To: The Microwave GroupFrom: Dick, K2RIW 23 Apr 2003.Re: Antenna Scattering Area, Where Does It Come From; Where Does It Go To?
Comments on Antenna Scattering Area INTRODUCTION -- After the
series of submittals on the subject of the WA5VJB Sci
Am Antenna article, I received a number of replies about Antenna Scattering
Area from K3JDC, NU8I, WA5VJB, W6OAL, WA2SAY, and WA1MBA. From these I have
formed some further opinions that might help (a little) to clear up this
difficult-to-understand subject. I'm not optimistic that my
thoughts will be the definitive answer for the reader; this is a very
controversial subject. For at least 30 years, many articles addressing it
have appeared in the IEEE Antennas and Propagation Proceedings, The AP
Magazine, among other respected journals. I believe that quite a few of the
articles left those readers quite unsatisfied. The syllabus used in the
course of many of the respected schools does not seem to cover the subject in
a way that allows a student to walk out of the classroom "whistling that
tune". I know it is time for a change in the syllabus. The following is a rather
heuristic, slightly visual, and non-mathematical approach. Some readers
prefer that approach. ANTENNA SCATTERING AREA, Part 1, DIPOLES. RECIPROCITY -- Every
reasonable, low loss, antenna (not containing a circulator or amplifier) obeys
the Law of Reciprocity. This means what ever
characteristics the antenna displays during transmission (Gain, Pattern, Polarimetry, Side Lobes, Impedance, Main Lobe Efficiency,
Radiated Phase (Time Delay), etc.), the antenna will
display the same characteristics during reception. THE DIPOLE -- Assume that
I'm doing an exhaustive study of all the characteristics of a Half Wave
Dipole Antenna, and that I want to pay particular attention to that antenna's
Effective Area, and it's Scattering Area. THE TEST -- I'll start out
with a well-matched Dipole, that has a good balun attached to it. I'll apply 1 watt of power to it at
the proper frequency. I'll carefully measure what happens in all of the three
dimensional space in the Dipole's far field, by using well-calibrated
instrumentation that performs non-invasive testing, while all the hardware is
located on my ideal antenna range (which might be in outer space). My
instrumentation will measure the radiated Power Flux Density (PFD), Polarimetry, and Time Delay, over every one of the 41,253
square degrees (4 Pi Steradians) that exists on the
"Sphere of Observation" that's located in the Far Field around the
Dipole. Assume that Sphere has a radius of 1 km, and that each of my 41,253
measurements are done in such a way that there is no overlap in the
measurements (they're all independent). TRANSMISSION -- During
transmission I'll note that the 1 watt of transmitter power was completely
absorbed (and supposedly radiated) by the Dipole, and none of it was
reflected back to the transmitter (in the steady state). During the Dipole's
rise time, there was some reflected energy. I next added up all of the power
that was observed over all of the "Sphere of Observation". I was
not surprised to find that the cumulative steady state power was exactly 1
watt. The Polarimetry measurements contained no surprises,
and the Time Delays (Phase measurements) contained no significant
differential delays. The Dipole seems to have a single phase center that
emits an Amplitude Pattern that looks like a Donut (with the Dipole through
the center), in the usual method of 3D display. TRANSMISSION ANALYSIS --
Next I'll take all the measured data and convert it into a computer generated
3D animation. The beginning of the slow motion movie will display the instant
of transmitter turn-on as a small, spherical, bubble of energy that emerges
from near the Dipole, and the bubble's radius expands at the speed of light
(which I've slowed down for the movie). (Some pretty strong bubble pattern
simplifications are being used in the beginning of this animation). The
surface of the first bubble represents the wave front of the first RF cycle emitted by the Dipole. The brightness (or
thickness) of various portions of the bubble's surface represents the PFD
that's transmitted in the direction of that portion of the bubble. THE BUBBLE "PATTERN"
-- In this animation the Dipole's antenna pattern is represented by a varying
bubble thickness (and surface brightness) in the various directions.
Broadside to the dipole the bubble is thick and bright. In the direction
closer to the ends of the Dipole the bubble is very thin and dim. In exactly
the Dipole co-linear direction, the bubble is so thin (and dim) that the
bubble appears to have a hole in that direction -- that's the Dipole's null
direction (the hole in the donut of the usual kind of 3D pattern display). CONCENTRIC BUBBLES --
Inside of the first bubble there's a second bubble, one wavelength smaller in
radius (the next cycle of energy), that has the identical pattern as the
first bubble, but this whole bubble is slightly brighter than the first,
because it represents a stronger cycle. The extra strength represents the
finite rise time of the Dipole's response since it has approximately a 10%
bandwidth. As these bubbles expand there grows a
third bubble inside of the pack that's slightly brighter again. Each of the
emerging bubbles start out brighter, until the
Dipole comes up to steady state response. 100 BUBBLES -- Assume that
the dipole is operating at 30 MHz (10 meter wave length), therefore at each
instant of time there are 100 concentric, expanding, bubbles located between
the emitting Dipole and the "Sphere of Observation" that's located
at a radius of 1 km from the Dipole, that's in the center of the sphere. RECEPTION TEST -- To test
the Dipole's reception characteristics, I'll choose one of the pieces of
instrumentation that's located on the Equator of the "Sphere of
Observation" (broadside to the Dipole) and I'll transmit a 1 watt ERP signal (+30 dBm) from that
location toward the dipole. Before the test I calculated that the Free Space
attenuation of a 30 MHz signal at a distance of 1 km is -61.99 dB. The Dipole
has a calculated Gain of +2.14 dB over isotropic, therefore my calculated
reception should +30 -61.99 +2.14 = -29.85 dBm. I
performed the reception test and I received exactly -29.85 dBm. SCATTERING AREA -- At first
the test looked great, until later I transmitted brief pulses toward the
Dipole to further investigate the Scattering Area. Each of the brief pulses
created a spherical wave front that started out from the chosen location on
the Equator of the "Sphere of Observation",
they progressed toward the Dipole, and eventually exited out the far side of
the "Sphere of Observation". Because of the known timing of the wavefront of the transmitted pulses, I could easily differentiate
the transmitted pulses from the Dipole's reflected pulses, because of their
extra time delay at most of the 41,253 observation points on the "Sphere
of Observation". I could also do a steady state test by observing all
the PFD's (and phase angles) at the 41,252
reception points, with, and without, the Dipole present, and resolving the
PFD differences (by an interferometry extraction
technique). EFFECTIVE AREA EQUALS
SCATTERING AREA -- When I added up all of the pulse energy that was scattered
by the Dipole, I noticed that it equaled the energy that the Dipole was
absorbing. In other words, the Dipole's Effective Area was equal to it's Scattering Area. At first,
this seems to be a paradox. How can the Dipole have 100% efficiency on
transmission (it wasted no energy that was applied to it), yet on reception
it seems to have 50% efficiency (it scatters 1/2 of the incident energy
that's applied to it). Is this a violation of the Law of Reciprocity? I say
no. RECIPROCITY EXPLANATION --
On transmission, the Dipole emitted spherical wave fronts that had a
particular amplitude distribution over the 41,253 square degrees of 3D space.
During the reception test we arbitrarily chose to take ONE of those 41,253
square degree locations and use it to send a signal back into the Dipole.
From the Dipole's point of view, that's the WRONG AMPLITUDE PATTERN -- 41,252
pieces of the desired pattern are missing. We violated one of the unstated
conditions of the Law of Reciprocity -- we didn't Reciprocate all of the original
conditions. FULL RECEPTION TEST -- If
we had the resources to emit 41,253 signals toward the Dipole, with all had
the correct relative amplitude (the original Dipole transmitter pattern),
than the Dipole will absorb ALL the reception energy, and the Scattering Area
will go to zero. SPACIAL IMPEDANCE MATCHING -- Up until now,
most RF mavens believed that impedance matching of
an antenna only concerns what we do at the feed point of the antenna
(electronic impedance matching). The second part of "impedance
matching" involves "Spacial Impedance
Matching", which means creating the correct amplitude pattern (in 3D
space), and phase pattern, that the antenna was really designed for. If you
mismatch either kind of these "Impedance Matches", then energy will
be scattered, and the possible efficiency will be compromised. SHRINKING BUBBLES -- If we
really performed the full Reception Test with the 41,253 properly tuned
emitters, analyzed the data, and converted this into a computer generated 3D
slow motion animation, what we would see is the same variable brightness
areas of the spherical bubbles as during the transmission test, except this
time each of the bubbles would be decreasing in radius, and they would each
appear to "collapse" on the Dipole. The Dipole would absorb 100% of
the energy of each of the bubbles, and there would be no scattered energy. We
would have performed a truly Reciprocal Test. CONCLUSION -- When it's
properly used, you could say that a Dipole doesn't have a significant
Scattering Area, it's Effective Area is proper, and it has 100% Reception
Efficiency. Unfortunately, the resources that are required to create a proper
Spacial Match to a Dipole are quite heroic, and
rarely realized. Therefore, we are forced to live with it's detriments, because there are few simplistic
antennas that are easier to use. NEXT INSTALLMENT -- In the
next installment concerning Dish Antennas (in the next few days), we'll need
to discuss Structural Scattering (cross section), as well as electronic
Scattering Area, and the variation of RCS versus
azimuth on all real dish antennas. It's a pattern that looks very different
from the antenna's Gain pattern. ANTENNA SCATTERING AREA, Part 2. INTRODUCTION -- Portions of
this memo will be controversial to some readers. The concepts that will be
expressed constitute a "Model" that has been created for the
purpose of predicting antenna reception characteristics, of both complex and
simple antenna types. Although the Model may be flawed, it has done a good
job of explaining the phenomena of Aperture Efficiency, and Scattering Area.
There probably will be other Microwave Reflector memos that question the
concept, or express a different point of view -- I welcome these (within
reason). When the smoke settles, I believe we will arrive at a higher
understanding of antennas. (1)
Electronic Impedance Matching -- That's what you do at the feed point of the
antenna to insure that all the transmitter power is absorbed by the antenna
(from the feed line). ITEM (2) IS MAJOR -- Most
antenna engineers seem to think that Item (1) is the one that causes the
magnitude of the Scattering Area that an antenna displays. But, in most
circumstances Item (2) has the greatest effect. POLARIMETRY ERROR -- If I was testing a
horizontally polarized antenna and I sent vertically polarized energy toward
the antenna during a reception test, few engineers would have any problem
accepting the notion that the AUT (Antenna Under
test) will have little response to that energy. For most low-mass types of
antennas (such as Dipoles and Yagis) the antenna is
rather transparent to the cross-polarized energy; that energy will pass
through the region of the antenna with little disturbance to the energy.
However, for more complex (or massive) types of antennas, such as a Horn or a
Dish, the antenna will present a kind of "short circuit" to the
cross-polarized energy, and that energy will be reflected, deflected,
diffracted, and generally scattered. It will appear as the antenna's
Scattering Area. SPACIAL PATTERN EFFECT, BACK LOBE RESPONSE
-- If I was testing a well-tuned Yagi antenna that
had a good Front-to-Back ratio, and I was applying the co-polarized reception
energy to the back of the antenna, obviously the antenna would have little
response. However, in this case the antenna is not transparent to that
energy. The Reflector, Driven Element, and nearby Directors are responding to
that energy, but their tuning is such that they are presenting an Interferometry Null, ar Phasing
Null at the Driven Element to the energy that arrives from that direction. A
well-constructed Yagi has no lossy
properties associated with it, so it can not
dissipate this energy. By the Law of Conservation of Energy, this energy must
be scattered in some other direction, or it must be generally reflected back
in the direction from where it came. I believe that the back of such a Yagi will present a Scattering Area that is at least
twice as large as that of a normal Dipole antenna. Under ordinary conditions,
a Dipole has a Scattering area that is equal to it's
Effective Area, but the Dipole that is part of this Yagi's
rearward reception has an Effective Area of near zero, therefore it's
Scattering Area must at least double, because the Dipole and other elements
involved. SPACIAL PATTERN MATCHING, FRONT LOBE --
Many Standard Gain Horn antennas have a gain of about 16 dBi.
The Amplitude Distribution across the mouth of the horn has a half
SIN-wave-like distribution, with the emitted signal strongest in the center,
and near-zero at the vertical edges (vertical polarization is assumed). If I
place a pair of identical horns together, mouth-to-mouth, the insertion loss
between them will be nearly zero dB (no loss). This condition will persist,
even if I create a space between the horns of quite a few wavelengths. This
nearly 100% energy transfer between the horns suggests that during this test,
each presents an Effective Area of nearly 100% to the other, and a Scattering
Area of nearly zero. NEAR FIELD COUPLING? --
There are antenna engineers who will simply dismiss this test by saying,
"the horns are within the Near Field Range of each other, they were
reactively coupled, therefore the 100% energy
transfer (although accurate) is not relevant". I disagree. There are
many situations where one portion of an antenna system is well within the
Near Field Range of another portion, and the system performs admirably, and
in a predictable manner. Here are a few examples: STANDARD GAIN HORN
RECEPTION -- If I use one of the Standard Gain Horns in a reception test
while receiving a planar wave that was emitted from a long distance, the
Standard Gain Horn will have a reception efficiency
(Aperture Efficiency) of about 60 to 70%. In other words, the Effective Area
(~ 65%) will be larger than the Scattering Area (~ 35%). At first, this may
seem to be a paradox. The horn has a radiating efficiency of nearly 100% when
it is used for transmitting a signal, yet it has an Aperture Efficiency of
about 65% during normal signal reception, and an Aperture Efficiency
(Effective Area) of near 100% when receiving the signal from an identical
nearby horn. AMPLITUDE DISTRIBUTION
MATCHING -- I say that the reason for this difference is the matching of the
Aperture Distribution from the nearby horn (the half SIN-wave-like
distribution), versus the mismatch of the far-field spacial signal (it presents a Uniform Distribution). At
first, the Uniform Distribution created by the spacial
wave sounds like the most desirable one for best reception. However, the horn
is not "tuned" for that kind of Aperture Distribution,
therefore it can not accept all of that type of spacial signal. The horn's half SIN-wave-like Aperture Distribution
created a particular Antenna Pattern (Spacial
Amplitude Pattern over the 4 Pi Steradians of
space) when the horn was transmitting. Only when that same (and complete)
Antenna Pattern is presented to the horn during a reception test will the horn
experience the same half SIN-wave-like Aperture Distribution, and thus be
able to accept 100% of the energy being presented to it -- which would be
displayed as a 100% Aperture Efficiency. There is a one-to-one Reciprocal
Relationship between an antenna's Aperture Distribution, and the Spacial Antenna Pattern it produces while transmitting. RE-READ? -- I believe that
many readers should re-read the last five paragraphs multiple times, in order
to absorb the concept. To most technologists who are exposed to this concept
for the first time, it sounds crazy. I know of no respected text book that
makes this concept clear. Among the antenna engineers who have had 20 to 30
years experience in the field, I believe that fewer than 20% of them
understand this concept. Yet, it is a very important concept if one desires
to understand the origin of Dish Antenna Aperture Efficiency, or Stealth A/C RCS (Radar Cross Section). For instance, any attempt to
create a Low Observable Aircraft (A/C) must entail a thorough understanding
of the concept of Antenna Scattering Area. It would be a
rather wasteful exercise to put forth a heroic effort to build a stealthy
A/C, only to have it become a "Gang Buster" echo on a Radar
screen because of the Scattering Area of the antennas that were installed on
that A/C -- this has happened many times. There are solutions to this
problem, but they are all very difficult to implement, and those solutions
would not improve the performance of our amateur microwave antennas. DISH ANTENNA APERTURE
EFFICIENCY -- For most Parabolic Dish Antenna systems a "well-designed
feed horn" will present a reflector edge illumination of -10 dB compared
to the dish center. This Illumination Taper results in a dish Aperture
Efficiency of about 55 to 65%. If it was possible to design an "ideal
horn" that presented an Amplitude Taper of 0.0 dB across the reflector,
with no wasted edge spill-over energy, then the antenna system could have an
Aperture Efficiency of nearly 100%. 60% APERTURE EFFICIENCY? --
Most Microwavers have trouble understanding where
the 55 to 65% Aperture Efficiency comes from in a "real" dish
antenna system. Often their flawed point of view is the following: "on
reception the parabolic reflector brings all the energy to a focus at the
center of the feed horn, therefore a good horn has no choice but to accept
all that energy." Unfortunately, the horn will not accept all the
energy, unless it is presented to the horn with exactly the correct three
dimensional amplitude pattern (primary horn pattern), that the horn was
designed for. DISH SPACIAL
PATTERN MISS-MATCH -- During boresight reception of
a far-off planar wave, the wave presents a Uniform Amplitude Distribution
across the parabolic reflector. After reflection, the portion of the wave
that hit the dish perimeter region will approach the horn with an almost equal amplitude as the portion of the wave that
hit the center of the dish. That total reflected signal now has a nearly uniform signal strength versus angle as it is
approaching the horn. The horn will not accept all the energy that has that
pattern; it was designed to transmit (or receive) energy that has a -10 dB
amplitude taper across the angles of the reflector. The portion of the signal
that is not accepted by the horn will be reflected back (Scattered) to the
parabolic reflector and re-transmitted back into space. This Spacial Pattern miss-match (and reflection [Scattering] ) phenomenon is a three dimensional characteristic of
antennas that is quite difficult for many engineers to accept. Therefore,
I'll present a pair of two dimensional examples, that may make it more clear. HYBRID COMPARISON, Example
(1) -- Assume that I'm using a Wilkinson Half Hybrid as a power divider. A 50
ohm Wilkinson is the Hybrid that has a pair of 1/4 wave 70 ohm lines
connected from the common port to the two output ports, with an internal 100
ohm isolation load resistor between the two output ports. I'll first apply 2
watts to the common port (I'll call this the "transmission" test),
and I'll notice that each of the two output ports will display a 1 watt
signal, that has a zero degree phase difference between the outputs. HYBRID COMPARISON, EXAMPLE
(2) -- Assume that I'm using a well-designed coaxial-type -10 dB Directional
Coupler as an unequal power divider -- it's somewhat like a -10 dB Taper Feed
Horn. During the "transmission" test I'll apply a 10 watt signal to
the Common port, and I'll realize a 9 watt signal (with a 90 degree lagging
phase shift) at the Straight Through port, and a 1 watt signal (at zero
degrees phase shift) at the Directional port. Notice that the efficiency is
100%. During the first "reception" test I'll completely reverse the
process by applying a 1 watt signal to the Directional port, and a 9 watt
signal (with a 90 degree leading phase angle) to the Straight Through port.
At the Common port I'll measure a 10 watt signal (100% efficiency), and most
of the Reciprocity process will be demonstrated. But, notice that the phase
angle must be reversed during the Reception Test -- both Antennas, Hybrids,
and Directional Couplers behave in this way. ANTENNA-HYBRID ANALOGY --
In both the Antenna and the Hybrid examples, power will be "wasted"
if the "transmission" conditions (amplitudes at the Hybrid ports,
or amplitudes at the various Antenna angles) are not 100% duplicated during a
"reception" test. The only difference between them is where the
"wasted" power goes. The "wasted" power becomes Scattered
in the case of an antenna Spacial Pattern Mismatch.
In a Hybrid mismatched "reception" test the "wasted"
power becomes dissipated in the Hybrid's internal termination. There have
been some "simplified" Hybrids that do not have an internal
termination (such as the 4:1 power divider that amateurs use to feed 4
antennas), these will Scatter the mismatched
"reception" power back to the antennas. Similarly the Hybrids in
the previous examples would scatter the mismatched "reception"
energy back to the sources, if their internal terminations were removed. Well
designed antennas have no lossy elements associated
with them, therefore they Scatter (and do not dissipate) the Spacial Pattern Mismatched energy that is presented to
them. ANTENNA/HYBRID DIFFERENCES
-- In the previous example a -10 dB Directional Coupler was being compared to
a -10 dB Illumination Taper Feed Horn. When the -10 dB signal (the 1 watt
signal) was omitted during the "reception" test, the Coupler's
efficiency decreased from 100% to 90% (-0.46 dB). However, in a Parabolic
Dish Antenna, the -10 dB illuminated area at the dish perimeter has much more
area that the 0.0 dB illuminated area near the center, thus it includes a
substantial amount of the dish "transmitting" energy. This would be
equivalent to using a Directional Coupler that had a number of -10 dB output
ports (each with an internal 50 ohm termination). If the
"reception" signals were omitted from all the -10 dB Coupler ports,
the impact on the Coupler's efficiency would be a decrease to 60% (-2.2 dB);
this would be similar to the dish Aperture Efficiency of 60% that occurs
during the reception of the Far Field wave that has the
"mismatched" Uniform Distribution signal that creates the Spacial Pattern Mismatch at the feed horn. PRELIMINARY CONCLUSION --
The concept expressed within this memo may seem quite radical to many
readers, and maybe the concept contains errors. However, over the years a
good number of technically-savvy people have come to accept the concept as
being real. I have used the concept to explain the measurements that I and my
colleagues have experienced while using an Instrumentation Radar to measure
the Scattering Area of various antennas under various conditions. As stated
earlier, the Model may be flawed, but it has (thus far) done an excellent job
of explaining the observed measurements. BORESIGHT ONLY -- Most of the main lobe
Aperture Efficiencies and Scattering Areas that have been discussed only
apply to the antenna's boresight response.
Beginning at angles that are slightly off of boresight,
some very different phenomena begins to happen; it's called Structural
Scattering. These characteristics, and some Scattering Area surprises, will
be discussed in a subsequent memo (next week). ANTENNA SCATTERING AREA, Part 3. INTRODUCTION -- I have
often been asked, "how big is the Scattering Area" of certain
antennas. Many amateurs and engineers think the Scattering Area is usually
quite small, and only of academic interest. As you are about to see, it can
sometimes be VERY LARGE. I'm about to describe the biggest one I've ever
measured, and I'll discuss the internal "war" that persisted within
a company (for 6 months) because of a technical difference of opinion on this
subject. I believe this incident can be quite educational concerning:
Scattering Area and RCS measurement, and a
characteristic of human nature that can hinder a project when a strong
difference of opinion occurs. But first, a few definitions are required. RADAR CROSS SECTION (RCS) -- In the Radar world, RCS
is defined by the "Projected Area" of a perfectly conducting
sphere, and the signal level that such a sphere will reflect back to the
Radar. For instance, a metal sphere that has a diameter of 1.128 meters has a
Projected Area of one square meter -- meaning that it will project a shadow
of one square meter on the ground at High Noon on a sunny day. That metal
sphere could be used as a one square meter (0.0 dBsm),
Standard Radar Target, for the purpose of calibrating the sensitivity of a
Radar system. RCS ASSUMPTIONS -- The first assumption
is that the projected area of the sphere will cause it to intercept a
predictable amount of the Radar signal -- one square meter's worth. The
second assumption is that the sphere will then scatter that signal energy
(equally) in all directions of free space. In reality, the sphere slightly
favors the Retro-Direction, but this difference is small, and usually ignored.
RCS CALCULATION -- The hard part to
understand is that many objects can have an RCS
that is MUCH LARGER (in RCS square meters) than
their physical area (in real square meters). This is particularly true for a
Corner Reflector, or a flat metal plate at it's normal angle. For each of these the: ·
A equals the Projected Area of the
plate (or Corner Reflector) at it's favored angle. ·
Lambda is the wavelength (in the same
units). ANTENNA SCATTERING AREA --
During a test of the reception characteristics of almost any antenna, a
portion of the energy that is presented to the antenna will not be absorbed.
That portion is sometimes quantified by the parameter called Scattering Area.
For instance, if a Parabolic Dish Antenna has a surface area of 100 square
feet, and if during a Far-Field boresight-angle
reception test it displayed a measured Aperture Efficiency of 60% (an
Effective Area of 60 square feet), then most likely that antenna has a
Scattering Area slightly above 40 square feet. The extra Scattering Area is
due to the fringing effect that occurs around the perimeter of the antenna,
and the scattering of other structural parts. That scattered signal energy is
usually scattered (to some degree) in all the three dimensional directions
around the antenna (almost isotropically). SCATTERING AREA VERSUS RCS -- A portion of the scattered energy is scattered
back in the direction from where it came; this retro-directed energy would be
available for reception at a Mono-Static Radar system. That portion could
become the target's Mono-Static Radar Cross Section (RCS),
after a conversion factor that normalized the range and the Radar's ERP. In general, the RCS energy
is less than the Scattering Area energy, since it only involves the energy
that is scatterer in a particular direction (back
to the Radar). However, a target's RCS rating (in
square meters) can be much larger than the Scattering Area (in square meters)
because of the peculiar definition of RCS, and it's relationship to the projected area of a metal sphere
that displays the same echo power. THE "RCS PROJECT" PROBLEM -- About 15 years ago I worked
on an Independent Research and Development (IRAD)
project that had to estimate the Boresight RCS of a particular Parabolic Dish Antenna (that was
unavailable to us), and the RCS had to be estimated
at a frequency that was far removed (much higher) than the antenna's normal
operating frequency. My colleagues and I came up with an estimated RCS number that was strongly challenged (by over 10,000:1)
by the project manager. The technical difference of opinion persisted for
over 6 months. This created some very frayed nerves
among the affected individuals. In order to resolve the dispute, the
company's Chief Scientist decided that a Proof-of-Concept measurement would
have to be made on a surrogate, real operating antenna system. THE TARGET -- The chosen
surrogate target was the antenna of an FAA ASR-8 Radar System that was
located at the local airport. It operated at 2.8 GHz, and we were going to make
an RCS measurement at 16.25 GHz (5.8 times higher
in frequency), at the frequency of a portable (Man-Pack) Radar system. The RCS measurement was going to take place from the balcony
of a 10th floor hotel room that was located 5 miles away. It had a line-of-sight
path to the airport radar. PERMISSIONS & SAFETY
REQUIREMENTS -- The FAA gave us permission for the test, and they had our
telephone number to cancel the test if any interference appeared on their
Radar screen. The company's Safety Department tested the Biohazard
characteristics of the portable Radar and declared it to be safe in the
manner in which we intended to use it. The hotel manager gave us permission
to rent the room and run the test, particularly because it was an off season,
and the top few floors of the hotel were empty. THE CANCELED TEST -- The
project manager developed a strong opposition to the test. He convinced the
company vice president to order a cancellation of the Proof-of-Concept test
by declaring that the measurement was a waste of company resources because
there would be too little echo power to measure, and there was a chance that
the portable Radar would cause an interruption of a hotel patron's heart
pacer, and the company would be sued. Thus, the project manager had succeeded
in getting the whole project canceled for the third time. This greatly
frustrated the involved employees. THE "K2RIW RADAR"
MEASUREMENT -- Because of my faith in the project I decided to proceed on my
own time. I prepared and calibrated a pair of 300 milliwatt
10.368 GHz "White Boxes" with the 29" dishes as a CW Radar. I and two fellow
employees went to the hotel on a weekend with about 500 pounds of test
equipment and we made the RCS measurement of the
FAA Radar by using my Ham Radio license to legalize the transmission. That
"flee powered CW Radar" made a
measurement that demonstrated significant 10.368 GHz echo power from the FAA
Radar antenna, and this result was reported to the company's Chief Scientist.
THE RESCHEDULED MEASUREMENT
-- Based on the encouraging measurement with the "K2RIW CW Radar" the Chief Scientist decided to take over
the management of the project, and he re-opened the project for the 4th time.
We were now authorized to use one of the company's portable Radars, and a
more formal Proof-of-Concept test was scheduled to take place at a US
Military Base (with their permission) that had both a FAA ASR-8 Radar and a
nearby mountain about 3 miles away. By locating the portable radar at various
points along the road that was on the nearby mountain, the RCS measurement of the FAA Radar could be made at various
elevation angles to the FAA Radar. THE RCS
MEASUREMENT PROCEDURE -- I was using a Man Pack AN/PPS-5 portable Radar that
put out 1 KW pulses at 16.25 GHz into a 42 by 14 inch Bat Wing antenna (38 dB
gain). This is a battery-powered transistorized
Radar with a Magnetron final transmitter that puts out 100 nanosecond pulses
at a PRF of 10 kHz (an average of one watt output).
I found the correct altitude along the road on the mountain (690 feet) that
put me in the peak of the 2.8 GHz beam from the FAA Radar (at +2.5 degrees
elevation). I found that I had to put a 20 dB pad in front of the PPS-5 Radar
receiver to keep it from saturating from the 16.25 GHz echo power I was
getting from the 2.8 GHz FAA Radar antenna, when it's rotating dish was aimed
at me. RADAR CALIBRATION FOR RCS MEASUREMENT -- I rechecked the RCS
Measuring Calibration of the PPS-5 Radar by performing three tests: a
calibrated corner reflector RCS test, a signal
generator receiver response test, and a directional coupler plus Spectrum
Analyzer transmitter power output test. During the RCS
measurements, the strength of specific echoes were
measured by using a pulsed signal generator to inject (with a calibrated
directional coupler) equal-amplitude pulses (at almost the same range) into
the portable Radar's receiver. The receiver responses were being observed on
an oscilloscope (an A-scope display). HOW BIG WAS IT? -- After
the on-site calibration tests, I confirmed that the FAA Radar (at it's boresight
angle) was generating a 16.25 GHs RCS of 57,500 square meters (+47.6 dBsm),
with a pair of brief peaks (on each rotation) that were even higher. There
are very few Radar discrete targets on Planet Earth that are much larger than
that. That's almost the RCS you will observe from a
full-sized Aircraft Carrier at a broadside angle. It would take a metal
sphere 271 meters high (888 feet) to equal the same Radar echo power. WHY THE >>BIG<<
DISAGREEMENT? -- The original project manager considered himself to be a
"Radar expert"; this caused him to arrive at the following three
(3) conclusions. He had heard that parabolic dish antennas are capable of
creating big echoes. Therefore, a few years earlier he had ordered the testing
of some of the parabolic dish reflectors that were in the company's
warehouse. (1) The feed horns were not present (he didn't think that
mattered), and thus the measured echoes were very small. (2) He reasoned that
the expanded aluminum mesh that made up the parabolic reflector of the 2.8
GHz FAA Radar antenna would allow most of the 16.25 GHz signal to pass
through without substantial focusing, or reflection. (3) He further reasoned
that the FAA Radar's 2.8 GHz feed horn would not respond to the 16.25 GHz
signal. He was extremely wrong on all three assumptions. WHAT HAPPENED NEXT -- The
project proceeded to the next steps toward developing a product. The original
project manager had earlier told many company officials that the employees on
this project (particularly Dick, K2RIW) were very foolish to believe that a
large echo would be realized. He lost much of his credibility after the
Proof-of-Concept measurement was finally made. Soon after the measurement he
resigned from the company (after 20 years of employment); many believe that
the embarrassment of this incident was a contributing factor. NEW RESPECT? -- For the
next 5 years my pronouncements and estimations (concerning RF and radio matters) went unchallenged. It is nice to be
respected and appreciated, and I received some company (and IEEE) awards
because of the results of this and two similar projects. But, I could no
longer obtain an objective opinion when I asked my fellow employees for a
confirmation of my speculative ideas -- they were afraid to challenge me.
They sometimes gave me credit for knowledge I did not have. WHAT MADE THE HUGE RCS? -- When a pulse from the 16.25 GHz portable Radar
arrived at the boresight angle of the antenna of
the FAA Radar, the 15 foot wide dish reflector (with 100 square feet of area)
experienced a nearly uniform Amplitude Distribution. Despite it's expanded aluminum surface
(with some energy feeding through the surface) and surface roughness, it
focused a certain portion of that energy at the center of the antenna's 2.8 GHz
feed horn. The horn was not "tuned" for that Amplitude Distribution
(particularly not at 16.25 GHz) and it rejected a good portion of the focused
energy. The rejected energy then become scattered by the horn, and a good
portion of that scattered energy was reflected back to the parabolic
reflector. The reflector "refocused" that energy back into a beam
that was directed at my portable Radar. THE HORN CREATES MOST OF
THE ECHO -- If the Radar's 2.8 GHz feed horn had not been present, then the
16.25 GHz focused energy would have simply passed through the focal point,
and been dispersed into space over the very wide angles that the parabolic
reflector subtends (from the viewpoint of the feed horn). You could say that
the feed horn created the Scattering Area that, ultimately, resulted in the
large RCS, because without it the maximum RCS would have been over 1,000 times smaller. EVEN STRONGER RCS? -- As the FAA Radar antenna rotated, the 16.25 GHz
signal that was focused by FAA Radar's parabolic reflector would sweep across
the region of the 2.8 GHz feed horn. At the proper boresight
angle, the focused 16.25 GHz signal would be located at the center of the 2.8
GHz feed horn, and a certain portion of that signal would proceed down the
horn, and be propagated by the 2.8 GHz wave guide to the Radar's receiver
(where much of it would be absorbed). However, at the two times during each
rotation that the antenna was aimed slightly to the left or right of the boresight angle to the portable Radar, the 16.25 GHz
signal would be focused on the outer edges of the 2.8 GHz feed horn. At these
times the feed horn could accept none of the focused energy, thus it was all
scattered. During these two very brief periods, the RCS
was considerably greater than the recorded value (57,500 square meters). The
instrumentation that I was using was not able to catch and record the value
of the two peaks; it is possible that they were almost 10 dB stronger. Thus,
a graph of the 16.25 GHz echo power versus antenna rotation would display a pair
of "rabbit ears". CONCLUSION -- I hope the Microwaver's find this saga to be educational, and maybe
they can use the information in some of their future projects. 73 es Good VHF/UHF/SHF/EHF Optical DX, |