The
classical W3DZZ antenna
in vertical installation designed by UL7GCC and shown at the Reference 1 is well known in Russia. Figure 1
shows the antenna. Diameter of sections A and B is 40 50mm.
How is it work?
40m band: The trap LC
cut out the upper section B from the antenna. So only section
A works as a radiator, and the section A has length in 10.1 meters,
i.e. has electrical length in 1/4l. Vertical
radiator having with the length of 1/4l
has a quarter wave resonance and works in very effectively way.
At the band the circuit LC works as a trap.
80m band: On the 80m band the antenna has summary physical length
of this two sections A+B a little less
than 1/4l. A + B = 16.47 meters, less then 20 meters OF quarter wave
length for the 80m band. A short vertical radiator has a capacity
part in its input impedance. 
But the circuit
LC at the 80m range has an inductance part in its impedance.
The inductance part compensates capacity part of the electrically
short vertical, and the antenna has a low SWR at the 80m range
too. In other words, the inductor of the LC works as a usual lengthening
spool.
10, 15 and 20m Bands: Visa
versa, at the 10,
15, and 20m the LC has a capacity part at its impedance that
goes the electrical length of the antenna to 1.75l
at 10m, to 1.25l at 15m and
to 0.75l
at 20m.
Do
not forget, verticals like counterpoises, so use several 1/4l
counterpoises for each bands.
Reference
M.
Chirkov, UL7GCC: Multi range vertical // Radio #12,
1991, p. 21.
