A. What are the important parameters of a whip antenna used for evaluating the antenna?
B. What creating antennas from wire, what is length and size of the wire to be used?
A. The most important parameter is the impedance (which should be 50Ohm at the operation frequency) and the VSWR (lower is better). Antenna gain is also important.
Datasheets of whip antennas usually state 0-2dBi gain but these values are usually measured with the antenna mounted on a large perpendicular ground plane. The optimal ground plane size for whip antennas is half-wavelength or higher. Below quarter-wavelength ground plane size significant performance degradation can be expected.
In general, whip monopole antennas are quarter-wavelength antennas so a straight wire antenna for example for 434MHz should be approx 17cm long. Of course if some coating material is used around the wire (with larger dielectric constant) and/or helical wire is used then the physical length of the antenna is shorter. From gain point of view it is better if the physical size of the antenna is larger.
B. The length of the antenna should be measured from the point where it leaves close proximity to the ground, or from the input/output. If a whip is mounted on a box, and connected with simple wire, then that wire becomes part of the antenna.
In the case of the whip, there must be a connection to a ground, even if the ground plane area is nothing more than circuit traces and a battery. The whip and ground plane combine to form a complete circuit. The electromagnetic field is set up between the whip and the ground plane, with current flowing through the field, which completes the circuit. Ideally, a ground plane should spread out at least a quarter wavelength, or more, around the base of the whip, ground planes can be made smaller, but this will affect the performance. The ground plane area must be considered when designing an antenna.
The length is inversely proportional to the frequency and may be calculated by: wavelength in cm = 30,000 / frequency in MHz. Thus, for a quarter wavelength:
l (cm) = ¼ ( 30,000 / freq (MHz) )
The formula should only be considered as a course measurement however since the length may actually be shorter if the whip is overly thick or wide, has any kind of coating, or is not fed close to the ground. It may also need to be longer if the ground plane is too small.