"field intensity meter will measure in terms of V/m, right? Thank you so much."
Yes, if it's a field strength meter, overall the instrument
will be able to give you a V/m reading through whatever
process of use its manual specifies.
"well, what are the factors that affect the signal strength in the transmitting antenna?"
Transmitting antennas have a radiation efficiency factor
that determines how efficiently they radiate. But that's
mostly relevant from the perspective of the transmitter.
Energy that the transmitting antenna does NOT radiate
into the far field is irrelevant to the question of the
field strength a given transmitting antenna does produce
at a given distant location.
It's most appropriate to measure the field strength of
a transmitting antenna at a distance that is at LEAST
several wavelengths away from the antenna, and at
least several times the geometric size of the antenna
away. This is what's known as the "far field" measurement
which indicates the field strength the transmitting antenna
produces at a distant location, and this is representative
of the propagating wave that is produced by the antenna.
If you measured the field strength too close to a
transmitting antenna you would get larger or smaller
than expected values due to the effect of "near field"
non-radiating or non-efficiently radiating radiation
components from the antenna. E.g. even a simple
LC circuit produces E and M fields NEAR itself, and hence
a field strength, but it is almost useless as an antenna
since almost no energy is radiated to distant locations,
it is all remaining concentrated very close to the
circuit / antenna.
So the effects that relate to a far field signal strength
measurement produced by a transmitting antenna are:
a) Geometric field pattern of the antenna in the direction(s)
that cause signal to be receivable at the measurement
location. The higher the geometric gain of the antenna
in directions that influence the receiving location, the
greater the received signal could be there.
b) Power being radiated by the transmitting antenna;
the more power being transmitted, the greater the
remote signal strength.
d) Obstruction loss between the transmitter and receiver;
i.e. is there an ocean or mountain in the path of the signal
that absorbs / reflects a given amount of it? Are there
obstructions like trees, buildings, etc. These will all cause
path loss.
c) Propagation absorbtion loss between the transmitter
and receiver; is some of the signal absorbed in the air,
the ionosphere, by the ground, by foliage, by
rain/clouds/snow, et. al.
d) Propagation path distance between the transmitter
and receiver; depending on the path of signal progagation
there will be some loss due to the geometric diffusion of
signal energy and hence field strength over that distance.
e) Multi-path or diffractive signal diminishment at the
receiving location. It's possible for signals to arrive at
the receiver from several different paths or reflections.
In the case of multiple signal paths the signal may not
'add up' in phase at the receiver and hence the overall
signal strength may become erratic or weaker at the
receiver, or in some cases it could be enhanced.
Obstacles that diffract the transmited wave between
the tx and rx will cause a diffraction pattern at the
receiver location and that interference/diffraction will
affect the signal.
