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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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Externet
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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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EHT
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PostPosted: Mon Jan 22, 2018 12:18 pm    Post subject: Reply with quote

With loudspeakers, it is common knowledge that using four speakers in series parallel 8 ohm configuration gives a 6db higher sound level than does a single speaker, for the same power input. The impedance on the input side of the transducer has not changed, but the radiation resistance of the output side has changed due to the increased cone area which better matches the air it is coupled to. In all that I have read so far, it is only the impedance on the INPUT side of an antenna that has been considered - and as we know, a 4 ohm speaker system is just as powerful as an 8 ohm or 16 ohm (all other non-dependent things being equal), same as with antenna impedances. Could it be that differing types of aerials "couple better" to space ?
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E Kafeman
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PostPosted: Mon Jan 22, 2018 5:30 pm    Post subject: Reply with quote

Antenna radiation resistance (part of total impedance) is a value which is depending on its environment. In free space should measured antenna resistance ideally be same as its feeding cable impedance while keeping loss resistance and reactive loss low.

Antenna radiation resistance is NOT a resistive property of the antenna in its self (in opposite to its resistive internal loss).
Antenna radiation resistance is a result of antenna as autotransformer or coupler to free space.
If that resistive value at cable side is seen as 5 Ohm or 500 Ohm is not relevant for its free space resistance, both values can be perfect for its respective antenna design, if both causes 377 Ohm coupling to free space.

Image borrowed from https://www.antune.net
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EHT
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PostPosted: Tue Jan 23, 2018 1:28 pm    Post subject: Reply with quote

Thank you, E Kafeman.
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R. Fry
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PostPosted: Fri Feb 16, 2018 12:45 pm    Post subject: Radiation Resistance Reply with quote

Radiation resistance is the result of an antenna acting to radiate e-m energy into space. It occurs whenever r-f current can be made to flow along the conductor(s) of an antenna.

Radiation resistance can be any ohmic value, including zero. Generally, longer conductors and greater r-f currents radiate more r-f energy.

A radiated e-m wave consists of two components: an electric and a magnetic field. In the far field of that antenna, those two components always are aligned 90 from each other in the directions of propagation.

In the far field of the antenna, the magnitudes of the electric and magnetic fields always are related to each other as if the impedance of free space is ~377 Ω.

However there is no physical requirement for the radiation resistance of an antenna also to be ~377 Ω.

For example, a Z-matched, λ/4 vertical monopole driven against a perfect ground plane has a radiation resistance of about 36 Ω. A λ/2 version of that system has a radiation resistance several times greater. Yet both of those versions will radiate e-m energy with equal efficiency.
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