On January 16, 2009 at 05:14, Ernie Bornn-Gilman said...
I'd like some info regarding "designed for 300 ohms."
If these antennas are designed for 300 ohms, but we use
75 ohm cable, why aren't they simply designed for 75 ohms?
I think the answer to that is that the antennas aren't
actually designed for 300 ohms, but instead they are designed
with dimensions that pick up signals, and they HAPPEN
to be about 300 ohms...so 300 ohm cable is used. Any
telecommunications engineer type comments?
The reason is actually that most TV antennas use a folded half wave dipole instead of a half wave dipole which has aprox 75 ohm impedance (73 exactly). Why do they use the folded version? ; very simple, because it has a wider bandwith than the half wave dipole that will allow the TV antenna to cover most of the TV frecuencies. BTW just for your info the simplest practical antenna is the quarter wave dipole (practical because the isotropic radiator is just a theoretical representation of an antenna) that uses a conductor surface like the ground and has aprox 36 ohms of impedance.
BTW impedance is a complex number (refer to complex numbering system for more info) in the form Z= R + Xj where R is the real resistance and Xj represents the imaginary part of the complex number Z which is the impedance.
I agree with you that "desing for" was not the proper wording, I guess I should have said most retail TV antennas have 300 ohm impedance at the dipole feeding point.