Dear all,
I have a circular polarized antenna under test (AUT) as transmitter and a linear polarized horn antenna as a receiver. I can only measure the magnitude with power meter and don't have the phase information. I can rotate the the LP horn antenna around its center, in the plane perpendicular to the direction of propagation. So I have the axial ratio measurements, which i have measured by rotating the horn around its axis.
Can you please guide how can i measure the radiation pattern of this CP antenna? Would measuring vertical and horizontal planes and using Pythagoras theorem will give the right result?
Regards
Imran
Spin the antenna continuously during the measurement, or take about 8 measurements or more at each direction. The variation of power with rotation allows you to calculate the axial ratio at each direction, and the angle of the elliptical polarization. Just measuring V and H without phase isn't enough.
actually, it is the only way for inlab wideband CP measurements, because metrology-grade CP antennas are very narrow band. And huge.
I think measuring the power (in Watt/milli-Watt) only in horizontal and vertical axis and then adding them in together will give the total power in all directions. When the linear polarized antenna is placed horizontally, it will receive the horizontal components of the signal polarized at different angles. And when the antenna is vertically oriented, it will receive the vertical components. And then adding them together will give the total power. I think it is the same reason that a linear polarized antenna receive 3db (half power) when transmitter is purely circular polarized.
Can anyone please comment on this?
Imran Aziz - I didn't mean to recommend, but to correct. A 45 degree polarized linear antenna will give the same result, so your measurement tells you nothing about circular polarization, only about the ratio of h and V polarizations.
Malcolm White - Thanks for your reply. In fact I have measured the axial ratio by taking around 30 samples in the complete circle (by rotating the LP horn antenna in the plane perpendicular to the direction of propagation). Now I want to measure the radiation pattern. And I think if I want to measure the total power at the receiver, I should measure the radiation pattern once by placing the LP horn in horizontal polarization, and then by placing it in vertical polarization. As fields in any direction has two orthogonal components (horizontal and vertical), I think adding these two measurements will give the total receive power.
Please correct me if I am wrong.
They may give the total power, but they won't tell you whether is is still circularly polarized off boresight. It might be enough to measure at 0, 45 90 and 135 degree linear polarization at each point on the polar diagram, then estimate the axial ratio. The four values should follow a simple analytic expression which you can derive or find out depending on your level of competence. Any elliptical or linear polarization at 45 degrees will give the same power in V and H. Do you want to know the total power radiated or the power available for a proper circularly polarized antenna to receive?
OK, now I somewhat understand. The method I mentioned will give the total power. But the power available for a proper circular polarized antenna to receive would be different and I don't fully understand how to measure this power. I can measure the axial ratio at each point on the polar diagram, but how to calculate the power available for a CP antenna to receive for this information?
Malcolm White - thanks for your valuable input. I have measured the attached radiation pattern with the axial ratio information as well.
Philosophy, diffraction, echo, and noise must be attested, but we ask ourselves why dont transmitting a sinsoidal signal analyze the recieving signal, take message signal, repeating that so many time and take the average of values.
From the data you have plotted - plus the data you haven't shown, which is the angle of the linear polarization, you can plot a graph of variation of linear polarization against angle for each position. From this graph you can get axial ratio and elliptical axis by fitting the points to the equations that describe the result of adding two circular polarized signals with variable phase and ratio. From the axial ratio you can get the powers in the two circular polarizations. What this method can never give you is which circular polarization has which power. This can cause problems because at places on the graph where the axial ratio is large the power can come out the other side with the opposite power ratio, or not, and you can't tell. It also can cause problems if you got your basic design wrong, because you will never find out that you have the opposite polarization until it is too late. This could happen with 3D antennas if designer and manufacturer worked with different drawing conventions, such as 1st angle projection and 3rd angle projection, which used to happen if Europeans made parts designed in the US, or vice-versa.
You need to read a chapter on circular polarization. What I am describing here is the very basics. If you don't know more than this you will not be a safe or reliable designer.
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