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Coaxial cable feeding the dipole helical antenna
should be placed perpendicular to the antenna and be perpendicular
at least in the open space of the window. RF choke should
be installed on the coaxial cable near feeding terminals.
It may be 10- 20 ferrite rings on the coaxial. RF-choke
made as inductor from the coaxial cable as well may be used.
This RF choke may have 5- 20 turns of the coaxial cable.
Diameter of turns may be 10- 20- cm. The inductor RF choke
may be placed on the window sill or attached to the wall
of the house. So, the inductor RF choke may be placed at
some distance from the feeding terminals.
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When the
helical antenna is
tuned to the resonance
frequency it should be
measured the antenna input impedance. RF- bridge, MFJ-259
or anyone other device may do the job. Shortened
dipole antennas that were made by me in the window opening
had input impedance near 20- 40 Ohm. So, the input impedance
for the dipole helical antenna was lower compare to the
asymmetrical helical antennas installed in the window opening.
Antennas with the low input impedance that is fed by the
50 Ohm coaxial cable without matching device will work with
high SWR.
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