7.
Harmonically Operated Antennas in Free Space.
a. Calculation of length. We learned earlier that an antenna in
free space is longer than an antenna erected over ground.
The
effects of the insulators and the earth made the antenna shorter.
The insulators are at the ends of the antenna.
In order to
compensate for the end effect, we must shorten the antenna by 5
percent using the formula below. The formula for a harmonic or long-
wire antenna is:
length = 492(H-.05) = 468
where H equals the number of half waves.
b. Directivity and gain. The gain of a long-wire antenna can be
seen from the chart below. As you can see there is very little gain
until the antenna is six wave lengths long. For each three db gain,
its like doubling your transmitter's output power.
c. In figure 71, we can see the radiation pattern developed from
various harmonic antenna lengths.
As the number of half waves are
increased, the number of lobes are also increased.
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