Electronics > Beginners
Why do we need antennas?
fonograph:
--- Quote from: PhilipPeake on August 15, 2018, 06:55:12 pm ---If you are looking for a simple answer (behind which things get very complex) the antenna is a device to match electrical impedance to free-space impedance.
With radio (and EM in general) its all about matching.
--- End quote ---
How can you possibly match electrical impedance to wave impedance? If I understand it correctly these two impedance types are completly different things hence why 377 ohm cable cut open at one end isnt going to radiate away 100% energy into air.
Isnt wave and electric impedance kind of a apples and oranges situation? So far I am understanding that they are two separate independent things.So when you write that antenna matches electric impedance to the impedance of free space,to my brain it appears as if you wrote about matching student debt pressure to water pressure inside pressurized tank,seems completly unrelated.
tautech:
--- Quote from: fonograph on August 15, 2018, 07:43:43 pm ---
--- Quote from: PhilipPeake on August 15, 2018, 06:55:12 pm ---If you are looking for a simple answer (behind which things get very complex) the antenna is a device to match electrical impedance to free-space impedance.
With radio (and EM in general) its all about matching.
--- End quote ---
How can you possibly match electrical impedance to wave impedance? If I understand it correctly these two impedance types are completly different things hence why 377 ohm cable cut open at one end isnt going to radiate away 100% energy into air.
Isnt wave and electric impedance kind of a apples and oranges situation? So far I am understanding that they are two separate independent things.So when you write that antenna matches electric impedance to the impedance of free space,to my brain it appears as if you wrote about matching student debt pressure to water pressure inside pressurized tank,seems completly unrelated.
--- End quote ---
There's two main things an antenna needs to provide and because of antenna reciprocity we design and prove transmission characteristics and it's easy to do with a VNA.
First the antenna needs be emissive at the chosen frequency and then how it's fed determines it's match to the feed line/system.
If you look at the last few pics I've posted in the SVA1015X thread you can see emissive properties in a Log Magnitude screenshot and it's match to the feeder in an SWR screenshot.
Just getting those 2 things close to optimum had the antenna performing close to ideal.
You could work in the reverse and tweak a receiving antenna with just a SA and an existing external transmission but good results would be more by chance.
RoGeorge:
"Why do we need antennas" is a very good question, and very deep, IMO.
Yet, here seems to be more a problem about words. As it was pointed out before, the "impedance of free space" and the "electrical impedance" are not the same type of beast.
The concept of impedance has a much broader sense than just the electrical impedance: The concept of impedance, by definition, means the ratio between cause and effect.. (Now, who's the cause and who's the effect, especially in electromagnetism, is a can of worms, and whoever dare to open it, will eventually end up eaten by those worms. ^-^)
fonograph:
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?
tautech:
--- 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.
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