Electronic Warfare and Radar Systems Engineering Handbook - Emission Control (EMCON) -

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 (4πR^{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}(4πR^{2}) = (10 mW/m^{2})(4π)(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:

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.

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 Caution

A 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(4πR^{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} /4π, 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}/4π = (G/4π)·(c/f)^{2} = (1/4π)·(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} 4πR_{1}^{2} = P_{D@2 m} 4πR^{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} 4πR_{1}^{2} = P_{D@2 m} 4πR_{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 Approach

Using 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}/4π 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:

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.

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