Tzitzikas wrote:
"I know that when we use an inverted L the radials increase the
antenna`s efficiency."

The same is true of any antenna which works against the earth to
complete its circuit. Radials at the surface of the earth may be
increased in number until there are more than 100 where the point of
diminishing returns is exceeded. U.S.A. government regulations require
medium-wave broadcasters to use 120 radials of at least 1/4-wave length
or else prove the installed grounding system meets or exceeds the
government`s efficiency standard. These radials are separated by 3
degrees for equal current distribution. Thousands of such installations
have been documentedand and are proved nearly lossless.

Tzitzikas also wrote:
"I would like to ask which antenna is better for 160m?"

The radiation which counts is along the surface of the earth because it
determines daytime coverage. Ionospheric coverage is useful at night
subject to many variations and even interferes with the surface wave at
some distance from the transmitter.

If some of the radiatinng antenna wire is horizontal, it may introduce
radiation which escapes to the ionosphere to interfere with the surface
wave at some distance from the transmitter unless it is carefully
balanced out as in the case of the T antenna or other antennas with
symmetrical balanced capacitive loading.

First, an antenna which is self-resonant presents no capacitive
reactance, which needs a lossy coil to balance out. The shortest
self-resonant antenna works against ground and is 1/4-wavelength. That
makes them popular. It`s a matter of getting maximum current into the
antenna. A resonant antenna is unimpeded by reactance.

If an antenna is between 1/4-wave and 1/2-wave in length it can be
resonated with a series capacitance. Capacitance is usually very low in
loss so it is efficient.

As an antenna grows from 1/4-wave to 1/2-wave it produces greater field
strength on the distant horizon, not because it is more efficient but
because its power is being radiated at lower angles near the surface of
the earth and less is radiated at angles above the horizon.

Best regards, Richard Harrison, KB5WZI