QUESTION

Question

I need help in designing a crossed dipole antenna at 435Mhz using microstrip lines for my interest. Currently, my reference is Gomspace ANT430. However, I am not sure how to calculate the antenna impedance, and how does it related to the microstrip line? Why is there a 90 degrees phase shift? How does the input signal look like with a single dipole and a crossed dipole without phase adjustment? How to adjust the phase factor and what do I need to look out for? Thank you for your help.

Answer 1

Hello Stephen,

The principles behind the crossed dipole or turnstile antennas are fairly straight forward.  Where I am seeing your question pertaining to is the feed structure and the 90 degree phase shift. The antenna impedance is controlled by the width of the feed trace and its proximity to ground. We have an trace impedance calculator available to the public on our website to help you determine the height above ground and trace width for a given impedance. Typically the wider the trace the lower the impedance or the closer the ground is to the trace the lower the impedance becomes.

The phase shift is due to the two dipoles being placed 90 degrees apart to receive horizontally and vertically polarized antenna signals and being fed axially to combine the two signals and for circular polarization. For more information on the desire and requirements of this you may want to look up turnstile antennas on Wikipedia since we do not currently manufacture any turnstile antennas.

Answer 2

Hello Stephen,
The impedance of an lambda/2 dipol is around 70 Ohms. However is stronly depends on the environment.
The 90° phase shift is used to achive cricular polarisation.
A single dipol has linear (horizontal) polarisation. Crossed dipols without phase shift have a diagonal (linear) polarisation.
Regards
Guenther