Sorting Out the Quest for a Compact Super Antenna

lbccomp.jpg (8771 bytes) Sorting Out the Quest
for a Compact Super Antenna

L. B. Cebik, W4RNL

omeone once asked me why folks keep searching for a compact super antenna when there are already so many good large antennas around. The question holds some interesting ambiguities. We might try to answer it in terms of investigator psychology. Challenge, mystery, and hope are powerful motivations for innovation, often as powerful as invention's mother, necessity. However, this line of answer is for others to take. I would not pretend to know investigators well enough to analyze their psyche's. Still, once some investigators think they have found an answer, if not THE answer, profit and/or fame do seem to play a role in the subsequent efforts to sell the answer.

A more logical and less psychological way to look at the question of why investigators pursue the compact super antenna is to think of this antenna as a goal. If the goal is pursued, it is because such an antenna would fulfill one or more important functions in the communications systems that we establish for broadcast, 2-way, and network purposes. Such an antenna would be applied within the LF to lower VHF range, where antenna real estate is relatively to intrinsically large. Hence, the chief function of a compact super antenna is to save space, to compress functions that require from many square feet to many square acres into something ranging from a few square inches to a few square feet.

Antennas have many performance parameters, not all of which are relevant to any one communications purpose. For some, gain is paramount. Indeed, that gain may be directional or non-directional, depending on the communications system requirements. In other cases, low noise or immunity to noise sources may be the most important performance factor. Interestingly, some of the more recent candidates for the role of compact super antenna have been very unclear about the performance parameter or parameters that support the candidacy.

Perhaps it may be useful to sort out some of the types of antennas that have historically entered the arena of compact super antennas to see what their benefits might be--or would be, if their principles prove sound.

1. Loops and other very small antennas. The compact loop, loosely illustrated in Fig. 1 and ordinarily under 0.3 wavelengths in circumference, is a classic example of a small antenna whose principles of operation are well understood. Design books and articles for this type of antenna abound. It makes no claim to gain, but employs a very narrow bandwidth to provide a relatively quiet listening window. Preamplification is ordinarily necessary, although many receivers have the surplus gain to handle the small loop by direct connection to the coupling arm.

The very narrow bandwidth of the small loop is typical also of a cluster of small antennas used predominantly as receiving antennas in the lower HF and upper MF range. Since there are numerous types of noise sources, different designs tend to suppress one type of noise better than another. The EWE (and its small diamond and delta offspring) and the K6STI small loop are examples of such antennas, each improving reception by noise suppression of one or another sort.

Unlike the Beverage, which requires a farmland fence line for support, the class of small antennas noted here all fit in suburban backyards for 160-meter operation. Hardly anyone attempts to use these antennas for transmitting purposes, except for the small loop, of course. Small transmitting loops have been tried since the 1920s, when some were designed as combined planar loop antennas and tank coils for keyed oscillators. In most cases, random wires and even metal drain spouts tended to yield higher signal strength. This accumulated experience has not prevented thriving entrepreneurs from touting variations of the loop as the answer to every ham's prayer for an indoor antennas that is as good as any large, ungainly outdoor antenna. The difficulty is that no one in the amateur radio experimenter's world appears to have the resources in terms of land, equipment, and candidate purchase price to perform the necessary range tests to either confirm or dispel the sales spiel.

Of all of the compact super antennas, the small loop remains the smallest, despite its gain deficit.

2. The loaded linear element. A second way of achieving some degree of antenna compaction is to load the element or elements in sundry well-developed ways. Fig. 2 illustrates some of the common loading techniques. Like the loop and its companion receiving antennas, loaded linear elements (or even loaded wire loop elements) rest upon well-understood principles easily traced to the theoretical roots of wire antenna operation. Moreover, there is a well-developed technology--almost as old as radio itself--for making these antennas to specification.

For example, the entire class of these antennas tends to sustain its gain into the region between 65% and 70% of normal resonant length, below which length, the gain tends to drop more rapidly. Inductive loading tends to suffer higher losses than other forms of loading due to resistive losses within loading coils of finite Q. Center-loading results in a more rapid drop of the feedpoint impedance than mid-element-loading, although the highly advertised gain advantage of mid-element-loading over center-loading is more theoretic than real. Because mid-element loading coils have a total reactance much greater than the reactance of a single center-loading coil for a given short antenna situation, for a fixed Q, their resistive losses are greater, which offsets a good bit of the advantage gathered by placing them farther out on the element.

Of the common loading methods, the "hat" tends to be the most efficient, since it sustains the element current levels of a full-size element out to where the hat structure begins. This feature of hat-loaded elements also holds true of multi-element parasitic arrays to the degree that the inter-element coupling can be more completely sustained. Hats, in their most ideal sense, simply divide the current at their starting point in such a way as to produce self-canceling fields at right angles to the main element. This arrangement tends to produce the highest gain and feedpoint impedance for a single element, as well as the widest operating bandwidth. However, the hat is inherently a mechanical problem, since it concentrates mass and wind resistance where we least wish them to be on an antenna element--at the unsupported end. Hence, a number of "near hat" structures have arisen to take up less room and provide almost complete cancellation of radiation from the element ends. Many of these near-hat schemes have been successful for specific applications, especially where the last measures of gain and operating bandwidth are not absolutely necessary.

However, as a venture designed to achieve the status of compact super antenna, the loaded element falls short--because it remains too long. Commercial short near-hat vertical dipoles are available and have their place in both the receiving and transmitting world of the radio amateur. Near an ocean, they are capable of surprising performance, but part of that surprise disappears if the beach location happens to be on a DX island. In short, a vertical dipole about 20% of full size can provide a signal that normally is considerably stronger than that of a small loop by a few S-units, although the signal strength will be quite noticeably weaker than that of a full size antenna element. Nonetheless, the shortest of the commercial ventures into hatted vertical dipoles might well be used as a measuring stick for some of the more recent technological introductions--a sort of secondary standard whose properties relative to full-size elements are well known.

3. Geometrically shrunken antenna forms. It once was fashionable to fold antenna elements into double and triple pathways and to assert that one obtained nearly full-size performance. I recall a ZL-Special variant from the 50s or 60s that had more S-curves than most mountain roads. Gradually, we came to have a better understanding of what is involved in linear loading--as the technique came to be called.

In its center-loaded form--at the top of Fig. 3--the linear load or folded portion of the overall antenna element tends to answer to equations for shorted transmission line stubs under a certain condition. The requisite condition is that the folded portion of the element have its two lines equally spaced from what has been called the main element. Under this condition, the two lines of the folded section have close to equal and opposite current phases at the same current magnitude. However, if we place the folded section in a planar arrangement relative to the main element, then the currents in the fold-back do not fully answer to transmission line characteristics. The net radiation from this region of the antenna--or from any point along it--tends to be a function of the sum of the currents on the various wires--and at least one of them has sizeable current at a phase angle that tends to cancel out with the current in one of the other wires. Hence, the linear load tends to act very much like a loading coil, but with a lower resistive loss.

Experiments suggest that the linear load, when placed at the outermost portion of the element, tends to be much more efficient than when placed at the element center. As shown in the figure, the folded section must point outward for this advantage. Some past commercial linear loaded elements claimed to start at mid-element for lower losses, but they folded inward toward the element center. Unfortunately, the misunderstanding connected with such systems is that they are no different than the center loading system: we still have three wires at the antenna element center section of highest current, and one of them goes in the wrong direction. Placing the loading fold-back section at the element outer end is actually a form of "near-hat" loading. Loading only has so many forms.

However, let's introduce an ambiguity into this picture. The repetitive geometric form, sketched in one of its incarnations at the bottom of Fig. 3, leaves open the question of which portion is the element and which part is the loading fold-back. The result has been a rather unproductive debate about the proper way to label the parts of the antenna, as if the labels then--in and of themselves--warranted any claims about the antenna element performance. Such debates have clouded our understanding not only of this particular element, but as well of the entire host of geometric antenna shape revisions based on fractal and other geometries. Fortunately, linear wire fundamentals govern all of these antenna efforts, and method-of-moments calculations do not look at labels. Consequently, it is possible to accurately analyze the performance potential of most of the antennas that comprise the geometric revision of classic antenna shapes.

Still, most of these efforts still fall short of the goal of a compact super antenna, one that retains very good gain in at least one sense of the term while reducing antenna size to something less than 20% of a full size element. The gain of most geometric revisions tends to decrease in normal curves for the reduced footprint of the antenna, although some of the losses associated with inductive loading are avoided. Hence, these antennas tend to approach the limits of gain for shortened antennas, subject only to wire losses. However, they remain relatively large among the candidates for the compact super antenna.

4. Special principle antennas. As Fig. 4 suggests, the present candidates for the role of compact super antenna from the "special principle" category number 2: the CFA and the Compact C. Interestingly, both antennas are volumetrically larger than full-size quarter wavelength monopoles against which their performance is usually measured--or at least verbally compared. Whereas the full size monopole is tall and requires extensive guying, these two antenna types are squat and require simple guying. Moreover, neither is said to require an extensive ground plane radial system.

Equally interesting, the hatted vertical dipole noted earlier is about the same height as the Super C, but uses a recommended mounting that is somewhat taller. The hatted dipole, when translated to the common frequencies used by the CFA is about twice as tall. However, in both cases, the hatted dipole structure is much simpler.

Given these comparisons, the question remains why there is such experimenter interest in these two antennas. I suspect, but would not assert with certainty, that the answer has something to do with the shroud of mystery that has surrounded--and continues to surround--these antenna types. It is a mystery looking for a solution, or more correctly, a collection of solutions.

It appears that the back engineering of the CFA has reached success or near success in recapturing the structure and networking that are necessary for the antenna to perform in ways coincident with inventor claims. Of course, serious range testing and comparisons with a standard monopole remain to be done on a basis that yields consistent and reproducible results. But it appears that experimenters have high confidence that such tests will be successful. However, success has many meanings. It remains to be seen whether independent field generation and joining at a distance from the assembly--without external coupling and reradiation--will in fact be achieved. As well, what the comparative gain will be, relative to a monopole, is still not known.

I suspect that, even with highly credible measurement and testing, not all skeptics will be satisfied. The best efforts at modeling show the antenna to be capable of only short monopole performance. If measurements turn up higher performance values and if it is clear that no coupling and reradiation can account for such measurements, we shall encounter two reactions. Some will assert that the CFA principle is vindicated. Others will look for other mechanisms to account for the radiation measurements.

In one sense, measured satisfactory radiation results will not comprise a satisfactory proof of the CFA principle, but will only prove device performance. Still lacking is the derivation from the claimed fundamental principle in Maxwell's laws of a set of equations that will ultimately yield a set of physical conditions that are clearly identifiable as a CFA device. Such derivations exist for wire antennas, giving them a solid theoretical basis. Performance success of a CFA device would still leave a gap between the device and its purported foundation.

Interestingly, the Super C suffers a similar gap, but within the body of wire antenna theory. Unlike the CFA, the Super C claims to be immune to magnetic coupling, and the antenna makers argue that they have a large body of experiential evidence supporting this claim. The bulk of the evidence consists of transporting the antenna into close proximity with metal structures that in fact detune monopoles and other standard antennas. The Super C remains unaffected.

Although the manufacturer has what must be termed experiential comparisons with monopoles--and the Super C appears to equal the monopole in overall performance--independent range testing has still to be performed. Indeed, in the realm of amateur antenna testing, it is unlikely that we shall ever see the marshalling of resources needed to perform such tests on an "as needed" basis. But range tests may in the long run be less critical to the Super C than its theoretical foundation.

The antenna operation claims for the Super C rest on the idea of there being between the basket and the plate a large capacitive field. Unlike the fixed and variable capacitors with which we are most familiar, the field is not tightly confined between two evenly spaced plates. Instead, the field exists largely between two plates at right angles to each other. The surface areas of the basket and the plate are quite large, but the spacing is variable.

What remains murky with the Super C is to what degree, if any, the field between the plates plays a role in the performance of the antenna--on the assumption that it performs up to the maker's claims. If the field plays a role, what role and quantitatively to what extent? If the field does not play a role in performance as an antenna (except for the claimed immunity to coupling with nearby metallic masses), then what factors do determine the performance and quantitatively to what degree? Perhaps only when there are some appropriate range measurements and comparisons with the monopole standard will we be in a position to account for the Super C performance as being a simple function of surface area or to conclude that other factors play a role. We shall still be left with the task of identifying and quantifying such factors.

In principle, the Super C questions ought to be simpler to resolve than those surrounding the CFA. Nonetheless, both antennas remain elusive. Certainly, both need rigorous range testing. But equally, both need some serious attention to basic theoretic questions that would connect them to fundamental principles.

5. Magical antennas. In the quest for the compact super antenna, devices emerge from time to time--some with the respectability of being patented--that claim to do everything with virtually nothing at all. What these claims lack is not merely a device which proves to live up to the claims of the inventor (or sales company). They also lack any clear connection with fundamental principles from which to derive the claimed performance of the device. Thus, the claims surrounding such magical antenna devices lack any logical cohesion with the body of concepts and mathematics by which we even recognize antenna action and events as going on.

It is the clear relationship with the fundamentals of antenna theory that gives sense to even mistaken claims about antenna devices. The relationships also provide us with the clues and the methods by which to find errors and to correct them. By having no logical or mathematical relationship to existing antenna theory, most magical antenna devices disqualify themselves from being called antennas at all. If a given device is to properly be called an antenna, it must be redescribed in terms that logically enfold it within the body of antenna principles.

I note the category of magical antennas only to illustrate the seriousness of logical gaps in relating claimed antenna performance to fundamental theory. So long as serious gaps exist in the fundamental theory of operation of both the CFA and the Super C antennas--and for any other antenna claiming to rest on special principles--skepticism (not cynicism) will remain a valid response. Granted that test results, if sufficiently rigorous, can provide clues to the direction required of theoretical explanations and derivations. However, until the theoretic work is completed, the skeptic can never be rationally silenced.

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