Mobile HF Loop Antennas - Part 2
Loop Antenna Modeling The ST940B Antenna System is a version of a Magnetic Loop Antenna (MLA) known as an ElectroMagnetic Ground-Plane Loop (EMGL). It also sometimes is called a half-loop antenna (partial-loop would be more descriptive, because the physical loop conductor forms more than half the total loop circuit), because part of the loop circuit includes conduction paths through a ground plane.
A simplified diagram of a typical full-loop antenna is shown in Figure 1 below.
Figure 1 - Full Loop
Figure 2 - "Half Loop"
The "half-loop" shown in Figure 2 is electrically similar, but easier to physically install on a vehicle. Furthermore, the ground plane helps to form a special radiation pattern that improves performance.
The full-loop in Figure 1 is excited by means of an inductively coupled smaller loop that connects to the coax feed-line. The "half-loop" in Figure 2 also could be fed that way, but the alternative shunt-feed method that is shown provides more uniform impedance matching between the transmitter output and the antenna across a wider range of frequencies. The shunt feed also is easier to implement physically in a mobile antenna. (The shunt-feed shown in Figure 2 often is referred to as γ-matching - "gamma matching")
The performance of a "half-loop" antenna can be modeled by means of the method of moments which allows for the precise definition of RF currents in antenna elements and the obtaining of the resulting radiation pattern, antenna gain, and antenna impedance characteristics. Figure 3 shows a 3-D wire model of the ST940B antenna constructed with the aid of the EZNEC 4 CAD modeling program.
Figure 4 shows the calculated 3-D RF radiation pattern produced by the antenna. The radiation pattern has less than 1 dB minimas in the horizontal plane and has only one slope directed upward in vertical plane. Therefore, except in the direction toward ground, the radiation shape approximates a sphere. An antenna that radiates almost equally in all directions obviously has a desirable pattern for use on moving vehicles that may frequently change course and head in any random direction.
In the vertical plane the radiation maximum is concentrated between 60 and 90 degrees. The combination of high vertical-angle and horizontal omnidirectional radiation makes the radiation pattern of this antenna ideal for typical HF mobile NVIS applications.
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