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free space impedence matching?

 
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jw
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PostPosted: Wed Jan 14, 2015 9:07 pm    Post subject: free space impedence matching? Reply with quote

My question is, why do we try to match our antennas to what appears to be arbitrary impedance's, for example 75ohm, 50ohm etc, rather than matching everything, antenna, cabling, and wave generating circuitry, to the impedance of free space, which is approximately 377ohms. Of course the next question is how can the impedance of free space be about 377ohm for all frequencies.

It seems at one time an attempt was made to match TV's to free space since they used twin lead 300ohm cabling, but more modern tv's have dropped this style of input altogether. WHY?
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PostPosted: Thu Jan 15, 2015 2:36 am    Post subject: Reply with quote

77 ohm is a very favored mathematical impedance calculation result for minimal losses in coaxial cables.
75 is a compromise to match a 73 ohm dipole with a 77 ohm low loss coaxial..

30 ohm is a very favored mathematical impedance calculation result to handle higher power.
A compromise between 30 ohm and 75 ohm resulted in another coaxial, the 50 ohm.

The 300 ohm is a compromise between the 288 ohm characteristic impedance a circular loop TV antena for UHF presents, and four times 75 ohms achievable by a simpler 4:1 balun in order to use the loop at a 75 ohm tuner input.
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jw
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PostPosted: Thu Jan 15, 2015 8:52 pm    Post subject: Reply with quote

So we don't attempt to match the impedance of free space because we don't know how to produce transmission lines and antennas of 377ohm?

Or is it that the transmission lines would eat up all the power in a long run?

Basically if my wave generating circuitry has an output impedance of 377ohm and I design an antenna with an impedance of 377ohm for the frequency I want to transmit, Assuming transmission lines are not a factor, will such a system more efficiently transfer power to free space then a 50ohm antenna system. According to the maximum power transfer theory it should.

I'd post an image to describe what I'm asking if I could figure out how.
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PostPosted: Thu Jan 15, 2015 9:36 pm    Post subject: Reply with quote

No - a 377 Ohm antenna will not radiate more power than a 50 Ohm antenna. 50 Ohm antennas can be very close to 100% efficiency with no constraints, and the mismatch loss does not exist. It is confusing far field concepts for E/H with transmission line equations for V/I. A lot of people think it should be matched to 377 Ohms, but htey have no idea what they are talking about.
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jw
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PostPosted: Fri Jan 16, 2015 8:18 pm    Post subject: Reply with quote

"the mismatch loss does not exist" I was one of those people who didn't know what they were talking about, which is why I kept my mouth shut for years, but I still want to understand. I still may not have fully wrapped my head around this concept but Wikipedia says "The electromagnetic field in the far-field region of an antenna is independent of the type of field radiated by the antenna. The wave impedance is the ratio of the strength of the electric and magnetic fields, which in the far-field are in phase with each other. Thus, the far-field "impedance of free space is resistive."

So if I interpret all of this correctly, the frequency has no bearing on the impedance of the medium we're radiating the wave into because once we get the electric and magnetic fields in the far field in phase, by simply tuning the antenna and matching it to the impedance of the transmitter, we have successfully matched the B and E fields in the far field and made the antennas resistance as real, rather than inductive or capacitive(Imaginary) as possible. Whatever that real resistance ends up being is the best we get and has nothing to do with 377ohm.
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