As a rule, we need taking some steps to make any antenna
to be resonant on several amateur bands. The body of the aerial
needs some special constructive elements to tune it into a resonance
within different amateur bands. These elements can be concentrated
(LC, L, C, for example, Cushcraft R7000
or distributed (loops, lines, for example, GAP-Titan (http://www.gapantenna.com/titan.html ). I.e. the antenna
is broken up into several parts in between which there are those
adjusting elements providing a resonance of the aerial. The more
such elements, the more difficulties with their optimum adjustment,
and reliability of a design as a whole leaves much to be desired
because it is cut by insulators.
because the aerial is a multiresonant one, it is enough to change
the band in the transceiver - simple and convenient, but not so
good if your neighbour HAM is on the air too - rustles and clicks
from the signals is usual business even if he drives the different
band. The multiband vertical can be made upon the other constructive
principals: the radiating part of the antenna through a switchable
matching network to the feed line. In the other word, input impedance
of a random wire is of a complex value, so the matching network
transforms input impedance of it into feed line impedance.
for the reason of an accurate matching on each amateur band it
is necessary to separate matching networks. In fact the common
multiband matching is not the best choice - it is very difficult
to achieve accurate matching (in fact for different bands matching
circuits may be different) and to provide necessarily good quality,
accordingly, will be more losses than for the separate network.
As for the similar designs it is of a rare issue (for example
though they have some advantages before the other verticals. For
1. Mechanical durability
of the vibrator because of absence of insulators.
2. An opportunity and convenience
of the optimum adjustment of VSWR at the antenna-feeder point
(i.e. network adjustment).
3. Simplicity of installation
due to lightweight aluminum tubing (except for capacitor loading
above in my case).
4. Greater selectivity due
to the switching network of the antenna, better suppression of
unwanted signals and harmonics of course.
May be it sounds crazy,
but this antenna realization is my former idea of using any random
wire as HF multiband, of course except for the telescopic aerial
of a household radio receiver 1 meter long, though I had similar
experience nearly 20 years ago - RLT (short distance field test)
on 3.5MHz... So, this time I'd like to share these results with
those who may be interested in. Of course it's not a panacea and
the full-size one band antenna would be the best choice, but in
a series of multiband aerials, in my opinion, the given design
obviously is worth considering of, especially for those who does
not have enough space to set up something extraordinary, but DX
chasing would be of desire.
Naturally, "random length"
means reasonable length at which theoretical efficiency on the
lowest frequency (1.8MHz) would be of at least ten percents, so
the total length should be at least 10 meters long. Further, by
means of separate LC matching network for every HF band, the aerial
will be matching 50-Ohm active load, then the 50-Ohm coax feed
line can be of any length. So the block diagram is just like this:
the mast itself - the switched matching networks - the coax feed
line. This vertical antenna is without traps, loops and similar
mechanically unreliable elements. In simple words it's just telescoped
aluminum tubing. And for some electric lengthening there're four
wires top-hat folded capacitive loading at the top of the mast