Radiation pattern measurements

Hello,

I have one question, that I cannot find answer anywhere. I need to measure radiation pattern of the spiral antenna in a cavity. How this should be done?

If I understand correctly, the spiral antenna in a cavity should be a receiver antenna, which will rotate 360 degrees in theta. I need two perpendicular cuts in phi=0 and phi=90. The problem is, which antenna should I use for the transmitting antenna? I only have this one and few that are linearly polarized.. Can be a transmitter antenna linearly polarized antenna? Is it necessary to use both antennas with circular polarization? How the transmitting antenna should be turned?


Thank you very much for your answer.

transmitting antenna used for testing process have to be standarized antenna's for both and polarization such as horn's. Transmitting antenna stays constant.

Linear polarization does the trick.

Regards
Elchiquito

Hello,

Using a directional antenna reduces influence of reflections. As elchiquito says, a horn can be used. You should be able to rotate the antenna to produce vertical and horizontal polarization.

If its gain is known (for example horn), you can use the friis formula for determining the gain (of course make sure you are in the far field region for both antennas and make corrections for cable loss, etc). When the gain is not known, then exchange the DUT for a good constructed dipole with balun (Gi = 1.64 without correction for cable and balun loss), or other known antenna. Based on the difference in output from DUT and dipole, you can determine the gain for each linear polarisation.

Your antenna will produce more or less circular polarisation. You need to sum the gains for both horizontal and vertical orientation to find the total gain (not using dB's). If you want to know how well the field is circular polarised, you need to rotate the horn (or other antenna) to find the axial ratio.

You may know that you can interchange receiver and transmitter, whatever is convenient for you.

To measure the radiation pattern, it's better to ensure that both of your antennas are co-polarized with each other in the chamber. To be honest, cross polarization doesnt seem to be that critical.

Co-polarization means that your antennas E-field and H-field are on the same plane.

Frodonet: He has a helix type antenna that will very likely produce circular polarisation at least in the main beam direction. So to find the total gain, he needs to measure in two orthogonal polarization planes (when using linear polarized "transmitting" antenna). Maybe moji123 can give some more details about the antenna.