The Crossed Field Antenna - Part 2
Figure 1 is a picture of a conventional wire antenna depicting the various elements required for radiation.
From Part 1 we learned that when we pass a current through a wire a magnetic field is created, which is explained by Equation 5:
We have not shown the feed for the antenna which would be the source of current into the wire. We also learned that an electric field is created from a change in flux density, as shown in eqation 3 from the previous section:
In other words, since the current in the wire is alternating at the desired frequency, the current is varying thus causing the magnetic field (flux density) to be varying, which in turn causes an electric field. This is often referred to as the simple generator principal.
Now that we have E and H to satisfy the Poynting theorem S = E X H, we must have the cross product (X) of E and H. There is a lot implied in X, including proper time, phase and position of the two fields with respect to each other. If Mother Nature had not been good to Marconi, we would have had to wait on Mr. Hately, et al, to create the Crossed-Field-Antenna rather than use a simple wire antenna.
For a wire antenna, the X in the Poynting theorem is accidentally satisfied as the formula below from the wire.
This implies large magnetic and electric fields at large distances from the wire (the CFA does not have this problem). As an example, I have a hard time operating on 160 Meters due to bothering the neighbors with telephone interference and causing their HI FI to jump off the table. On 160:
To say that another way is to say that it is the electric and magnetic fields from an antenna that cause local interference, not the electromagnetic wave that radiates. (Again, the CFA virtually eliminates interference to other systems due to its ability to concentrate the electric and magnetic fields in the very near proximity of the antenna).
Numerous items can interfere with the E and H fields between the place at which they are created and the place at which they combine. This contributes to the inefficiency of a wire antenna and to the shape of the radiated pattern. For example, if there are wires near an antenna, there will be currents and voltages induced on those wires from the electric and magnetic fields of the antenna. We usually notice this as the lights flicker when we get on the air. To examine the electric field of your antenna, hold a florescent bulb in the vicinity and watch it light up. One of the severe limitations to the use of a small loop antenna is the proximity to objects made of materials other than good conductors such as copper and aluminum, notably ferrous (iron) materials such as chain link fences.
The antenna is small, but the fields are large out to:
and usually is noticed as a change in VSWR as the loop is rotated near a ferrous object.
Eddy currents on the surface of the ferrous object are composed of excited electrons that get confused and cause heat due to their confusion. A more simple explanation is that the small loop can be thought of as the primary winding of a transformer. The currents in the ferrous material are as if there were a heater connected to the secondary of the transformer, and that resistive load is reflected back into the primary of the transformer, thereby consuming energy from the transmitter rather than being radiated. This not only applies to the small loop but to any wire antenna. Now it is easy to understand why a 75 meter mobile antenna gives poor performance when mounted on a car made of ferrous material, and that a small loop will just not work when close to a car body. All of this can be eliminated if we can contain the electric and magnetic fields to a very small distance from the antenna – as we can in the CFA.
You need to be aware that the polarization of an antenna is specified in terms of the E and H fields for engineering applications. For normal Ham antennas it is simply the orientation of the wire (vertical or horizontal).
It is important to note that the equations used to develop the radiation are reciprocal, i.e., if electromagnetic radiation impinges on an antenna, E and H fields will cause current flow in the wire. Therefore, there will be RF signals flowing from the antenna to a connected receiver. To further clarify the concept of polarization, if the E and H fields in the received radiation are not properly aligned with the physical wire, the amount of energy received will be reduced. Look at Figure 1 again and imagine the received E and H fields are interchanged—no coupling will occur from a cross polarized electromagnetic wave from another antenna.
Magnetic Field Associated with a Simpler Capacitor
With that background we now turn our attention to the development of a magnetic field without the use of current flow in a wire. Referring back to the reversed form of Maxwell’s 4th equation and, in particular, the feature of D‘ creating an independent magnetic field from J, consider the practical illustration of circular capacitor plates as shown in Figure 2 with an applied voltage from an RF source.
Free charges flowing into and out of the capacitor, and also within the capacitor plates themselves, are a source of J. Also, due to a buildup of free charge in the capacitor, E lines and therefore D lines exist between the capacitor plates.
As the D lines vary in strength due to sinusoidal charge variation on the plates at the operating frequency, D² will create a sinusoidal magnetic field through:
Since HD²’ is in time-phase with D² then HD² is 90 degree phase advanced from D.
Also, since J flowing into and out of the plates is sinusoidal then J=>DXHJ produces a sinusoidal magnetic field HJ which is in phase with J. It can be shown (or as my professor used to say—it is intuitively obvious) (I prefer to say—have faith) that in the vicinity surrounding the capacitor gap, the magnetic field lines from J into and out of the plates and the magnetic field lines from D² will be concentric circles surrounding the gap and in-phase.
Now, J flowing within the plates themselves will create a magnetic field HP. Applying the rules of Biot-Savart to the geometry of the plates, many components of magnetic fields produced from individual J contributions within the plates will cancel, resulting in reduced strength circular field lines surrounding the plates. We should expect the created field HP to be in phase with HJ, but taking into account the geometry and the current motion within the plates, then HP is directed in the opposite direction to HJ. This is equivalent to a 180-degree phase change between HP and HJ.
Experimental Proof
Keep in mind that, until now, there has been a lot of theory about producing a magnetic field without current flow in a wire. However, since it is not common practice to need a magnetic field created in this manner, you could not run to the local store and buy one. A simple experiment was carried out to verify that HD’ does exist surrounding circular capacitor plates, an experiment that is easily duplicated. The main equipment required is an RF signal source capable of a frequency in the range of 10 to 100 MHz with an output voltage up to 20 volts and an output current up to 3 amps, and secondly a triggered dual-trace oscilloscope.
As shown in Figure 3, two circular, flat plate conductors (made from wire mesh) 18 inches (47 cm) in diameter were positioned as a capacitor with an air gap spacing of approximately 8 inches (20 cm). The capacitor was placed on top of a large conducting ground sheet. The top plate was then connected to coax terminated by two 100-ohm resistors paralleled between the live inner core and the shield of the coax. The entire volume surrounding the capacitor gap was then Faraday shielded using a second large conducting sheet such that no HJ contributions from the coax cable could extend into the region around the capacitor gap. The Faraday shield is also connected to the coax shield. The magnetic fields within and surrounding the capacitor were measured using a circular, balanced, Faraday shielded coax loop with a diameter of 2.5 inches (6 cm) which was matched and connected to one of the inputs of the oscilloscope, thus eliminating standing wave problems on the coax. To provide a reference phase signal for the measured magnetic fields from the Faraday loop, a small resistor (4.7 ohms) was placed in the live coax at the signal source, and the voltage monitored across the resistor using the second input to the oscilloscope.
Experimental Results
A peak to peak voltage of 15 volts was chosen at a frequency of 40 MHz. Positioning the Faraday loop in the middle between the two plates, the measured voltage and phase from the loop was plotted as a function of distance r (radius = 25 cm) from the center of the capacitor and is presented in Figure 4. Referenced to HJ (taking into account path length, etc.) then between the plates HP is strongest even though mutual effects will always exist between the loop and the plates. Moving outwards, HP decreases and HD’ takes over, hence the 180-degree phase change. The crossover takes place near the edge of the plates. Outside the capacitor plates, the magnetic field is therefore due mainly to D’ between the plates. For this article, the actual value of the magnetic field plotted in Figure 4 is not significant, only the relative magnitude and phase that defines the shape of the curve.
That simple experiment was a significant step in the development of the CFA. It proved that Maxwell’s law:
Is alive and well and that it is functioning between the capacitor plates, and also that D’ is an additional and significant source of magnetic field surrounding circular capacitor plates at the operating frequency. Though some textbooks comment on the existence of D’ within capacitor plates, the authors fail to realize that it creates its own magnetic field which can extend well outside the capacitor plates. Don’t believe everything you read – the author may not be informed.
For future reference, note that the majority of HD² is in a region beginning at the edge of the capacitor plates and diminishes to a small value at a distance equivalent to twice the diameter from the center of the plates. This large concentration of a magnetic field in a small region surrounding the capacitor plates allows the CFA to be small and efficient (and not interfere with other items). Remember the broadcast antenna at 1 MHz that is only 21 feet tall? The capacitor plate in that antenna is only 4 meters (13 feet) in diameter.
Before proceeding to Part III of this series, you need to reflect on the magnificent work done by the inventors Maurice Hately, GM3HAT, a former professor in Scotland, and student named Fathi Kabbary from Egypt. Until now, only Mr. Hately and Dr. Kabbary have been mentioned, but there is a third party by the name of Dr. Stewart, MM1DVD, who made significant contributions early on, then left the “team.” After four years, Dr. Stewart returned and is now a significant player again. The fact he was not mentioned before was due to an oversight by this ghostwriter (Ted Hart) an omission for which I apologize.
We will never know all the steps taken by these gentlemen, only the results as reported here. In the scientific world, this work can only be described as elegant. Now you can begin to see why it is my personal opinion that this is the greatest work done in the field of antennas since Maxwell. Marconi simply had an accident, and all others have followed trying to explain that accident and add Band-Aids. Even the analysis programs for antennas all assume current flow in a wire which is an appreciable portion of a wavelength. This work is fundamental and ingenious.
Part 3 of this series will detail a CFA configuration that could be used on the Ham bands. Until then – stay tuned.
Originally posted on the AntennaX Online Magazine by Maurice C. Hately, GM3HAT and Ted Hart, W5QJR
Last Updated : 20th March 2024