The antenna
was mounted on the clothes drier installed near wooden window
frame and was placed near 100 cm out of the concrete wall of the
building. Form of the antenna was made from wooden sticks. The
sticks were fixed by the nails to the wooden triangles.
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Each side of
the wooden tringle had diagonal of 70- cm. Length of the sticks
was 2.3- meter. Aluminum stick was installed on the wall of the
building on the old antenna insulator from VHF radio. I used chip
bare aluminum wire in diameter of 2- mm. For me it was very convenient
to use the bare aluminum wire because it was possible very easy
to connect to the wire to find resonance of the antenna on the
desired bands. Table 1 shows length of the helix where the helical
antenna has resonance at the amateur bands.
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When the helical antenna with grounding is installed
and coaxial cable (do not forget about RF chokes on the both sides
of the cable) is placed from the antenna to the transceiver, the
helical antenna should be tuned to the resonance on the desired
bands. It is very simple to find the resonance frequency of the
antenna with help of RF-bridge. By choosing the tap from the antenna
find the resonance frequency of the antenna. Device Artificial
Ground helps eliminate the antenna reactance. So, it is needs
two steps- first choose the tap, then, mark the tuning on the
device Artificial Ground where the helical antenna has exactly
resonance.
It is possible straight away connect to the taps shown
in the Table
1 and then tune the antenna in the resonance with help
of shortening capacitor or lengthening inductor. If the helical
antenna has the resonance lower the desired frequency in serial
with the antenna turn on a shortening capacitor, if the helical
antenna has the resonance higher the desired frequency in serial
with the antenna turn on a lengthening inductor.
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However, helical antenna tuned to the resonance with
help of a shortening capacitor or lengthening inductor has narrow
passband compare to antenna tuning by choosing tap.
The helical antenna is sensitive to position of the counterpoises and
coaxial cable going from antenna to transceiver. So after tuning
the antenna the things should be unchangeable in the place.
Antenna was installed at the first
floor of the 9- story building. Antenna was
tested in the Air and showed good result. However, I used the
antenna (second variant, with helix in 22- meter length) only
one month. Then the antenna was demolished because it was too
bulky and prevent from drying the clothes (and curiosity of my
neighbors)...
References
1. http://www.antentop.org/qrp.htm
2. http://www.antentop.org/024/ZS1JHG_024.htm
73! de VA3ZNW
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