The antenna system of Fig 62 uses the principles
of cophased verticals to produce a broadside, bi-directional pattern providing
approximately 5.8 dB of gain over a single element. The antenna performs
as three in-phase top-fed vertical radiators approximately 1/4 wavelength
in height and spaced approximately 1/2 wavelength. It is most effective
for low angel signals and makes an excellent long-distance antenna for
either 3.5 or 7 MHz.
The three vertical sections are the actual
radiating components, but only the center element is fed directly.
The two horizontal parts A, act as phasing lines and contribute very little
to the radiation pattern. Because the current in the center element
must be divided between the end sections, the current distribution
approaches a binomial 1:2:1 ration.
The vertical elements should be as vertical
as possible. The height for the horizontal portion should be slightly
greater than B, as shown in Fig 62. The tuning network is resonant
at the operating frequency. The L/C ration should be fairly low to
provide good loading characteristics. As a starting point, a maximum
capacitor value of 75 to 150 pF is recommended, and the inductor value
is determined by C and the operating frequency. The network is first
turned to resonance and then the tap point is adjusted for the best match.
A slight readjustment of C may be necessary. A link coil consisting
of a few turns can also be used to feed the antenna. |
