Sigma wrote:
"This design os already working with a quarter wavelength antenna. My
task is to modify it to a smaller (shorter) one."

An antenna shorter than resonant length uses a lossy loadong ciil(a) in
most cases to neutralize its capacitive reactance. The antenna system
must be resonant so that reactance does not limit energy into the
antenna.

A 1/4-wave of wire is only 1/2 of an antenna. The other 1/2 of the
antenna must be supplied by another 1/4-wavelength of wire or by an
image-maker such as a large conducting sheet or a ground plane. The
image maker is larger than a lone 1/4-wavelength of wire.

A 1/2-wavelength of wire is resonant and it is a complete antenna. A
1/4-wavelength is not.

A shorter than resonant-length of wire is less directional than a
full-length antenna, but not by much.This means there is less "gain" in
it at its best direction. Its real problem is its fast rising capacitive
reactance as it is shortened from resonance. Another problem is its
dropping radiation resistance which causes its loss to heat resistance
to take a larger share of its total energy.

There is a way to slightly reduce the length of a resonant dipole, and
that is to make it fat instead of thin. This is usually costly,
inadequate, and inconvenient. The length of a 1/2-wavelength conductor
is only 94% of a 1/2-wavelength in free-space if its diameeter is 1/1000
of a wavelength. See Fig 8 on page 2-5 of the 19th edition of the ARRL
Antenna Book.

Another way to reduce the length of a resonant dipole is to attach large
conductive areas to its ends as capacitive hats for loading. This is
almost lossless as compared with coils.

The antenna may not be the problem if you are working in the near field.
You may have an advantage in the much higher energy very close to the
antenna. The antenna`s induction fields are much larger than its
radiation field up close. See page 865 of Terman`s 1055 edition.

You might not use a salient antenna at all. You could use a metal
1/2-wavelength slot. See Chapter 9 of Kraus` 1950 edition of "Antennas".

A practical solution may be a 1/4-wavelength, end to end, folded dipole.
This consists of a resonant 1/2-wavelength of wire. It takes only 1/2
the space of a !/2-wavelength open-ended dipole. Its gain is only 0.5 dB
less than the 1/2-wavelength dipole. Its problem is a feedpoint
resistance of 6000 ohms which may be hard to match. See page 509 of TV
and Other Receiving Antennas" by Arnold B. Bailey.

Best regards, Richard H. Harrison, KB5WZI