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ANTENTOP- 03- 2003, # 004

Half-Loop Antennas

 

The equivalent circuit aided in the calculation of the voltages and the currents developed over each electronic component.

II. 4. Modelisation of the antenna type B

The tuning capacitor is positioned in the primary of the autotransformer (FIG 8).

 

 

Figure 8

 

Its equivalent electrical scheme is given on FIG 9.

 

 Figure 9

 The Type B antenna is modeled in the same way as the Type A antenna, and using the same physical parameters. An additional capacitor may be added in the feed rod to optimize the radioset matching impedance.

II.5. Compared performances type A and type B antennas

Comparative simulations

 The compared simulations gave a clear advantage to the Type A antenna type. As an example, FIG 10 shows a +12 dB gain advantage for the Type A antenna at 12 MHz

 Comparative measurements

 

The comparative simulated results were confirmed by the comparative measured bandwidths Using 2 antennas having the same radiating surface, the compared measured bandwidths were 5 to 10 times

 

Figure 10

higher for the Type B than for the Type A antenna. In a tuned circuit, bandwidths (B) are inversely proportional to the quality factor (Q), and Q is proportional to the efficiency (h ); when Q >> 1, h x B = Rr/2p La = constant If ha and hb are the Type A and Type B antenna efficiencies , and Ba et Bb their bandwidths respectively, the applying formulas are ha Ba = hb Bb, and ha / hb = Bb / Ba When the measured bandwidth ratios is Bb / Ba = 10 , the efficiency ratio becomes ha/hb is 10.

Explanation

Observing that Type B antenna optimizes the tuning in the primary circuit, and that the Q-factors of primary and secondary are quite different, the energy transfer in the secondary is not maximized. On the contrary, in the Type A antenna the tuning brings a maximum Q-factor and the current is the highest in the radiating resistor.

Conclusion

 The Type A antenna design brings the best antenna efficiency.

II.6 Improvement of the design

Increasing the bandwith

 Trials on vehicles were made under strong rain. Modifications of the tune positions were observed at the highest frequencies (FIG 11 )

Such modifications can give an operational problem with no possible reset in transmission (in FH mode principally).

This shifting problem was resolved by widening the bandwidth by using two radiating elements in parallel and electrically linked. The simulation of this structure

 

 

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