How do yagi antennas work?

[AgentCOPP1]

I've always wondered this about this mysterious little antenna (well actually it's not very little) and WHY it does what it does. I know that there's a driven element with one parasitic behind it that reflects and multiple in front of it that direct, but why does it do that? What are the physics behind it?

I need to know or else I might go insane.

 

[zz0468]

Simple question, with a really complex answer.

The simplest way to describe it is that the parasitic elements absorb, then re-radiate the energy coupled into it from the driven element. The spacing and element length dictates how that energy is re-radiated. With careful spacing and length, that parasitic element re-radiation will be additive in one direction, and destructive in another, which is where the gain and directivity comes from.

Digging a bit deeper... making the parasitic elements shorter or longer alters the reactive components of the currents flowing in the elements, and causes the currents to lead or lag the voltage, which causes them to re-radiate out of phase with the field that exites them.

 

[k3cfc]

Simple question, with a really complex answer.

The simplest way to describe it is that the parasitic elements absorb, then re-radiate the energy coupled into it from the driven element. The spacing and element length dictates how that energy is re-radiated. With careful spacing and length, that parasitic element re-radiation will be additive in one direction, and destructive in another, which is where the gain and directivity comes from.

Digging a bit deeper... making the parasitic elements shorter or longer alters the reactive components of the currents flowing in the elements, and causes the currents to lead or lag the voltage, which causes them to re-radiate out of phase with the field that exites them.

Your last paragraph is what i am trying to wrap my head around studying for my extra.

K3CFC

 

[AgentCOPP1]

Simple question, with a really complex answer.
Digging a bit deeper... making the parasitic elements shorter or longer alters the reactive components of the currents flowing in the elements, and causes the currents to lead or lag the voltage, which causes them to re-radiate out of phase with the field that exites them.

Can you please clarify this paragraph? I don't understand how the reactance in the elements directs the energy.

 

[ab3a]

The question you ask is not an easy one to answer. If you are not intimidated by mathematics, I suggest you look up the book Antennas by John D. Kraus. Though later editions are still under copyright, the Second edition is now available for free.

See
Antennas 2nd edition

Chapter 11-9, Arrays with parasitic elements on page 476 is where the discussion begins.

 

[zz0468]

Can you please clarify this paragraph? I don't understand how the reactance in the elements directs the energy.

The reactive characteristics in the parasitic elements cause the element to re-radiate out of phase from the RF that excites it that's coming from the driven element.

Depending on the element spacing, and the phase angle of the re-radiated energy, it will have either an additive effect, or a canceling effect. Make it longer, and space it right, and it cancels the RF coming from the driven element. That makes it a reflector. Make it shorter, and space it right, and the re-radiated RF reinforces the energy from the driven element. That makes it a director.

That's a very simplistic idea of what's going on, but hopefully you get the idea now.

 

[AgentCOPP1]

The reactive characteristics in the parasitic elements cause the element to re-radiate out of phase from the RF that excites it that's coming from the driven element.

Depending on the element spacing, and the phase angle of the re-radiated energy, it will have either an additive effect, or a canceling effect. Make it longer, and space it right, and it cancels the RF coming from the driven element. That makes it a reflector. Make it shorter, and space it right, and the re-radiated RF reinforces the energy from the driven element. That makes it a director.

That's a very simplistic idea of what's going on, but hopefully you get the idea now.

I understand what is happening to the radio waves on a superficial level, but as for the spacing of the elements and the length of them, I still have a hard time understanding WHY it does what it does. I know that the more director parasitic elements you have, the more concentrated the signal becomes, but I don't know why. I guess I'm just gonna have to give it some time and let me think it through haha.

 

[zz0468]

Without a firm foundation of knowledge of antennas and radiation and RF theory, you won't ever get more than a superficial idea of what's happening.

 

[k3cfc]

Without a firm foundation of knowledge of antennas and radiation and RF theory, you won't ever get more than a superficial idea of what's happening.

So what you are saying that we can't learn antenna theory. Hmmmm. okay

 

[zz0468]

So what you are saying that we can't learn antenna theory. Hmmmm. okay

No, that's not what I said.

 

[KD5SPJ]

What he said was that you need to know more about antenna theory to grasp the answer to your questions.

 

[k3cfc]

What he said was that you need to know more about antenna theory to grasp the answer to your questions.

Yes Kevin i know. i just read a book and now i have that knowledge.

 

[KD5SPJ]

I have a nice 13 element beam for my 2m base as well as a common vertical. I usually run the vertical but finally got a rotator so I am putting up the beam and going to try hitting some stations further away than I can on my vertical. My vert is only 5db gain, and the beam is 25 or so.