| Electronics > Beginners |
| Why do we need antennas? |
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| IanB:
--- 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 --- Think about electricity. You can transmit the same amount of power using high voltage and low current (requiring thinner cables), or using lower voltage and higher current (requiring thicker cables). With electromagnetic radiation something similar happens with frequency. At higher frequencies and shorter wavelengths photons have more energy, so you can transmit a certain amount of power using smaller antennas. At lower frequencies and longer wavelengths photons have less energy, so you need bigger antennas to transmit the same amount of power. Once again, size comes into it. |
| bsfeechannel:
I love these noob questions. They're disconcerting and embarrassing. Like, where do babies come from? The simple answer will let your inquirer confused. The complete version with the "sordid" details will let them horrified. According to this article, we do not use coax with the exact impedance of the air because that is not convenient from the standpoint of power and voltage. Waveguides have their restrictions too. If you leave a transmission line open, you will have some propagation, but the impedance mismatch will make part of the radio wave be reflected, causing all kinds of nasty problems, some of them catastrophic in the case of power signals. Antennas match the line with the air by simply propagating all or almost all the signal that is fed to them. So no signal is reflected. To do this several tricks are employed. In resonant dipoles, the standing wave will induce currents and voltages that will match that of the cable. Feeders of parabolic antennas behave much like a speaker horn, by gradually changing the impedance of the guide. The design of an antenna defines its bandwidth, directivity, polarization, and many other parameters, some of them legally required. As for the antenna gain, the reference is an ideal isotropic antenna. It radiates power uniformly in all directions. If your antenna concentrates power in a specific direction, you will have a "gain" in that direction. So parabolic antennas have high gain, but they are very directive. Dipoles have low gain but they propagate in all directions perpendicular to the axis of the dipole. |
| PhilipPeake:
--- 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 --- But they are related. If it helps, look up single wire transmission lines: https://en.wikipedia.org/wiki/Single-wire_transmission_line That maybe helps, because there there isn't the same return path that you get wirh twin wire or co-ax transmission lines, its more pure EM wave. All you have to do is couple that into free space to allow the RF to flow outwards. Without the matching doo-dad at the end, you still get reflections, standing waves and dissipation as heat. Put a terminator on the end, and all that vanishes and the power dissipates in the terminator. Put an antenna in place of the terminator and the energy still flows cleanly, but radiates outwards instead of being dissipated as heat. The antenna is coupling the energy into free space impedance - its matching your transmission line to free space. |
| xrunner:
Just as a side note - Why do we use 50 Ohm Coax? |
| RoGeorge:
--- Quote from: xrunner on August 15, 2018, 11:29:35 pm ---Just as a side note - Why do we use 50 Ohm Coax? --- End quote --- This is the only paper I could find about 1929, Lloyd Espenschied and Herman A. Affel: The U.S. Patent 1,835,031 for "Concentric Conducting System" https://patentimages.storage.googleapis.com/04/dd/87/9c9d8a899ef3f1/US1835031.pdf |
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