Ham knows and widely used to Symmetrical Trap Dipole Antennas.
Classical example of such antenna is W3DZZ antenna. At my opinion
such antennas has just one benefit- patterns of the antennas are
almost the same at the different bands. However, the antennas
have lots deficiency. There are heavy weight, complexity in design,
large enough windage and narrow bandwidth at the lower bands,
high SWR at some bands.
Asymmetrical Dipole Antennas (that are used
by hams) are free from some of lacks of the Symmetrical Trap Dipole
Antennas. However the main lack of the Asymmetrical Dipole Antennas
is the main lobe of the diagram directivity at lower bands is
toward to main lobes at the other upper bands.
An interesting antenna was design by me by combination of these two
types of antennas- Symmetrical Dipole Antennas and Asymmetrical
Dipole Antennas. I called it "Asymmetrical Trap Dipole Antenna."
The antenna has pattern almost similar to pattern of Symmetrical
Trap Dipole Antennas. However the antenna needs twice less traps
compare to Symmetrical Trap Dipole Antennas. So, Asymmetrical
Trap Dipole Antenna is more easy to tune up and has less windage
(compare to Symmetrical Trap Dipole Antennas).
Figure 1 shows the design of
the Asymmetrical Trap Dipole Antenna. Dimensions of the antenna
are given for height 15 meter over the ground (there are in the
brackets dimension for height 15 meter over the ground)
How the antenna works: At the 40- meter Band there are left
wire and wire up to trap on 7- MHz take part in the antenna radiation.
So it is asymmetrical dipole with the ratio of the sides 1:2.
At the 80- meter Band there are left wire and wire up to trap
on 3.5- MHz take part in the antenna radiation. So it is asymmetrical
dipole with the ratio of the sides 1:2, however, the left wire
of the dipole is the less wire of the antenna. At the 160- meter
band all right part of the antenna works. It is not classical
asymmetrical dipole with ratio 1:2. However because of the inductors
in the right wire and low height above the ground the antenna
may be matched with 110- Ohm with SWR 1.25:1 .
The antenna has input impedance close to 110- Ohm at its
working Bands. Antenna may be match with 50- Ohm coaxial cable
by binocular transformer 1:2.56. References 1,
2 are described such transformers. Do not use autotransformers. As a rule autotransformer does not provide cut-off
RF current from the outer braid of the coaxial cable.
So the cable would be radiated and very possible RFI and TVI around
of the antenna. It would be useful to install RF choke on the
coaxial cable before the entering the cable in to the room.
breakdown of the transformer by static electricity it should be
installed resistor on 100- kOhm (or more) in bridge with antenna
wire (any of them, or, that is better, between the middle point
of the transformer) and the braid of the coaxial cable. The braid
should be grounded in the shack. I made traps for the antenna
accordingly to Reference 3.
Picture of the similar traps are at Reference 4.
shows pattern of the antenna installed at height 30 meter above