A 3/4 λ Collinear Groundplane Antenna
Source: Antenna Theory by Stutzman.
Abridged by: abcd567.
In many instances it is desirable to obtain more gain from a monopole than that provided by a 1/4 wavelength groundplane.
To obtain more gain, a Collinear 3/4 λ or 7/8 λ groundplane monopole may be used as Fig.1 illustrates. The 1/4 λ stub provides the necessary phase shift to place the current on the upper and lower portions of the monopole in phase.
In practice, the 1/4 λ phasing stub is usually replaced by a phasing coil. Both the stub and its coil substitute provide the necessary phase shift to place the current on the upper and lower portions of the monopole in phase. This phasing coil may be roughly thought of as the wire in the stub coiled into a coil.
The purpose of the loading coil at the base of the 3/4 λ Collinear monopole is to improve the VSWR. Fig.3a shows the input impedance with the base coil in place. Note that a zero reactance occurs at 860 MHz. Fig.3b shows the VSWR referenced to 50 Ohms. The VSWR curve could be made more symmetrical by slightly increasing the inductance of the coil at the base.
The far-field pattern is shown in Fig. 2c. Note the favourable strong broadside radiation of the 3/4 λ Collinear monopole (i.e. the whip with phasing stub & loading coil).
For comparison, Fig. 3 shows the far-field pattern of the 3/4 λ simple monopole (i.e. a 3/4 λ whip without stub & without coil). This pattern is generally highly undesireable.
The gain of the 3/4 λ monopole with phasing stub & loading coil is at least 3dB greater than the 1/4 λ monopole.