The Crossed Field Antenna - Part 3
1) The Poynting theorem S = E X H and,
2) that D’ = > D X HD‘, which was experimentally proven, results in a concentrated magnetic field H surrounding circular capacitor plates.
It is assumed that everyone knows when a voltage is placed between two plates that an electric field E will exist between those two plates, and D the electric displacement flux is just D = eE, where e is the dielectric constant of the insulator. For our purposes the surrounding dielectric is air where e = 1.
Now that we have E and H to satisfy the Poynting theorem, we only need to manage X to have a crossed-field-antenna. The curl, i.e., the cross product X, is a geometrical expression. It relates the field geometry of E and H to produce radiated power. E X H = EHsin (theta) where “theta” is the geometrical angle between the crossed stressed fields. In other words, if E and H are at geometrical right angles, then radiated power is maximized.
CFA and Poynting
By now it should be obvious to the casual reader that the name of the CFA is derived from the Poynting theorem, i.e., the crossed product of E and H. This remarkable series of developments has resulted in the direct synthesis of S by combining separate E and H fields in a small volume. The success of the CFA is a direct consequence of reversing in particular the 4th Maxwell equation to gain a full understanding of the physical reaction or field production nature. This leads us directly to the design of the CFA. Please note there are several variations, and each significant one will be described in this series. There are others noted in the patents referenced earlier.
Since the “barrel-shaped CFA” was the first experimental model leading to the current evolution, we shall cover it first. In 1992, results of tests on a groundplane version were released. After exhibiting each of these we will discuss various improvements on the concepts and early experimental models. There will also be information on a current mode CFA, which is a simple small wire loop properly phased to provide good performance on the HF Ham bands.
Remember, we are studying history, and we are currently in an era about 1988. This first experimental antenna worked successfully, but was not the optimum design—don’t run out and build one before reading the rest of the story.
The Firstarrel Shaped CFA
“Large” circular capacitor plates, when supplied with RF, will produce strong magnetic fields around the plates through D’ => D X H. In the Crossed-Field-Antenna these capacitor plates are referred to as D-plates for obvious reasons. As shown in Figure 1A, two large cylinders of short length are positioned one above and one below the D-plates. When the cylinders are driven by a RF power source, they produce high-frequency E fields (due to the voltage across the plates). These cylindrical plates are therefore called E plates.
The total height of this antenna was only 70 cm (28 inches) and operated over the HF spectrum. Note the plates are analogous to a dipole antenna but much, much smaller than any practical wire dipole, capable of being < l/200. Whoa !!! Think about that !!! How about a 75 meter antenna that is only l/200 = 75/200 = 0.375 meters (a little over a foot) tall ???? Now you can appreciate an earlier statement that said the small loop antenna was the only way to build a small high efficiency antenna until 1988 when these fellows did this exceptional work.
Feeding
To properly feed this antenna, the power from the transmitter is split roughly in half between the E-plates and the D-plates. Through suitable design considerations and delay arrangements (that is a simplification if ever there was one) between the E and D plate voltages, a toroidal field volume surrounding the D-plates is crossed stressed with properly phased E and H field components such that E/H matches space impedance. Radiation is then produced through S = E X H and power flows out to space as vertically polarized radio waves of intense power density.
The electromagnetic wave (Poynting Vector) begins where the E and H fields are cross stressed. This is very close to the CFA antenna, not at a distance of l/2p, as it does in wire antennas. As the energy spreads out from the source, it can be thought of as the surface of an ever-expanding sphere. The farther from the source, the greater the size of the sphere, thus the less energy per square meter on the surface of the sphere. To put this into proper perspective, consider for example a CFA to be 1 meter in height, thus the surface of the electromagnetic wave is a sphere with a nominal diameter of 1 meter. The surface area is therefore about 4pr^2 = 3 square meters.
For a wire antenna operating on 160 meters, the surface area of the sphere where the electromagnetic radiation begins (at a distance [radius] of l/2p) is 4pr^2 = 8148 square meters. Therefore, the power density from the CFA at this frequency for the example antennas would be greater by a factor of 2716, or a power ratio of about 34 dB. This is what was meant by the statement above referring to radio waves of intense power density. With that in mind, ponder the reduction in the strengths of the electric and magnetic fields at a distance from the CFA, which cause the local interference problems—there are virtually no E and H fields beyond 1 meter for the example size CFA.
Figure 1-B is included to show relative physical dimensions of the original CFA A unit having a E plate barrel diameter of 8 inches works well on all HF Ham bands from 160 thru 10 meters. Activity is currently underway to optimize the physical configuration and the results will be presented in this series of articles—don’t sharpen the hacksaw just yet.
Pause for a moment and consider that we have thus far in our history lesson learned that the important features of the CFA are:
- Extremely Small
- Efficient Radiator
- The CFA Plates are physically Configured to Form Field Lines of equal curvature.
- The E and H Fields are Concentrated in Close Proximity to the Antenna
- Broad Bandwidth – Limited Only by the Phasing/Matching Network
Extremely Small
To say these antennas are extremely small is an understatement. Their size is independent of the radiated wavelength – an unprecedented concept in antenna theory and design. The designers say “In fact, there appears to be no restriction in the physical size of CFAs and they can be made as small as desired.” To fully appreciate this, consider that small wire antennas (fraction of a wavelength) have very low radiation resistance. Therefore, loss in matching networks and conductor loss cause them to be impractical due to poor efficiency. Just the opposite is true for the CFA, where the impedance increases as the size decreases for a given frequency. High efficiency phasing/matching networks are limited to medium magnitude impedances, thus setting the boundary limit on the minimum size. The input impedance of all CFAs, regardless of size (within the range of reasonableness), can be readily set to 50 ohms by properly adjusting the phasing/matching network.
It bears repeating that a fully operational broadcast antenna in Egypt is only 21 feet tall compared to a 211 foot 1/4 wavelength vertical that was used before the CFA became available. We hope Dr. Kabbary will provide additional insight later in this series of articles that led him to choose that particular physical implementation.
The initial “barrel shaped CFA” had a total length (or height if you prefer) of approximately 3 feet (1 meter) that exhibited performance comparable to a conventional dipole when tests were performed at 5, 15 and 27 MHz. We are talking small—a wavelength at 5 MHz = 196 feet, the CFA was 3 feet, therefore 3/196 = 0.015 wavelengths. This was a test model, and it has been stated by the authors that the CFA could be much smaller. Hurrah!!!! No more big antennas required by us city dwellers, even on 160 meters.
There are physical limitations to the maximum size of the CFA. One of the first such noted by the author was the inherent capacity of the E and D plates. When these become large, it is not practical to develop a phasing/matching network that provides the desired results. In addition, when the physical dimensions of the CFA become an appreciable fraction of a wavelength, the phasing between E and H in the near field is not constant thus causing variations in the interacting zone. The maximum height must be less than 10% of a wavelength. Therefore, relative to the CFA, smaller is better. Too small and the impedances are too high to make a practical phase/matching network. Any size within these boundary conditions should radiate well. Note of caution: be aware the physical configuration is critical, i.e., the ratios of dimensions must be correct to effect appropriate matching. Not just any old cans and lids put together in the shape of a CFA will perform as an antenna. Much more on this later.
Efficient Radiators
The CFA, as indicated above, is capable of producing a radiated field whose amplitude compares to that of a conventional dipole. Tests were conducted on the Dipole version by the British Army. For each test frequency, a 1/2 wavelength wire dipole was cut to resonance and the CFA was held at the same height. The results: same signal strength within measurement tolerance. Therefore, the efficiency of this antenna is very high. It is obvious from the drawing in Figure 1 that there are no lossy components in the antenna itself. We will show later that the phase/matching network is conventional and the loss in the network is negligible (even at 30,000 watts).
The CFA may be Physically Configured to Enhance Gain
Referring back to the statements made earlier about a groundplane version of the CFA for AM broadcast service, below is a photograph of an early version of the AM broadcast antenna in Egypt. Its size is apparent compared to Dr. Kabbary standing beside the unit. An improved version has a specifically shaped cone appended to the top of the antenna that increased the ground wave pattern gain 6 dB over a 1/4 wave vertical broadcast tower, with an associated reduction in the skywave pattern gain. This writer just learned that plans are being made to measure the exceptionally strong ground wave signal in England coming from Egypt, and the results from those measurements may be presented at a conference in the spring of 1999.
One section of this series of articles is being researched and prepared by Kent Holt who is using a simulator that allows varying the shape of the CFA and monitoring the electric and magnetic fields as well as the Poynting vector. The goal of this effort is to further optimize the physical design to enhance the pattern gain. Remember, the CFA is a relatively new concept, leaving a world of potential improvements to be discovered. I continue to be amazed that the group was able to develop the design without a simulator.
The E and H Fields are Concentrated in Close Proximity to the Antenna
Figure 1 does not do justice to the portrayal of the true electric and magnetic fields. Refer back to the second article in this series and look again at Figure 4. Note the magnetic field is concentrated such that the majority of the field is within one plate radius of the edge of the D plates. The length of the E plates is chosen to provide an electric field that provides maximum cross stress of the magnetic field. Therefore, it can be said that the majority of the electric and magnetic fields are contained in a sphere with an equivalent diameter of the length of the dipole version of the CFA.
It needs to be said that the electric field lines must be perpendicular to the surfaces of the cylindrical E plates where they contact the surface. This is best shown in the following drawing, Figure 2. The concentric arcs represent electric field lines between the two plates. Notice that each line is perpendicular to the surface of the plate. I hope this makes it easier to visualize the need to set the E plate length to properly encompass the magnetic fields from the D plates, and have roughly similar curvature. The antenna is practically a “point source”—the waves must be initiated as spherical wave-fronts.
All of that was said to ensure the reader understands the electric field lines are contained very close to the CFA. The electric and magnetic fields of conventional wire antennas (this includes broadcast verticals) are so large they do include earth in the fields, which contributes to the loss of the wire antennas. Also, in the near field of a wire antenna, the E and H fields are 90 degrees out of time phase, thus producing reactive power. Again, the E and H fields of the CFA are virtually self-contained in the sense they do not encompass earth in the fields if the CFA is off the ground by only a small amount. By the same reasoning, the separate E and H induction fields from the CFA do not extend to large areas and thus do not cause interference to other electronic systems as do wire antennas (shields on coax cables are more effective for electric fields than for induction fields).
Inventor Paths Converge
Next month, our history lesson diverges into three separate paths. Dr. Kabbary completed his studies in Scotland and returned to Egypt to develop the ground plane version of the CFA for AM Broadcast. Mr. Hately retired from the University and formed his own company to continue development of the CFA, which led to the development of the current mode CFA. Dr. Stewart left to study for the Christian ministry. He has now returned to the academic world as a full time lecturer at the University in Glasgow. All three of these very talented and dedicated men have now come back together and are pooling their talent to continue development of the CFA.
Since Dr. Kabbary produced the ground plane CFA mentioned in the 1988 patent application and released the results in 1992, we will dedicate next month’s article in this series to that activity.
Please be Patient – Much More Explanation Essential.
I will produce reports and evidence from Fairlight, Arqiva and OfCom of the Installation in the UK and then Later in the Isle of Man – MD0MDI
The mail from this series has been heavy, with most asking when will we present the complete details to allow individuals to build their own. Please be patient. All great things are worth waiting for.
I want to apologize for using the Greek symbol delta which is an inverted “nabla’ as the symbol for curl. However, there are some limitations to the fonts in these computers and we do the best we can.
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Also, please note that this writer began to work with Mr. Hately several years ago to develop an article for publication. Now, the other two gentlemen have offered their comments and this author is extremely pleased to be a “joint” author with three very distinguished gentlemen, as indicated in the title of the article.
Originally posted on the AntennaX Online Magazine by Maurice C. Hately, GM3HAT and Ted Hart, W5QJR
Last Updated : 21st March 2024