| Electronics > Beginners |
| Why do we need antennas? |
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| metrologist:
--- Quote from: fonograph on August 14, 2018, 10:36:23 pm ---But why gain? Antennas are passive devices,not amplifiers,they cant output more power that is put into them.If the transmission line and air are both 377 ohm,100% of energy from the coax should go into air,right? ( probably not right,I know I know... ) --- End quote --- I was going to explain it by using optical lenses. They too are passive, but can concentrate light or magnify images. Antennas have aperture just as lenses. A lens and mirror, for example, can take an omnidirectional light and focus it onto a single beam, a flashlight. A Yagi works similarly, with a reflective element and directive elements, that focus the radio energy from/to the driven element. |
| IanB:
--- Quote from: fonograph on August 14, 2018, 10:21:40 pm ---I read and heard that its all about impedance matching,so I dont see any reason why electromagnetic field should not smoothly transition from any kind of 377 ohm transmission line like microstrip or coax into air.And if that would work,then I dont see the reason to make antennas. --- End quote --- You've got to think about it in terms of the amount of energy transferred. An antenna is an energy harvester or radiator. For example, solar panels harvest sunlight. Big panels harvest more sunlight and generate more electricity than smaller ones. Why? Loudspeakers radiate sound energy. Big loudspeakers radiate more sound energy than small earphones. Why? |
| PhilipPeake:
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. |
| IanB:
--- 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. --- End quote --- While partly correct, that is not the whole story. An antenna is also a device for transferring power from one medium to another. --- Quote ---With radio (and EM in general) its all about matching. --- End quote --- It's not all about matching. It's also about power handling. Impedance matching affects efficiency. Size affects power handling. Consider: a TV broadcast antenna is very large, a home TV receiving antenna is quite small in comparison. Both correctly match impedance, yet one is much bigger than the other? Why might that be? (And the same can be asked about the transmitting antenna on a cellphone mast and the receiving antenna in your cellphone.) There is something else that matters here apart from impedance matching. |
| PhilipPeake:
--- Quote from: IanB on August 15, 2018, 07:22:35 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. --- End quote --- While partly correct, that is not the whole story. An antenna is also a device for transferring power from one medium to another. --- Quote ---With radio (and EM in general) its all about matching. --- End quote --- It's not all about matching. It's also about power handling. Impedance matching affects efficiency. Size affects power handling. Consider: a TV broadcast antenna is very large, a home TV receiving antenna is quite small in comparison. Both correctly match impedance, yet one is much bigger than the other? Why might that be? (And the same can be asked about the transmitting antenna on a cellphone mast and the receiving antenna in your cellphone.) There is something else that matters here apart from impedance matching. --- End quote --- Power handling is in the physical size of conductors and antenna elements. The basics stay the same. The difference on the cell phone tower antenna is that it is designed to direct all its energy (and reciprocally, receive gain) in a particular direction. Each antenna only covers a small segment of the 360 degrees coverage. It doesn't waste energy going up into space, down into the ground, or behind it. That is why it is bigger and more complicated. The antenna in your phone needs to radiate in 360 degrees. Having it radiate only at, say, 0 to 30 degrees above the horizon would be nice but reality is that people use their phones in all sorts of orientations. The antenna also has to fit in the phone. In general, aesthetics in cell phones counts for much more than any sort of efficiency. Cell phone antennas mostly are not even matched that well. They can get away with it because of the much better antennas and TX/RX in the cell towers. |
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