GSM Spring Magnetic Antenna comparison based on physical dimensions
Kindly find the attached picture for more details.
The left antenna (A) has:
1. Smaller in height.
2. More number of small radius turns
3. Small base (magnetic mount)
The right antenna (B) has:
1. More height
2. Less number of turns which larger radius comapared to A.
3. Large base (magnetic mount)
4. Extra Single turn above multiple turns.
Q1: How can i determine the dB gain of antenna?
Q2: If there is no relevant measuring instrument available, can i choose the best among two just on mere physical dimensions?
Q3: What is the purpose of helix/spring in both the antenna?
Q4: What is use of extra helix in Antenna-B?
Unless you are using an expensive 3D EM simulator as HFSS or CST, all the antenna gain calculations I guess would not be accurate for this kind of antennas.
The most precise (and better than any simulator) would be to do an antenna replacement method measurements (referenced to a dipole antenna measurements done in an antenna chamber).
Using a signal generator, a spectrum analyzer, and a test antenna (which could be a simple Yagi antenna made for the desired frequency) you record first the gain of your antenna, and after that replace the antenna with a reference dipole.
The antenna gain that you get will be in dBd (dB referenced to a dipole).
Q1: See post by vfone.
Q2: physical dimensions - Longer length allows for better performance at lower frequencies. These antennas, when measured, are they placed on a rather big ground plane as that is part of the design.
If minimal size of ground, antenna performance will drop a lot. A important difference between these antenna is often length and quality of coaxial cable. A poor and long cable can reduce performance at higher frequencies with several dB. There are antennas on the market that are sold with 3-4 m RG58 and claiming 50% efficiency above 2 GHz. That is not possible as cable losses alone are more then 50%. Assume they did measure antenna without cable, and then did forgot to add cable losses in total performance.
Q3: Helix function - It have several functions. As these are monopole antennas must there be a length that is corresponding of at least quarter wave length to reduce impedance mismatch. A way to do that is to add a coil as antenna else had been needed to be longer. The coil do then gives better impedance match but somewhat reduces antenna efficiency. For higher frequencies can a coil act in a way that concentrate radiation in horizontal plane att cost of radiation in higher elevations.
Q4: Extra coil - Same function as other coils at the antenna. Upper coils are usually related to antenna impedance at higher frequency range. Another thing that confirms this for this antenna, is that it have few turns and for that reason will it have minimal effect of the lower frequencies.
These antennas are often designed to be extremely wide in frequency coverage and are for that reason a compromise relative antennas built for a narrow coverage.
If you know what frequency range you need, select a such antenna for best performance.
A good coaxial cable, not unnecessary long, can be a much more important factor then the antenna element for a total better performance.
Cheap antennas do often have cheap coaxial cable, as good cables are expensive.
This type of antennas are intended for big metallic ground planes, such as a car roof. Without this ground is antenna performance very unpredictable if it performs at all.