4 ms·

"but the receiver has no gain, the energy it traps depends only on its area" Really no directionality, no resonance? Just make bigger area antenna out of whate

"but the receiver has no gain, the energy it traps depends only on its area" Really no directionality, no resonance? Just make bigger area antenna out of whatever and it collects more energy regardless of geometry? Please go on this is new information to me.

deleted 5y ago
[deleted]
CamperBob2 5y ago
An antenna is just a passive impedance-matching device, nothing more or less. It matches 377 ohms to whatever the feedline impedance is. Making the antenna's effective area larger is the only way to capture more arriving energy from the transmitter. At the same time, as its size grows, the area of the antenna that's not exposed to the transmitted wavefront increases even more rapidly than the area that is, making it a losing proposition to keep increasing the size alone. Instead, we use directive and/or reflective elements whose geometry allows a smaller antenna element to receive more energy from one direction at the expense of others. Just like a lens, in other words. A larger lens or mirror captures more light than a smaller one (focal length considerations aside), but that's not all there is to making a good camera or telescope. The right combination of smaller optical elements can do almost as well as a single huge lens, while being much more practical to work with. One key point is that in neither case is any amplification or "gain" present. It's just a matter of which approach captures more of the limited number of photons available. Another is that the size of the lenses or antenna elements has to be expressed in terms of wavelength, not absolute spatial dimensions, if you want to compare them directly. That's ultimately what the path loss equation describes, albeit awkwardly. Path loss increases at shorter wavelengths simply because the exposed areas of the antennas get smaller while the wavefront doesn't. We can sometimes compensate for that by making the antennas bigger in terms of the number of wavelengths. Mathematically, the path losses involved in the Deep Space Network would be much lower if they could use LF/HF frequencies instead of microwaves, for instance... but to take advantage of that, the antenna sizes would have to increase to ridiculous dimensions, on the order of miles. Microwaves end up being a much better fit in practice. Even if lower frequencies were usable, the resulting increase in atmospheric noise (and decrease in available bandwidth) would swamp any advantages.
- SigmundA 5y ago
  > At the same time, as its size grows, the area of the antenna that's not exposed to the transmitted wavefront increases even more rapidly than the area that is, making it a losing proposition to keep increasing the size alone. And this is based on frequency correct? So frequency is direct component. >One key point is that in neither case is any amplification or "gain" present. It's just a matter of which approach captures more of the limited number of photons available. Gain: the factor by which power or voltage is increased in an amplifier or other electronic device, usually expressed as a logarithm. Gain does not require amplification, the transmission gain of antenna is not based on it, nether is reception. In both transmission and reception it is based on how efficient the antenna is and it's directionality. Both are directly tied to frequency and geometry together, you cannot ignore frequency as the GP said. This paper seems to be a great resource and helped reaffirm my understanding of frequency being a key component in antenna design and it not being "shoehorned" in. Gain and effective area are proportional to each other and therefore the term gain is used for receiving antennas because it matches it transmission characteristics, hence reciprocity: "Many antenna properties are the same for both transmitting and receiving. It is often easier to calculate the gain of a transmitting antenna than the collecting area of a receiving antenna, and it is often easier to measure the receiving power pattern of a large radio telescope than to measure its transmitting power pattern. Thus this receiving/transmitting “reciprocity” greatly simplifies antenna calculations and measurements. Reciprocity can be understood via Maxwell’s equations or by thermodynamic arguments. Burke and Graham-Smith [20] state the electromagnetic case for reciprocity clearly: “An antenna can be treated either as a receiving device, gathering the incoming radiation field and conducting electrical signals to the output terminals, or as a transmitting system, launching electromagnetic waves outward. These two cases are equivalent because of time reversibility: the solutions of Maxwell’s equations are valid when time is reversed.” https://www.cv.nrao.edu/~sransom/web/Ch3.html https://www.cv.nrao.edu/~sransom/web/Ch3.html
  - CamperBob2 5y ago
    And this is based on frequency correct? So frequency is direct component. Right, frequency and wavelength are inverses of each other. You can turn one into the other by dividing the velocity by either, f=c/m or m=c/f. Of course c isn't really c, but depends on the transmission media. Gain does not require amplification, the transmission gain of antenna is not based on it, nether is reception. By default, power gain is what an RF engineer refers to when s/he talks about "gain." And that does require amplification. You can increase voltage or current through impedance transformation, but never both at once.

deleted 5y ago

[deleted]

CamperBob2 5y ago

An antenna is just a passive impedance-matching device, nothing more or less. It matches 377 ohms to whatever the feedline impedance is. Making the antenna's effective area larger is the only way to capture more arriving energy from the transmitter. At the same time, as its size grows, the area of the antenna that's not exposed to the transmitted wavefront increases even more rapidly than the area that is, making it a losing proposition to keep increasing the size alone. Instead, we use directive and/or reflective elements whose geometry allows a smaller antenna element to receive more energy from one direction at the expense of others. Just like a lens, in other words. A larger lens or mirror captures more light than a smaller one (focal length considerations aside), but that's not all there is to making a good camera or telescope. The right combination of smaller optical elements can do almost as well as a single huge lens, while being much more practical to work with. One key point is that in neither case is any amplification or "gain" present. It's just a matter of which approach captures more of the limited number of photons available. Another is that the size of the lenses or antenna elements has to be expressed in terms of wavelength, not absolute spatial dimensions, if you want to compare them directly. That's ultimately what the path loss equation describes, albeit awkwardly. Path loss increases at shorter wavelengths simply because the exposed areas of the antennas get smaller while the wavefront doesn't. We can sometimes compensate for that by making the antennas bigger in terms of the number of wavelengths. Mathematically, the path losses involved in the Deep Space Network would be much lower if they could use LF/HF frequencies instead of microwaves, for instance... but to take advantage of that, the antenna sizes would have to increase to ridiculous dimensions, on the order of miles. Microwaves end up being a much better fit in practice. Even if lower frequencies were usable, the resulting increase in atmospheric noise (and decrease in available bandwidth) would swamp any advantages.

SigmundA 5y ago
> At the same time, as its size grows, the area of the antenna that's not exposed to the transmitted wavefront increases even more rapidly than the area that is, making it a losing proposition to keep increasing the size alone. And this is based on frequency correct? So frequency is direct component. >One key point is that in neither case is any amplification or "gain" present. It's just a matter of which approach captures more of the limited number of photons available. Gain: the factor by which power or voltage is increased in an amplifier or other electronic device, usually expressed as a logarithm. Gain does not require amplification, the transmission gain of antenna is not based on it, nether is reception. In both transmission and reception it is based on how efficient the antenna is and it's directionality. Both are directly tied to frequency and geometry together, you cannot ignore frequency as the GP said. This paper seems to be a great resource and helped reaffirm my understanding of frequency being a key component in antenna design and it not being "shoehorned" in. Gain and effective area are proportional to each other and therefore the term gain is used for receiving antennas because it matches it transmission characteristics, hence reciprocity: "Many antenna properties are the same for both transmitting and receiving. It is often easier to calculate the gain of a transmitting antenna than the collecting area of a receiving antenna, and it is often easier to measure the receiving power pattern of a large radio telescope than to measure its transmitting power pattern. Thus this receiving/transmitting “reciprocity” greatly simplifies antenna calculations and measurements. Reciprocity can be understood via Maxwell’s equations or by thermodynamic arguments. Burke and Graham-Smith [20] state the electromagnetic case for reciprocity clearly: “An antenna can be treated either as a receiving device, gathering the incoming radiation field and conducting electrical signals to the output terminals, or as a transmitting system, launching electromagnetic waves outward. These two cases are equivalent because of time reversibility: the solutions of Maxwell’s equations are valid when time is reversed.” https://www.cv.nrao.edu/~sransom/web/Ch3.html https://www.cv.nrao.edu/~sransom/web/Ch3.html
- CamperBob2 5y ago
  And this is based on frequency correct? So frequency is direct component. Right, frequency and wavelength are inverses of each other. You can turn one into the other by dividing the velocity by either, f=c/m or m=c/f. Of course c isn't really c, but depends on the transmission media. Gain does not require amplification, the transmission gain of antenna is not based on it, nether is reception. By default, power gain is what an RF engineer refers to when s/he talks about "gain." And that does require amplification. You can increase voltage or current through impedance transformation, but never both at once.