Values
The antenna factor can also be developed in terms of the receiving antenna's effective area. This can
be shown as follows:
Or in log form:
20logAF = 20logE - 20logV =
While this relation holds for any antenna, many antennas (spiral, dipole, conical etc.) which do not have a true
"frontal capture area" do not have a linear or logarithmic relation between area and gain and in that respect the
parabolic dish is unique in that the antenna factor does not vary with frequency, only with effective capture
area. Consequently a larger effective area results in a smaller antenna factor.
A calibrated antenna would
be the first choice for making measurements, followed by use of a parabolic dish or "standard gain" horn. A
standard gain horn is one which was designed such that it closely follows the rules of thumb regarding area/gain
and has a constant antenna factor. If a calibrated antenna, parabolic dish, or "standard horn" is not available, a
good procedure is to utilize a flat spiral antenna (such as the AN/ALR-67 high band antennas). These antennas
typically have an average gain of 0 dB (typically -4 to +4 dB), consequently the antenna factor would not vary a
lot and any error would be small.
EXAMPLE:Suppose that we want to make
a very general estimation regarding the ability of a system to meet EMCON requirements. We choose to use a spiral
antenna for measurements and take one of our samples at 4 GHz. Since we know the gain of the spiral is relatively
flat at 4 GHz and has a gain value of approximately one (0 dB) in that frequency range. The antenna is connected
to a spectrum analyzer by 25 feet of RG9 cable. We want to take our measurements at 2 meters from the system so
our setup is shown below:
Our RG9 cable has an input impedance of 50Ω, and a loss of 5 dB (from Figure 5, Section 6-1).
First, let's assume that we measure -85 dBm at the spectrum analyzer and we want to translate this into the
equivalent strength at 1 NM. Our power received by the antenna is: P
_{r} = -85 dBm + 5 dB line loss = -80
dBm
also P
_{D} = P
_{r}/A
_{e} and A
_{e} = Gλ
^{2}/4
π
= (G/4
π)·(c/f)
^{2} = (1/4
π)·(3x10
^{8}/4x10
^{9})
= 4.47x10
^{-4} m
^{2}in log form: 10 Log P
_{D} = 10 Log P
_{r} - 10 Log A
_{e}
= -80 dBm + 33.5 = -46.5 dBm/m
^{2} at our 2 meter measuring point
To convert this to a value at 1
NM, we use
P
_{t}G
_{t} = P
_{D@1 nm} 4
πR
_{1}^{2}
= P
_{D@2 m} 4
πR
^{2} and we solve for P
_{D@1 nm}in log
form after cancelling the 4B terms:
10 Log P
_{D@1 nm} = 10 Log P
_{D@2 m} + 10 Log (R
_{2m}/R
_{1nm})
= -46.5 dBm/m
^{2} - 59.3 dB = -105.8 dBm/m
^{2} which is more power than the maximum value of -110
dBm/m
^{2} specified.
If we are making repetitive measurement as we might do when screening an
aircraft on the flight line with numerous systems installed, or when we want to improve (reduce) the leakage on a
single system by changing antennas, lines, connectors, or EMI gaskets or shielding, this mathematical approach
would be unnecessarily time consuming since it would have to be repeated after each measurement. A better approach
would be to convert the -110 dBm/m2 value at 1 NM to the maximum you can have at the measuring instrument (in this
case a spectrum analyzer), then you could make multiple measurements and know immediately how your system(s) are
doing. It should be noted that -90 to -100 dBm is about the minimum signal level that can be detected by a
spectrum analyzer, so you couldn't take measurements much further away unless you used an antenna with a much
higher gain.
In order not to exceed EMCON, the power density must not exceed -110 dBm/m
^{2} at 1
NM, which is
10
^{-11} mW/m
^{2}.
P
_{t}G
_{t} = P
_{D@1 nm} 4
πR
_{1}^{2} = P
_{D@2
m} 4
πR
_{2}^{2} and we solve for P
_{D@1 nm}
we solve for P
_{D@2 m} = 10
^{-11} (1852m)
^{2}/(2m)
^{2} = 8.57 x 10 mW/m
^{2}
= -50.7 dBm/m
^{2}We'll be using a spectrum analyzer, so we want to compute what the maximum power
or voltage may be.
Method 1 - Using the Power Density ApproachUsing
logs/dB and the values of P
_{D@2 m} and A
_{e} determined previously:
10 Log P
_{r} = 10
Log P
_{D}
+ 10 Log A
_{e} = -50.7 - 33.5 = -84.2 dBm
taking line loss into account we have: -84.2 - 5 dB = - 89.2
dBm as the maximum measurement reading.
If we wanted to calculate it in volts, and take into account our
line impedance we would have the following:
P
_{r} = P
_{D}A
_{e} = V
^{2}/R =
V
^{2}/50Ω also A
_{e} = Gλ
^{2}/4
π so solving for V we have:
since our line loss is 5 dB, we have -5dB = 20 Log V
_{2}/V
_{1}. Solving for V
_{2}
we get 7.79x10
^{-6} volts or -89 dBm as a maximum at our measurement device input. We can see immediately
that our value of -85 dBm that we measured on the previous page would not meet specifications, and neither would
any signal with more power than -89 dBm.
Method 2 - Using the Antenna Factor
Approach
Starting with the same value of power density that we obtained above (8.57x10
^{-9} W/m
^{2}),
we find the field intensity from Table 1, Section 4-1 to be approximately 65 dBμv/m. Also from Figure 3 in this
section, AF = 43 dB @ 4 GHz. (by calculating with equation [6], the exact value is 42.3 dB)
From equation
[6]:
20log V = 20log E - 20log AF
20log V = 65 - 43 = 22 dBμv/m.
Since dBμv/m = 20 log (V)(10
^{6})
= 20 log V + 20 log 10
^{6} = 20 log V + 120 , we see that to get an answer in dBv we must subtract 120
from the dBμv/m value so: V
_{dB} = 22 - 120 = -98 dBv. We then subtract our line loss (-5dB) and we have:
V = -98 - 5 = -103 dBv = 17 dBμv/m = 7.1x10
^{-6} volts
using the fact that P = V
^{2}/R and
for the input line R = 50Ω, P = 1x10
^{-12} W = -120 dBW = -90 dBm
Although this method is just as accurate as that obtained using method 1, the values obtained in Table 1,
Section 4-1, and Figure 3 must be interpolated, and may not result in values which are as precise as the
appropriate formulas would produce.
Sample Problem: What is the approximate
transmit power from a receiver?
A. 1 nanowatt (nW) F. 100 μW K. 10 W
B. 10 nW G. 1 milliwatt
(mW) L. 100 W
C. 100 nW H. 10 mW M. 1 kilowatt (kW)
D. 1
microwatt (μW) I. 100 mW N. 10 kW
E. 10 μW J. 1 watt
(W) O. 100 kW
The question may seem inappropriate since a receiver is supposedly a passive
device which only receives a signal. If the receiver was a crystal video receiver as shown in Section 5-3, it
wouldn't transmit power unless a built-in-test (BIT) signal was injected after the antenna to periodically check
the integrity of the microwave path and components. The potential exists for the BIT signal to leak across
switches and couple back through the input path and be transmitted by the receiver's antennas.
If the
receiver uses a local oscillator (LO) and a mixer to translate the signal to an intermediate frequency (IF) for
processing (such as a superhet shown in Section 5-3), there is the potential for the CW LO signal to couple back
through the signal input path and be transmitted by the receiver's antenna. Normally a mixer has 20 dB of
rejection for the reverse direction. In addition, the LO may be further attenuated by receiver front end filters.
In both cases, the use of isolators described in Section 6-7 could be used to further attenuate any signals going
in the reverse direction, i.e. back to the antenna. A good receiver design should ensure that any RF leakage
radiated by the receiver will not exceed the EMCON level.
In answer to the initial question, "transmit" leakage power should be less than -34 dBm (0.4 μW) to meet
EMCON. Therefore, the real answer may be "A", "B", or "C" if EMCON is met and could be "D" through possibly "G" if
EMCON is not met.
Table of Contents
for Electronics Warfare and Radar Engineering Handbook
Introduction |
Abbreviations | Decibel | Duty
Cycle | Doppler Shift | Radar Horizon / Line
of Sight | Propagation Time / Resolution | Modulation
| Transforms / Wavelets | Antenna Introduction
/ Basics | Polarization | Radiation Patterns |
Frequency / Phase Effects of Antennas |
Antenna Near Field | Radiation Hazards |
Power Density | One-Way Radar Equation / RF Propagation
| Two-Way Radar Equation (Monostatic) |
Alternate Two-Way Radar Equation |
Two-Way Radar Equation (Bistatic) |
Jamming to Signal (J/S) Ratio - Constant Power [Saturated] Jamming
| Support Jamming | Radar Cross Section (RCS) |
Emission Control (EMCON) | RF Atmospheric
Absorption / Ducting | Receiver Sensitivity / Noise |
Receiver Types and Characteristics |
General Radar Display Types |
IFF - Identification - Friend or Foe | Receiver
Tests | Signal Sorting Methods and Direction Finding |
Voltage Standing Wave Ratio (VSWR) / Reflection Coefficient / Return
Loss / Mismatch Loss | Microwave Coaxial Connectors |
Power Dividers/Combiner and Directional Couplers |
Attenuators / Filters / DC Blocks |
Terminations / Dummy Loads | Circulators
and Diplexers | Mixers and Frequency Discriminators |
Detectors | Microwave Measurements |
Microwave Waveguides and Coaxial Cable |
Electro-Optics | Laser Safety |
Mach Number and Airspeed vs. Altitude Mach Number |
EMP/ Aircraft Dimensions | Data Busses | RS-232 Interface
| RS-422 Balanced Voltage Interface | RS-485 Interface |
IEEE-488 Interface Bus (HP-IB/GP-IB) | MIL-STD-1553 &
1773 Data Bus |
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