Electronics > Beginners
Why do we need antennas?
tautech:
--- Quote from: fonograph on August 16, 2018, 09:50:09 pm ---
--- Quote from: tautech on August 15, 2018, 09:34:51 pm ---
--- Quote from: fonograph on August 15, 2018, 09:28:33 pm ---If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?
--- End quote ---
No.
Poor SWR will rob the high impedance version of performance.
--- End quote ---
Why would there be SWR problem?
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Unless the signal is absorbed into the antenna and emitted, the result is poor SWR and poor antenna performance.
Two issues are relevant here, feedline matching and antenna resonant frequency.
Yes it is black magic until you get your head around a few simple principles however without the right tools getting an antenna to perform is mostly trial and error. Sure we can just feed KW into it to overcome poor design but the result would swamp out others attempting to use RF bands too and that's precisely why there's country specific regs on which bands are available and how much EIRP is permitted.
IanB:
--- Quote from: fonograph on August 16, 2018, 09:53:00 pm ---While I am thankful for your informative post it was information completly unrelated to my question.At no point did I mention antenna size.I know lower frequencies need bigger antenna,my question was about antenna electric impedance.
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But you did mention antenna size. You clearly implied the size of the antenna would be the size of the open end of your coaxial cable floating in the air. For various reasons (size and geometry among them), the cut end of a coaxial cable is not able to act as an efficient antenna (but it can of course act as a very inefficient antenna with low power handling capability).
xrunner:
--- Quote from: fonograph on August 16, 2018, 10:01:32 pm ---
Can this 377 ohm wave impedance be achieved no matter what electric impedance the antenna have? Can there be high electric impedance antennas like 1 M ohm,high voltage and still have 377 ohm wave impedance and as result be efficient?
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Well I didn't specialize in antenna design theory as a career, but if you have a perfect dipole cut for 10 MHz and a 50 ohm transmission line, and connect it to a dummy load of 50 ohms, then all the energy will be absorbed into the load. So far so good.
Now if you have the same system and put a perfectly cut dipole for 10 MHz on the end, the energy will be radiated away into free space. Free space has that impedance of 377 ohms as a consequence of what? The system designer? Well, because it's one of those fundamental physical things that is what it is. There is no matching required because once its radiating into free space it has been transformed. It has left one system and entered another. The act of radiating is the transformation. A perfectly radiating dipole radiates it's energy into free space, which happens to have an impedance of 377 ohms. It's not "matched" as you seem to think it needs to be, it simply goes into the only medium it can go into.
The only matching has to be impedance of the coax and then to have a perfect dipole at the end which radiates all energy into space. It goes the only place it can go, into free space.
I'm sure others will correct me as needed of course. :popcorn:
fonograph:
--- Quote from: IanB on August 16, 2018, 10:42:27 pm ---
--- Quote from: fonograph on August 16, 2018, 09:53:00 pm ---While I am thankful for your informative post it was information completly unrelated to my question.At no point did I mention antenna size.I know lower frequencies need bigger antenna,my question was about antenna electric impedance.
--- End quote ---
But you did mention antenna size. You clearly implied the size of the antenna would be the size of the open end of your coaxial cable floating in the air. For various reasons (size and geometry among them), the cut end of a coaxial cable is not able to act as an efficient antenna (but it can of course act as a very inefficient antenna with low power handling capability).
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Thats interesting take on my post,I didnt think about it that way.It was more of a irrational fantasy scenario to serve as bread that I can spread my buttery question on,I guess you native English speakers call it painting the picture,I did not believe in it nor did I expect anyone to view it from a angle that would make it seem like I am implying size doesnt matter.I never thought that antenna size doesnt matter.
IanB:
--- Quote from: fonograph on August 16, 2018, 11:58:07 pm ---I never thought that antenna size doesnt matter.
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That's good, as that is one obstacle out of the way.
The next step is to believe that antenna shape matters.
Once you believe that antenna size and antenna shape both matter, then it becomes clear why we need antennas.
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