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.
1 shows schematic of a combined three-band antenna.
Figure 1 A combined
The 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
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