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Dipole antennas can be reduced in size by means of a novel design technique. The approach adds two short-circuited cylindrical covers to the extremes of what would have been conventional dipole arms. The increased inductive load reduces the antenna’s resonant frequency without compromising the radiation pattern.

To demonstrate this approach, the resonant frequency of a dipole antenna was reduced by 35% (the technique can reduce resonant frequency by as much as 64% if bandwidth requirements are reduced). As the experimental results show, the wire size of the antenna can also be reduced to bring about the resonant-frequency reduction, without folding or resorting to circuit materials with high dielectric permittivity. All of these factors indicate an antenna design approach suitable for practical applications.

As is well known, the properties of electrically short cylindrical dipole, monopole, and whip antennas can be improved by adding inductive loads to these structures. This is particularly useful for applications where antenna size must be minimized. The effects of series inductive loads have been presented in a number of studies.

For example, by superposition of asymmetrical dipoles, one researcher determined the resulting efficiency of a short monopole for various locations of the load.1 Another researcher built upon this work by developing an approximate solution for the current distribution on doubly loaded short antennas, and computing the impedences and radiation patterns.2 By using a piecewise sinusoidal moment method, yet another researcher computed the radiation resistance improvement and the load point for maximum efficiency.3,4 Since then, a number of studies have been performed on the benefits of inductively loaded antennas.5-13

As noted above, this report details a novel approach for reducing the size of a dipole antenna by adding a distributed inductive load. The load consists of a short-circuited cylindrical cover with less than one-quarter-wavelength overall electrical length. Of course, it is useful to know the effects of the load on various antenna parameters, including bandwidth, radiation pattern, and input impedance; various parameters for this modified antenna will be compared to a conventional dipole.

Good agreement was achieved between the measurements and computer simulations. Both sets of results show that the antenna can exhibit one-half-wavelength dipole characteristics with reduced size.

Theoretical Analysis

Generally, the input impedance of a one-half-wavelength dipole is capacitive below its resonant frequency. Any attempt to reduce the resonant frequency of the dipole antenna must therefore add an inductive load in such a manner that the capacitive impedance of the dipole is exactly canceled at the reduced resonant frequency. According to transmission-line theory (TLT), an inductive load could consist of a short-circuited transmission line with overall length less than one-quarter wavelength, with reactance as given in Eq. 1:

jXin = jZ0tanβl   (1)

where:

l = the length of the short-circuited transmission line;

β = the phase-shift constant of the short-circuited transmission line; and

Z0 = the characteristic impedance of the short-circuited transmission line.

Since this transmission line is considered short, the losses in this model are assumed to be negligible; they are not taken into account and the attenuation constant is set to zero. Therefore, this configuration can modeled as a short-circuited lossless transmission line as depicted in Fig. 1.

Approach Trims Size Of Dipole Antennas, Fig. 1

Also based on TLT, the impedance of the dipole can be seen approximately as an input impedance of lossy open-circuit transmission lines. As a result, the reactance of the dipole can be expressed in terms of Eq. 2:

jX′in = (-jZ′0)[(α′/β′)sh(2α′l′) + sin(2β′l′)]/[ch(2α′l′) – cos(2β′l′)]   (2)

where:

α′ = the equivalent attenuation constant of the dipole;

β′ = the equivalent phase-shift constant of the dipole;

Z0′ = the characteristic impedance of the dipole; and

l′ = the length of the dipole arm. When l′ < 0.35λ, Eq. 2 can be simplified to Eq. 3: 

jX′in = -jZ′0cotβ′l′   (3)

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