Practical_Antenna_Handbook_0071639586

236 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
Non-Quarter-Wavelength Vertical Monopoles
If now we gradually increase the length of our grounded monopole, we observe the
same flattening of the doughnut shape as we saw with the dipoles of Chap. 6. Lengthening
the monopole beyond l/4 results in additional RF energy in the low-angle main
lobe of the pattern, in exchange for reduced radiation at higher elevation angles. Figure
9.5A shows the approximate patterns for vertical monopoles of three different lengths:
l/4, l/2, and 5 ⁄8l. Note that the main lobe of the quarter-wavelength antenna has reasonable
gain over the widest range of elevation angles, but also the lowest maximum
gain of the three cases. The 5 ⁄8-wavelength antenna, which is the grounded monopole
equivalent of the extended double Zepp (EDZ), enjoys both the lowest angle of radiation
and the highest maximum gain.
The patterns shown in Fig. 9.5A assume a perfectly conducting ground surrounding
the antenna. The effect of real-earth ground losses is to eliminate any hint of the radiated
field at distant receiving sites for elevation angles near the horizon (Fig. 9.5B).
The feedpoint impedance of a grounded monopole is a function of the length of the
radiator. For the standard quarter-wavelength antenna over perfect ground, the feedpoint
radiation resistance is a maximum of 37 Ω (i.e., one half that of a l/2 dipole), with
only a very small reactance component. Figure 9.6A plots the reactive and resistive
components of the vertical monopole’s feedpoint impedance for radiator lengths from
60 degrees to 120 degrees, while Fig. 9.6B shows the radiation resistance for antennas
shorter than l/6 (90 degrees corresponds to l/4, so 60 degrees is for a l/6 radiator).
Note that the radiation resistance for such short antennas is extremely small. For
example, a monopole that is 30 degrees long ( 30 ⁄ 360 = 0.083 l) has a resistance of approximately
3 Ω. General practice for such antennas is to use a broadband impedancematching
transformer to raise the impedance of such antennas to a higher value (Fig.
9.7), but the biggest problem with such low-radiation resistances is that it is extremely
difficult to avoid dissipating most of the transmitter power in lossy grounds, matching
5 8
2 4
Figure 9.5A Vertical radiation pattern versus a function of element length over perfect ground.