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Antenna theory is a pet peeve of mine. The problem is that it is universally taught with the "Path Loss Equation" and that equation leads to a very poor unders

Antenna theory is a pet peeve of mine. The problem is that it is universally taught with the "Path Loss Equation" and that equation leads to a very poor understanding of how antennas work. Almost every technically trained person understands that electromagnetic radiation energy dissipates according to the inverse square law in the far field. But the Path Loss Equation jumps through hoops to shoe horn in a frequency term that clearly, if you think about it, has no reason to be there. The distance term is necessary of course but the frequency term scientifically illiterate. This all comes down to the fact that at some point someone decided they would deem the gain transmitting and receiving antennas as "equal and reciprocal" if they are physically the same. In order to make this delusion work out mathematically a phantom frequency term has to forced into the equation to make the maths work out. For those who care (not many I expect) the way things really work is that the gain of the transmitter can be function of frequency (geometry and all that) but the receiver has no gain, the energy it traps depends only on its area. Ok I'm stepping of my soap box, thanks for listening.

spookthesunset 5y ago
> but the receiver has no gain, the energy it traps depends only on its area. Is that true though? Isn’t this one of those “wave / particle duality” things? Like if you treat the photons as particles the “area” thing makes sense but as a wave, does it?
enchiridion 5y ago
Nah this is cool. Where’s a good place to learn more?
tyingq 5y ago
It is misleading, but it's there because otherwise you would have to assume the receiving antenna size would change along with the distance. People practically calculating loss are usually doing so with a specific (or a few specific) antennas in mind. It's not often they would be calculating based on an antenna large enough to not take it into account.
felipemnoa 5y ago
Quite interesting. As someone else mentioned, could you recommend any materials where one can learn more about this?
- CamperBob2 5y ago
  The ARRL Handbook is a good place to dig into the basics. You can go in several directions from there, but that'll get you started.
SigmundA 5y ago
"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.
mNovak 5y ago
I assume you're referring to the Friis transmission formula? It only references frequency because that's usually more convenient than wavelength. I hardly think it's fair to call that scientifically illiterate. Also converting from wavelength to frequency is trivial, not jumping through hoops. To be clear, given two identical isotropic antennas (zero gain) in the far field, there absolutely needs to be a frequency^2 term (or 1/wavelength^2) in the "path loss". This is the reason AM radio has so much greater range than FM, for instance. Transmit power, and gain of either or both antennas simply adds (dB) to that path loss. Note that receiving antennas absolutely can have gain. While it does tend to be proportional to area, the "receiving area" concept is actually a somewhat imprecise simplification (consider, what's the area of a Hertzian dipole? or compare the areas of a Yagi and simple dipole).
pjc50 5y ago
> the receiver has no gain That implies that everyone from Yagi onwards has been wasting their time designing better antennae?

spookthesunset 5y ago

> but the receiver has no gain, the energy it traps depends only on its area. Is that true though? Isn’t this one of those “wave / particle duality” things? Like if you treat the photons as particles the “area” thing makes sense but as a wave, does it?

enchiridion 5y ago

Nah this is cool. Where’s a good place to learn more?

tyingq 5y ago

It is misleading, but it's there because otherwise you would have to assume the receiving antenna size would change along with the distance. People practically calculating loss are usually doing so with a specific (or a few specific) antennas in mind. It's not often they would be calculating based on an antenna large enough to not take it into account.

felipemnoa 5y ago

Quite interesting. As someone else mentioned, could you recommend any materials where one can learn more about this?

CamperBob2 5y ago
The ARRL Handbook is a good place to dig into the basics. You can go in several directions from there, but that'll get you started.

SigmundA 5y ago

"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.

mNovak 5y ago

I assume you're referring to the Friis transmission formula? It only references frequency because that's usually more convenient than wavelength. I hardly think it's fair to call that scientifically illiterate. Also converting from wavelength to frequency is trivial, not jumping through hoops. To be clear, given two identical isotropic antennas (zero gain) in the far field, there absolutely needs to be a frequency^2 term (or 1/wavelength^2) in the "path loss". This is the reason AM radio has so much greater range than FM, for instance. Transmit power, and gain of either or both antennas simply adds (dB) to that path loss. Note that receiving antennas absolutely can have gain. While it does tend to be proportional to area, the "receiving area" concept is actually a somewhat imprecise simplification (consider, what's the area of a Hertzian dipole? or compare the areas of a Yagi and simple dipole).

pjc50 5y ago

> the receiver has no gain That implies that everyone from Yagi onwards has been wasting their time designing better antennae?