Figure 56. Electric field around a grounded quarter-wave
antenna.
2.
Radiation Characteristics.
a. The radiation pattern of a quarter-wave grounded antenna is
similar to that of a half-wave vertical antenna that is not grounded.
A grounded quarter wave vertical radiation pattern favors long
distance communication (750 miles) with low take off angles, while
short distance communications 0-100 miles requires a radiation
pattern straight up. See figure 57. The difference is the radiation
An ungrounded half-wave antenna has an impedance of 73
ohms, while a grounded quarter-wave antenna has an impedance of 36
which normally requires a 50 ohm load.
This causes a mismatch
between the antenna and transmitter, which results in a loss of
radiated energy.
The transmitter's loading coil matches the
transmitter to the antenna but uses up some of the RF power while
doing so.
The shorter the antenna, the more matching required, and
the more loss, until you have only a fraction of the original RF
power left to radiate into space.
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