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 (http://www.cushcraft.com/amateur/r7000.htm ) 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 example:
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
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 is used.