When is a Compact Antenna Compact?
By Publisher, Jack L. Stone
n interesting thought occurred to me the other day than pertains to the
question presented by the title of this column. When is a "Compact Antenna
Compact?" The simplicity of the thought was provocative as it also occurred to me
that there was no single answer. I believe the real answer is: when it suits a particular
For quite some time, I have had an arbitrary concept of a compact antenna, mostly as it applies to the radio amateurs. Its a very simple concept and one we fondly refer to here as the Holy Grail as in search of. Mind you, its a goal, as one must have one so we know what to work toward.
Perhaps this picture from the front cover of a book on compact antennas depicts it better than words will ever do. This setup of Dick Jones, K9ZNZ is a classic now. The antenna you see is mounted in his attic connected with 30 feet of cable running down to his room dedicated to his radio shack. Thus, its completely concealed from the outside neighborhood and any hall monitors that may be lurking out there.
A ROSE BY ANY OTHER NAME...
A QRP buff, Dick happily operates 40 meters with 5 watts on CW and makes his regular contacts between 500-1,700 miles distance. He has been using this setup for more than eight years now and his needs are satisfied. Argue that the device is just a dummy load and feedline, or something else is doing the radiating if you will, but that doesnt matter to Dickhes still quite happy just the same. A rose by any other name smells the same.
So, hurray for radiating cables and dummy loads! With that said,
however, if one is building the device and knows it is a dummy load, then okay, but, if
some manufacturer is making lofty claims about some mysterious device at a price, then if
the coax or dummy load is doing the work, it should be disclosed as such. Take special
notice when a certain size length feedline is dictated as being essential. Also, if one is
told that the device needs to be mounted on a very tall mast, then how does that qualify
as a compact antenna? One might as well have a tall, efficient vertical tower!
I do not mean to sound overly simplistic in describing Dicks arrangement. Quite the contrary, I am quite aware of the efforts and the great amount of focus Dick placed on this setup and he is to be commended. Many would envy the results and his arrangement. Many dont even have an attic! This is a dream setup and may all of you be as lucky to find such slick solutions to your particular operating needs. BTW, this book with Dicks picture is found in the BookShelf and contains all of the setup and operating details.
DEFINING A COMPACT ANTENNA
Back to the question of defining a compact antenna, I decided to ask a few other radio people this question just to see if my theory was correct that one compact does NOT fit all, as logically it should not. It depends on whether one needs an inside or outside antenna, DX or QRP and everything in between. Perhaps in some cases, we may be talking about volume of an antenna when ascertaining its size. In other words, rather than building upwards when high profile is restricted, we build outwards to gain efficiency, assuming we have enough space.
Moreover, it has been observed and commented that if one considers that it takes the whole antenna system to make the antenna perform, a compact is not really very compact. Conversely, if it meets the NEEDS, and can operate within the limits of space (volume?) given, then the more compact we can make that volume (whether up or out) then more condensing or compacting still remains the goal.
Here is a definition of a small antenna as was given in an excerpt by Harold A. Wheeler from the Antenna Engineering Handbook, third edition. This excerpt defining a "small antenna" submitted to me shall be taken to be synonymous with the meaning of the compact antenna:
.A small antenna is here defined as an antenna occupying a small fraction of one radiansphere in space. Typically its greatest dimension is less than one-quarter wavelength (including any image in a ground plane). Some of its properties and its available performance are limited by its size and by the laws of nature. An appreciation of these limitations has proved helpful in arriving at practical designs.
The radiansphere is the spherical volume having a radius of 1/2pi wavelength. It is a logical reference here because, around a small antenna, it is the space occupied mainly by the stored energy of its electric or magnetic field .
LET'S ASK FOR OTHER RECENT OPINIONS
Ive taken a small but fresh poll of opinions from a group of radio friends and heres a few of the answers I would like to share, some brief, some with greater detail, but almost all differ in some respect, while I believe most agree to the one thing in commonsmaller. As you will see, they vary widely, some are brief and others come with more thought. I imagine there are many more opinions of this out there as well.
My definition of a compact antenna is an antenna having a length of less than 1/4 of its full-size counterpart. A 10-meter tall vertical must become < 2.5 meters. A loop 10 meters on a side must become less than 2.5 meters on a side.
As I see it, we have three performance parameters to play with: size, efficiency, and bandwidth. As the first part of a design one or two of these parameters can be set as a performance specification. Physics then dictates the remaining parameter(s).
For example, if we set our efficiency goal to 1% then we have wide latitude with size and bandwidth. These two parameters can be traded off - 1% efficient with small and narrowband or large and wideband. A -12 dB spec equates to an efficiency of 6%. This seems reasonable to me. One can obtain 6% efficiency with an antenna of about 1/25 normal size or a one-meter tall 80-meter antenna for example.
I know we are always fighting the issue of feedline radiation. For a small antenna I like to set a feedline radiation maximum of 6 dB below the antenna radiation. The feedline radiation can then be largely ignored.
I should point out that the 1/25 size antenna for 6% efficiency I give is figured for a top hat of about the same diameter as the height. If we are free to build out instead of up then nearly 100% efficiency can be achieved in a very short antenna. Take a 1-meter tall 80-meter vertical for example. If we make it into 1-meter tall Inverted-L with a 19-meter horizontal section then efficiency will be nearly 100% over perfect GND. No lossy loading coil is needed. Bandwidth will be narrow. Now let's maximize the bandwidth. Bandwidth is proportional to Q, which is X/R. The R, or radiation resistance, will be maximized when the current-area of the 1-meter vertical section is maximized. The X, which is the capacitive reactance of the top hat must then be minimized. The farthest we can go is to make a solid top hat with a radius of around 19 meters. We then have a very short vertical with no loading coil, essentially 100% efficiency, and maximum bandwidth.
My 28-MHz spiral vertical is built along these lines. Although it is only 9 inches tall it is 24 inches across. You can imagine what the 80-meter version will look like - 6 feet tall and 16 feet across! A bit different from our usual idea of a small antenna but it does meet my specification of being 6 feet tall (so it's not visible outside of my back yard). I will see if the spiral can be shrunk without taking too big of a hit in efficiency and bandwidth. It might be small but it's not compact.
Perhaps the definition of a small or compact antenna has to involve volume as defined by Harold A. Wheeler. It could be fun to see how small an antenna could go and still meet the -12 dB efficiency spec. The 40m Isotron is around -16 dB and it's mighty small. I think the efficiency could be upped to -12 dB with a couple of improvements. It fits in a sphere having a radius of 34 cm or a volume of 52,000 sq. cm. In contrast, a 40-meter spiral vertical, scaled from my 28 MHz spiral, has a radius of 130 cm and fits in a sphere having a volume of 2,900,000 cm. This is 56 times the volume of the Isotron but the efficiency is 13 dB better than the Isotron. It's a question of designing for "near full-sized antenna" performance rather than a really compact antenna, which suffers more loss of efficiency.
Smaller, yes. As small as possible, to deliver acceptable performance. To me, "acceptable" is no more than 2 S units down from optimum. That would put comms at approximately the Ham QRP level using a "compact" antenna.
Efficiency is the key. All else pales when compared to the desired effect, the maximum received signal possible at the far end of the radio circuit. Grounding may help RFI issues and afford safety, and power is a relative term.
I would think possibly two classes of antenna could be considered: one that is designed to take the average power of a Ham station (100 Watts), and another class to handle the full legal limit (1500 Watts). Commercial designs could be designed around similar constraints; those that are pertinent to the service at hand.
No reliance on tuned feeders/coax lengths. Ground independence would be ideal. Directivity would be awesome, but beggars can't be choosers. Electrically safe to stuff into an attic, etc. Sturdy enough for mobile mounting. Not overly complex to build if desired or tune.
For me, a compact antenna is an 80-meter antenna that fits in my 20 meter yard! ( and still radiates)
Here's my wish list from my experimental notebook for 20 meters
1. Small size, 0.75 meter turning radius on 20M, not more than 1.5 meters tall
2. Efficient: (85% minimum)
3. Wideband: (5% 2:1 SWR)
4. Available as Omnidirectional or Directional design
5. Directionality: 8dBi gain, 15 dB F/B ratio
6. No need for counterpoise, 1/4 W stub or other large external devices
7. Environment: No more than 5% change in parameters with 0-deg C to 40-deg C temperature, relative humidity from 10% to 100% condensing
8. Power: 100W continuous with not more than 5% change in any parameter in 5 minute TX at 20 deg C.
9. Height: Less than a 5% change in any parameter when HAG varied from 1 meter to 15 meters
10. Proximity: Less than a 5% change in any parameter when placed within 1.5
meter of any metal object
11. Grounding Vs ungrounded should not change any parameter by more than 5%
12. Cost: Manufacturing cost (materials) Not To Exceed US $25
1. Efficiency -- Very difficult to achieve in a small antenna -- resistive losses become a much larger percent of the overall impedance. In our portfolio of tricks we need to develop means to reduce losses in small antennas. Tricks used in mobile applications, UHF etc., include large diameter coil inductors, silver plating, etc. No doubt there is much more to learn.
2. Small antennas should be designed to handle power as a minimum 100 watts continuous and 400 watts PEP.
3. With balanced antennas, grounding is not important. However, a good ground allows for the foreshortening of a dipole-like antenna to ground plane-like antenna. A consequence is that with a short ground plane-like antenna, ground losses become large, unknown and unmanageable. We should avoid small, grounded antennas when possible.
The question of what a compact antenna is rests upon the additional specifications brought to the design project. Here are a few samples of "compact" antennas and some of the added specifications.
1. A compact beam: the Hex beam, the HF5B Butterfly beam, and a number of other commercial and home-made projects are considered compact relative to full size Yagis (normally, 2-element, but sometimes 3). Among compact beams, then, the competition is the most gain and/or best front-back performance possible in the smaller size. Mono-band vs. multi-band considerations get mixed in here, as do changing performance figures from one band to another.
2. The compact QRP field antenna: Here the notion of compact applies first and foremost to being able to transport the antenna to/from the field site, but with the stipulation that assembly/disassembly must be convenient, and the antenna should be easily supported if not self-supporting. The most common antenna breakdown is into 2' to 3' sections. The weight versus sturdiness of the antenna then become a prime consideration. Note that the antenna in this context can be full-size or less when assembled.
3. Confined-quarters antennas (such as attic/room/etc.): These antennas are the ones for which efficiency becomes one of the prime considerationsbut not the only one. As well, one needs to consider relative freedom from unwanted interactions with objects and structures in the operating environment. The compactness may be measured against a standard antennasuch as a 1/4-wavelength monopoleor internally compared among compact antennas designed for essentially the same service.
In this category, we must beware of illusions. If analyses of some "compacts" are correct, then the compact elements sold are only the antenna termination and feed, not the entire antenna.
4. Space-limited antennas: The CFA and Super-C, regardless of operating principles, are both designed as antennas that function in the same environment as a standard antenna, but occupy far less room in at least one critical dimension. Volumetricallyand relative to the frequency of operationsuch antennas occupy more room. Hence, it is the critical dimension that is at question. At AM-BC wavelengths, it is the matter of height, plus all of the non-radiating support paraphernalia that is saved by going to the squat design. Once one settles this question, we can ask for efficiency comparisons, where the efficiency can be measured by a) signal strength at specified distancesground wave or skipor b) power radiated vs. power supplied.
We can generate more categories, but these are sufficient to show how context-bound the basic question is. Hence, one must ask not the simple question "What is a compact antenna?" Rather, we must ask "What will count as a compact antenna relative to a set of operating needs, performance specifications, and design constraints?"
The last answer again sums it up in a similar manner as
I stated at the beginning one size does not fit allthere will be more than ONE
compact. The right one will be according to a particular need!
Now, not everyone needs a compact of course and theres plenty of room for more good, efficient antennas out there in the marketplace. However, the trend in the marketplace toward smaller should not be ignored as with any product. To do so is to get left behind. The customer is always right!
THE ISOTRON REVEALED
Speaking of compacts, the Isotron fits within that category and it has been around for some 22 years. Yet, it still remains a mystery as to how and if it worksthat is, up until this month. If you noticed the front cover story this month, we have an excellent article about the Isotron. The article is entitled The Isotron Revealed and is Part 1 of a two-part series written by Dave Cuthbert, WX7G. Dave and other members of the GARDS have been working hard to investigate the various capacitor-type devices in use today. What better way to conduct research into the development of a better compact antenna than to first look closely at existing technology. Do they work and if so, how do they work? Can we learn from some of the good ideas found and re-employ them in a more efficient manner that will achieve our goals? The answer is: certainly! ...and what NOT to employ. Maybe a whole new direction will emerge.
Dave has done an excellent job and this is probably the most thorough investigation into the Isotron made to date. He not only determines if and how it works, but how it may best be used to the extent that time allowed for the investigation. Finally, after 22 years of presence, we now know about the Isotron that so many have asked us about over the years.
RADIATION MECHANISM OF TRANSMITTING ANTENNAS
We have the pleasure again this month of presenting more of Dr. Kabbarys theories on the subject of antennas and how they should work within his own interpretation of such theories. Ever since he published his paper on the corrections to Maxwells fourth equation, we have been trying to understand how this is supposed to make the CFA work. Accordingly I have urged Dr. Kabbary to present more information about his own theories so that perhaps we could all better comprehend and perhaps provoke thought. This is something that has been needed all along and this month, Dr. Kabbary presents Part 2 of his theories in his article Radiation Mechanism of Transmitting Antennas.
This article will be followed by Part 3 next month where Dr. Kabbary will present yet another installment of this discussion, including the subject of Far Field Radiation. As Dr. Kabbary states, "...radiation at far field can be described as vortices carrying energy pockets, having the frequency higher than the transmitter frequency, but less than microscopic frequency. This mechanism of far field radiation will be shown in our next article...." Moreover, Dr. Kabbary will explain more about his definition and distinction between the macroscopic and microscopic references made in his articlesin other words, when does macroscopic become microscopic?
IN THIS ISSUE
This month is our 66th online issue! We again include many fine articles by our great writing team. Now, allow me now to introduce this month's line-up of content:
THE OCTOBER 2002 ONLINE ISSUE NO. 66 CONTENTS:
OUR MONTHLY COLUMNS (plus this one by yours truly):
FEATURE ARTICLES IN THE LIBRARY:
Basics of Very-Wide-Band Yagi Design
|In Part 1 of this small study of very-wide-band (VWB) Yagis, we examined an 8-element parasitic array using crossed elements and quadrature feed with potential application to satellite communications in the 250-317-MHz region. In the course of our examination, we encountered some of the principles underlying VWB Yagi design. However, those principles in part were compromised by the bandwidth-broadening effect of the quadrature feed system. The Yagi described there, if set out as a planar structure without the crossing elements shows a significant reduction in operating bandwidth. In this Part 2 of our investigation, we shall pose a simple question: can the bandwidth of a planar Yagi be extended to cover the same 67-MHz bandwidth with usable performance?|
LAB NOTES: Updating the Gieskieng Transmission Line II
|Last month Joel described the effect of using a loading capacitor to lower the resonant frequency of a quarter wave open-ended line section. The capacitor can be considered as a variation of the lines open end. The line transforms its impedance into a short circuit at resonance. In a similar way of thinking, an inductor can be considered a variation of a short circuit. An inductor, too, can be used to lower the resonant frequency of a half wave section. The half wave section transforms the inductive reactance into a short circuit at resonance. Since the antenna analyzer cannot measure high impedance, only transformations into a short can be measured accurately using a MFJ analyzer. Placing a capacitor across the exact center of an inductively loaded half wave section uses both a quarter wave and a half wave effect at the same time. With the correct inductive input and a coupling link to the drive cable, the 8.94-meter length of twinlead can be tuned from below 3.5 MHz to above 7.4 MHz.|
Sferics in the Mountains
|Igor made his first observations of whistler sferics in the 1970s on a regenerative receiver, when he first began to be engaged in radio amateur. At that time he did not know anything about the existence of whistler sferics" and did not take notice of these strange short whistles that sometimes appeared in headphones. Later, during his studies at the university, he read about this phenomenon in special literature and he would have liked to repeat my first experiences in whistler sferics reception. But unfortunately for a long time it was only a dream. Recently, after reading Harold Allens (W4MMC) articles (Harold Allen gave in his articles perfect exposition of whistler sferics phenomenon!), and after looking through different websites, which were devoted to this phenomenon, this idea again drew Igor's attention. But how can he catch the sferics? So, lets look at the first equipment Igor used for hearing whistler sferics.|
2.44 GHz Standard Gain Antenna
As the author asks: Why would you want Standard Gain Antennas? Well, most likely for use on an antenna range to determine the actual gain of other antennas you are testing. My employer (RFSAW, Inc of Dallas, TX) asked me to put together a small Antenna Range for 2.44 GHz since we would be doing some work in that ISM Band. (Passive RFID Microtags) Being a dedicated Amateur Radio Operator I immediately protested this assignment. This would be a dream job and I intended to take full advantage of it. There would be a lot to learn that most Amateur Radio Operators (including myself) had never needed to consider. I needed to find practical answers to several questions. First, how do you build antennas with gains that are known to a small fraction of a dB? Second, how is Path Loss calculated? Third, what would be the minimum size room I would need to get reliable answers? Fourth, How can I make valid pattern measurements and what equipment must I buy or build to do so? Find out more about this subject in this fine article by Ed...!
Vertical Antenna and Some Variations
|The most basic low frequency antenna is the quarter wavelength vertical antenna. A quarter wave is practical and reasonable above about 1000 kHz. Below roughly 1000 kHz, the vertical requires considerably greater economic resources. However, this article is about economic vertical antennas and concentrates on antennas less than approximately 150 feet, or 50 meters, in height. Vertical antennas tend to radiate along the surface of the earth giving the best distance coverage for the money. Included, is a discussion and drawings of a 140-foot vertical loaded with Top Hat.|
|Have you ever looked at the Bilal Isotron advertisements and wondered how these tiny antennas workedor if they really did? Is an Isotron a tiny miracle antenna or just a dummy load? Several of the GARDS are hard at work exploring capacitive antennas and GARDS member Harold Allen has been sharing some very interesting information on his study of the 20-meter Bilal Isotron, which is often sited as an example of a capacitive antenna. A couple of months ago antenneX publisher, Jack Stone, asked if any of the GARDS would like to examine a different Isotron. Dave jumped at the chance as he has always been intrigued by the Isotron advertisements and wondered if this little antenna would really work as claimed. The results of Dave's very thorough investigation of the Isotron are also intriguing!|
Well, there you have it, folksthanks 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. October
2002 antenneX Online Issue #66
reGARDS, Jack L. Stone, Publisher firstname.lastname@example.org
Send mail to email@example.com
with questions or comments.
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