10 ms·

Antenna Theory (2016)

345 points by dosshell 7y ago

tyingq 7y ago
"I am a practicing antenna engineer, with a PhD in antennas and I have worked for many years in defense, university and the consumer electronics field as an antenna engineer." Nice that some of the "old web" soldiers on. Firsthand info from actual experts.
amingilani 7y ago
There is a surprising lack of easily digestable antenna/ham radio related material on the internet. I know because it took me 3 weeks to learn the basics of antennas when I expected to finish in 2 nights. Some of the best information I read was from old Royal Canadian Airforce videos, atleast several decades old[0] I still haven't been able to find a general equation for a have wavelength dipole antenna explained in simple English. I do have one based on empirical evidence, though[1]. I've even bought a copy of the ARRL Handbook, but I find that it goes from 0 to OMG-language-is-this too quickly. Thank you, I wish I'd found this site earlier. [0]: https://www.youtube.com/watch?v=7bDyA5t1ldU https://www.youtube.com/watch?v=7bDyA5t1ldU [1]: https://ham.stackexchange.com/questions/12996/what-is-the-equation-for-the-arm-lengths-of-a-half-wavelength-dipole-antenna https://ham.stackexchange.com/questions/12996/what-is-the-eq...
- dbcurtis 7y ago
  >I still haven't been able to find a general equation for a have wavelength dipole antenna explained in simple English. Equation for what? Length at resonance? Feed point impedance? Other interesting parameters? Antennas are one of those things where it takes a long time to develop intuition, and there are no simple formulas for anything, just useful models that get you close, and simulation programs that work well enough to give an answer within your manufacturing tolerances. (All models are wrong, some are useful.) So start with a couple of fundamental ideas: 1) Accelerate an electron, get a photon. 2) An antenna is a transformer that couples the end of your feed line to free space. The reason the general family of dipole antennas is efficient is that the Ohmic resistance is usually around an Ohm or less, and the "radiation resistance" can be raised to around 70 to 80 Ohms. So 80/(80+1) is the ratio of energy coupled to space versus total energy input. Pretty good efficiency. In a center fed dipole, the driving voltage creates an electrostatic force that attempts to slosh electrons in the conductor one way or the other. At resonance, a small amount of energy input creates lots of sloshing, because the driving voltage just needs to give a boost to the resonant sloshing. Off resonance, effectiveness is much lower. Actual length at resonance depends on the length:diameter ratio of the conductor, the dielectric constant of the surrounding medium, the height above ground, and the dielectric constant and conductivity of the ground. If you know all of those, the dipole can be modeled as a just barely tractable boundary value problem. The empirical formulas that you see usually assume a practical conductor diameter and practical height. You might enjoy playing with one of the NEC2-based antenna modeling programs out there. NEC2-family solvers do "method of moments", where each wire is chopped up into segments, and then for an N-segment model, an NxN matrix of mutual inductances models the coupling among wire segments. The ARRL Antenna Book takes more time to explain fundamentals than the Handbook. The ARRL also publishes an antenna physics book that I haven't read. I notice that PhD committee chair for the author of the Antenna Theory web site we are yakking about was Balanis, who wrote a pretty good book called "Antenna Theory" -- but the book assumes you are an EE graduate student with at least a semester of multi-dimmensional DiffEQ beyond the elementary DiffEQ course.
  - amingilani 7y ago
    That's the problem I suppose. Most of the material is either geared for EEs, or assumes I need a refresher. Not someone completely new to the hobby. >Equation for what? Length at resonance? Feed point impedance? Other interesting parameters? I should have clarified: The arm length of a half wavelength dipole and all the variables that go into it. I assumed it was 1/4 wavelength, but while building mine, I discovered calculators that gave calculations different from mine. >So start with a couple of fundamental ideas: 1) Accelerate an electron, get a photon. 2) An antenna is a transformer that couples the end of your feed line to free space. I don't mean to sound thick, but you've already assumed too much. Before I signed up for a membership with my local radio society, I didn't even know Ohm's law. I pride myself in being technical. If I can teach myself to code and, in a few years, craft tested API's and decoupled front-ends that are tested through CI pipelines and deploy through CD, I can surely teach myself enough physics to build an antenna — no. I still struggle to understand basic concepts like: + Baluns + Why does the height of an antenna effect its effectiveness? + Gain + Circuit design + Transistors + Honestly, I still think radio waves are magic sometimes, even though I think I've seen the effects of electrically generated magnetic fields on coils >You might enjoy playing with one of the NEC2-based antenna modeling programs out there. Tried playing with CocoaNEC 2.0, but the lack of documentation left me feeling like an air head. I'm hoping a little more exposure to electrical systems will help.
    - rfdave 7y ago
      You're trying for a huge breadth of material, stuff that is covered in multiple specialities in electrical engineering. Multivariable calculus is really the entry point for engineering level antenna design. Balanis is the book I used in my antennas class, https://www.amazon.com/Antenna-Theory-Analysis-Constantine-Balanis-ebook/dp/B01A0393XG/ref=sr_1_1?crid=H3F47WRBE8RA&keywords=balanis+antenna+theory&qid=1555810869&s=gateway&sprefix=balanis%2Caps%2C207&sr=8-1 https://www.amazon.com/Antenna-Theory-Analysis-Constantine-B... but you'll need multivariable calculus to get through it.
      - Junk_Collector 7y ago
        Balanis' book is a great reference but I wouldn't recommend it for learning. Honestly the ARRL antenna book and antenna handbook are the best practical books on putting an antenna together without getting bogged down in the details. The real hard part is getting some radio equipment so that you can experiment and learn since Spectrum and Network analyzers are waaaay out of most people's hobby budgets.
- ac29 7y ago
  I really liked the free Anritsu e-learning course "RF Fundamentals", which I did earlier this year: https://us.anritsu-learning.com/course/categorylist.php?viewtype=course https://us.anritsu-learning.com/course/categorylist.php?view... I work in the radio industry, and it was very useful when putting together training materials. You need a free account to access it.
  - amingilani 7y ago
    This might just be the most helpful thing someone has pointed me to! Thank you! Here's the syllabus for anyone else looking to check it out: Radio Frequency: Module 1 Decibels: Module 2 Modulation: Module 3 RF Impairment: Module 4 Antennas: Module 5 Coaxial Cables: Module 6 Components: Module 7 Propagation: Module 8 Microwave: Module 9
obviuosly 7y ago
Am I overlooking something or is the "cantenna" not considered a fundamental antenna type? It does not seem to be listed in the page about different antennta types. It seems to be quite easy to build once you find a suitable can (easier than a Yagi-Uda antenna) and it seems it can easily keep pace with a Yagi-Uda antenna of similar size.
- tyingq 7y ago
  It would be a waveguide antenna. He shows a slotted one on the page you're talking about.
ChuckMcM 7y ago
This is a great summary. I too have struggled to get decent information on the web about antenna theory and design. As it turned out, I was searching wrong :-) The keyword is 'electrodynamics' and the canonical text is "Classic Electrodynamics" by Jackson. I am told that if you can understand the contents of this book, antennas are pretty straight forward. I started in on it, got whacked upside the brain a number of times, then backed off to "Introduction to Electrodynamics" by Griffth which is the undergraduate version and does a bit more math review, which was essential in my case. My plan is that once I am through that I'll go back and re-start Jackson. The fun bit here is that if you look for computer code to simulate this stuff you will run into a lot of Fortan code. So if you ever wanted to learn Fortran this will give you some code to puzzle over.
- dbcurtis 7y ago
  > you will run into a lot of Fortan code. By which you probably refer to the NEC2 code base. NEC2 is public domain, so that is what most hackers use. The native UI is column-senstive punch cards. Blessedly, there are people that have put more modern front-ends on the NEC2 back-end. NEC2 is OK-ish, as long as you avoid the well-known bugs. NEC4 fixes some of the bugs, but falls under ITAR, so requires a license and can't be exported (last I knew, anyway). There are also multi-kilo-dollar-per-seat antenna modeling packages available commercially. NEC2 is pretty old, and an interesting story I heard about the validation of the model was that the DOD, having helicopters handy, stuffed a helicopter full of instruments and flew it around an antenna range to capture ground-truth data for antennas that had been built from models. Last week I was talking with my friend N6BT, who has been in the antenna business for decades. For 3 or so years he has had a quad-rotor that he flies around with a signal generator, and uses the GPS time from the quad-rotor to correlate GPS time-stamped data from his ground-based spectrum analyzer to collect actuals. He is finding MANY discrepancies (primarily at low angles) between NEC4 and actual, due to the sketchy ground models.
  - ChuckMcM 7y ago
    That has been my experience as well, at work we have a multi-kilobuck simulation package but we still put the antenna on our range to test it. The range consists of basically a robot arm that can hold the antenna under test (AUT) in any orientation, a transceiver/spectrum analyzer that can move forward or back to get into the near, Fresnel, and far ranges, and a transceiver/spectrum analyzer that is connected to the AUT. Comparing simulations to actual always yields some interesting nuggets of information.
dazhbog 7y ago
I once went to an antenna tuning/design/factory for my startup's product and I told the guy there, I want an antenna to support X frequencies, this that dBs, good VSWR, hopefully achieve X kilometers range, etc etc. Two days later he had 3 designs of antennas ready to be made into a flexible PCB and two days after that we got the FPCB samples. I was amazed so I asked to see how he works. He took copper tape, and with a boxcutter he carves the antenna, and adds solder blobs to tune with the network analyzer. Then, once he was satisfied, he shoved them into the anechoic chamber and boom, done. Black magic stuff. Antenna intuition is really hard to attain, it takes years playing with the right equipment in the right environment..
- js2 7y ago
  > Antenna intuition is really hard to attain For some reason this reminded me of musician Carolina Eyck playing a theramin on this Tiny Desk Concert: https://www.npr.org/2019/01/10/683943414/carolina-eyck-and-clarice-jensen-tiny-desk-concert https://www.npr.org/2019/01/10/683943414/carolina-eyck-and-c... It’s worth watching to see her demo how she plays the different notes and also changes the volume. And here’s more of a theramin tutorial by her: https://youtu.be/MJACNHHuGp0 https://youtu.be/MJACNHHuGp0
- btashton 7y ago
  I remember this from a professor in college as well. I spent weeks modeling an antenna element and then once I had it made, he brought out the copper tape and a knife and tuned it to closer match what we wanted. It is humbling to see this kind of craftsmanship in technology.
- etaioinshrdlu 7y ago
  I think with good simulation tools, optimization processes, maybe even deep learning, we could make it a lot less of black magic. Make it boring and approachable instead. It kind of reminds me of chemistry. You have physical laws that are fairly simple but the interactions are hard to describe without a bunch of computation.
  - nisuni 7y ago
qndreoi 7y ago
Andrew McNeil has a good set of videos on building 2.4 and 5 GHz antennae on youtube: https://www.youtube.com/channel/UCHqwzhcFOsoFFh33Uy8rAgQ https://www.youtube.com/channel/UCHqwzhcFOsoFFh33Uy8rAgQ
robochat 7y ago
As a physicist rather than an EE (although I'm neither now), antennna always led to confusion. In particular antenna effective area and reciprocity. Trying to imagine a 1d dipole antenna funneling some 2D part of the incoming wavefront into its output waveguide just felt like magic. As did the trying to intuitively see that an antenna's gain is the same in transmission and reception when the wavefronts seem totally unequivalent. Would have liked to have really studied it more but antenna don't get covered in a typical physics course.
- madengr 7y ago
  Think of it like this: The wavefront around a dipole is comprised of near and far fields, with the near field being spherical and the far field planar. That near field has a reactive component (stored energy) that does not propagate, but falls off at 1/r^3 or faster. So an incoming plane wave induces charge motion, which builds up the reactive near field over many cycles, generating that spherical wavefront. So that seems to indicate reciprocity is only valid for a steady state, but it’s still valid. If your transmit antenna were fed a monocycle (i.e. not time for the near-field to build up), the receiving antenna wouldn’t have enough time either.
  - Junk_Collector 7y ago
    Evanescent fields exist sure but Far field is really just a useful mathematical construct. It is typified by a wave-front where you can approximate no phase difference wherever it lands on a planar antenna. It's like how you can approximate the Earth as flat when making a platting because it is very large and you are very small. If you look at the Farfield approximation calculation for a large antenna or phased array, you'll see that the equation is a function of distance, wavelength, and aperture size. Edit: I should point out that Evanescent waves do not carry power (no net energy flow) so the power transfer is always reciprocal between 2 antennas.
- sunstone 7y ago
  As an EE with sense of physics I ran into the same confusion. Your instincts regarding reciprocity, effective area and all that were in the right direction and with a few more steps you would have had your satisfying 'Eureka' moment. Most of these comes from the Pathloss Equation which it turns out is a stitch up to make things simpler and easy (but wrong). It's best explained with respect to parabolic antennas but applies to all antennas. The key point to picking this apart is to consider reciprocity which states that the that an antenna is "the same" as either a transmitter or a receiver. In particular the gain is is the same. So gain is the increased power with respect to an isotropic radiator. With a parabolic antenna the focus (ie beam width) of the transmitting beam does depend on frequency due to geometric concerns and as such, in the path loss equation the the antenna gain appears as frequency dependant as it ought to. However, reciprocity requires that the receiver also have an identical, frequency dependant gain. The gain of the receiver though depends only on its physical (or effective) area and not on its frequency. In the pathloss equation you can see that the loss goes as the reciprocal of the square of the distance, which it should, but also goes the reciprocal of the square of the frequency. This frequency term (which causes the equation to violate conservation of energy, normally a bad thing) is there to cancel out the bogus frequency term incorporated into the gain of the receiving antenna due to the also bogus reciprocity law. So to simplify, in an electromagnetic link between two antennas, the gain of the transmitting antenna depends upon the frequency of the transmitting carrier, because the focus of the beam varies with frequency. The signal then drops off as 1/r^2 in the normal way (with no frequency component) and the gain of the receiver depends only on its size. A bigger receiver antenna captures more energy from the receiver. That's it, simple and sensible. Effective area is a separate topic for long wavelength transmissions but also sensible in the end.
jimnotgym 7y ago
I have an etymological question. The chap who supervised my ham tests insists that bugs have antennae and radios have aerials. I wonder if it is a US/UK thing? He is British.
- NeedMoreTea 7y ago
  It's always an aerial in the UK, where the radio was once called the wireless. I suspect the British chap being picky about transatlantic language exchange probably called them insects, not bugs. ;)
  - jimnotgym 7y ago
    Well even funnier than that he said bugs and then corrected himself (although I am pretty sure the two words have a strict definition that means they are not interchangeable). He also used the word antenna lots of times too! Most ham textbooks in the UK use the term antenna, but common usage is definitely still aerial
    - NeedMoreTea 7y ago
      We'll definitely always use aerial for the TV or radio receiving thing on the roof, or the thing you pull out the back of the set. Seeing the sibling comment with a distinction, and yeah I have heard it occasionally for transmission capable. A 30 foot pole in the back yard might indeed get called antenna. We don't seem to use bugs as a catch all much any more, though it's coming back, and we always kept it for bed bugs. Not especially consistent - but few of these transatlantic complaints are!
- Sharlin 7y ago
  > I wonder if it is a US/UK thing? Primarily, yes. Though Wiktionary has the following usage note: > Some make a distinction between an antenna and an aerial, with the former used to indicate a rigid structure, and the latter consisting of a wire strung in the air. For those who do not make a distinction, antenna is more commonly used in the United States and aerial is more commonly used in the United Kingdom.
sunstone 7y ago
It's slightly ironic that the source page cites the Einstein quote about simplicity and yet, one of the central equations of antenna link design is the "Path Loss Equation" which is in fact, simpler than possible. :)

345 points by dosshell 7y ago

"I am a practicing antenna engineer, with a PhD in antennas and I have worked for many years in defense, university and the consumer electronics field as an antenna engineer." Nice that some of the "old web" soldiers on. Firsthand info from actual experts.

amingilani 7y ago

There is a surprising lack of easily digestable antenna/ham radio related material on the internet. I know because it took me 3 weeks to learn the basics of antennas when I expected to finish in 2 nights. Some of the best information I read was from old Royal Canadian Airforce videos, atleast several decades old[0] I still haven't been able to find a general equation for a have wavelength dipole antenna explained in simple English. I do have one based on empirical evidence, though[1]. I've even bought a copy of the ARRL Handbook, but I find that it goes from 0 to OMG-language-is-this too quickly. Thank you, I wish I'd found this site earlier. [0]: https://www.youtube.com/watch?v=7bDyA5t1ldU https://www.youtube.com/watch?v=7bDyA5t1ldU [1]: https://ham.stackexchange.com/questions/12996/what-is-the-equation-for-the-arm-lengths-of-a-half-wavelength-dipole-antenna https://ham.stackexchange.com/questions/12996/what-is-the-eq...

dbcurtis 7y ago
>I still haven't been able to find a general equation for a have wavelength dipole antenna explained in simple English. Equation for what? Length at resonance? Feed point impedance? Other interesting parameters? Antennas are one of those things where it takes a long time to develop intuition, and there are no simple formulas for anything, just useful models that get you close, and simulation programs that work well enough to give an answer within your manufacturing tolerances. (All models are wrong, some are useful.) So start with a couple of fundamental ideas: 1) Accelerate an electron, get a photon. 2) An antenna is a transformer that couples the end of your feed line to free space. The reason the general family of dipole antennas is efficient is that the Ohmic resistance is usually around an Ohm or less, and the "radiation resistance" can be raised to around 70 to 80 Ohms. So 80/(80+1) is the ratio of energy coupled to space versus total energy input. Pretty good efficiency. In a center fed dipole, the driving voltage creates an electrostatic force that attempts to slosh electrons in the conductor one way or the other. At resonance, a small amount of energy input creates lots of sloshing, because the driving voltage just needs to give a boost to the resonant sloshing. Off resonance, effectiveness is much lower. Actual length at resonance depends on the length:diameter ratio of the conductor, the dielectric constant of the surrounding medium, the height above ground, and the dielectric constant and conductivity of the ground. If you know all of those, the dipole can be modeled as a just barely tractable boundary value problem. The empirical formulas that you see usually assume a practical conductor diameter and practical height. You might enjoy playing with one of the NEC2-based antenna modeling programs out there. NEC2-family solvers do "method of moments", where each wire is chopped up into segments, and then for an N-segment model, an NxN matrix of mutual inductances models the coupling among wire segments. The ARRL Antenna Book takes more time to explain fundamentals than the Handbook. The ARRL also publishes an antenna physics book that I haven't read. I notice that PhD committee chair for the author of the Antenna Theory web site we are yakking about was Balanis, who wrote a pretty good book called "Antenna Theory" -- but the book assumes you are an EE graduate student with at least a semester of multi-dimmensional DiffEQ beyond the elementary DiffEQ course.
- amingilani 7y ago
  That's the problem I suppose. Most of the material is either geared for EEs, or assumes I need a refresher. Not someone completely new to the hobby. >Equation for what? Length at resonance? Feed point impedance? Other interesting parameters? I should have clarified: The arm length of a half wavelength dipole and all the variables that go into it. I assumed it was 1/4 wavelength, but while building mine, I discovered calculators that gave calculations different from mine. >So start with a couple of fundamental ideas: 1) Accelerate an electron, get a photon. 2) An antenna is a transformer that couples the end of your feed line to free space. I don't mean to sound thick, but you've already assumed too much. Before I signed up for a membership with my local radio society, I didn't even know Ohm's law. I pride myself in being technical. If I can teach myself to code and, in a few years, craft tested API's and decoupled front-ends that are tested through CI pipelines and deploy through CD, I can surely teach myself enough physics to build an antenna — no. I still struggle to understand basic concepts like: + Baluns + Why does the height of an antenna effect its effectiveness? + Gain + Circuit design + Transistors + Honestly, I still think radio waves are magic sometimes, even though I think I've seen the effects of electrically generated magnetic fields on coils >You might enjoy playing with one of the NEC2-based antenna modeling programs out there. Tried playing with CocoaNEC 2.0, but the lack of documentation left me feeling like an air head. I'm hoping a little more exposure to electrical systems will help.
  - rfdave 7y ago
    You're trying for a huge breadth of material, stuff that is covered in multiple specialities in electrical engineering. Multivariable calculus is really the entry point for engineering level antenna design. Balanis is the book I used in my antennas class, https://www.amazon.com/Antenna-Theory-Analysis-Constantine-Balanis-ebook/dp/B01A0393XG/ref=sr_1_1?crid=H3F47WRBE8RA&keywords=balanis+antenna+theory&qid=1555810869&s=gateway&sprefix=balanis%2Caps%2C207&sr=8-1 https://www.amazon.com/Antenna-Theory-Analysis-Constantine-B... but you'll need multivariable calculus to get through it.
    - Junk_Collector 7y ago
      Balanis' book is a great reference but I wouldn't recommend it for learning. Honestly the ARRL antenna book and antenna handbook are the best practical books on putting an antenna together without getting bogged down in the details. The real hard part is getting some radio equipment so that you can experiment and learn since Spectrum and Network analyzers are waaaay out of most people's hobby budgets.
ac29 7y ago
I really liked the free Anritsu e-learning course "RF Fundamentals", which I did earlier this year: https://us.anritsu-learning.com/course/categorylist.php?viewtype=course https://us.anritsu-learning.com/course/categorylist.php?view... I work in the radio industry, and it was very useful when putting together training materials. You need a free account to access it.
- amingilani 7y ago
  This might just be the most helpful thing someone has pointed me to! Thank you! Here's the syllabus for anyone else looking to check it out: Radio Frequency: Module 1 Decibels: Module 2 Modulation: Module 3 RF Impairment: Module 4 Antennas: Module 5 Coaxial Cables: Module 6 Components: Module 7 Propagation: Module 8 Microwave: Module 9

obviuosly 7y ago

Am I overlooking something or is the "cantenna" not considered a fundamental antenna type? It does not seem to be listed in the page about different antennta types. It seems to be quite easy to build once you find a suitable can (easier than a Yagi-Uda antenna) and it seems it can easily keep pace with a Yagi-Uda antenna of similar size.

tyingq 7y ago
It would be a waveguide antenna. He shows a slotted one on the page you're talking about.

ChuckMcM 7y ago

This is a great summary. I too have struggled to get decent information on the web about antenna theory and design. As it turned out, I was searching wrong :-) The keyword is 'electrodynamics' and the canonical text is "Classic Electrodynamics" by Jackson. I am told that if you can understand the contents of this book, antennas are pretty straight forward. I started in on it, got whacked upside the brain a number of times, then backed off to "Introduction to Electrodynamics" by Griffth which is the undergraduate version and does a bit more math review, which was essential in my case. My plan is that once I am through that I'll go back and re-start Jackson. The fun bit here is that if you look for computer code to simulate this stuff you will run into a lot of Fortan code. So if you ever wanted to learn Fortran this will give you some code to puzzle over.

dbcurtis 7y ago
> you will run into a lot of Fortan code. By which you probably refer to the NEC2 code base. NEC2 is public domain, so that is what most hackers use. The native UI is column-senstive punch cards. Blessedly, there are people that have put more modern front-ends on the NEC2 back-end. NEC2 is OK-ish, as long as you avoid the well-known bugs. NEC4 fixes some of the bugs, but falls under ITAR, so requires a license and can't be exported (last I knew, anyway). There are also multi-kilo-dollar-per-seat antenna modeling packages available commercially. NEC2 is pretty old, and an interesting story I heard about the validation of the model was that the DOD, having helicopters handy, stuffed a helicopter full of instruments and flew it around an antenna range to capture ground-truth data for antennas that had been built from models. Last week I was talking with my friend N6BT, who has been in the antenna business for decades. For 3 or so years he has had a quad-rotor that he flies around with a signal generator, and uses the GPS time from the quad-rotor to correlate GPS time-stamped data from his ground-based spectrum analyzer to collect actuals. He is finding MANY discrepancies (primarily at low angles) between NEC4 and actual, due to the sketchy ground models.
- ChuckMcM 7y ago
  That has been my experience as well, at work we have a multi-kilobuck simulation package but we still put the antenna on our range to test it. The range consists of basically a robot arm that can hold the antenna under test (AUT) in any orientation, a transceiver/spectrum analyzer that can move forward or back to get into the near, Fresnel, and far ranges, and a transceiver/spectrum analyzer that is connected to the AUT. Comparing simulations to actual always yields some interesting nuggets of information.

dazhbog 7y ago

I once went to an antenna tuning/design/factory for my startup's product and I told the guy there, I want an antenna to support X frequencies, this that dBs, good VSWR, hopefully achieve X kilometers range, etc etc. Two days later he had 3 designs of antennas ready to be made into a flexible PCB and two days after that we got the FPCB samples. I was amazed so I asked to see how he works. He took copper tape, and with a boxcutter he carves the antenna, and adds solder blobs to tune with the network analyzer. Then, once he was satisfied, he shoved them into the anechoic chamber and boom, done. Black magic stuff. Antenna intuition is really hard to attain, it takes years playing with the right equipment in the right environment..

js2 7y ago
> Antenna intuition is really hard to attain For some reason this reminded me of musician Carolina Eyck playing a theramin on this Tiny Desk Concert: https://www.npr.org/2019/01/10/683943414/carolina-eyck-and-clarice-jensen-tiny-desk-concert https://www.npr.org/2019/01/10/683943414/carolina-eyck-and-c... It’s worth watching to see her demo how she plays the different notes and also changes the volume. And here’s more of a theramin tutorial by her: https://youtu.be/MJACNHHuGp0 https://youtu.be/MJACNHHuGp0
btashton 7y ago
I remember this from a professor in college as well. I spent weeks modeling an antenna element and then once I had it made, he brought out the copper tape and a knife and tuned it to closer match what we wanted. It is humbling to see this kind of craftsmanship in technology.
etaioinshrdlu 7y ago
I think with good simulation tools, optimization processes, maybe even deep learning, we could make it a lot less of black magic. Make it boring and approachable instead. It kind of reminds me of chemistry. You have physical laws that are fairly simple but the interactions are hard to describe without a bunch of computation.
- nisuni 7y ago

qndreoi 7y ago

Andrew McNeil has a good set of videos on building 2.4 and 5 GHz antennae on youtube: https://www.youtube.com/channel/UCHqwzhcFOsoFFh33Uy8rAgQ https://www.youtube.com/channel/UCHqwzhcFOsoFFh33Uy8rAgQ

robochat 7y ago

As a physicist rather than an EE (although I'm neither now), antennna always led to confusion. In particular antenna effective area and reciprocity. Trying to imagine a 1d dipole antenna funneling some 2D part of the incoming wavefront into its output waveguide just felt like magic. As did the trying to intuitively see that an antenna's gain is the same in transmission and reception when the wavefronts seem totally unequivalent. Would have liked to have really studied it more but antenna don't get covered in a typical physics course.

madengr 7y ago
Think of it like this: The wavefront around a dipole is comprised of near and far fields, with the near field being spherical and the far field planar. That near field has a reactive component (stored energy) that does not propagate, but falls off at 1/r^3 or faster. So an incoming plane wave induces charge motion, which builds up the reactive near field over many cycles, generating that spherical wavefront. So that seems to indicate reciprocity is only valid for a steady state, but it’s still valid. If your transmit antenna were fed a monocycle (i.e. not time for the near-field to build up), the receiving antenna wouldn’t have enough time either.
- Junk_Collector 7y ago
  Evanescent fields exist sure but Far field is really just a useful mathematical construct. It is typified by a wave-front where you can approximate no phase difference wherever it lands on a planar antenna. It's like how you can approximate the Earth as flat when making a platting because it is very large and you are very small. If you look at the Farfield approximation calculation for a large antenna or phased array, you'll see that the equation is a function of distance, wavelength, and aperture size. Edit: I should point out that Evanescent waves do not carry power (no net energy flow) so the power transfer is always reciprocal between 2 antennas.
sunstone 7y ago
As an EE with sense of physics I ran into the same confusion. Your instincts regarding reciprocity, effective area and all that were in the right direction and with a few more steps you would have had your satisfying 'Eureka' moment. Most of these comes from the Pathloss Equation which it turns out is a stitch up to make things simpler and easy (but wrong). It's best explained with respect to parabolic antennas but applies to all antennas. The key point to picking this apart is to consider reciprocity which states that the that an antenna is "the same" as either a transmitter or a receiver. In particular the gain is is the same. So gain is the increased power with respect to an isotropic radiator. With a parabolic antenna the focus (ie beam width) of the transmitting beam does depend on frequency due to geometric concerns and as such, in the path loss equation the the antenna gain appears as frequency dependant as it ought to. However, reciprocity requires that the receiver also have an identical, frequency dependant gain. The gain of the receiver though depends only on its physical (or effective) area and not on its frequency. In the pathloss equation you can see that the loss goes as the reciprocal of the square of the distance, which it should, but also goes the reciprocal of the square of the frequency. This frequency term (which causes the equation to violate conservation of energy, normally a bad thing) is there to cancel out the bogus frequency term incorporated into the gain of the receiving antenna due to the also bogus reciprocity law. So to simplify, in an electromagnetic link between two antennas, the gain of the transmitting antenna depends upon the frequency of the transmitting carrier, because the focus of the beam varies with frequency. The signal then drops off as 1/r^2 in the normal way (with no frequency component) and the gain of the receiver depends only on its size. A bigger receiver antenna captures more energy from the receiver. That's it, simple and sensible. Effective area is a separate topic for long wavelength transmissions but also sensible in the end.

jimnotgym 7y ago

I have an etymological question. The chap who supervised my ham tests insists that bugs have antennae and radios have aerials. I wonder if it is a US/UK thing? He is British.

NeedMoreTea 7y ago
It's always an aerial in the UK, where the radio was once called the wireless. I suspect the British chap being picky about transatlantic language exchange probably called them insects, not bugs. ;)
- jimnotgym 7y ago
  Well even funnier than that he said bugs and then corrected himself (although I am pretty sure the two words have a strict definition that means they are not interchangeable). He also used the word antenna lots of times too! Most ham textbooks in the UK use the term antenna, but common usage is definitely still aerial
  - NeedMoreTea 7y ago
    We'll definitely always use aerial for the TV or radio receiving thing on the roof, or the thing you pull out the back of the set. Seeing the sibling comment with a distinction, and yeah I have heard it occasionally for transmission capable. A 30 foot pole in the back yard might indeed get called antenna. We don't seem to use bugs as a catch all much any more, though it's coming back, and we always kept it for bed bugs. Not especially consistent - but few of these transatlantic complaints are!
Sharlin 7y ago
> I wonder if it is a US/UK thing? Primarily, yes. Though Wiktionary has the following usage note: > Some make a distinction between an antenna and an aerial, with the former used to indicate a rigid structure, and the latter consisting of a wire strung in the air. For those who do not make a distinction, antenna is more commonly used in the United States and aerial is more commonly used in the United Kingdom.

sunstone 7y ago

It's slightly ironic that the source page cites the Einstein quote about simplicity and yet, one of the central equations of antenna link design is the "Path Loss Equation" which is in fact, simpler than possible. :)