Ceiling helical antenna may have input impedance near 30- 60 Ohm, so,
usual 50- Ohm coaxial cable may be connected to the antennas without
any matching device between antenna and the cable. The high input
impedance of the ceiling shortened helical antenna installed could
be explained by losses of the RF energy by nearest objects.
If the input impedance is different at least to 20 percent from the wave
impedance of the feeding coaxial cable it should be installed
simple L- matching device between the antenna terminals and coaxial
cable. Good choice if the electrical length of the coaxial cable
is multiplied by lambda/2. Then it should be measured antenna
resonance frequency at the end of the coaxial cable. Most common
method is by measuring the SWR at the coaxial. In some cases helical
antenna (or simple L- matching device between the antenna terminals
and coaxial cable) may need some additional tuning. Free program
MMANA (References 10) has option for calculation
of the L- matching circuit.
Form
of the Helical Antenna
The ideal form for the helical antenna should be lightweight,
mechanically strong, have low dielectric losses at working frequencies,
and preferably have low cost.
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Form for the HF shortened helical antenna, if it is
possible, should have diameter of 5- 10- cm. Very often plastic
water tubes are used for form of the helical antenna. The tube
has almost all mentioned above points- the tubes are lightweight,
mechanically strong, have low dielectric losses at working frequencies,
and low cost.
Helical antenna with form on the water pipe may be
installed at the place with help of synthetic rope. The rope going
inside to the pipe, knotted at the ends and the rope fixed at
the place of installation of the helical antenna. Figure 28 shows installation of the helical antenna
with help of synthetic rope.
Shortened helical antenna with form on the water pipe
may be hang up for one point if the vertical
installation is required.
If the vertical helical antenna is small and form of
the antenna has sufficient strength it, could be mounted on some
heavy base and then, if it is required, fixed with guys from synthetic
ropes. Figure 29
shows the vertical installation of the helical antenna.
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