Performance of Short, Vertical, Top-Capacitance-Loaded AntennasAuthor: R.J.Edwards G4FGQ © 3rd October 2002
Capacitance loading of a short vertical antenna can improve radiating efficiency more than inductance loading. Best possible improvement for a 2/3-height loading coil relative to base loading is typically 3 dB. But a large top hat can improve performance by more than 6 dB. In this program the hat is a number of horizontal radial wires or rods optionally surrounded by a wire circle.
Top-capacitance loading improves radiating efficiency in two ways:
- A more uniform distribution of antenna current versus height results in an increase in radiation resistance for no loss in the top-hat. Whereas the high loss resistance of a raised coil can outweigh the benefit of increased radiation resistance.
- A tuning coil is always necessary. But its inductance is less when a capacitance hat is fitted and if overall coil dimensions are unchanged then loss in the coil is much smaller.
A more uniform current due to a top hat lessens the advantage of fitting the coil part-way up the antenna. The coil can then be combined with the Z-match network located between antenna and the transmitter as in this program.
A variable or switched inductor in a separate tuner, rather than a fixed coil at the base or part-way up the antenna, also assists multi-band operation.
All short-antenna L-network tuners need large capacitor values. But voltages are low and capacitors can consist of several smaller paralleled fixed values. Current-carrying capacity is also improved. (T-match tuners have higher loss.)
The improvement in dB due to Top-C loading compared with the same antenna without the hat is computed. Analysis includes the smaller coil loss due to less inductance being needed for Top-C loading.
Values of ground loss resistance and coil Q are the most uncertain of the input data. Because coil resistance affects values of the impedances to be matched it reacts back on the inductance value needed. The end result is more uncertainty in computed matching network L and C values and on efficiency. Such an interaction matters when coil loss is an appreciable fraction of 50 ohms. But with antenna and coil proportions normally used this extra uncertainty is unimportant.
Antenna feedpoint impedance, R+jX, is included in computed results. This may be be useful if an alternative matching network is considered.
NOTE: Height + top hat radial length is limited to roughly 1/4 wavelength.
Guesstimation of Ground-Loss Resistance Referred
to Antenna Feedpoint
Ground loss of a vehicle roof-mounted antenna can vary between 3 and 12 ohms, being greater for small vehicles. Resistivity of the surface soil underneath a vehicle will have an effect. An arid soil may actually reduce loss. Soil conditions in the surrounding area will also affect hat capacitance to ground.
A shallow-buried circular close-mesh mat has a loss resistance not less than 0.5*(Soil Resistivity)/(Mat Diameter in metres) ohms. Diameter << lambda.
The table below is for a soil resistivity of 100 ohm-metres = 10 milli-Siemens. N = Number of shallow-buried 12-gauge = 2mm dia radials. Length is in metres. Ohms = a crude estimate of input resistance referred to the system's centre.
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