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Vertical helical antenna required good RF grounding.
It may be metal roof, metal rail or artificial grounding
made with counterpoises. Three counterpoises with length
of lambda/4 will provide excellent ground for the helical
antenna.
It should be taken measure both for vertical and
for dipole helical antenna for elimination of the influence
of the outer braid of the feeding coaxial cable. So RF choke
should be installed at the feeding terminals. It may be
10- 20 ferrite rings on the coaxial or just 5- 20 turns
of the coaxial cable. Diameter of turns may be 10- 20- cm.
RF choke especially is needed for helical antennas with
high (more the 5) shortened factor.
When helical antenna is made we need tune the antenna
to the resonance frequency at the operation band.
Tuning
the Helical Antenna for the 40 meter Band
When a shortened helical antenna is ready and installed
at the operation place the antenna should be tuned to the
resonance frequency. At first we need find out what the
resonance the antenna has. The resonance frequency may be
found with GDO or RF- Bridge. Figure
6
and 7
are illustrated the method.
If the resonance frequency is significant differ
from the desired- up to 10 percent, the antenna is tuned
by changing the length of the wire in the helix. If the
resonance frequency is lower, the length of the wire should
be decreased, if the resonance frequency is higher, the
length of the wire in helix should be increased. The experiment
will show what the length should be changed to find resonance
on the desired frequency. It is initial tuning the antenna.
The resonance frequency after that tuning may differ from
the desired one up to 1- 2 percent.
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Fine tuning make the antenna resonance frequency
matched with what we needed. The tuning may be done by changing
the pitch in the parts of the helix. Such tuning may be
applied for shortened helical antenna with high distance
pitch. If the resonance frequency of the antenna is lower
the needed so parts of the turns of helix should be moved
to open end(s) of the antenna. Figure 15
shows the tuning. If the resonance frequency of the antenna
is higher the needed parts of the turns of the helix should
be moved to feeding terminals of the antenna. Figure 16 shows the tuning. The new pitch of
the helical antenna and places for the pitch is experimented
to find.
When the
helical antenna is
tuned to the resonance
frequency it should be
measured the antenna input impedance. RF-bridge, MFJ-259 or
any other device may do the job. 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 feeding 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 9) has option for theoretical calculation
of the L- matching circuit based on measurement of antenna
input impedance and impedance on the end of the feeding
coaxial cable. Be truth I always use practical adjustments...
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