9 ms·

Antenna Theory

128 points by bsilvereagle 5y ago

mikewarot 5y ago
Vector Network Analyzers have come down in price to the $100 range, which is amazing in and of itself, so the home experimenter, likely a Ham radio operator, has a lot more capability to precisely measure things that were pure guesswork in the past. Of course, the anechoic chamber is a nice thing to have. I once got to see the Anechoic chamber at Northrup Grumman near Chicago during a open house... a theater size room they used for various measurements. I'll never forget it. This is a nice collection of information. The other place to get this type of data in the past was the Amateur Radio Handbook from the ARRL.
- madengr 5y ago
  You can always make impedance measurements with a slotted line, 6-port reflectometer, or vector voltmeter if you don't have access to a VNA. My undergrad microwave professor had us learn on the slotted line before letting us touch the VNA.
  - bsder 5y ago
    Any good materials/references for this? I've got some steady-state 60GHz stuff I'd love to try (non-radar), but the equipment is prohibitively expensive. Being able to analyze things with much less expensive, even if more labor intensive, tech would be welcome.
    - mikewarot 5y ago
      With a free space wavelength of 5 mm, any mechanical probing is going to involve mechanical stages and micrometers. Yikes!
- topspin 5y ago
  There is a rapid cycle of development occurring with these low end VNAs. Cheap spectrum analyzers are beginning to appear as well.
cf100clunk 5y ago
Also, here's a good place for design theory and construction practice of consumer-grade TV and FM radio antennae: https://www.digitalhome.ca/forums/antenna-research-development.186/ https://www.digitalhome.ca/forums/antenna-research-developme...
sizzzzlerz 5y ago
Antennas are weird beasts. Their behavior and performance can be predicted and measured in very technical ways but, at the end of the day, throwing a random length of wire over a tree branch, cut to 1/2 or 1/4 of the operating frequency wavelength can be all one needs to communicate with another station, even a very lower transmit power. This applies primarily to HF and VHF. Frequencies reaching up into the microwave range is where antenna design starts becoming a dark art, practiced by wizards and neuromancers.
- EdwardDiego 5y ago
  I've used old school HF portable radios when I was a ranger and in a valley with no line of sight to our VHF repeaters in the park. The fun part was finding a clearing big enough to string the antenna out. Usually involved ascending to the bushline or descending onto the river flats. Being able to listen to Scottish life guard boats from the other side of the world, at dusk as the ionosphere layers changed, was cool. Zimbabwean cricket broadcasts could be listened to up until about 9:30am in the morning.
omeze 5y ago
Antennas are super fun, they still feel like magic to me every time I see one. In college I took a microwaves theory course where we had to design antennas for some problem sets, and we gave them to some dude in a basement lab who then fabbed them in like an hour for us to test. Kind of mind boggling that with a few tools and handiwork you could produce real electronics with reasonable characteristics!
- bopbeepboop 5y ago
  A $500 mill will let you mill out one or two sided circuit boards without anything particularly dangerous used. The future is wild: $500 and some free software let’s you build electronics at home.
- EdwardDiego 5y ago
  When I worked with radio engineers, and they told me it's impossible to fully model a Yagi antenna, I was dubious. But sure enough, developing a new Yagi really depends on trial and error at the end of the day.
  - docfort 5y ago
    R.W.P. King studied wire dipoles for years and supposedly produced over 100 PhD students doing the same. The details are really really hard to get right and absolutely require empirical work. There are two core problems. One is that while Maxwell’s equations do model practically all EM phenomena, and therefore should make everything amenable to simulation, it requires detailed knowledge of material properties everywhere. Dealing with pure vacuum (or crisp, clean air over short distances) is therefore totally easy because it is all homogeneous and well-measured. But antennas convert electrical activity to traveling waves in space, so they are necessarily made up of different matter than the thing you want to transmit or receive into. Now you need to know the material properties of the antenna structure in a way that is incredibly hard to know. So we just approximate it as a series of uniform copper cylinders and say that one of them is connected perfectly to a cable using an infinitesimally thin gap. Yagi-Uda antennas depend upon complex interactions among the wires to achieve surprising levels of gain, and these details actually matter strongly. But they are practically unmodelable. So we simulate something and work hard to reconcile both the simulation (requires a lot of theoretical knowledge) and the actual antenna. The other problem is also hard. In Maxwell’s equations, you can book it down to a single equation describing how energy is converted to a traveling wave. That energy conversion involves a Dirac delta function (our shorthand for “itty bitty thing the rest of the system doesn’t otherwise interact with “). The delta function goes to infinity at some point. A real energy source is just some weighted collection of these deltas. In other words, I have a bunch of singularities. At some nominal distance from each singularity, the math is well-behaved. Close-up, however, things fall apart. Numerical precision is exhausted quickly, field strengths go off the rails, phase velocities exceed the speed of light (Hertz has some fun wtf moments at the end of one of his notebooks grappling with this), and other fun effects. Software hates this. So theoreticians come up with ways to model these singularities at a close, but non-zero distance for a bunch of special cases. Software must decide among the cases with hints from the human operator and match up with the desired geometry. It’s super hard to get right in all cases. My advisor and others at OSU spent decades coming up more special cases to enable rapid design of dish antennas, and certain shapes of wire antennas, and even to accelerate stealth F-117 design. Compared to all of this craziness, even a theoretical person like me will still spend a lot of time in the lab because it is just faster and strictly more accurate. Antenna design is often characterized as black magic. It really isn’t. We just don’t know literally everything about the world and it sometimes matters for these devices. The mystery is not inward towards the mind, but outward towards the universe at small and large scales.
adamrezich 5y ago
(2016), previously: https://news.ycombinator.com/item?id=19708982 https://news.ycombinator.com/item?id=19708982
quickthrower2 5y ago
Site looks infested with ads
- squarefoot 5y ago
  With Ublock Origin I see only one, which would be very easy to kill using its element zapper tool.
  - quickthrower2 5y ago
    On mobile i see them all and I normally i don't mind but on this site they show ads as navigation links making it pretty hard to navigate the site. Especially when the images the ads use are designed to make them not look like ads.
  - pmontra 5y ago
    Firefox Android with uBO and not even that single ad. A nice text only site with GIFs for formulas and diagrams.
- deleted 5y ago
  [deleted]
squarefoot 5y ago
A very interesting site full of downloadable papers on various subjects including antennas. https://www.jpier.org/PIER/pier.php https://www.jpier.org/PIER/pier.php This one for example may turn useful if one needs to build a WiFi multiband planar internal antenna. https://www.jpier.org/PIERL/pier.php?paper=11073004 https://www.jpier.org/PIERL/pier.php?paper=11073004 ...yes, that stuff is black magic/voodoo
monkeycantype 5y ago
Once upon a time, when I was very young, I travelled to a faraway island with an antenna in my bag that I had put together myself. Everything I knew about antennas I had learned in a single evening from an elderly retired naval engineer, a no nonsense expert in his field, shaped by lifetime in the military, we sat in his den - a clean office with medals and oil paintings of grey steel warships hung square and level. His serious visage beamed with love and delight when his partner - whose projected persona was the polar opposite of his own reserve - a flamboyantly camp man in trailing colorful silks who brought us cups of tea and exquisite little cakes, and would look at the the notes I was taking an theatrically shudder, swooning with the the back of hand to his forehead that ‘he never would never understand all that stuff’ I wasn’t entirely sure I did either - the retired engineer was able to explain things in the lingo of his trade, but couldnt confirm my attempts to connect the terms he was using to my first year university physics concepts. But a month later I was sending an email from the jungle via length of unspooled cable
- quinnjh 5y ago
  I love this story !!! Dawww. Also naval engineers IME have been some of the best no-B.S. teachers for engineering stuff.
  - monkeycantype 5y ago
    Thanks quinnjh
entropicgravity 5y ago
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
I may be mistaken, but this site is by a former Apple antenna engineer, isn't it? If he's who I seem to recall he is, he must have some incredible Steve Jobs war stories.
- amelius 5y ago
  "You're holding it wrong"
  - CamperBob2 5y ago
    Exactly. For his sin, exiled from the Garden.
mNovak 5y ago
Another great site in this area is microwaves101.com A huge range of topics, and pleasantly snarky anonymous editor.
mypastself 5y ago
Excellent stuff, surprised I’ve never stumbled upon the site before. It’s not just about antennas, it also offers concise yet intuitive explanations of related concepts such as Fourier analysis.
rob_c 5y ago
`Complexity is not a sign of intelligence; simplify.` couldn't agree more...

128 points by bsilvereagle 5y ago

Vector Network Analyzers have come down in price to the $100 range, which is amazing in and of itself, so the home experimenter, likely a Ham radio operator, has a lot more capability to precisely measure things that were pure guesswork in the past. Of course, the anechoic chamber is a nice thing to have. I once got to see the Anechoic chamber at Northrup Grumman near Chicago during a open house... a theater size room they used for various measurements. I'll never forget it. This is a nice collection of information. The other place to get this type of data in the past was the Amateur Radio Handbook from the ARRL.

madengr 5y ago
You can always make impedance measurements with a slotted line, 6-port reflectometer, or vector voltmeter if you don't have access to a VNA. My undergrad microwave professor had us learn on the slotted line before letting us touch the VNA.
- bsder 5y ago
  Any good materials/references for this? I've got some steady-state 60GHz stuff I'd love to try (non-radar), but the equipment is prohibitively expensive. Being able to analyze things with much less expensive, even if more labor intensive, tech would be welcome.
  - mikewarot 5y ago
    With a free space wavelength of 5 mm, any mechanical probing is going to involve mechanical stages and micrometers. Yikes!
topspin 5y ago
There is a rapid cycle of development occurring with these low end VNAs. Cheap spectrum analyzers are beginning to appear as well.

cf100clunk 5y ago

Also, here's a good place for design theory and construction practice of consumer-grade TV and FM radio antennae: https://www.digitalhome.ca/forums/antenna-research-development.186/ https://www.digitalhome.ca/forums/antenna-research-developme...

sizzzzlerz 5y ago

Antennas are weird beasts. Their behavior and performance can be predicted and measured in very technical ways but, at the end of the day, throwing a random length of wire over a tree branch, cut to 1/2 or 1/4 of the operating frequency wavelength can be all one needs to communicate with another station, even a very lower transmit power. This applies primarily to HF and VHF. Frequencies reaching up into the microwave range is where antenna design starts becoming a dark art, practiced by wizards and neuromancers.

EdwardDiego 5y ago
I've used old school HF portable radios when I was a ranger and in a valley with no line of sight to our VHF repeaters in the park. The fun part was finding a clearing big enough to string the antenna out. Usually involved ascending to the bushline or descending onto the river flats. Being able to listen to Scottish life guard boats from the other side of the world, at dusk as the ionosphere layers changed, was cool. Zimbabwean cricket broadcasts could be listened to up until about 9:30am in the morning.

omeze 5y ago

Antennas are super fun, they still feel like magic to me every time I see one. In college I took a microwaves theory course where we had to design antennas for some problem sets, and we gave them to some dude in a basement lab who then fabbed them in like an hour for us to test. Kind of mind boggling that with a few tools and handiwork you could produce real electronics with reasonable characteristics!

bopbeepboop 5y ago
A $500 mill will let you mill out one or two sided circuit boards without anything particularly dangerous used. The future is wild: $500 and some free software let’s you build electronics at home.
EdwardDiego 5y ago
When I worked with radio engineers, and they told me it's impossible to fully model a Yagi antenna, I was dubious. But sure enough, developing a new Yagi really depends on trial and error at the end of the day.
- docfort 5y ago
  R.W.P. King studied wire dipoles for years and supposedly produced over 100 PhD students doing the same. The details are really really hard to get right and absolutely require empirical work. There are two core problems. One is that while Maxwell’s equations do model practically all EM phenomena, and therefore should make everything amenable to simulation, it requires detailed knowledge of material properties everywhere. Dealing with pure vacuum (or crisp, clean air over short distances) is therefore totally easy because it is all homogeneous and well-measured. But antennas convert electrical activity to traveling waves in space, so they are necessarily made up of different matter than the thing you want to transmit or receive into. Now you need to know the material properties of the antenna structure in a way that is incredibly hard to know. So we just approximate it as a series of uniform copper cylinders and say that one of them is connected perfectly to a cable using an infinitesimally thin gap. Yagi-Uda antennas depend upon complex interactions among the wires to achieve surprising levels of gain, and these details actually matter strongly. But they are practically unmodelable. So we simulate something and work hard to reconcile both the simulation (requires a lot of theoretical knowledge) and the actual antenna. The other problem is also hard. In Maxwell’s equations, you can book it down to a single equation describing how energy is converted to a traveling wave. That energy conversion involves a Dirac delta function (our shorthand for “itty bitty thing the rest of the system doesn’t otherwise interact with “). The delta function goes to infinity at some point. A real energy source is just some weighted collection of these deltas. In other words, I have a bunch of singularities. At some nominal distance from each singularity, the math is well-behaved. Close-up, however, things fall apart. Numerical precision is exhausted quickly, field strengths go off the rails, phase velocities exceed the speed of light (Hertz has some fun wtf moments at the end of one of his notebooks grappling with this), and other fun effects. Software hates this. So theoreticians come up with ways to model these singularities at a close, but non-zero distance for a bunch of special cases. Software must decide among the cases with hints from the human operator and match up with the desired geometry. It’s super hard to get right in all cases. My advisor and others at OSU spent decades coming up more special cases to enable rapid design of dish antennas, and certain shapes of wire antennas, and even to accelerate stealth F-117 design. Compared to all of this craziness, even a theoretical person like me will still spend a lot of time in the lab because it is just faster and strictly more accurate. Antenna design is often characterized as black magic. It really isn’t. We just don’t know literally everything about the world and it sometimes matters for these devices. The mystery is not inward towards the mind, but outward towards the universe at small and large scales.

adamrezich 5y ago

(2016), previously: https://news.ycombinator.com/item?id=19708982 https://news.ycombinator.com/item?id=19708982

quickthrower2 5y ago

Site looks infested with ads

squarefoot 5y ago
With Ublock Origin I see only one, which would be very easy to kill using its element zapper tool.
- quickthrower2 5y ago
  On mobile i see them all and I normally i don't mind but on this site they show ads as navigation links making it pretty hard to navigate the site. Especially when the images the ads use are designed to make them not look like ads.
- pmontra 5y ago
  Firefox Android with uBO and not even that single ad. A nice text only site with GIFs for formulas and diagrams.
deleted 5y ago
[deleted]

squarefoot 5y ago

A very interesting site full of downloadable papers on various subjects including antennas. https://www.jpier.org/PIER/pier.php https://www.jpier.org/PIER/pier.php This one for example may turn useful if one needs to build a WiFi multiband planar internal antenna. https://www.jpier.org/PIERL/pier.php?paper=11073004 https://www.jpier.org/PIERL/pier.php?paper=11073004 ...yes, that stuff is black magic/voodoo

monkeycantype 5y ago

Once upon a time, when I was very young, I travelled to a faraway island with an antenna in my bag that I had put together myself. Everything I knew about antennas I had learned in a single evening from an elderly retired naval engineer, a no nonsense expert in his field, shaped by lifetime in the military, we sat in his den - a clean office with medals and oil paintings of grey steel warships hung square and level. His serious visage beamed with love and delight when his partner - whose projected persona was the polar opposite of his own reserve - a flamboyantly camp man in trailing colorful silks who brought us cups of tea and exquisite little cakes, and would look at the the notes I was taking an theatrically shudder, swooning with the the back of hand to his forehead that ‘he never would never understand all that stuff’ I wasn’t entirely sure I did either - the retired engineer was able to explain things in the lingo of his trade, but couldnt confirm my attempts to connect the terms he was using to my first year university physics concepts. But a month later I was sending an email from the jungle via length of unspooled cable

quinnjh 5y ago
I love this story !!! Dawww. Also naval engineers IME have been some of the best no-B.S. teachers for engineering stuff.
- monkeycantype 5y ago
  Thanks quinnjh

entropicgravity 5y ago

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

I may be mistaken, but this site is by a former Apple antenna engineer, isn't it? If he's who I seem to recall he is, he must have some incredible Steve Jobs war stories.

amelius 5y ago
"You're holding it wrong"
- CamperBob2 5y ago
  Exactly. For his sin, exiled from the Garden.

mNovak 5y ago

Another great site in this area is microwaves101.com A huge range of topics, and pleasantly snarky anonymous editor.

mypastself 5y ago

Excellent stuff, surprised I’ve never stumbled upon the site before. It’s not just about antennas, it also offers concise yet intuitive explanations of related concepts such as Fourier analysis.

rob_c 5y ago

`Complexity is not a sign of intelligence; simplify.` couldn't agree more...