antenna-theory.com

[raphael]

Good morning everybody
I have a question:
assume you have to feed, via a 50 ohms microstrip line, a printed antenna on a PCB. I wonder if the length of the microstrip line can influence the electrical length of the antenna, modifying (decreasing) its resonant frequency. Or maybe, because the line is well matched,
it only affects the electrical delay?
I understand that the problem is the same with a 3D antenna fed by a (pretty long) cable (ex: 50ohms coaxial)

Does anybody know the answer?
Thank you very much in advance for your reply

[Schubert]

Basically, the cable itself can act as part of the antenna. The transmission line itself does not radiate - that is, the job of the tx line is to have balanced currents/voltages flowing between the two wires.

However, at RF (particularly with coaxial cables), when a tx line is connected to an antenna there is a new path for current to flow: down the outside of hte back of the outside of the cable. To understand this, read up on baluns:

http://www.antenna-theory.com/definitions/balun.php

Hence, the transmission line will extend the length of the antenna, and therefore it is possible to have the transmission line lower the resonant frequency of the antenna (because the antenna acts larger).

This occurs whether or not the antenna is well impedance matched - you can't cure the problem with matching.

[raphael]

thank you very much!