combined three-band antenna
Three band antenna fundamentals:
At a lack of the place for installation of a separate vertical
antenna for each of three upper HF ranges it is possible to use
a combined three-band antenna that works at the ranges itself.
Figure 1 shows schematic of a combined three-band antenna.
Figure 1 A combined
antenna consists of from three quarter-wave verticals that are
resonated for each of working ranges. The verticals are connected
in the bottom together. Two quarter-wave counterpoises should
be use for each operation range of the antenna A coaxial cable
with 50-Ohm characteristic impedance will do well for the antenna.
A coaxial cable with 75-Ohm characteristic impedance also would
be work with the antenna, but a SWR in the coax will be higher
compare to 50-Ohm coaxial cable. Table
1 shows the combination of ranges where a mutual influence
of vibrators against each other is minimum.
Design of the Antenna: Three various
designs of the three- range antenna are shown below. Figure 2
shows a simple design suitable for 6 - to 15-M. The three
vibrators are placed on a small distance from
each other. The distances
between the vibrators are fixed with the help of small plastic
insulators. The design has very strong mutual influence for every
vibrator against each other.
Figure 2 Simple design
of a three ranges antenna