, WK0O, BSEE, MSEE
and sometimes Webpage Curator
Here is a question from a recent email that I answered. I have obscured the writers name and email address:
********* wrote:> Dear Mr. Cross, > May be you can recommend the articles and (or) books about the noise > cancellation in > the ferrite rod antennas or at least best sources about design of the > ferrite rod antennas altogether. > > Best Regards >
Mr. ***,
I have divided my answer into two parts: that dealing with noise
reduction and that dealing with sources of ferrite loop antenna design
information.
Part 1: noise reduction
I am not sure about your question on "the noise cancellation in the
ferrite rod antennas" as to precisely what you mean by the question. I
will assume that you are talking about the apparent reduction in local
man-made noise that is often found when using that kind of antenna. The
reduction of noise is entirely due to the use of a loop as a magnetic
field probe instead of a wire or plate as an E-field probe.
The ferrite used in "beads" for EMI noise reduction are working on a
different principle. The ferrite bead reduces EMI by effectively placing
a lossy series inductance in series with the circuit that needs noise
reduction. At low frequencies signals are not impeded, but at high
frequencies the ferrite looks similar to an inductor is parallel with a
resistance. Consequently, the ferrite bead is useful for
blocking/absorbing the higher frequency noise.
That is not the role of the ferrite in a loop antenna. In the antenna
case the ferrite rod is used to concentrate the field so that a very
small loop can have performance similar to larger air core loops. The
noise reduction occurs as a result of the nature of most man-made noise
sources and the way the field impedance varies with distance away from
the source. (All of the following is assumes that we are talking about
a frequency low enough for the length of wire contained in the loop
antennas to be only a very small fraction of a wavelength.)
Many local noise sources at low frequencies come from high voltage
switching circuits, or arcing etc. that generate "high impedance"
electromagnetic waves. This means that the ratio between the Electric
field and the magnetic field is much higher than the 377 ohms ratio that
is found in the far field. Since the far field is at a large distance
from the source of noise at low frequencies, the amplitude of "local"
sources usually become small by the time the receiving antenna is that
far away -- consequently those noise sources are no longer local.
For the local noises that are closer to the source than the far-field
distance, the magnetic probe antenna has an advantage over the electric
field probe antenna because the ratio of noise in the electric to
magnetic field is so high. When comparing the signal-to-noise ratio of
the electric field probe antenna to the magnetic field probe antenna, the
magnetic probe antenna will often have better performance. (The
exception would be if the local noise is being generated by large current
loops.) If the noise is not local the signal-to-noise performance of
the two antennas (assuming sufficiently noise free amplifier electronics)
should be the same.
The **reduction** in man-made noise would occur if a non-ferrite loop
antenna (i.e. air core loop) were to be used as a magnetic probe antenna
assuming the loop was well balanced. Good balance is necessary in order
for the loop antenna not to act just as a big hunk-of-wire that would end
up acting like an E-field probe in addition to the magnetic field probe.
Getting the loop well balanced is the trick. Frequently larger air-core
loops have an E-field shield on them to help. A ferrite loaded loop is
usually much smaller than the equivalent air loop and the amount of wire
is significantly less than an air-core loop. This makes the ferrite loop
antenna easier to balance to start with. Still, when used at higher
frequencies (such as short-wave) a E-field shield is still sometimes used
(see the "Direction Finding" chapter of a recent The ARRL Antenna Book
published by the ARRL Newington CT for an example of a shielded ferrite
rod. Mine is a 15th edition published in 1988 but I believe the newer
editions still have this information).
It should be noted that because the loop antenna is a "low-impedance"
type antenna, its electrical properties and signal pickup is often less
effected by the local environment than the "high-impedance" E-field type
antenna.
Part 2: Design Information and References
The "best sources" for ferrite rod antenna design? This is a good
question. That is what I was eventually hoping to achieve in the web
pages I was (am) planning on putting together. I slapped the page that
currently exists together in hopes that I would be making incremental
progress in its completion. This last year was just too busy and I have
just recently begun work on it again.
One can find a little bit here and a little bit there about ferrite rod
(ferrite loop) antennas but most articles are slightly oversimplified
from the view point that simplifying assumptions have usually been made
-- it is fine to make simplifying assumptions; what isn’t always clear is
whether the correct ones for a particular problem have been made. I hope
to have a longer treatment of the design issues than what most of the
following references have when I get done but that is no help until I
finish. If you have a specific design issue I might be able to guide you
to the right reference
Below I have listed a few key references. I should note that in my goals
for the webpage information, what I am doing is not really creating
anything new; I am really only evaluating past work while condensing
information and orienting the results toward design rather than primarily
analysis. To that end, I am developing some computer tools (such as the
moment method stuff on the web page) to help in the evaluation. The
computer tools are being developed from fundamental principles so that
they are not dependent on any particular reference.
I have already mentioned The ARRL Antenna Book published by the ARRL
Newington CT. In the 15th edition published in 1988 the information was
located in chapters 5 (under small loops) and chapter 14 (Direction
Finding Antennas). It is probably in some other chapter in the newer
editions. This reference gives the very basics of ferrite loop antennas
but won’t give much guidance for new designs on figuring out SNR vs.
size, Q, what to expect in the way of inductance, etc. The information
given is in the form of a practical guide as to what has worked before
for the specific situations of interest to those writing the chapters.
Perhaps the first place to go is what is sometimes called the Jasik
Antenna Handbook. The newer one is actually:
Johnson, Antenna Engineering Handbook, Third Edition, Richard C. Johnson
editor of third edition, Henry Jasik editor of First Edition,
McGraw-Hill, New York 1993
On pages 5-6 through 5-9 has a section of the small loop treatment
dedicated to ferrite loaded loops. There is enough information to get
started trying to design an antenna from scratch but it doesn’t get into
SNR or Q issues. The final design would likely still need considerable
empirical tweaking since my research has shown that the curves for Fv,
FL, and Fr are also dependent on the material permeability. This
reference (and others) show these parameters as a single curve.
{Although I know this parameters vary, I have not yet got to the point in
my research where I know how important the variation is to practical
engineering design considerations.}
Stutzman and Thiele, Antenna Theory and Design, John Wiley and Sons, New
York, 1981,
This has a short section on pages 99-105 talks about small loop antenna
with ferrite loops mentioned specifically on page 102. He gives a
formula for the radiation resistance of a small loop and for a ferrite
loaded loop. Unfortunately the formula for the ferrite loop depends on
the effective magnetic permeability (which is not the material
permeability). No procedure is given to find the effective permeability
although he has a reference to Polydoroff (which I mention later) about
this. Not enough information to start a design from scratch but enough
theory so that you could get started with some ferrite make some
measurements and get going designing one empirically.
John D. Kraus, Antennas, 2nd Edition, McGraw-Hill, New York, 1988
On pages 259- 263 this reference (along with the preceding pages) present
the basic formulas for a small ferrite loaded loop and gives a specific
example of an effective permeability for one particular rod. He also
does some example calculations for receiving an AM radio station with one
example antenna. Still not enough information for a complete design from
scratch.
W.J. Polydoroff, High-Frequency Magnetic Materials, Wiley , New York,
1960,
Pages 53-59 deal with some of the factors of effective rod permeability.
Subsequent pages in chapter 9 starting at page 113 deal with the "General
Case of Ferroinductor" including some basic stuff on loss in coils. The
information on rod permeability and demagnetizations factors is more
complete than some references (but Bozorth’s paper is more complete -- see
below)
The paper:
Bozorth R. M. and D. M. Chapin, "Demagnetizing Factors of Rods”, Journal
of Applied Physics, Volume 13, May 1942, pp. 320-326
Has some of the most complete work on magnetic rods and the factors that
lead to the effective permeability of all of the papers I have. Many of
the later papers derive results from this paper. The data from this
paper is what I use to compare other results (including the moment method
code). This paper does not, however, deal with any antenna or inductor
issues and is solely concerned with the permeability, demagnetization
factors, and flux distributions. All of which is important for the
antenna case but does not include all factors.
E. C. Snelling, Soft Ferrites, Properties and Applications, Butterworth
and Company Ltd. , London, Second Edition 1988.
This reference contains more pages of potentially useful information than
just about any single reference. I say potentially because once again
the need to check assumptions and applicability. Ultimately some of the
factors get a little assumed when he talks about antenna. (Since I
haven’t finished looking at everything yet, it is a little early to be
talking about assumptions because my final results may end up no better).
>From pages 149-157 this book deals with open magnetic cores (i.e. rods),
the effective permeability (rehashed from Bozorth but good), curves which
show measured flux distributions from different rods in an uniform field,
and flux in rods from coils on the rods. The fact that these flux
distribution curves showed variation with the intrinsic material
permeability was one of the motivations for writing a moment method
program; the ones given don’t necessary cover the cases everyone would be
interested. (I may be wasting my time in the long run if they end up
covering the range of important practical cases.) There are also pages
starting at page 155 dealing with the inductance of a coil on the rod
which looks like it is simple and general enough to use I am not sure he
mentions that the formulas associated with figure 4.14 are in millimeters
until the bottom of the next page however. I have not yet tried to apply
this section; I am waiting to evaluate it more fully until some of my
other fundamental stuff is finished so I can compare it to some
models. Chapter 10 pages 303-312 is completely devoted to Ferrite
Antenna (i.e. small loaded loop antennas) including voltage pickup
factors (effective height), signal to noise, bandwidth and impedance, Q
factors, inductance, and temperature factors. This all sounds very
complete but, I think someone starting a design from scratch is likely to
be frustrated a tad by what seems to me to be analysis-oriented-design
rather than design-oriented-analysis. This will be one of the principle
references that will serve as a basis of critique and presentation
concept framework once I get done enough to start cranking out web pages
etc.
Special note: This book is still in print because of its general
usefulness to those involved in all things ferrite. You probably want to
find this book in a technical library someplace as the price (the last
time I checked) was $289.00.
Some other interesting papers:
C. A. Grimmett, "Ferrite Cored Antennae", Convention Record of the IRE
1954 National Convention, Part 7, Broadcasting and Television, pages 3-7
Mostly a report on the "new" ferrite materials making it possible to
rediscover the principle of using loaded coils as antennas. Nice intro
to general principles but mostly valuable now as a source of one persons
experimental curves of pickup, flux distribution, inductance, Q, for the
particular problem Grimmett was interested.
Block, H. and Rietveld, J. J. " Inductive Aerials in Modern Broadcast
Receivers", Philips Technical Review, Vol 16, No. 7 (1955), pp181-194
On page 186-194 there is information relevant to the ferrite loaded loop
antennas. Considers Phillips work on ferrite antenna and some
observations on coil positioning for maximizing effective height and
signal to noise ratio. This article also briefly considers the use of
multiple separated rods with series connected coils to increase signal
pickup.
V. H. Rumsey and W. L. Weeks, Electrically Small, Ferrite-Loaded Loop
Antennas, 1956 IRE Conv. Record, Part 1, p165-170
This is a fairly technical paper starting with basic electromagnetics,
field equations and integrals to find a means of calculating all of the
basic loaded coil properties. In order to make the treatment tractable,
the authors use a prolate spheroid (for which the field distribution is
mathematically know) as an approximation to the rod shape. After finding
some basic equations for inductance, loss, Q, etc. they evaluate their
concept against a small set of measured data.
I have many other papers but, in the interest in getting back to you in a
reasonable amount of time, I have selected a few. I hope to eventually
get the whole list published on the website anyway so your question got
me to start the process of typing it in.
Ray
Enjoy,
Ray L. Cross, , WK0O,
BSEE, MSEE
and sometimes Webpage Curator
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Last modified on: Saturday 3 September 2007