impedance matching to free space

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The answer is simple. Stop listening to simplistic answers to complex questions.  Translation, most of what you will hear about technical issues will be watered down mathematical models of something. This is the part that really confuses alot of people.  Math models are simplifications of some subject to be used  within very narrow limits. You will hear daily on this site arguments start over somebody using a simple model they think is absolutely factual about a subject they read in a handbook.  The reality is that the model is mostly useless for any serious discussion and the model was really intended to give some kind of answer to satify non or low technical ability readers.

Marten Beels:
Thanks for the variety of responses.  LOL to the Hoover upright...

Yes, I understand that mathematical models can only be applied to limited situations.  After all, according to the math, a half wave end fed antenna should have an infinite impedance, and you can't match anything to that!  But they do work...

Anyway, it makes sense that we only have to match to the INPUT impedance of the antenna, that doesn't say anything about the OUTPUT impedance of the antenna.  I've got "Foundations of Electromagnetic Theory" sitting here, but wow, the math is HEAVY!

Something for me to pore over.


Allen C. Ward:
Matching is important!  Remember even lighting puts out less into a short.
Allen KA5N

William J J Hoge (I go by my first middle name "John"):
The main thing I learned in my Fields & Waves class was that we communicate in spite of our antennas not because of them.

Yes, most antennas operate with a high SWR into free space, but, even with a high SWR, some power can be transferred.  Some current flow, so there is an H-field.  Some potential difference is present, so there is an E-field.

One of the nice properties of the folded dipole is that its radiation impedance is nearly the same as the characteristic impedance of free space.  I'm using on on 60m with my Argonaut V with good results.

Jeff Sanchez:
Q: So my question is, why do we not have to match the impedance of an antenna to that of free space?
According to the Antenna book, the theoretical intrinsic impedance of free space is about 377 ohms.

A: In a sense you are through the antenna.  The energy you are transferring from the source (transmitter) to the load (antenna) is in the form of electrical current.  Opposition from the load in the form of conductor resistance and radiation resistance is the impedance of the antenna (1/2-wave dipole is 73 ohms).  When electrical current is fed to a resonant antenna, it converts the electric current to an electro-magnetic field.  Opposition to the electro-magnetic field traveling into free space is your "free-space" impedance (377-ohms).

Q: Is it because an antenna operating in free space is not working like a classic generator/load combination?

A: No, it is working like a classic generator/load combination, you’re simply converting the energy into a different form.  An antenna operating in free space will have a feed point impedance of 73-ohms (1/2-wave dipole) but the EM field emitted from the antenna will see a free space impedance of 377-ohms.  Think of it as performing an impedance transformation from 73-ohms to 377-ohms.  

Another way to look at it is in the case of an impedance matching transformer.  Primary sees 73-ohms but the secondary sees 377-ohms.  Coupling is performed through a magnetic field; source (transmitter) is electrically isolated from the load (free space). Your source always sees 73-ohms on the primary as long as the secondary sees 377-ohms.  Seventy-three ohms is the antenna feed point impedance and 377-ohms is free space impedance.

I hope this will help you understand the phenomena.  There's really nothing tricky about the mathematics behind this concept.  Simple ohms law.


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