EXTERNAL COAX CURRENT
CAUSE AND EFFECT
This document was prepared to explain why there is current on the shield of the coax and how to mitigate its affects.
Tuning and matching an EH Antenna can be difficult unless there is a full understanding of the antenna and the design is properly implemented. The purpose of this document is to provide the necessary information.
Figure 1 presents an equivalent circuit of the concept of the radiating portion of an antenna (any antenna), which is simply a capacitor in series with the radiation resistance developed by the antenna. To be able to pass current through the capacitor and the radiation resistance it is necessary to provide a very high voltage. This is typically done by adding an inductor to bring the circuit to resonance. The inductor (coil) can be tapped to provide a convenient match to a transmission line of any desired impedance including a coaxial transmission line. The complete antenna is depicted in Figure 2.
The antenna capacity of an EH Antenna is developed by the use of two (2) large cylinders. Because they are large in diameter they can be short in length, typically less than 4% of a wav elength.
To understand the concept of the EH antenna, it is convenient to look at the Electric (E) and Magnetic (H) fields of the antenna as shown in Figure 3. It is important to realize that the total length of the antenna is a small fraction of a wavelength. For this reason the cylinders are capacitors with negligible inductance. When a high voltage is applied between the two antenna elements an E field is developed. The voltage is high at the feed point but must be zero at the end of the elements. Therefore, there is a large voltage gradient between the two ends of each cylinder. That voltage gradient (differential voltage) causes a current to flow on the cylinders. In turn, that current causes a magnetic (H) field to surround the cylinders. Now we have the necessary ingredients to develop radiation, i.e., the E and H fields are physically orthogonal and they are in time phase. This occurs because the E and H fields are created by the applied voltage. In other words, while the RF voltage is present (an alternating sine wave) an E field exists and conduction current flows on the cylinder and that current creates the H field. Note that the E and H fields are contained within a sphere defined to have a diameter the same as the antenna length, and there are no reactive fields. This is in contrast to conventional antennas having large reactive fields sufficient to create radiation at the far field distance of the antenna.
There is another component to the EH Antenna that was not included in Figure 2, only to simplify the drawing. With reference to Figure 4, note that two (2) more capacitors were added to the schematic diagram.
The one to the right of the tuning coil is the distributed capacity of the tuning coil. The one to the right of the antenna capacity + radiation resistance is a capacity between the antenna and ground. “Ground” may be the coax or a tower the antenna is mounted on or some other object that forms the other “plate” of the capacitor. Current through that capacitor must return to the antenna and unless the return path is provided the return will be on the outer shield of the coax. Because this capacity is smaller than the antenna capacity this stray current will be smaller. In addition, it has little effect on radiation because it is not associated with an E field. However, if the antenna is tuned then moved to a different mounting location, a change in the “ground” capacity could affect the tuning (resonant frequency). In addition, a part of the internal current of the antenna will be diverted from the antenna capacity which in turn affects the impedance as seen at the coax connector. This will be seen as a change in VSWR.
To alleviate all tuning and matching problems, the process should be done in two steps. When the antenna is first tuned and matched (while in the shop) a good ground wire should be added to the antenna ground terminal and connected to a significant shop ground. This will stabilize the antenna while performing the initial tuning. The second step is to mount the antenna on a tower or metal pipe and complete the tuning and matching, or mount the antenna on a plastic pipe and add a good ground wire. The support pipe could be tilted to allow the antenna to be close to ground for tuning and matching. If the antenna is mounted on a vehicle, a ground wire connected to the vehicle will serve the purpose.
Coax current is not unique to the EH Antenna. Typical Ham antennas are large and therefore fixed. After they are installed then they are tuned and matched. However, if the coax line is then changed the same effect will be noted. The effect has been magnified by the fact that a small EH Antenna and the associated coax can readily be moved.
Ted Hart CEO EH Antenna Systems