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The plots from Figure 10 may be applied for both
dipole and vertical helical antennas. However vertical helical
antennas have lots unpredictable results in the Q factor.
It is depends on the antenna ground. When the vertical helical
antenna has good grounding system it could be expected that
the Q would be close to shown on the Figure 10.
If the vertical antenna has poor grounding system and losses
objects nearby the antenna would have Q lower the shown
on Figure 10.
Figure
8 and Figure 10 could suppose that optimal shortened
helical antenna should shortened factor from 5 to 8. Helical
antenna with shortened factor from 5 to 8 still needs lots
place for installing and antenna with higher shortened factor
(more the 8) is very critical to free space near the antenna.
Capacitive
Top Loading at Shortened Helical antennas
Radio-amateurs may find descriptions of shortened
helical antennas with the length of the wire in the helix
less the lambda/2 and sometimes close to lambda/4. However,
it is usually applied to capacitive top loading vertical
helical antennas. Addition of the capacitive load at the
top of the helical antenna allows reduce the antenna resonance
frequency (compare to this one without top load) by almost
in twice times. Figure 11 shows design of a vertical capacitive top loading
shortened helical antenna. Just take attention that the
helical antenna Tesla (Picture 6)
and shortened helical antenna of old broadcasting radio
station (Figure 2)
- both of them are capacitive top loading helical antennas.
I did research to find plots for determining the
parameters of the capacitive top loading helical antennas.
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Unfortunately, it was not successful. The behavior
of these antennas is very unpredictable. Parameters of the
capacitive top loading helical antenna is very depends on
quality of the grounding and the structure of the top loading.
However I may do some recommendation to the design of the
capacitive top loading helical antennas.
The length of the wire forming the helix may be equal
(but not less) to the lambda/4. To the resonance frequency
the antenna is tuned with the help of the wires in the Top
Loading. Resonance frequency depends on structure of the
Top- loading- the dimension of the umbrella, quantity of
the wires in the umbrella and placing the umbrella relative
to the helix.
The input impedance of the capacitive
top loading helical antenna usually is less (up to two times) compare to usual
helical antenna having the same height. It makes such antennas
more complicated in the matching with feeding coaxial cable.
As usual it should be some matching circuit between the
feeding terminals of the antenna and the feeding coaxial
cable.
The Q factor of a capacitive
top loading helical antenna usually is less (up to two times) compare to usual
helical antenna having the same height. Therefore, the bandwidth
of a capacitive
top loading helical antenna is greater than the bandwidth of helical antenna
having the same height. Top loading helical antenna is not
so critical to ambient subject as usual helical antenna
having the same height.
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