When Minds Meet

To do easily what is difficult for others is the mark of talent.
To do what is impossible for talent is the mark of genius.
— Henri-Frédéric Amiel (1821–81), Swiss philosopher, poet.

$fractent20.jpg (24369 bytes)$ INTRODUCTION

ardly a month goes by that antenneX does not introduce a new antenna design or other type of product design for the antenna system, including test and/or measuring equipment. Most of the antennas are experimental in nature as a product design in progress. In this last month of the year 2004, this is no exception. Just take a look at this beautiful piece of workmanship by Werner Hödlmayr DL6NDJ on his latest antenna design he calls the “Fractent20:”

Over the past several years Werner has come up with some very interesting antenna designs as well as circuits used for matching/feeding that have been presented here in the various monthly publications of antenneX. I for one always look forward to Werner’s next idea because I know they will be well-worth time to analyze. Reproducing an antenna like his latest one above will present quite a challenge for most however.

Werner is interested in this particular vein of research because he believes the Fractal antennas can have a small size with large bandwidth and can be operated at more than one resonant frequency. Obviously, it is certainly a challenge to build such antennas for lower frequencies like 14MHz and below. This month's article of Werner's shows one version using the Tent function in a multi fractal loop configuration.

In previous articles, Werner demonstrated the feasibility of small, efficient fractal antennas based on a "Tent function," working in a frequency range from 140 to 500 MHz. The accumulated experience has led to further developments aiming at the lower frequencies where the size of classical antennas represents a limiting factor and barrier for effective design.

$fractent202.jpg (13303 bytes)$ First trying different configurations with the simulation program, NEC-Win Plus, Werner found the best efficiency/size relation when using a dipole with distributed fractal loop structures along the conductor. He believes this unique shape reduces the size of conventional dipoles by a factor of 8...!

The general problem when designing antennas for such wavelengths is the difficulty for testing the performance of the prototype. Usually, some designers rely on the various computer-modeling programs and certify the values computed this way while others build real models at reduced scale and measure the performances of these models on the assumption that the results are transferred linearly to the real size.

Werner chose the latter choice and builds a model for the 2-meter band, scaled down ten times. In this month’s article by Werner, some theoretical considerations about the “Fractent®” are discussed and test results of the 2-meter Model shown.

MEETING OF MINDS
I was extremely and warmly pleased to hear from one of our antenna-discussion list members, David Jefferies that a "meeting of minds" was held on November 7, 2004 at the residence of David's in the UK. Attending this unique meeting were two other list members, Kirk T. McDonald and Alan Boswell.

Host, Dr. David J. Jefferies is an educator at the School of Electronics and Physical Sciences University of Surrey Guildford - Surrey, England. Dr. Kirk T. McDonald is a Professor of Physics at Joseph Henry Laboratories, Princeton University, Princeton, NJ USA. Alan of Essex, UK is an Engineer with BAE Systems that employs world-wide more than 90,000 people.

Two fine academic minds and another from the "trenches" where theory becomes reality. What could be better for a "meeting of minds?"

Many on our antenna-discussion list are familiar with the fine exchanges on various subjects by these members. Indeed, they stimulate some very interesting discussions for us all to enjoy as readers and/or participants. If you aren’t a member of this discussion list yet, what are you waiting for?

I asked David for a bit of info about their discussions during the dinner meeting and he replied:

"....It was very interesting yes, we had quite wide-ranging and intensive discussions on a number of antennas topics. The general feeling was that any compact antenna has to radiate like a dipole if it is sufficiently small, and that then there isn't any doubt about the Chu limit.

I aired my ideas about capacitative coupling to nearby larger structures provoking additional radiation, and also the inductance of a perfect ground plane meaning that it could support currents which radiate.

They discussed the PV computer experiments they are doing; if an antenna structure actually doesn't source radiated power what is it there for at all? There seemed to be a consensus that antenna models are "horses for courses" and you don't explain all situations with a "one size fits all" theory.

We went around the interesting problem of what happens if you cut a one-wavelength-perimeter loop antenna at the points where the current falls to zero: does anything change? ....and it was suggested I go and do the experiment....."

BTW: This is not the first meeting between those who have met with common interests originating through activities at antenneX. Here was an article about another meeting between two GARDS' members:
http://www.antennex.com/library/shack/shack.htm

A WEALTH OF KNOWLEDGE
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During early 2003, we decided the discussion forums had served their purpose well, but it was time for a change to something better. That replacement was the launch of the antenna-discussion mail list where our readers (subscribers or not) could join and exchange ideas via this classic Majordomo mail management system. Initially, this was a regular posting list for individual threads along with a digest list of the daily messages.

Then, we were urged to also provide a web-based archive of the messages by date and by thread. This too was done and now the combined memberships of all three resources are in excess of 2000. And, since its launch in mid-April 2003, short period of just a few months, some 4600 messages have accumulated covering a wide variety of antenna subjects from theory to how to make them and make them work better.

So, as I stated above, if you are yet a member of our antenna-discussion list, what are you waiting for? You are missing out on some great discussions. You can be just a reader if you like, or when something strikes a "nerve" join in with a post of your own.

AN INVITATION TO CONTRIBUTORS
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antenneX thrives on the contributions of antenna experimenters, ranging from the informal home shop construction project to the theoretical investigation of basic antenna, feedline, and propagation phenomena. Over the years, we have published articles on the use of new or newly adapted materials, known antennas adapted to new circumstances, modifications of antenna structures, basic explorations of both common and unusual antennas, antenna modeling exercises, design improvements, antenna matching techniques from both a physical and mathematical perspective, evaluations of mini-antennas and their underlying theory of operation, new and patentable designs, propagation tutorials, and.... The list goes on, since no antenna-related topic is irrelevant to the readers of antenneX.

At the same time, antenneX has experienced continuous growth in its readership—for which we are appreciative. However, all readers can help us do even better. How? By submitting an article every now and then based on your current antenna work that may be useful at any level to other readers.

Among the engineering and researching readers, there are undoubtedly a number of unclassified and non-proprietary findings that antenneX readers would like to know. Among the practical antenna designers, there are ideas, tests, and numerous other practical findings to benefit our readers. Antenna builders very likely have some techniques to share with other readers. Besides the regular articles, we always have the home work shop column for shorter practical ideas and we always have the invited news and editorial column for information about new technologies, future advances, lost old but good ideas, and personal views on the good to bad things that are happening in the world of antennas and propagation.

If you are uncertain about whether your ideas merit an article, please feel free to send an outline to the general editor/publishers at submissions@antennex.com . Do not feel that you must be ready to be a regular submitter to write for antenneX, because we welcome the individual contribution as much as monthly articles. As well, do not believe that the slots in each issue are already spoken for—we shall always make room for a worthy article.

An SWR-Feedline-Reactance Primer:
Part 2. Some Interesting Antennas and Matching Systems
By L.B. Cebik, W4RNL

In this Part 1, we begin to develop a more visual and hopefully a more intuitively correct understanding of the behavior of resistance and reactance along a transmission line. As well, we saw from the models that we used to explore this territory the ways in which the line length, even apart from line losses, can affect resistance and reactance as we move away from the design frequency toward band edges. Along the way, we encountered some interesting uses of feedline transformers that are an odd multiple of a quarter-wavelength. In this episode, we shall do more of the same, but with some differences. As well, we shall examine further the quarter-wavelength of feedline as an impedance transformer used to effect a match between an antenna and a main feedline that do not initially match. This exploration will lead us to look at at least two other matching systems: the beta or hairpin match and the match line-and-stub system. In each case, we shall be interested in the behavior of resistance and reactance as we place these systems between the antenna terminals and the main feedline.

The Very-High C/L Tuned-Circuit Tesla Dipole
By Joel C. Hungerford, KB1EGI

For this month’s episode Joel built the very high C/L ratio parallel tuned circuit drive for the Tesla dipole, using a “U” shaped length of #6 copper wire and a 2000 pF high voltage mica capacitor. Joel continues with his experiments with his "Tesla Dipole" configurations.

Antenna Tuning and the Conjugate Match, Another Look
By Fred M. Griffee, N4FG (EE Retired)

Here, Fred first refers to Maxwell’s (W2DU) statement that recognizes a matching network not only provides a conjugate impedance match but also effectively lengthens or shortens a given transmission line length. Indeed, he finds it curious when reading articles that describe antenna tuning by varying the transmission line length and wonders why an antenna tuner is not used? But it could be simply a matter of improved efficiency. However, the question arises, how much of an increase in efficiency would be experienced if present at all? In this article, Fred demonstrates that Maxwell’s statement in his excellent book “Reflections II” is what happens in addition to presenting a conjugate impedance match and determines whether increased efficiency is experienced when using transmission line tuning.

The BIG 160-Meter Vertical Project
By Dave Cuthbert, WX7G

Have you always wanted to put out a great signal on 160 meters but thought that you couldn’t because you live on a small city lot? Or, are antenna covenants keeping you away from serious work on “top band?” Well here is 38-foot fold-over, self-supporting vertical project that will let you put out a respectable 160-meter signal and not upset the neighbors. During the day, it remains tilted down and hidden from view. But, when nightfall arrives, you tilt it up and you’re on the air in less than a minute. And best of all, you can build it with easily available materials using only simple hand tools.

Red and Green Lights to Insight: A Review
Two Recent Books from Thomas H. Lee
By David J Jefferies

DJ says whilst there are many established classic texts in the general area of RF, microwave, and antenna systems, readers of antenneX magazine may, like me, always be on the lookout for notable new books which have something to offer the reader which is not available elsewhere in the Public Domain. One might think that this is rather difficult, given the maturity of the discipline, and so new books that stand out from the generality of those on offer don't come along very often. This month seems to be an appropriate time to review two books by the Stanford University professor Thomas H Lee, both published by Cambridge University Press. Your reviewer hopes to make a case for the accession of these books to the library of keen amateurs, as well as RF professionals.

A Fractal Antenna for 14MHz: the “Fractent20”
Theoretical Considerations & Testing of a 10x Scaled-Down Model
By Werner Hödlmayr, DL6NDJ

Werner believes Fractal antennas have a small size, large bandwidth, and can be operated at more than one resonant frequency. It is found to be a real challenge to build such antennas for lower frequencies like 14MHz and below. This new article by Werner shows one version using the Tent function in a multi fractal loop configuration. See above for more on this article.

Well, there you have it, folks—thanks for listening and remember, the reading lamp is always on for you in the reading rooms. If I can be of further help, I'm just a Stone's Throw! away. December 2004 antenneX Online Issue #92
reGARDS, Jack L. Stone, Publisher jack@antennex.com