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Helical antennas
with shortened factor
more the 10 has unpredictable
behavior so it is impossible to make simple plot for such antennas.
As you can see from the Figure 8 for feeding helical antennas
it is possible use 50 Ohm coaxial cable. However for best
match of the helical antenna with transmitter this cable
should have electrical length multiplied to half of the
resonance wavelength. Some of the simulation antenna program
shows the very low input impedance of the short helical
antenna. However in my practical measurements I have seen
the input impedance in the border of the 30- 60 Ohms (not
in the 3- 10 Ohms like the program predicted). I do not
argue why it is, bad grounding, losses around or something
else... In antenna practice it means that in most of cases
you may feed the helical antenna through usual 50- Ohm coaxial
cable, is not it?
In some cases, the vertical helical antennas initially
had input impedance greater than that shown in the plot
from Figure 8. Tuning
the antenna ground to resonance by length of the counterpoises
lowered the input impedance of vertical helical antenna.
To reduce influence of coaxial cable connected to
shortened helical antenna to parameters of the antenna an
RF choke should be installed at the end of the coaxial cable
near the antenna feeding terminals. It is may be near 10
ferrite rings installed on the coaxial cable. RF choke made
as inductor from coils of coaxial cable as well good choice.
Diameter of the coils may be 10- 20- cm and the inductor
may contain 5- 20 turns.
To increase the efficiency of the helical antenna
it is necessary to use wire of the largest possible diameter
for the helix, and the diameter of the helix should be the
maximum possible for the practical implementation of the
antenna. The form of the helix should have low dielectric
losses. It is desirable to use even winding of the helix.
Q
Factor of Helical Antenna
Important parameter of helical antenna is the quality
(Q) factor. Knowing the Q of the antenna allows approximate
the bandwidth and place where the antenna should be installed.
The higher the Q of the antenna is the narrower the antenna
bandwidth is and the reactive field has high intensity near
the antenna during the operation on transmitting. Any subject
with RF losses that sitting near the helical antenna will
absorb the RF energy when the antenna will transmit. Helical
antenna with high shortened factor has narrow bandwidth
thereof high Q.
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So helical antenna with high shortened factor will
require lots free space nearby otherwise the antenna will
heat the subjects that placed near the helical antenna and
the RF losses should be present.
It is possible easy verified. Just hook- up helical
antenna with shortened factor more the 10 to transmitter
that provide power more the 50- W. If coaxial cable going
to the antenna has ferrite rings at the antenna terminals
that the ring will be warm through some times.
However the ferrite rings are warm not due the reflected
wave on the outer side of the braid. The reactive field
near the short helical antenna heat the ferrite rings. Just
place the ring near the antenna and the ring (as well as
any RF absorption stuff) will be warm.
For that reason the ferrite rings should not be used
for RF choke with helical antenna with shortened factor
more the 5. Usual inductor on 5-20 coils made of the coaxial
cable feeding the antenna would be good replacement of the
choke with ferrite rings on the coaxial cable.
I did measurement of Q of rows of my experimented
helical antennas. The antennas were operated at 7- 28- MHz.
The antennas had form in diameter 2- 10 cm. Antennas had
even winding of the helix.
Antennas that were wound on form in 2-5 cm there
were wound with wire in diameter of 0.5- mm and more. Antennas
that were wound on form in 5- 10 cm there were wound with
wire in diameter of 1.0- mm and more. Q factor of the antenna
was found as follow:
Q=F1/F2.
Where:
Q is the quality factor of the antenna;
F1 is the resonance frequency of the antenna;
F2 is the antenna bandwidth.
F1, resonance frequency of the antenna, I defined
as frequency of the first resonance where SWR of the antenna
has minimum value. I used to my home-made RF bridge to measure
the resistance of the antenna. The RF -bridge does not show
reactance, it shows that sum of the active resistance plus
reactance. F2, bandwidth of the antenna I also defined with
help of the home- made RF -bridge.
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