Axial Ratio

[mirok]

Hi All,

Dose any one of you have any information about the calculation of axial ratio for examining the cicular polarization of antenna pattern.

Thanks for your attention.

 

[VEBill]

Axial ratio is the difference between the 'horizontal' and 'vertical' (or otherwise orthogonal) components. The most appropriate 'units' would be dB, but a straight factor could be used as well (it is just a ratio, a unitless number).

Imagine that you create the circular polarization using two properly phased and spaced orthogonal dipoles. Imagine that one dipole isn't working as well as the other - perhaps due to additional cable loss in the phasing network. The circle will not be 'round'. One axis will be 'bigger' than the other axis.

As far as calculating it, it will typically be based on measurements of each axis using a linear sense antenna. A linear antenna should detect equal signal at any orientation in the plane of the wave - if the incoming signal was perfectly circular.

Obviously the imperfect circularity could be introduced in the environment (after the signal leaves the source) - especially reflections.

 

[fmradio]

Just to note that, neglecting reflections, h-pol and v-pol boresite radiation will be equal from a linear dipole at a 45 degree angle to the horizon -- yet this is not a c-pol antenna. Likewise, an antenna that is intended to radiate c-pol may have equal h-pol and v-pol radiation, but radiation at other rotation angles can be quite different.

C-pol axial ratio has to be a measure of the uniformity of the fields at all rotation angles, not just those in the H&V planes.

Several papers at

http://rfry.org

include h-pol and v-pol radiation patterns from typical FM broadcast transmit antennas in practical applications. Although these antennas use elements that are intended to be c-pol, their true c-pol axial ratios in reality are rather poor.

In FM and TV broadcast transmit antennas, the cavity-backed radiator has about the lowest (true) c-pol axial ratio, as installed.

 

[VEBill]

I was trying to hint at the same thing by putting the words 'horizontal' and 'vertical' in single quotes and adding the clause "or otherwise orthogonal".

Your posting has certainly clarified the point.

 

[Higgler]

If you're not measuring the antenna by spinning a linear antenna in a circle then you must measure it using a dual polarized antenna with good phase tracking between the two polarizations, or just rotate one horn 90 degrees and take data a second time.

The formula 4.5 found on this website shows how to calculate axial ratio from the two orthogonal theta and phi antenna measurements. The formula uses voltage(E), not power in the calculations.

http://scholar.lib.vt.edu/theses/available/etd-02102000-19330046/unrestricted/09chapter4.PDF

A note on axial ratio- left hand versus right hand. There are two conventions. If someone specifies your antenna to be RHCP, ask them if that is RHCP similar to GPS, or have them clarify how the RHCP is defined.

kch

 

[fmradio]

RE: "or just rotate one horn 90 degrees and take data a second time."
__________

However this method may not detect defects in c-pol axial ratio unless the receive antenna is rotated in increments of less than 90° -- probably 45° or less. This method also requires free-space, or at least qualified test range conditions, as reflections of h-pol and v-pol may be quite different, and lead to inaccurate results/conclusions.

 

[AlWingy]

Sometimes not so straight forward as those fellas make out. You want to look up the actual equtions in books.... you can do it with orthogonal measurements using linearly polarized antennas or you can make the measurement using LHCP and RHCP antennas using the appropriate mathematics. But obtain the correct equations first!!!!!
It's best to get the magnitudes and phases for your answer as this will give you the most information.