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 the 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.
Practical
Installation of Dipole Helical Antenna in House Made of
Radio- Transparent Stuff
If house is built of radio-transparent stuff the
dipole helical antenna may be installed under the ceiling,
parallel to the wall of the house, or diagonally on the
ceiling or wall of the house. The length of the dipole helical
antenna will be limited only by the size of the room. Check
the wall or ceiling on to electrical wires and metal tubes.
The electrical wires and metal tubes (water and ventilation)
may cause serious influence on the operation of the helical
antenna.
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For example take a look to installation of a dipole
helical antenna in 5 meter length installed under the ceiling.
Figure 27
shows the antenna. Antenna with the length may be installed
at lots houses. The antenna may be designed for range from
20 meter (shortening factor equal 2) to 100 meter (Shortening
Factor equal 10). The range is included amateurs bands 20,
30, 60 and 80 meter.
Coaxial cable feeding the
antenna is going perpendicular to the antenna as much distance as
possible. RF choke should be installed on the coaxial cable
near feeding terminals. It may be 10- 20 ferrite rings on
the coaxial. Inductor RF- choke 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. Balance/Unbalance transformer installed at feeding
terminals of the ceiling helical antenna as well may improve
the antenna operation.
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.
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