use of the 41” rod antenna in electromagnetic interference

Use of the 41” Rod Antenna in Electromagnetic Interference Testing; A Brief Chronology EMC Compliance, February 2010

The purpose of this monograph is to describe the use of the 41” rod antenna in electromagnetic interference testing from its first appearance sixty years ago, through the present day (2010). While the rod itself hasn’t changed, its use, matching networks and test configuration have changed over the years, and the whys and wherefores are discussed herein.

Historical Development – The Beginning Use of the 41” rod antenna in EMI test work dates back to 1953, with the release of MIL-I-6181B, ” Interference Limits, Tests and Design Requirements, Aircraft Electrical And Electronic Equipment.” That specification, and all further revisions through MIL-I-6181D, “Interference Control Requirements, Aircraft Equipment” in 1959, placed the rod antenna at a 12” distance from the test sample. The purpose of the 12” placement, per NADC-EL-5515, “Final Report, Evaluation of Radio interference Pickup Devices and Explanation of the Methods and Limits of MIL-I-6181B,” dated 10 August 1955, was to simulate the pick up of open wire antenna lead-ins in common use during WWII and the decade following.

Communication radio connections to antennas at this time used unshielded wiring, as shown in the following picture. The antenna lead is the white wire emanating from the lower right hand side of the BC-348Q radio. The copper wire below it is the ground reference; it was normally taken to aircraft structure as close as possible to the radio installation.

BC-348Q WWII era radio covering 0.2 – 0.5 MHz and 1.5 – 18 MHz in six bands.

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The input impedance of the mixer tube was very high, and it was necessary to separate the antenna lead wire from aircraft structure to limit capacitive loading. That is the function of the porcelain standoff seen just to the right of the radio. The high impedance wire was very susceptible to capacitive crosstalk, and pains were taken to keep it separate from other wiring. Notice the separation of the wiring along the top of the picture suspended on the porcelain standoff from other cable assemblies. A BC-348Q radio may be seen on the second tray from the top of the rack in the center of the picture.

Radio room of WWII-era B-26 bomber

William Jarva, the author of the NADC-EL-5515 report cited above, realized that the main radiated emission problem was due to the unshielded antenna lead-in inside the fuselage being just as sensitive as the actual long wire antenna that ran from the top of the vertical stabilizer to some point near the nose of the aircraft. The long-term fix for the resultant rfi was to eliminate the unshielded antenna lead-in from future procurements. MIL-I-6181B banned such procurements. But there was a very large inventory of such radios, and the aircraft that had them installed, and so MIL-I-6181B still had to protect those installations. Mr. Jarva picked the 41” rod antenna as provided with the AN/PRM-1 meter (new at the time) as a reasonable simulation

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of the unshielded antenna lead-in at frequencies below 20 MHz. Above 20 MHz, a tunable dipole was used.

NADC-EL-5515 describes a measurement made by Mr. Jarva to develop a radiated emission limit to protect the BC-348Q radio installation. The following picture is a re-enactment of his set-up.

NADC-EL-5515 re-enactment test set-up.

In the above picture are seen, from left to right, a BC-348 Q radio, an impulse generator, an AN/PRM-1 EMI meter, and its ac power supply. The antenna lead in wire is just visible emanating from the BC-348Q, and it runs up the vertical 2x4 stud just adjacent to the radio, and behind it. The AN/PRM-1 41” rod antenna and the BC-348Q antenna lead-in vertical section are both equidistant from the rod connected to the impulse generator. With the impulsive source operating, the BC-348Q was tuned to frequencies where interference was audible, and the AN/PRM-1 was tuned to the same frequency and recordings were made of the rf potential measured by the AN/PRM-1 at each frequency of interest. When that was completed, the data looked like the figure below. While the numbers are clearly as obsolete as the unshielded antenna lead-in wire, what is notable about the limit is the units. They are units of rf potential, not field intensity. The 41” rod antenna was an integral part of the AN/PRM-1 antenna, and the quantity of interest was not the field intensity illuminating an antenna, but the rf potential

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coupled capacitively to a wire. A limit such as this, based on the rf potential at the antenna terminal of a specified type antenna is termed an “antenna-induced limit.”

101.1

0

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FREQUENCY (MC)

MIC

RO

VO

LTS

PE

R K

C

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O O

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O SIGNAL INDUCED IN A 41 - INCH ROD BY SOURCES WHICH EMIT SIGNALS EQUAL TO THE THRESHOLD OF A TYPICAL AIRCRAFT RECEIVER SET-UP A – MIL-I-6181B STANDARD LIMITS FOR A 41 - INCH ROD ANTENNA

_ _ _ _ BACKGROUND LEVEL OF THE AN/PRM-1

A

Rod antenna limit data from NADC-EL-5515

The following excerpt from NADC-EL-5515 explains the physics of the situation in Mr. Jarva’s own words:

“ANTENNA SYSTEMS FOR RADIO INTERFERENCE MEASUREMENTS

In the frequency range 0.15 to 20 mc, radiating elements, pick-up antennas and distances, generally used for radiated radio interference measurements, are small compared to wavelength. The amount of energy transferred from field to antenna depends on the nature of the signal source and the type of receiving antenna used. For instance, if the radiating interference source is a single, small closed loop of wire, a great deal of current can flow without developing much voltage across the loop. Consequently, a large magnetic component is developed in the induction field in conjunction with a comparatively small electric component. To extract a large amount of energy from such a field, a similar loop antenna, correctly matched to a receiver, should be used as the pick-up device to provide what may be compared to a good impedance match in ordinary circuit theory. If a short rod antenna, sensitive to the electric component of the field, were used as the pick-up device very little energy transfer would result and a situation comparable to a condition of impedance mismatch would exist. When a short rod antenna is the signal source, a large voltage can be developed on the rod, but with very little current flow. Consequently, the field developed is composed of a large electric component and a small magnetic component. In this case, another rod used as a pick-up device would indicate the presence of an intense field, whereas, a loop antenna would give very little indication. Typical radio interference sources in aircraft include the extreme cases described and all other variations. In general, the ratio of the electric to the magnetic components surrounding an unshielded lead will vary directly as the impedance of the load terminating the lead, and the apparent impedance presented to the various pick-up antennas will vary in the

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same manner. This statement applies to radial and tangential field components as contrasted with the more usual concept of wave impedance encountered in shielding theory, which applies only to the components tangential to the line of propagation.

Although it would be desirable to require the measurement of both the electric and magnetic components of the interference field, it is felt at the present time that such requirements would make specification testing excessively complex. Experience has indicated that aircraft electronic equipments, which operate in the lower frequency ranges (0.15 to 20 mc), are more sensitive to the electric field because of the unshielded high impedance antenna lead-in, which has been in general use. Present practice is to control the electric field by radio interference measurements. This is done by utilizing a 41-inch rod antenna and treating any difficulties arising from equipments generating strong magnetic fields as special cases which require particular attention when the equipment is installed in the plane. Reference (e) requires that all equipment used with antennas be designed for use with a shielded antenna lead. If and when the unshielded antenna lead is completely eliminated from use in aircraft, a review of present methods and limits in the frequency range 0.15 to 20 mc will be required. Radio interference meters using the 41-inch rod antenna are so constructed and calibrated that they read directly the microvolts which are induced in the antenna by the interference field.”

Note: The reference (e) cited is MIL-I-6181B.

With that background as to how the 41” rod antenna came to be used for EMI testing, we progress to its implementation in the EMI test chamber.

Rod antenna use diagram from MIL-I-6181

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Recreation of MIL-I-6181 radiated emission test set-up, with rod antenna

Note that in the MIL-I-6181 drawing and the recreation, the rod connects directly to the EMI meter. As bands were selected, the antenna was internally properly matched to the mixer input. Note further that the mixer tube presented a high input impedance, so that the 41” rod was not loaded as it would be by a modern mixer with an input impedance more nearly approximating fifty Ohms.

Note also the very short bond strap between EMI meter and ground plane. The rod antenna was only 12” from the test sample front face. That reflected achievable wire separation in that era’s aircraft. The purpose of the bond strap was to make the ground plane the reference for the rod antenna’s pickup potential. The EMI meter was battery-powered in this application; the ground plane is the sole ground reference.

As time went by, complaints arose about the difficulty of using the AN/PRM-1 meter in immediate proximity to the test sample. While a remote meter was provided with the AN/PRM-1, the controls still had to be adjusted on the meter face itself. Stoddart Aircraft Radio Company, the inventor of the AN/PRM-1, then provided a more modern version of the rod antenna, with its own base, passively tuned. This allowed remote use of the EMI meter itself. A test set-up using the rod antenna with its own base is shown in a picture of an EMI test from the 1950s or early ‘60s, together with a recreation showing more detail.

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Picture of MIL-I-6181 radiated emission test using rod antenna prior to 1963

Recreation of MIL-I-6161 test set-up picture

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Close-up of remote rod antenna, showing tuning bands, and also counterpoise of roughly 12” square dimension

MIL-I-6181B specified the rod antenna only to 20 MHz, with dipoles used at higher frequencies. Later revisions pushed the breakpoint to 25 MHz (viz. 25 or 30 MHz for MIL-STD-461/-462, all versions).

Historical Development –Beginning of the Modern Era MIL-I-6181B was the first modern EMI specification – meaning they recognized the radiated emission mechanism and scientifically adapted the test procedure to protect the real world victim. MIL-I-6181 and similar derivative specifications such as MIL-I-26600, “Interference Control Requirements, Aeronautical Equipment” dated 1958 were superseded by MIL-STD-461, “Electromagnetic Interference Requirements for Equipment” and MIL-STD-462, “Electromagnetic Interference Characteristics” in 1967. The Tri-Service MIL-STD-462 placed the rod antenna at a one meter distance from the test sample. This was in recognition of the NADC-EL-5515 observation (quoted earlier) that as the open-wire lead-in radios were phased out of use and replaced by 50 Ohm coaxial input radios, the test method using the rod antenna at 12” would have to be revisited. The use of modern coaxial shielded lead-ins removed the high impedance unshielded sensitive victim to the exterior of the aircraft where the antenna was mounted. Increasing the antenna-test sample separation was a simulation of the newer radio-to-antenna connection technique.

Another change related to the removal of the sensitive victim wiring from within the aircraft was the consequent attention placed on protecting the antenna from rfi. This resulted in a change from the antenna-induced limit to a modern field intensity limit.

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The 1967 release of MIL-STD-462 used a counterpoise that was floated from the ground plane supporting the test sample. The counterpoise was only grounded through its coaxial connection to the EMI receiver, which was very important because at 14 kHz, the bottom of the RE02 band, a single point ground was necessary for measurement integrity when using a passive (octave band tuned) rod antenna.

Rod antenna use per MIL-STD-462 basic release (1967)

Another change introduced in MIL-STD-462 was the use of the biconical antenna instead of a tuned dipole at frequencies above the rod band. Whereas MIL-I-6181 and similar specifications required a dipole tuned as low as 28 or 35 MHz (on the order of 5 meters or sixteen feet end-to-end), the biconical, at 137 cm (54”) tip-to-tip can be tilted to be used vertically as well as horizontally. Pre-MIL-STD-461 EMI standards only required control of horizontally polarized coupling or fields, above 20-30 MHz, but MIL-STD-461/-462 controlled both polarizations above 30 MHz.

And finally, it was about this time that active base rod antennas became commonly available. This was a technology development, not a specification or standard requirement. Instead of tuning a rod antenna through octave bands that tracked those of the remote receiver, the rod antenna drove a FET gate that acted as a near open-circuit. This means that the 41” rod antenna’s inherent open-circuit effective height of 0.5 meters (or 6 dB antenna factor) was achievable. Compared to tuning out the rod’s 10 pF source impedance (full discussion elsewhere in this report) with one inductor per octave, the improvement in antenna factor was on the order of 50 dB at 10 kHz. This development facilitated the use of spectrum analyzers for EMI testing when

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they became available. The analyzer’s sensitivity wasn’t as good as that of the EMI receiver, but it didn’t need to be, using an active rod antenna. The downside of course was that the active circuitry had a limit on dynamic range for high-level signals. Its response to a broadband signal could be quite limited, and if there was a strong out-of-band signal, that could diminish the ability to receive a low-level signal. The latter issue wasn’t important inside a shielded test chamber.

Rod Antenna Calibration Problem Issue Fixed in MIL-STD-461F

The characteristics of an electrically short wire antenna have been well-understood since the beginning of 41” rod antenna use. The radiation resistance is negligible, and the source impedance is dominated by capacitance (from the EMCO 3301B rod antenna manual):

C = [55.63 * h] / [ln (h/a)]

where,

C is the rod capacitance in picofarads,

h is the physical length in meters, and

a is the rod radius, also in meters.

This results in a source impedance of around 10 pF. In order for a 41” rod antenna to be useful, that capacitive source impedance must either be tuned out, or matched, as discussed previously. Regardless of whether the antenna matching network is of the passive tuning or active matching type, the calibration procedure is to remove the rod antenna element, and inject an rf signal relative to the case, through a 10 pF capacitor simulating the rod antenna source impedance.

Empire Devices VA-105 rod antenna bases, with 41” rod on far left and simulating 10 pF capacitor at center. Close-up of injection capacitor on left.

In order to not load the rod antenna, the area around the rod antenna mounting point must be free of the grounded case, in order to limit parasitic capacitance between rod and case to a fraction of a picofarad. Note in the above photos the entire top piece is plastic. In the next picture of a

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passive rod antenna with a metal enclosure, there is a large diameter cutout around the rod element receptacle, and it is clear they used printed circuit board material to cover it.

Stoddart 92198-3 Antenna Coupler

Clearly the 10 pF series calibration injection device must provide the same clearance, for the same reason. If the calibration device loads the 10 pF source impedance, then the calibration becomes inaccurate, making the antenna look less efficient (higher antenna factor), because the rf potential measured on the 50 Ohm side of the 10 pF capacitor is loaded on the other side, and that loading is not factored into the antenna factor calculation.

Unfortunately, this is exactly what occurred with one manufacturer’s calibration device, resulting in about a 3 dB error in calibrated antenna factor. Once this was conveyed to the manufacturer, in the fullness of time a new device was developed, but in the meantime the DoD Tri-Service Working Group developing MIL-STD-461F, “Requirements For The Control Of Electromagnetic Interference Characteristics Of Subsystems And Equipment,” dated 2007 had decided to ban all manufacturers’ devices, and require a jig per Figure RE102-8 of that standard, and new in that standard.

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Views of rod antenna top showing roughly 1” diameter cut out around rod insertion point, and the mating cal fixture with a small grommet insulating the simulated rod input. The picture below shows the same sort of isolation between the bolt head and the grounded case.

Figure RE102-8 of MIL-STD-461F and a homemade fixture built to the drawing (courtesy of NTS Boxborough, MA)

As discussed above, the change in calibration fixture design introduces a frequency dependent offset, because it reduces the loading parasitic shunt capacitance on the 10 pF source impedance. Two calibration measurements, one using the MIL-STD-461F Figure RE102-8 fixture, and one using the calibration fixture shown above it are compared below. Except for the offset due to the

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box fixture, there is no difference in characteristic. What is plotted below is raw data, not corrected for cal fixture or rod antenna characteristics. These plots theoretically should be 15 dB apart, and are very nearly that.

Tracking generator swept from 10-30 MHz into MIL-STD-461F Figure RE102-8 fixture on left, and box calibration fixture on right. Offset due to construction of different devices, but it is clear the differences are frequency independent.

Politics Introduces A Problem The Army, Navy and Air Force had their own Service-unique standards up to 1967. The purpose of MIL-STD-461/-462 was to provide a single Tri-Service standard, with attendant economies of scale. But the unwilling Services had to be led to the bargaining table, and to keep some degree of contentment and minimize inter-service rivalry, the requirements, test methods and definitions all previously contained in any one specification, were now broken out into three separate standards. The Navy had responsibility for the requirements in MIL-STD-461; the Air Force was responsible for the procedures in MIL-STD-462, and the Army owned the definitions in MIL-STD-463, “Definitions and Systems of Units, Electromagnetic Interference Technology,” dated 1966. But within three years of the release of MIL-STD-461/-462, the Air Force had come up with Notice 2, and a year later the Army had their own Notice 3. Unlike Notice 1 and typical Notice practice, Notices 2 and 3 were not cumulative; they were entirely different standards. The letter of the law had been observed, but the spirit had been flouted.

One area of commonality between Notices 2 and 3 was a change to the rod antenna configuration: whereas previously the counterpoise was floated from the ground plane, now it was bonded to it. This change, effective in 1970 (Notice 2) and 1971 (Notice 3) found its way unimpeded into MIL-STD-462D, “Measurement Of Electromagnetic Interference Characteristics,” (1993) and the consolidated MIL-STD-461E, “Requirements For The Control Of Electromagnetic Interference Characteristics Of Subsystems And Equipment,” (consisting of both requirements and procedures) (1999). Notice 2 wording is as follows: “4. Paragraph 4.2.3.2 Add this sentence: When a counterpoise is used with a rod antenna, it shall be bonded to the ground plane with a strap at least 30 cm wide.” Note that the 1967 set-up is similar to that of the 2007 MIL-STD-461F RE102 rod antenna with the exception of the lack of the lossy ferrite bead increasing the impedance of the bond path.

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Recall that before MIL-STD-462, there was a change in antenna type and polarization at somewhere between 20-30 MHz, depending upon the particular specification and vintage. The efficiency of the vertical rod and horizontal dipole at 30 MHz was quite different, so the limit was discontinuous at the breakpoint, and the signatures were as well.

Typical antenna-induced radiated emission limit showing limit discontinuity when switching from vertical rod to horizontal dipole

But with MIL-STD-461 going to a field intensity limit, and MIL-STD-462 requiring both horizontal and vertical polarization of the biconical antenna, it is reasonable to expect some degree of continuity at the antenna change breakpoint for vertical biconical polarization. In fact, the RE02 and RE102 limits of all versions of MIL-STD-461 are continuous at the breakpoint. The slope may be changing, but the limit amplitude is continuous. But after Notices 2 and 3 were released, it was not always the case that the signatures were continuous at the antenna breakpoints even for vertical biconical polarization. This is even more obvious if an overlap of data is taken between 20-30 MHz, where both antennas are calibrated for operation. Another and related issue that comes to light is that a surprising number of totally different test items all seem to have a broad peak between 20-30 MHz.

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MIL-STD-461 basic (1967) RE02 limit, showing discontinuous slope but continuous limit at rod antenna/biconical breakpoint frequency of 25 MHz.

Problem Recognition Mr. Steve Jensen “Measurement Anomalies Associated with the “41 Inch Rod” antenna when used in Shielded Enclosures, dated 17 July 2000 and Dr. Luke Turnbull “The Groundplane Resonance: Problems with Radiated Emissions Measurements below 30 MHz” (Automotive EMC Conference 2007) independently identified shortcomings of rod antenna measurements in the last octave of use. These were critiques not just against MIL-STD-461E RE102, but many other standards that use the 41” rod antenna below 30 MHz. The issue was a great discrepancy between measured fields at the 30 MHz breakpoint between the rod antenna and the biconical antenna, vertically polarized. While one would not expect precise agreement, due to quite different physical apertures, the 20 dB difference in the data below is problematic. Mr. Jensen showed by overlapping biconical and rod antenna from 20-30 MHz that the biconical always returned much lower levels.

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Data from Jensen, Steve. “Measurement Anomalies Associated with the “41 Inch Rod” antenna when used in Shielded Enclosures,” dated 17 July 2000.

Dr. Turnbull took a more theoretical approach, and compared the measurements made with a rod antenna to what theory would predict the field to be from a particular radiating stub excited by a 1 uV signal. The ordinate scale units are dBuV/m, based on that 1 uV excitation. The green trace is the prediction, and the other colored traces are various configurations of grounding the counterpoise and ground plane. The pink and blue traces most closely model RE02 Notice 2 through RE102 in MIL-STD-461E grounding practice. The black and red traces are closer to MIL-STD-461F RE102 practice.

Data from Turnbull, Luke. “The Ground Plane Resonance - Problems with Radiated Emissions Measurements Below 30 MHz,” published in the Automotive EMC Conference 2007 Record

Dr. Turnbull’s solution that was close to the eventual MIL-STD-461F change is shown below. He used radar absorbing material in lieu of the ferrite loaded coax of -461F.

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Picture from Turnbull, Luke. “The Ground Plane Resonance - Problems with Radiated Emissions Measurements Below 30 MHz,” published in the Automotive EMC Conference 2007 Record

Mr. Jensen flagged this issue in a critique of the draft MIL-STD-461F, and the Tri-Service Working Group went to work to define, understand, and fix the problem. The main investigator was Mr. John Zentner, who at this time (2007) was retired from the Air Force, but who had acted as the chair of the TSWG when developing MIL-STD-461E. Mr. Zentner’s solution became the new instructions for using the rod antenna in MIL-STD-461F.

Mr. Zentner’s Approach Given a large discrepancy between the rod and biconical measurements, it is not immediately clear which, if either, are giving the “correct” value. By correct is meant the most accurate assessment of the average field over the physical length of the 41” rod element, placed one meter in front of the radiating source. Note that the theoretical effective height of the 41” rod antenna is 0.5 meter, but that is only going to give the expected answer for a field which is oriented parallel to the rod, and is constant over the rod’s 41” length. The real fields from test samples and attached wiring are going to have strong curvature, so that the fields intercepted along the length of the rod are going to be different, and the result measured at the base of the rod will represent the average of those intensities along its length. Mr. Zentner needed both a source of a reasonably well-behaved field, and an accurate standard with which to measure it. The field source and measuring device are shown in the picture below. The field source is a horizontal copper tube suspended above ground and excited relative to ground. The measuring device is an electrically short field probe; the same type as is used during field intensity monitoring for radiated susceptibility (RS103) testing. This device was picked because it is very well understood theoretically, and the field curvature over its physical aperture is negligible: regardless of the actual field curvature, the field gradient over its aperture is essentially constant.

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Mr. Zentner’s test set-up at the EMI test facility at Wright-Patterson Air Force Base; this room is the same one that was used to determine and then define absorber requirements under MIL-STD-462D and follow-on revisions of MIL-STD-461.

Given the above set-up, Mr. Zentner’s problem reduced to calibrating the field intensity using the field probe, and then comparing the rod antenna’s measurement, and tweaking the ground plane set-up until acceptable agreement was obtained.

Bump in the Road One small problem has to be mentioned; that is the relative ranges of field intensity measured by the field probe and the rod antenna. Today all rod antennas in use are actively matched; the amplifier in the rod antenna base will saturate around 1 V/m, sometimes less. Field probes in contrast measure from 1 V/m on up, and for some 1 V/m may be pretty close to the noise floor. So to calibrate the field using the probe, an amplifier had to drive the field generating copper tube to 2.5 Volts/meter. That level was too high for the rod antenna, so it had to be accurately scaled back for the rod measurement. This was done using a directional coupler; the same one used for MIL-STD-461E/F CS114. The level measured from the forward power port (50 dB down) while generating 2.5 V/m was used to drive the copper tube for use with the rod antenna. Thus if the field probe had measured 2.5 V/m, the rod antenna ideally would return 78 dBuV/m:

20* log (2.5) + 120 – 50 = 78.

Mr. Zentner’s Findings

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What Mr. Zentner found was that between 10-30 MHz the counterpoise was in some degree of resonance and was “hot”. In fact, the rf potential between the end of the counterpoise and the ground plane exceeded the potential measured at the rod antenna output. This clearly was affecting the measurement integrity. Mr. Zentner solved this problem by going back to the pre-Notice 2 configuration, and additionally placing a ferrite bead on the coaxial cable connection between rod antenna base and EMI receiver to provide a lossy path to the chamber ground.

The eventual MIL-STD-461F configuration of rod antenna and counterpoise was found to provide the following correlation between field probes and rod.

Practical Results of the Ground Plane Configuration Change Mr. Derek Walton of LF Research, a commercial EMI test facility in Poplar Grove, Illinois, performed a set of measurements to evaluate the effectiveness of the MIL-STD-461F change to the ground plane configuration. The results of the measurements show very close correlation between the -461F rod antenna configuration and a vertically polarized biconical antenna between 20-30 MHz, and much poorer correlation when using the -461E rod antenna configuration.

Test Description A standard site source was mounted on the ground plane in an LF Research absorber-lined chamber that more than meets the requirement for absorber treatment as described in MIL-STD-461E/F section 4.3.2.1. The site source was connected to a comb generator with 1 MHz harmonic spacing. Measurements were made of the resulting radiated field intensity between 20 to 30 MHz using three different techniques: A 41” rod antenna configured for MIL-STD-461E, the exact same rod antenna, no changes, but configured per MIL-STD-461F, and a biconical antenna developed particularly for MIL-STD-461A, polarized vertically. Pictures of the three

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set-ups are shown below. The overall signature from 20 to 30 MHz is also shown below for all three techniques. Following that, a superposition of all three measurements is presented on a frequency by frequency basis to make comparison and contrast easier.

It is pretty clear that the -461F rod and vertical biconical track quite closely, while the -461E rod configuration is much farther off.

41” rod antenna set-ups for measuring site source; -461E on left; -461F on right

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MIL-STD-461A biconical antenna measuring standard site source emissions

Spectrum of site source as measured using MIL-STD-461E rod antenna configuration

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Spectrum of site source as measured using MIL-STD-461F rod antenna configuration

Spectrum of site source as measured using biconical antenna per MIL-STD-461E/F

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The data below is the same as above, but all three measurements are superimposed, and each harmonic is isolated, so the correlation, or lack thereof, is immediately perceived.

At 20 MHz, the -461E/F configurations yield identical results, and are 10 dB above the biconical value.

At 21 MHz, the -461F configuration yields a result midway between that returned by the -461E rod configuration and the biconical.

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At 22 MHz, the -461F configuration much closer to that of the biconical than that returned by the -461E rod configuration.

At 23 MHz, the -461F configuration and the biconical yield identical field intensities, while the -461E rod configuration measurement is about 13 dB higher.

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At 24 MHz, the -461F configuration and the biconical measurements are within 1 dB, while the -461E rod configuration measurement is about 15 dB higher.

At 25 MHz, the -461F configuration and the biconical measurements are within 2 dB, while the -461E rod configuration measurement is about 13 dB higher than the -461F value.

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At 26 MHz, the -461F configuration measurement exceeds that of the biconical by 4 dB, but the -461E rod configuration measurement is about 13 dB higher than the -461F value.

At 27 MHz, the -461F configuration and the biconical yield identical field intensities, while the -461E rod configuration measurement is about 13 dB higher.

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At 28 MHz, the -461F configuration and the biconical measurements are within 3 dB, while the -461E rod configuration measurement is about 11 dB higher than the -461F value.

At 29 MHz, the -461F configuration and the biconical measurements are about 5 dB apart, with the rod antenna measurement for the first time lower than that from the biconical. The -461E rod configuration measurement is about 10 dB higher than the biconical value.

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At 30 MHz, the -461F configuration and the biconical measurements are about 4 dB apart, with the rod antenna measurement again lower than that from the biconical. The -461E rod configuration measurement is about 6 dB higher than the biconical value.

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use of the 41” rod antenna in electromagnetic interference

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