ANTENTOP- 03- 2003, # 004

A 125W radioset combined with a tuned loop antenna is sufficient to fulfill the mission requirement using the Near Vertical Incident Signal (NVIS propagation). This will be further improved due to frequency management and the new generations of HF modems which will bring a lower threshold of sensibility.

II DEVELOPMENT OF A NEW MOBILE TUNED FRAME ANTENNA

II.1 Principle

The mobile tuned loop antenna is a "half-loop" set-up vertically on a metal surface which achieves a full loop equivalence. The metal surface like a mobile platform (truck or shelter, ship's cabin,...) must have a good electrical continuity. Thie half-loop is half the size of a full loop and makes installation possible on small vehicles on the move.

The half-loop is folded and joined at each end to the platform's earth. One end is loaded by a variable capacitor.

A feed rod ("the feed coil") links the radioset RF access to a precise point of the half-loop. It is equivalent to a fixed reactive element, and the whole system acts as a loss-free autotransformer whose primary circuit can be set to 50 W.

II.2 Modelisation of the antenna

The modelisation purpose is the definition of the electrical circuit and the parameters of the antenna. It is made by the wire methods of moments.

The radiating element is represented by a radiating impedance (Rr, La) with a loss resistance Rp

The tuning capacitor is represented by a serial circuit (C, Rc), C being the capacitor value and Rr its loss resistance.

The 50 matching is figured by a loss-free transformer M with a matching ratio K, and a parallel or serial inductance L at the RF input.

Establishment of the equivalent circuit parameters:

-The radiating element (Rr, La) is calculated by an electromagnetism software based upon the method of moments.

- The radiating element loss Rp is determined according to the antenna material and section

- The capacitor's losses Rc are determined through the manufacturer's data

- The matching ratio K is a function of the primary to secondary radiating surface ratio

- The inductance L is a function of the spiral surface comprised between the feed bar and the platform.

Two types of antennas have been compared, type A and type B, differing by the positions of their capacitors.

II.3. Modelisation of the antenna type A

The capacitor is positioned in the secondary of the transformer, at the end of the line (FIG 1).

Figure 1

The electrical equivalent sheme is given FIG 2

Figure 2

The results are computed by a specific C.A.D. radiofrequency device and compared to the values measured on full scale antenna mock-up.

As an example, FIG 3, FIG 4, FIG 5 show the

Figure 3

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