Log periodic antenna intuitions for its region properties

Thread Starter

yef smith

Joined Aug 2, 2020
186
Hello,a log periodic antenna shown In the diagram shown bellow. As you can see there are reflective region and non radiative region. there are phase shift of 180 degrees between neighbor "wings".
I have read that the not radiative region is not radiative because it hase capcitative profile
. I can see why it has capacitate profile? And why a capacitative profile doesnt allow us to radiate?
Same this for the reflecting region,its says that its reflective because its an inductive profile. why inductive profile reflects the wave?
Thanks.
1617132561332.png
 

nsaspook

Joined Aug 27, 2009
8,158
You could think of it as similar (at one operational frequency of the LPDA) in electrical response to a three+ element Yagi (traveling wave) antenna with director elements, a driven element and a reflector element.

The directors are capacitive: The current leads the voltage.
The driven element is resistive: Current and voltage in phase.
the reflector is inductive: The current lags the voltage

This phase shift across the properly spaced elements causes the combined emissions from all the elements to be in-phase and constructive in some direction while being out of phase and destructive in others.
235px-Yagi_antenna_animation_16_frame_1.6s.gif

I don't mean to say they are the same in physical construction or design.

Only that the electrodynamics of the radiative regions directivity are similar.
 
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Thread Starter

yef smith

Joined Aug 2, 2020
186
Hello Nsaspook, How we know that reflector is inductive?
Why inductive profile causes to reflect the radiation?
Thanks.
 

Thread Starter

yef smith

Joined Aug 2, 2020
186
Hello Nsaspook,reflection coefficient acts as the formula shown bellow
for the wave to be reflected the LOAD has to be eigher 0 or very large impedance.
Z_L=jwL so it depends on the L and frequency value,i cant see why they say its automatic reflection of the wave?
and why its inductive profile?
Thanks.

1617172487617.png
 

Thread Starter

yef smith

Joined Aug 2, 2020
186
Hello,Can i have an intuition regarding how exactly we get a constructive sum in the main direction?
capacitative part make -90 degrees phase inductive part make +90 degrees phase so how they make 360 degrees in phase?
Thanks.
 
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