4 ms·

At the transmitter there are two kinds of gain. The first is simply crack up the power and more signal energy will be received at the receiver. The other "gai

At the transmitter there are two kinds of gain. The first is simply crack up the power and more signal energy will be received at the receiver. The other "gain" is where the physical shape of the transmitting antenna directs more of the energy it receives in one direction (usually straight ahead) rather than a different directions. The amount of gain due to the physical shape will vary with the frequency of radio wave relative to the physical shape the antenna. Think of a flashlight. The beam is brighter in the middle and falls off towards the sides. That's "gain" compared to a flashlight that beams light in all directions equally, like a sphere. So while the transmitter can "shape the beam" to create gain, the receiver cannot (though the path lose equation implies that it does). The receiver can physically just trap the amount of watts per square metre that it receives and this amount depends only on its size and not on the frequency of the incoming radiation. So the transmitter and receiver antennas are very different in their physics of how the operate but the Path Loss Equations fudges things to make it look like they are the same ie that the gain of either is equally dependent on the frequency/wave length when in fact that's false and the two antennas are asymmetric in how they respond the carrier frequency.

SigmundA 5y ago
So while the transmitter can "shape the beam" to create gain, the receiver cannot (though the path lose equation implies that it does). The receiver can physically just trap the amount of watts per square metre that it receives and this amount depends only on its size and not on the frequency of the incoming radiation. So radio telescopes don't shape the incoming radiation? You can't just have a big flat antenna with a lot of area, they use dish reflectors and wave guides to shape the incoming radiation down to the actual antenna that has a size determined by guess what, FREQUENCY. "Figure 3.3: Most high-frequency feeds are quarter-wave ground-plane verticals inside waveguide horns. The only true antenna in this figure is the λ/4 ground-plane vertical, which converts electromagnetic waves in the waveguide to currents in the coaxial cable extending down from the waveguide. According to the strict definition of an antenna as a device for converting between electromagnetic waves in space and currents in conductors, the only antennas in most radio telescopes are half-wave dipoles and their relatives, quarter-wave ground-plane verticals. The large parabolic reflector of a radio telescope serves only to focus plane waves onto the feed antenna. (The term “feed” comes from radar antennas used for transmitting; the “feed” antenna feeds transmitter power to the main reflector. Receiving antennas used in radio astronomy work the other way around, and the “feed” actually collects radiation from the reflector.) https://www.cv.nrao.edu/~sransom/web/Ch3.html https://www.cv.nrao.edu/~sransom/web/Ch3.html
- entropicgravity 5y ago
  What ever it is you think might be happening at the receiver, if it violates conservation of energy it's unlikely to be correct.
  - SigmundA 5y ago
    Conservation of energy directly leads to the role of frequency in antenna reception and reciprocity: Figure 3.5: A cavity in thermodynamic equilibrium at temperature T containing a resistor R is coupled to an antenna, also at temperature T, through a filter blocking electromagnetic radiation but passing currents having frequencies in the range ν to ν+dν. Imagine an antenna inside a cavity in full thermodynamic equilibrium at temperature T connected through a transmission line to a matched resistor (whose resistance equals the radiation resistance of the antenna) in a second cavity at the same temperature (Figure 3.5). A filter between the cavities passes only currents in a narrow range of frequencies between ν and ν+dν. Because this entire system is in thermodynamic equilibrium, no net power can flow through the wires connecting the antenna and the resistor. Otherwise, one cavity would heat up and the other would cool down, in violation of the second law of thermodynamics.

SigmundA 5y ago

So while the transmitter can "shape the beam" to create gain, the receiver cannot (though the path lose equation implies that it does). The receiver can physically just trap the amount of watts per square metre that it receives and this amount depends only on its size and not on the frequency of the incoming radiation. So radio telescopes don't shape the incoming radiation? You can't just have a big flat antenna with a lot of area, they use dish reflectors and wave guides to shape the incoming radiation down to the actual antenna that has a size determined by guess what, FREQUENCY. "Figure 3.3: Most high-frequency feeds are quarter-wave ground-plane verticals inside waveguide horns. The only true antenna in this figure is the λ/4 ground-plane vertical, which converts electromagnetic waves in the waveguide to currents in the coaxial cable extending down from the waveguide. According to the strict definition of an antenna as a device for converting between electromagnetic waves in space and currents in conductors, the only antennas in most radio telescopes are half-wave dipoles and their relatives, quarter-wave ground-plane verticals. The large parabolic reflector of a radio telescope serves only to focus plane waves onto the feed antenna. (The term “feed” comes from radar antennas used for transmitting; the “feed” antenna feeds transmitter power to the main reflector. Receiving antennas used in radio astronomy work the other way around, and the “feed” actually collects radiation from the reflector.) https://www.cv.nrao.edu/~sransom/web/Ch3.html https://www.cv.nrao.edu/~sransom/web/Ch3.html

entropicgravity 5y ago
What ever it is you think might be happening at the receiver, if it violates conservation of energy it's unlikely to be correct.
- SigmundA 5y ago
  Conservation of energy directly leads to the role of frequency in antenna reception and reciprocity: Figure 3.5: A cavity in thermodynamic equilibrium at temperature T containing a resistor R is coupled to an antenna, also at temperature T, through a filter blocking electromagnetic radiation but passing currents having frequencies in the range ν to ν+dν. Imagine an antenna inside a cavity in full thermodynamic equilibrium at temperature T connected through a transmission line to a matched resistor (whose resistance equals the radiation resistance of the antenna) in a second cavity at the same temperature (Figure 3.5). A filter between the cavities passes only currents in a narrow range of frequencies between ν and ν+dν. Because this entire system is in thermodynamic equilibrium, no net power can flow through the wires connecting the antenna and the resistor. Otherwise, one cavity would heat up and the other would cool down, in violation of the second law of thermodynamics.