Antenna polarization an loss factor

Hi all! I have another newbie question... If i'm not mistaking GSM base stations (in Europe) usually have a linearly polarized antenna for transmitting GSM signal. What are the practical consequences of using a circularly polarized antena in a connected device (cell phone) ? I presume there will be some polarization loss factor, but is such concept feasible ?



Thanks in advance.

It sure is!

The (theoretical) loss resulting from cross polarisation is simply determined by the projection of the incident polarisation onto the polarisation of the receiving antenna (zero for orthogonal polariations, unity when they're optimally aligned).

In the case of a circularly polarised antenna, the projection of a linear incident field onto the circular basis of the antenna results in a gain (loss) factor of 0.5. i.e. the received signal will be -3 dB weaker than if a vertical antenna was used.... which is (usually) insignificant compared to the free space patch loss. So yes, it will certainly work

(Why don't we see more circularly polarised cell phone antennas then? Primarily because they're more *complicated*. Obtaining the omnidirectional pattern so easily achievable with a straight [vertical] piece of wire is much harder in the circular domain. While an antenna such as the quadrifilar helix is a possible solution, it is much bulkier [and fiddlier] than a simple monopole).

Thank You for the answer thylacine1975 ! Ok, i have asked this question in one thread before, but i would like to hear another opinion about it. So the question is is does rotation of the device (GPRS and GPS) have influence on signal strength (physical model - imagine a cell phone attached to a 10 cm (4 in) long cord on one end and the other end of the cord is in your hand spinning the phone around 200 rounds per minute) ? Does circularly polarized antenna solves the problem ?

Thanks.

Yes! ...but only if linear polarisation is being transmitted.

When transmitting linear polaristion, having a circularly polarised antenna at the other end certainly helps alleviate periodic variation in signal strength related to relative rotation. This is one of the reasons directional circularly polarised antennas (such as "crossed yagi's" and helicals) are popular in the amateur satellite community. [Aside from being the ideal antenna for circularly polarised communications, another reason is to combat the (variable) Faraday rotation experienced by the signal on it's journey through the upper atmosphere].

If the transmitted polarisation is circular however, then the projection onto either a linear or circular measurement basis (receiving antenna) will be constant for all angles of rotation: 0.5 for linear, 1 for the same sense of circular polariation and 0 for the opposite sense. Assuming perfect 'circularity' (i.e. no elliptical component) and a uniform radiation pattern, no polarisation-angular-dependant fading will be observed.

An interesting caution though, is that we have only considered the magnitude of the received (transmitted) signal and neglected the phase. For communications links this is (usually) OK, but for phase-sensitive applications (such as GPS), relative rotation between circularly and linearly polarised antenna systems modulate the received phase angles which (potentially) complicate the navigation solution calculation.

So if i understand correctly GSM is no problem and GPS is a little more coplicated but doable. Good news

Thnak You very much for the (quick) answers, Your help is much appreciated.

systems with mixed polarizations, linear and circular in the case you posed have both advantages and deficits. For example some signal is lost between a linear and circular antenna (typically -3 db). On the credit side rotating either antenna does not add additional loss as one would see with two linear antennas. There the signal could vary from 0 dB to very negative when "cross polarized". -40 dB or much more would not be a surprise.

Cost and complexity are also issues. Circular antennas are generally larger and more complex than linear counterparts (when they exist).

This is a subject that is discussed in most microwave antenna text books.

Ok, thanks for the answer.

In theory, circularly polarized antennas would have better performance for my aplication described by the physical model in post #3 (my conclusion-not neceserrily true, but in practice, i can't seem to find right handed circ. polarized embedded GSM/GPRS antennas on the market. Are there any ? If not, will linearly polarised antenna work in rotating conditions ?

And, i'm wondering...what about the doppler effect?


Thank You in advance.