Construction Techniques

conSCIENCE: Conscience and reputation are two things.
Conscience is due to yourself, reputation to your neighbor.
—St. Augustine (354-430)

INTRODUCTION
wpeB.jpg (1219 bytes)

s I look back through the many issues of antenneX, I am struck by the very large number of construction techniques our writers have used to build their prototypes and their finished antennas. The antenneX archives are a large resource for ideas on the electrical properties of antennas. However, they are equally a place to look for innovative construction methods.

Many writers and experimenters try to emulate commercial practices. For example, in the upper HF region, where large Yagi arrays are common, we find extensive use of aluminum tubing. One interesting facet of the stepped-diameter elements used for these arrays is the regional and manufacturing differences that we find in antenna physical design. In Europe, where aluminum is available in metric sizes, we often see heavier elements and structures, sometime called the "European Oak Standard." The most common element structure in the U.S. employs the convenient 1/8" increments in the outside diameter of tubing, where the 0.058" thick walls allow convenient size-to-size nesting. At the extreme, a few U.S. manufacturers use thinner-wall tubing to produce the "American Willow Standard." As a result, most U.S. HF arrays use a steeper set of element diameter reductions to yield the same wind-loading specifications. One consequence of the differences is that U.S. antennas tend to be lighter but more flexible in the wind, while European versions for the same performance tend to be more rigid but much heavier. Each style has advantages and disadvantages.

WIDE SKILLS
Some of our experimenters have shown consummate machine shop skills to produce custom parts for their antennas. Home machine shops appear to be more common in Europe than in the U.S. But wherever the work occurs, it results in mechanically superior antennas, especially where a design may require moving parts. The range of materials available for custom milling continues to grow with the appearance of new metal alloys and new synthetics suitable for RF as well as structural use. At the opposite extreme—but still within very proper experimenter limits—are a number of writers who rely on common parts available from a local radio store or a mail/web outlet. We can easily obtain antenna wire, insulators, connectors, and transmission line materials, and those materials are often all that some writers need to produce very interesting and serviceable antennas.

Between the individuals who use off-the-shelf parts and those who fabricate everything from scratch we find "the adapters." This large group of experimenters takes pride in transforming materials intended for other purposes to the needs of antenna projects. The most obvious and widespread practice is the use of PVC in all of its forms and with all of its connectors. Some builders encase wire elements inside lengths of tubing. One classic case is a J-pole antenna made from 300-Ohm TV twinlead and then suspended inside a PVC tube for weather protection. PVC designed for plumbing or electrical conduit service provides masts for antennas, support arms for quad loops, and a host of fittings to house connections and to tie the entire structure together.

Polycarbonate was once a material that we could only find at mail/web sites. Today, it has replaced brittle acrylics at home centers as protective windowpane stock. Polycarbonate works well with common woodworking tools to form plates. Modern adhesives let us build boxes where necessary. VHF and UHF antenna builders sometimes prefer Delrin and other custom synthetics, but polycarbonate serves well for antenna prototypes. In terms of RF properties, we have come a very long way from the days of Bakelite. For all plastics, of course, UV protection is a must, and sometimes those mail-order sources (and their catalogs) are the only way to be sure that our materials will not degrade in the sun. Fiberglass is another popular material, especially in tubular form. Occasionally, we find fiberglass plates that are thick enough to be structurally sound.

KEEPING AN EYE OUT
There is a set of experimenters who like to adapt antennas to casual field outings. These builders are always on the lookout for lighter sturdy materials that will support at least thin wire elements. They have discovered that long fishing poles under tension can handle #20 wire. As well, they assemble and disassemble easily, allowing more operating time during an outing. Their ventures are a good lesson in keeping our eyes open. Whenever we visit a home center, a hardware store, or a sporting-goods shop, we should be alert to the possibilities for adapting what we find to antenna projects. Even the kitchen utensil store may yield some interesting and useful metal items for antennas.

The term "plumber's delight" arose in the early days of Yagi antennas and designated assemblies in which the elements attached directly to metal booms. Among experimenters, the term means something different: the use of plumbing pipe to form an entire complex antenna structure. When working with electrically small antennas, mechanical fasteners do not ensure low-loss junctions, and so the torch and solder replace the screw and the rivet. Smaller pipe sizes now generally use a softer copper that allows bending into shapes that common plumbing fittings will not allow. In fact, the softer tubing has become a common "crafting" material for making home decorations. We may someday see antennas disguised as copper garden features. Some builders even forego aluminum's lighter weight to use copper that they can directly solder without running into problems of electrolysis

Home-center aluminum stock includes U-channel, square, flat, and L-stock shapes. Many builders prefer the flat surfaces for ease of assembly relative to the special needs of round stock. The wind resistance is higher when the pieces are flat, but they can save some costs in terms of avoiding special fixtures to join elements to a boom. In some cases, these materials are also the best electrical choices, for example, when trying to form a short section of low-impedance parallel transmission line.

These observations only sample the range of materials brought to the worktable by various antenna builders and experimenters. There are potential antenna materials available for every level of shop skill. In fact, there are materials for antennas that we have not yet discovered—and most of them are already in home building centers or supply houses waiting for the right person with an active imagination to uncover their rightful place in an antenna. So, keep those experiments coming. One never knows when that new breakthrough might just happen to you!

A NEW BOOK
One of our contributors, Doug Miron has a new book "Small Antenna Design" on which he sent me an announcement about the release. Looks like a very interesting book and I have included info from the book's Preface sent to me by Doug, plus a link to a website that distributes the book. The website contains more info about the chapter titles and topics covered.

wpeA.jpg (5062 bytes) Preface
Miniaturization of electronic systems has accelerated over the last few decades and this process feeds expectations for even smaller components and systems in each new generation of equipment. Antennas have not been exempt from this pressure to be made smaller. Often, the result has been the use of antennas that are reduced in size without regard to their performance. This has led to needlessly poor system efficiency and reduced range. In this text we discuss the limitations on small antenna performance, possible tradeoffs, recent developments, detailed design and optimization.

Antenna performance is fundamentally a function of size measured in wavelengths at the operating frequency. "Electrically small" antennas are those that are small compared to the wavelength, not necessarily small compared to the people who use them. The wavelength at the middle of the AM broadcast band is 300 m, so a tower 30 m tall is called electrically small even though it's 15 times the height of a tall man. "Small" in the antenna business can mean "electrically small", "low profile", or "physically small".

Historical applications have included mine communications, broadcast transmission and reception, and mobile radio communication for both military and civilian uses. Present and future applications include the historical ones plus mobile telephones and handheld combinations of telephones and wireless data links for video and computer mobile networks, and wireless data networks that include both stationary and mobile elements. Versions of these networks are being designed and deployed in the obvious area of personal communications, and areas as diverse as medical monitoring and industrial production. The performance and efficiency (battery life, for example) of any of these systems depends in a very basic way on each device's ability to get its signal out and capture the signals from the other elements in the network. The antenna is the component that does it.

The intended reader for this book is the design engineer with a B.S.E.E. degree. Chapter-end problems have been written so that the book can also be used in a senior-level elective course. Most people graduating from such a program have an exposure to electromagnetic theory but no experience in rf circuits or actual antennas. Some mathematics is necessary to understand the concepts presented, but few derivations are given. Each topic will begin with a discussion of the physical principles involved. The simplest possible illustration will be used first, one with analytic results available if possible. Then more complex and more practical versions of the topic will be presented. The emphasis is on the intelligent use of formulas where available and applicable, and numerical modeling. The analytic results for simple structures not only provide useful guidance in themselves, they also serve as checks or standards for the numerical modeling process. The ideal situation is achieved when analysis, numerical simulation, and experimental results all agree, and published results are used whenever possible.

A major concern of this work is to bring small antenna design into the current computational environment. Familiarity with the Windows operating systems, C++, and MATLAB is assumed, but not entirely essential. A reader familiar with C will not find it difficult to read the program listings in the book. Some historically-important data has been converted from graphical form to curve-fit equations so that the entire design process can be done on the computer. Many original modeling programs have been written for both traditional and novel antennas and supporting structures.

Teaching is the third way of learning. First, one learns as a student, then as a practitioner. To teach, one has to understand even better to pass the art and science on to another. Unlike this Preface, the book is written mostly in the style of informal lecture and conversation. It is as much a story as a textbook.

The CD-ROM
Numerical methods are the main tool used in this book. The main numerical tool is NEC2, the U.S. Government-developed code for modeling wire antennas. The folder nec has source code and executables for the current user-modified version of this code. The root folder also has a public-domain GUI in a .zip file. In addition, the nec folder has many C++ programs to generate NEC input files for various antennas and structures. These programs read text files with numbers describing the geometry and operating conditions for each antenna and structure. Examples of such text files and other utility programs are also in the nec folder.

MATLAB was used for data analysis in many of the examples in the book. Utility programs to find the equivalent circuit Q, bandwidth, resistance components, and antenna voltage for a given power input are in the matrf folder. This folder also contains programs for impedance-matching design, both narrow-band and wide-band. Also, there are programs for generating data used in some of the book's examples, including the NEC basis functions and curve-fitting with these functions.

A link to a vendor's website description is HERE.

If any of our readers should purchase this book, I would very interested in a review article. Just let me know.

REMEMBER SHROPSHIRE?
It has been years since the test CFA situated in Shropshire came to an abrupt end. Last reported was that the test unit was far from a decent level of performance and co-inventor/promoter, Kabbary had departed the scene not to return. Other technical help was solicited but to no avail. This was followed by squabbles about the patent ownership, or at least rights to use it.

To refresh our memory, below is an excerpt of the last summary I reported in a portion of my column of August 2001 about most of the (known) existing CFAs outside of Egypt — almost 5 years ago!

[...] BUT FRUSTRATIONS ABOUND (Dateline August 2001)
Although a number of new broadcast CFAs have been installed within the past two years, the CFA program's image was beginning to suffer because none were working. According to last information, here is a summary of where things stand for the CFA's program outside of Egypt:

Sydney, Australia - CFA mode shut down and operating in non-CFA mode. CFA is for sale, but no takers yet. Frustrations.
Kiel, Germany - Station discontinued operation and CFA not is use. While shut down of station may not be because of the CFA, no apparent interest in using it.
San Remo, Italy - CFA never worked up to par and plans to purchase a larger CFA put on hold. Present CFA waiting for Kabbary to return and continue efforts to tune. After several months of waiting, no known date set for return. Frustrations.
Brazil - CFA is not working. Tried three different cones to no avail. Engineers are frustrated. Again, several months have passed waiting for Kabbary to return. Project engineer searching for ways to tune absent any available help from Kabbary, but no success yet nor good signals. More frustrations.
IoM CFA - Was to be the largest CFA in the world. Permission to build denied by the Planning Commission. This CFA was never able to have a chance to prove itself, but substantial funding and effort lost as a result of the denial. Some part of that approval process dealt with the lack of understanding how the CFA worked and its amount of radiation.
UK Test CFA - It is not working and transmitter shuts down on startup. Discouraged, more than a month ago, Kabbary left the site and went home to Egypt. As of today (July 31, 2001) no date is set for Kabbary to return and make additional efforts to tune. Setbacks are costly and great frustration has set in here too.

brazilmap.jpg (13589 bytes) ABSENCE OF TECHNICAL SUPPORT
Obviously, from the above, the CFA program is suffering the loss of more credibility as each day passes from the much-needed lack of technical attention on so many existing projects. Although I cannot speak for those involved in the projects, certainly there must exist a great deal of frustration by not knowing how to make those idle CFAs work nor when or IF they ever will work as hoped at the beginning of the projects. This still does NOT mean that the CFA cannot be made to work at some level, only that at the CFA projects outside Egypt are in dire need of the proper technical support needed to tune them. As I pointed out in an earlier column a few months ago, it is a scary thing to think that so much depends on the special abilities of a single person, and what could happen if that person is not available for whatever reason. This is what happens!

For example, more than a half dozen attempts have been made to tune the one at Santos, Brazil to the "CFA mode". At all times the CFA has been "sensitive" but has not yet produced a good signal. By "sensitive" is meant that every time it seems to be about to reach the "CFA mode", it jumps out to a low efficiency mode similar to a very short monopole's characteristic. This is the most common problem found in all of the dozens of antenneX experiments with the CFA and we have always noted that tuning is the most tricky part of the CFA. This stage is where would-be CFAers either succeed or fail. Statistically, a large percentage fail. So, this is where Kabbary's magic is applied. However, it seems even he does not have it down to a complete science. It has been observed that why can't a program be developed so that once one of these stations are set up, a Laptop computer could be employed for the necessary tuning part. Of course such a program would need to possess all of the variables that pertain to the location of the site and be factored in, e.g., soil conductivity, terrain, surrounding objects, just to mention a few that can have a bearing.

DELAYS DEFY LOGIC!
Our inspection trip, were it to have turned out with positive results about the CFA's performance could have been just the "shot in the arm" needed to give the CFA program a much-needed boost. If the Sentech members of our inspection team saw that the CFA worked well enough to solve their needs, it could mean a tremendous amount of potential business—and antenneX could report the positive results to 175 countries (now 203) in the blink of an eye! The fact that this trip hasn't happened yet seems to defy logic on the surface if indeed the promoters believe its claims and want to promote sales of the CFA. Of course if we did the inspection and things did not turn out well, that "sword cuts both ways" and the bad news would get out just as fast! But, our team from the start has maintained an open mind about the CFA and have no preconceived notions about the outcome. It is truly hoped the CFA does work! The world NEEDS an efficient compact antenna and there have been high hopes that this is the answer, or at least part of the answer. The device doesn't have to be perfect—it just needs to work well enough. We don't even care if we don't understand "Poynting Vector Synthesis" as long as it works and we can verify that it truly does work using conventional measurements to the extent possible under clinical conditions.

Kabbary attributes the delay of our inspection trip largely to a major change in the management at ERTU (Egyptian Radio and TV Union), the company that owns the CFAs (or at least operates them) in Egypt and for whom Kabbary works as a consulting engineer. It is unclear as to what testtime.jpg (24983 bytes) motivation if any that ERTU has regarding the expansion of the CFA program outside of Egypt. In fact, considering this "shakeup" in the management hierarchy, one wonders about the future of the CFA program within Egypt! Word has it that not all members within the ERTU are in support of Kabbary's CFA program—in fact, some may be opposed and Kabbary is reported to have been marshalling his supporters. Clearly, a major change in the management of any organization has its potential affect on all programs of the earlier management and creates some uncertainty about the future. But, in this particular case, is it really a management change? My understanding is that the spouse of the previous ERTU head of engineering has taken over the day-to-day management activities as a result of the husband's poor health.

Thus, as far as any further plans our team may have for an inspection of the CFAs in Egypt, such will require the approval, in writing, from the new management as well as the guarantee of full cooperation from Kabbary and his participation throughout the inspection. Any and all agreements would need to provide for full access to the CFA site during inspection and allow adequate time to carry out such inspection. Our team estimates about seven days to do the tests properly.

But, I must add that our group is losing interest in continuing the same "roller coaster" ride that has stretched out over so many months. Something so important shouldn't take so long or be met with so much resistance, especially when considering an important customer is involved in the effort -- not just antenneX whose interest is more of a fact-finding journalistic effort. It is a normal expectation for a customer to see and test a product before buying a "pig in a poke". What a shame—an opportunity may have been missed—but "The Test of Time" continues…..[...]

Below follows the last of the pictures received from all angles at the Shropshire test site. These pictures are provided by the courtesy of Prof. David Jefferies who visited the site along with Prof. Mike Underhill to conduct independent tests in 2001. As I recall, those tests indicated very poor results — single digit % efficiency. The excellent pictures and comments are published here with the permission of David. Our thanks to him. Distant view from a half wavelength from the "antenna"	View from a quarter wavelength away
Telephoto view showing height above large body of water	View under trees - from surface of "radian sphere"
Another close angle	Tuning workbench - note tape conductors (skindepth loss reasons)
Feeding the CFA	Feed Wire