Small Antenna Design
Keeping mathematics to the absolute minimum, this guide describes the theory behind effective small antenna design while providing design techniques and examples for small antennas for different operating frequencies.
TABLE OF CONTENTS Small Antenna Design
6.4: Using Resonance
6.4 Using Resonance
We have presented a few examples of resonant small antennas, most recently Example 6.4 for volume-loading. In that example, the wire s inductance was resonated against the capacitance of the end volumes. It is clear from the top-loading section that an antenna made with wire radials can be made resonant either by using thin enough wire in the radiator, or adding a lumped inductor at the junction between the radiator and the radials. Example 6.2, the inverted-L on a radio box, shows that the impedance can be greatly improved when the radio box is part of the antenna. It seems reasonable that a more effective top-load would achieve resonance in such a case. In this section, I present several examples to explore the practical application of these ideas. Also, a couple of examples using open-ended resonant coils are given.
Example 6.4: A Volume-Loaded Dipole at 40 m
Figure 6.13 shows the picture of the NEC wire list for a dipole that is 4 m tall by 4 m wide. The end bodies are octagonal frames made from " copper water pipe. The central wire radius is 1.6 mm, approximately AWG #8, and it is 3.6 m long. In free space, the dipole resonates at 7.1525 MHz, 42 m, with a resistance of 6.13 ?. The uniform-current radiation resistance for 3.6 m is 5.8 ?, so there is a little help from the verticals of the end frames. The Q is 38.36. The equivalent L
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