8 ms·

U.S. Marine Corps Antenna Handbook (1999) [pdf]

310 points by joos3 3y ago

ulnarkressty 3y ago
I find these guides a bit disappointing in that they always give the example of a wire loop generating an EM field, which one could sort of imagine given day to day experience with direct current and conductors. However this all breaks down when looking at regular monopole antennas - how can the conductor conduct when one of the ends is just hanging in the air? AC magic I suppose. Similarly they are very short on details on what exactly is going on when the EM field is generated. I guess it consists of photons, but where exactly do they come from and how are they generated using (in some cases) only milliwatts of power?
- amelius 3y ago
  Circuit theory breaks down when it comes to antennas.
  - rusk 3y ago
    Circuit theory breaks down at high frequencies and the appropriate generalisation of Kirchoffs is embodied in Maxwell’s equations.
    - jacquesm 3y ago
      Indeed, it's a bit like Newton vs Einstein. The one is a special case of the other, it doesn't make the former wrong but the latter gives better answers in situations where the special case constraints aren't satisfied.
  - H8crilA 3y ago
    Circuit theory is demonstrably wrong. The energy doesn't flow inside of a wire, it flows around the wire. https://en.wikipedia.org/wiki/Poynting_vector https://en.wikipedia.org/wiki/Poynting_vector https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY It is a very useful tool in practica.
    - jacquesm 3y ago
      Energy does flow 'inside' a wire at low frequencies. The skin effect gets higher and higher as the frequency goes up until you reach a point where it is all skin effect. But even that 'skin' isn't idealized it definitely has a thickness, about 30 u at 5 MHz and 6.5 at 100 Mhz. There is a neat little calculator here: https://www.omnicalculator.com/physics/skin-depth https://www.omnicalculator.com/physics/skin-depth
      - H8crilA 3y ago
        No. DC energy also flows around the wire. And I mean around, entirely outside of the conductor, not in the skin or anything like that. In particular it's not the electrons that carry the energy, though they can receive it from the fields and depose inside the conductor (like in a light bulb). Wires, or conductors in general, are so useful because they allow us to manipulate the EM fields and channel the energy very efficiently. It's an very common misconception coming from circuit models. (the movement of electrons, and thus the deposition of the energy via "resistance", may indeed be limited to just the surface of the conductor; this is what that calculator shows)
        jacquesm 3y ago
        Complete nonsense, the current carrying capacity of a wire is directly proportional to the surface area of the cross section of the wire. If it were the skin it would be proportional to the circumference and it clearly is not. I have no idea where you came into this idea but it is just completely wrong. You can test it for yourself with $50 worth of gear.
        colanderman 3y ago
        @H8crilA is talking about energy, you are describing current. Energy flow is the product of the electric and magnetic fields [1]; electric field within a conductor is zero, therefore energy flow within a conductor is zero. [1] https://en.wikipedia.org/wiki/Poynting_vector https://en.wikipedia.org/wiki/Poynting_vector
  - jacquesm 3y ago
    It's at best a leaky abstraction but for some domains it is very useful. Antennas are 'magic' from the point of view of Ohms law and so on but once you move to the electromagnetic domain you will find equivalents for most of the elements in the original abstraction: impedance, reactance and so on.
    - SAI_Peregrinus 3y ago
      Yep, and even common nonlinear elements like diodes and transistors are non-Ohmic.
      - jacquesm 3y ago
        No, they are just non-linear: as the voltage goes up the current goes up in jumps because the resistance changes in a non-linear way, but it is still resistance and you can still express the system at any point of the curve using Ohm's laws and you can still compute the power lost in any part of that system using the simplest equations. A coil or a capacitor (or even a piece of wire, but there the effects are very small) are components that do not follow Ohm's laws, you need to add more parameters than just current, voltage and resistance to work out what's happening. Tricky stuff!
- Frummy 3y ago
  Photons are created when an electron drops to a lower energy level. Like your voicebox and the sound you make when you speak, it doesnt come from somewhere but is created right then
  - eternauta3k 3y ago
    No, the "photons" here are not created by that phenomenon but rather by accelerating electrons. https://en.wikipedia.org/wiki/Larmor_formula https://en.wikipedia.org/wiki/Larmor_formula
    - Frummy 3y ago
      Cool, thanks for letting me know.
- max_ 3y ago
  Its abit outdated. But it think you may enjoy reading through Oliver Lodge's book, "Signalling Through Space Without Wires" [0] https://catalog.hathitrust.org/Record/001617948 https://catalog.hathitrust.org/Record/001617948
- amadsen 3y ago
  It doesn't really "generate" a field. The EM field is always there, permeating the universe. An acceleting electron will _disturb_ the EM field (depositing some energy and momentum into it), and this disturbance will propagate through the field at the speed of light (naturally, since at the right energy level such a disturbance is what we call light). At high energies the disturbance will be finely localized in space and behave like a particle, which we call a photon. It's fine to refer to it as such also at lower energies, but slightly misleading because at the very low energies that we talk about here ("radio") it is very spread out in space and behaves more like a wave (with a wavelength of ~meters). In the case of AC, electrons are moving "back and forth" over a short distance (somewhat simplified but useful picture) with the same effect. Think about moving your hand up and down through water - you will create a wave.
  - programmer_dude 3y ago
    >The EM field is always there Did you mean to say "the E field is always there"? Loved this explanation otherwise.
    - computerfriend 3y ago
      It's one field, which can be split into E and B components if you like.
      - georgeg23 3y ago
        The B field is just the E field's relativistic effects to an observer not in the same motion frame. https://wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity https://wikipedia.org/wiki/Classical_electromagnetism_and_sp...
    - H8crilA 3y ago
      Both are always there if the circuit is running. Also, both E and M fields are required to transport any power using electricity. This is also true for DC current. See https://en.wikipedia.org/wiki/Poynting_vector https://en.wikipedia.org/wiki/Poynting_vector - the formula multiplies E by B, therefore if any of the two is zero no power (or information) transmission can occur. This is true over the air, as well as over a wire.
- jcims 3y ago
  >AC magic I suppose. Yep. Think of a long pipe open on one end and capped on the other. If you seal the end with your mouth and blow, it will quickly pressurize. But if you seal a loudspeaker to it and sweep through the frequencies you will find that the loudness of the speaker varies proportionally to how close the tone is to a resonant frequency of the tube. The elections in a monopole antenna have a sort of elastic relationship to one another where the influence propagates at the speed of light rather than the speed of sound. They are also able to move quite freely within the conductor like gas molecules in the tube. So if you have an antenna that’s two meters long and play a ‘tone’ with the equivalent of the loudspeaker at 150 million hz, the tube of electrons resonates. Just like the atmosphere can couple resonant cavities of the same frequency, antennas are coupled by the electromagnetic field to resonate at the same frequency. This provides a certain ‘gain’ of the energy at the end of the ‘tube’ relative to other frequencies that aren’t resonant, which is what feeds our ears or amplifiers with something differentiable from the rest of the noise. Now, imagine you have your ear glued to another tube just like the loudspeaker tube and your other ear is plugged. You’re going to hear the world around you, but it’s mostly going to be at harmonics of that resonant frequency. So when your buddy asks ‘can you hear me?’ a you get this really ringy ‘mwaa mwoo mwee mwee?’. Now imagine he turns on the speaker tube on the other side of the room? It’s going to brightly stand out among all of the other things you hear, ooooooooooooooooo. Ok, now, he hands you a roll of paper and says draw along the roll relative to how loud it is. Up is louder down is softer, and he goes back and starts messing with the volume knob: oooooOOOOOOoooo....oooooOOOoo..ooOoOo The thing you draw on that roll of paper looks like a sound wave. And now you know how AM radio works.
- ajsnigrutin 3y ago
  Yep, AC is magic. And not just antennas.... look at computers... at 3GHz (DDR4+ ram), the wavelength is ~10cm, so looking at a single sine wave, you have 1.8volts at the cpu, zero volts 2.5cm away from the cpu, -1.8v 5cm away, zero 7.5cm, and again 1.8 volts 10cm from the cpu... now look at the distance between the cpu and ram, the other frequencies that come with the square(-ish) waves, and all the math to make a basic RAM read/write work. Just a normal wire (or cable - the simplest component in DC electronic circuits, just a line, that does nothing) changes everything... you send a (voltage) signal in, and the current has to flow at some rate, even before the "signal" (field) reaches the other side of the cable, to "see" if the other end is "open" or soldered together or if some sort of a resistor is soldered there.
  - jacquesm 3y ago
    That's not how current propagates. Current propagates - broken analogy warning - in the same way that marbles would propagate through a tube. You push a marble in on one end and assuming the tube is full another marble pops out the far end. So the flow rate of the electrons is vastly lower than the flow rate of the signal itself!
    - ajsnigrutin 3y ago
      Of course, but currents here are a result of a field that moves those electrons one way or another, and electrons move, before the field reaches the other end (where a resistor is.. or is not.. or a short circuit is), so in your analogy, marbles move down the tube, before they "realize" that the tube is closed on the other end.
      - jacquesm 3y ago
        Fair enough. It's an analogy anyway and it is a broken one in more ways than one (for instance: it doesn't deal with propagation of pulses very well because these can be reflected from the end of the tube, you'd have to model it as a tube full of marbles and springs to account for that) but it serves for some discussion purposes.
- NovemberWhiskey 3y ago
  >regular monopole antennas There is no real monopole antenna - there's always another "half", like a ground plane, a counterpoise system or even just the body of the person holding a handheld radio.
- myth2018 3y ago
  > how can the conductor conduct when one of the ends is just hanging in the air? AC magic I suppose I struggled with these matters too. I wouldn't call it "AC magic", but "RF magic". Notice that you need a different mental model to understand those circuits. In DC and low-freq AC, it's enough to think that voltages and currents manifest themselves instantaneously in a conductor. But that is a (useful) simplification. Roughly speaking, the energy supplied by the power source takes time to propagate over the wire [0]. Having that in mind, take a look at the animated gif in this page: https://en.wikipedia.org/wiki/Dipole_antenna https://en.wikipedia.org/wiki/Dipole_antenna. I believe it's gonna make more sense now. This mental model also helps to understand why a magnetic loop isn't simply a piece of short-circuited wire -- for a DC or low freq AC circuit, it is indeed a short-circuit; for an RF circuit, it is not. [1] [0] https://en.wikipedia.org/wiki/Velocity_factor https://en.wikipedia.org/wiki/Velocity_factor [1] this mental model is not enough to fully understand any antenna. Many designs also depend on other phenomena, like the electromagnetic interactions between its components, and interactions with its surroundings. But I think that, equipped with this mental model, you'll be able to research further in case you feel interested.
- eternauta3k 3y ago
  > how can the conductor conduct when one of the ends is just hanging in the air? At frequencies where the wire radiates, you can't model a wire as an open resistor. You need to model it as a bunch of Rs, Ls and Cs, and there you will see the current flow.
- jerry1979 3y ago
  Quick answer: The antenna is like a tube, and the energy stuff is like liquid that sloshes back and forth in the tube generating a kind of wiggle. This Canadian military video should help: https://www.youtube.com/watch?v=-F7KYLO4Bkg https://www.youtube.com/watch?v=-F7KYLO4Bkg
squarefoot 3y ago
Also worth mentioning, the US Navy Electricity and Electronics Training Series (NEETS), plus other interesting documentation one can find from the top menu here. https://maritime.org/doc/#neets https://maritime.org/doc/#neets
- andromaton 3y ago
  NEETS is a treasure trove of practically.
8bitsrule 3y ago
I liked to play with longwires (sect. 4-22) back in the day ... the longer, the more directional they get. Fun to experiment with (back when copperweld was cheap). But when I did eventually wind up living on a farmstead for a couple of years, I didn't have the time (or the nearby trees) to try out really really long wires.
- jacquesm 3y ago
  You can try barbed wire fences! On a dry day they can work very well, obviously you won't have much say in their direction but if you're near a fence it can't hurt to hook up a sensitive receiver to see what you get.
aaron695 3y ago
There is a MCRP 8-10B.11 (Formerly MCRP 3-40.3C) (2016) but running a diff the text is exactly the same except the first title pages - https://w5sc.org/wp-content/uploads/2020/06/MCRP-8-10B.11.pdf https://w5sc.org/wp-content/uploads/2020/06/MCRP-8-10B.11.pd... It's hard to believe nothing has changed in 20 years, we've move on a lot passed 1999 aka pre-internet society.
- ianburrell 3y ago
  The basics or antenna design have been known for a long time. Any advances have been in the small antennas used in devices, not the large antennas strung up by Marines.
  - jacquesm 3y ago
    That's not really true though, phased arrays are a fairly recent development and now that we have the computational power to use them more effectively are being used in many more practical applications.
nunez 3y ago
This is really well written and surprisingly accessible! I know nothing about communication systems but was able to read this pretty easily.
- throwanem 3y ago
  Well, it is meant for Marines. Hence also the wide margins, so that even after perfect binding and trimming there's still plenty of space for making notes with a half-eaten crayon. Joking aside, lots of military manuals share the trait of introducing their subject matter well, in order to ensure that whoever's using the material has at least a reasonably strong basis in the "what" and "why" as well as the "how" - I suppose ideally that's something that'd be covered in training, but also that manual authors don't assume they are writing for an audience that is guaranteed to have been trained. The result is often a supremely useful resource in whatever subject it treats. Frankly, I can think of worse models for the technical documentation we as software engineers produce...
  - jabroni_salad 3y ago
    I started using their template for writing a mini ops manual as a read-only-friday project, and it's still in use and the format became the business's standard for other documentation. feels nice when I see a new hire reading it. Also, I like the cookies. https://quartermaster.army.mil/jccoe/publications/recipes/section_h/H02000.pdf https://quartermaster.army.mil/jccoe/publications/recipes/se...
- sdfgsrget 3y ago
  Military training stuff has to be accessible. Here's Frank Wilczek: WHEN I WAS ABOUT TO BEGIN TEACHING at Princeton, my friend and mentor Sam Treiman called me into his office. He had some wisdom to share. Sam pulled a well-worn paperback manual from his desk and told me, "During World War Il the Navy had to train recruits to set up and operate radio communications in a hurry. Many of those recruits were right off the farm, so bringing them up to speed was a big challenge. With the help of this great book, the Navy succeeded. It's a masterpiece of pedagogy. Especially the first chapter. Take a look." He handed me the book, opened to the first chapter. That chapter was titled "Ohm's Three Laws." I was familiar with one Ohm's law, the famous relation V = IR that connects voltage (V), current (I), and resistance (R) in an electric circuit. That turned out to be Ohm's first law. I was very curious to find out what Ohm's other two laws were. Turning the fragile, yellowed pages, I soon discovered that Ohm's second law is I = V/R. I conjectured that Ohm's third law might be R = V/I, which turned out to be correct.
leroy_masochist 3y ago
Pretty surreal to see this trending on HN! "The NVIS loop: so easy, a marine could do it"
antegamisou 3y ago
For those with some fundamental undergrad math background, check out the gold standard book on antennas Antenna Theory: Analysis and Design, Constantine Balanis https://www.amazon.com/Antenna-Theory-Analysis-Constantine-Balanis/dp/1118642066/ https://www.amazon.com/Antenna-Theory-Analysis-Constantine-B...
teeray 3y ago
The ARRL antenna book is also worth your while: https://www.arrl.org/arrl-antenna-book https://www.arrl.org/arrl-antenna-book
stall84 3y ago
radio is probably one of my all time fav concepts. thanks for this
stall84 3y ago
Say what you will about American militarism... but the branches put out some exceptionally thorough, clear, and practical training documents... in most cases. (Same goes for the FAA from personal experience)
kepler1 3y ago
Two observations, one mundane, one serious. Funny how even in reference textbooks on radio/EM/etc, for some reason people cannot help but use half circles on a plot about a sine function, instead of the proper function shape. Don't know why that is! See Figure 1-2. And more seriously, I think that all of our useful military education texts date from pre-1990s, when the military was one part an able research and development (and educational) and contracting institution. Now today most all of that capability has been outsourced to the military contractors and anyone who could write such a text (or properly design an aircraft on the military side) has long left.
- zrail 3y ago
  At least in terms of aircraft I think you have some camp tinted glasses on. Aircraft, vehicles, ships, and equipment of all sorts has always been developed by private industry. Often in very close partnership with the military, of course. Any manufacturing capacity directly owned by the US Government has always been extremely specialized. Think Los Alamos and Oakridge vs Boeing.
  - kepler1 3y ago
    Of course they were developed largely by industry in putting designs into execution and production, but decades ago, the in-house knowledge to spec and actively participate in the detailed technical/engineering design stages still existed in large measure within the military. Think of most aircraft types up to the 70s -- people in the military (engineers, designers) worked side by side with Grumman, Northrop, etc. in the designs and saying what kind of technical envelope was possible. Actually drawing up designs. In large part (generalization of course, except for some pockets of expertise) the Pentagon is a bunch of project managers. Skilled I'm sure, but the depth of technical knowledge that used to exist has largely moved to the contractors. And this migration/dispersal of responsibility -- which notably caused a loss of single-minded direction in creating aircraft/ships that meet a specific desired need, driven by deep expertise -- has led to the hugely expensive but bland/mediocre/too many chefs in the kitchen projects that you often think of today. Just my opinion of course.
entropicgravity 3y ago
Ya but what's that 'f' (or lamba) term doing in the path loss equation? I'm pretty sure Einstein or Feynman would cast a jaundiced eye at that.
- shotnothing 3y ago
  where?
diracs_stache 3y ago
During my time in the Navy doing radio comms and intelligence, I quickly learned a mastery of Maxwells equations, information theory, Fourier series etc. was much less important than learning and applying practical knowledge from manuals like this. For the folks discussing transmission lines I think the telegraphers equation is a useful tool.
nier 3y ago
Reminds me, the other day I came upon this article that was written about military planes using 5 mile long antennas to communicate with submarines. https://www.thedrive.com/the-war-zone/31477/heres-why-an-e-6b-doomsday-plane-was-flying-tight-circles-off-the-jersey-shore-today https://www.thedrive.com/the-war-zone/31477/heres-why-an-e-6...

310 points by joos3 3y ago

I find these guides a bit disappointing in that they always give the example of a wire loop generating an EM field, which one could sort of imagine given day to day experience with direct current and conductors. However this all breaks down when looking at regular monopole antennas - how can the conductor conduct when one of the ends is just hanging in the air? AC magic I suppose. Similarly they are very short on details on what exactly is going on when the EM field is generated. I guess it consists of photons, but where exactly do they come from and how are they generated using (in some cases) only milliwatts of power?

amelius 3y ago
Circuit theory breaks down when it comes to antennas.
- rusk 3y ago
  Circuit theory breaks down at high frequencies and the appropriate generalisation of Kirchoffs is embodied in Maxwell’s equations.
  - jacquesm 3y ago
    Indeed, it's a bit like Newton vs Einstein. The one is a special case of the other, it doesn't make the former wrong but the latter gives better answers in situations where the special case constraints aren't satisfied.
- H8crilA 3y ago
  Circuit theory is demonstrably wrong. The energy doesn't flow inside of a wire, it flows around the wire. https://en.wikipedia.org/wiki/Poynting_vector https://en.wikipedia.org/wiki/Poynting_vector https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY It is a very useful tool in practica.
  - jacquesm 3y ago
    Energy does flow 'inside' a wire at low frequencies. The skin effect gets higher and higher as the frequency goes up until you reach a point where it is all skin effect. But even that 'skin' isn't idealized it definitely has a thickness, about 30 u at 5 MHz and 6.5 at 100 Mhz. There is a neat little calculator here: https://www.omnicalculator.com/physics/skin-depth https://www.omnicalculator.com/physics/skin-depth
    - H8crilA 3y ago
      No. DC energy also flows around the wire. And I mean around, entirely outside of the conductor, not in the skin or anything like that. In particular it's not the electrons that carry the energy, though they can receive it from the fields and depose inside the conductor (like in a light bulb). Wires, or conductors in general, are so useful because they allow us to manipulate the EM fields and channel the energy very efficiently. It's an very common misconception coming from circuit models. (the movement of electrons, and thus the deposition of the energy via "resistance", may indeed be limited to just the surface of the conductor; this is what that calculator shows)
      - jacquesm 3y ago
        Complete nonsense, the current carrying capacity of a wire is directly proportional to the surface area of the cross section of the wire. If it were the skin it would be proportional to the circumference and it clearly is not. I have no idea where you came into this idea but it is just completely wrong. You can test it for yourself with $50 worth of gear.
        colanderman 3y ago
        @H8crilA is talking about energy, you are describing current. Energy flow is the product of the electric and magnetic fields [1]; electric field within a conductor is zero, therefore energy flow within a conductor is zero. [1] https://en.wikipedia.org/wiki/Poynting_vector https://en.wikipedia.org/wiki/Poynting_vector
- jacquesm 3y ago
  It's at best a leaky abstraction but for some domains it is very useful. Antennas are 'magic' from the point of view of Ohms law and so on but once you move to the electromagnetic domain you will find equivalents for most of the elements in the original abstraction: impedance, reactance and so on.
  - SAI_Peregrinus 3y ago
    Yep, and even common nonlinear elements like diodes and transistors are non-Ohmic.
    - jacquesm 3y ago
      No, they are just non-linear: as the voltage goes up the current goes up in jumps because the resistance changes in a non-linear way, but it is still resistance and you can still express the system at any point of the curve using Ohm's laws and you can still compute the power lost in any part of that system using the simplest equations. A coil or a capacitor (or even a piece of wire, but there the effects are very small) are components that do not follow Ohm's laws, you need to add more parameters than just current, voltage and resistance to work out what's happening. Tricky stuff!
Frummy 3y ago
Photons are created when an electron drops to a lower energy level. Like your voicebox and the sound you make when you speak, it doesnt come from somewhere but is created right then
- eternauta3k 3y ago
  No, the "photons" here are not created by that phenomenon but rather by accelerating electrons. https://en.wikipedia.org/wiki/Larmor_formula https://en.wikipedia.org/wiki/Larmor_formula
  - Frummy 3y ago
    Cool, thanks for letting me know.
max_ 3y ago
Its abit outdated. But it think you may enjoy reading through Oliver Lodge's book, "Signalling Through Space Without Wires" [0] https://catalog.hathitrust.org/Record/001617948 https://catalog.hathitrust.org/Record/001617948
amadsen 3y ago
It doesn't really "generate" a field. The EM field is always there, permeating the universe. An acceleting electron will _disturb_ the EM field (depositing some energy and momentum into it), and this disturbance will propagate through the field at the speed of light (naturally, since at the right energy level such a disturbance is what we call light). At high energies the disturbance will be finely localized in space and behave like a particle, which we call a photon. It's fine to refer to it as such also at lower energies, but slightly misleading because at the very low energies that we talk about here ("radio") it is very spread out in space and behaves more like a wave (with a wavelength of ~meters). In the case of AC, electrons are moving "back and forth" over a short distance (somewhat simplified but useful picture) with the same effect. Think about moving your hand up and down through water - you will create a wave.
- programmer_dude 3y ago
  >The EM field is always there Did you mean to say "the E field is always there"? Loved this explanation otherwise.
  - computerfriend 3y ago
    It's one field, which can be split into E and B components if you like.
    - georgeg23 3y ago
      The B field is just the E field's relativistic effects to an observer not in the same motion frame. https://wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity https://wikipedia.org/wiki/Classical_electromagnetism_and_sp...
  - H8crilA 3y ago
    Both are always there if the circuit is running. Also, both E and M fields are required to transport any power using electricity. This is also true for DC current. See https://en.wikipedia.org/wiki/Poynting_vector https://en.wikipedia.org/wiki/Poynting_vector - the formula multiplies E by B, therefore if any of the two is zero no power (or information) transmission can occur. This is true over the air, as well as over a wire.
jcims 3y ago
>AC magic I suppose. Yep. Think of a long pipe open on one end and capped on the other. If you seal the end with your mouth and blow, it will quickly pressurize. But if you seal a loudspeaker to it and sweep through the frequencies you will find that the loudness of the speaker varies proportionally to how close the tone is to a resonant frequency of the tube. The elections in a monopole antenna have a sort of elastic relationship to one another where the influence propagates at the speed of light rather than the speed of sound. They are also able to move quite freely within the conductor like gas molecules in the tube. So if you have an antenna that’s two meters long and play a ‘tone’ with the equivalent of the loudspeaker at 150 million hz, the tube of electrons resonates. Just like the atmosphere can couple resonant cavities of the same frequency, antennas are coupled by the electromagnetic field to resonate at the same frequency. This provides a certain ‘gain’ of the energy at the end of the ‘tube’ relative to other frequencies that aren’t resonant, which is what feeds our ears or amplifiers with something differentiable from the rest of the noise. Now, imagine you have your ear glued to another tube just like the loudspeaker tube and your other ear is plugged. You’re going to hear the world around you, but it’s mostly going to be at harmonics of that resonant frequency. So when your buddy asks ‘can you hear me?’ a you get this really ringy ‘mwaa mwoo mwee mwee?’. Now imagine he turns on the speaker tube on the other side of the room? It’s going to brightly stand out among all of the other things you hear, ooooooooooooooooo. Ok, now, he hands you a roll of paper and says draw along the roll relative to how loud it is. Up is louder down is softer, and he goes back and starts messing with the volume knob: oooooOOOOOOoooo....oooooOOOoo..ooOoOo The thing you draw on that roll of paper looks like a sound wave. And now you know how AM radio works.
ajsnigrutin 3y ago
Yep, AC is magic. And not just antennas.... look at computers... at 3GHz (DDR4+ ram), the wavelength is ~10cm, so looking at a single sine wave, you have 1.8volts at the cpu, zero volts 2.5cm away from the cpu, -1.8v 5cm away, zero 7.5cm, and again 1.8 volts 10cm from the cpu... now look at the distance between the cpu and ram, the other frequencies that come with the square(-ish) waves, and all the math to make a basic RAM read/write work. Just a normal wire (or cable - the simplest component in DC electronic circuits, just a line, that does nothing) changes everything... you send a (voltage) signal in, and the current has to flow at some rate, even before the "signal" (field) reaches the other side of the cable, to "see" if the other end is "open" or soldered together or if some sort of a resistor is soldered there.
- jacquesm 3y ago
  That's not how current propagates. Current propagates - broken analogy warning - in the same way that marbles would propagate through a tube. You push a marble in on one end and assuming the tube is full another marble pops out the far end. So the flow rate of the electrons is vastly lower than the flow rate of the signal itself!
  - ajsnigrutin 3y ago
    Of course, but currents here are a result of a field that moves those electrons one way or another, and electrons move, before the field reaches the other end (where a resistor is.. or is not.. or a short circuit is), so in your analogy, marbles move down the tube, before they "realize" that the tube is closed on the other end.
    - jacquesm 3y ago
      Fair enough. It's an analogy anyway and it is a broken one in more ways than one (for instance: it doesn't deal with propagation of pulses very well because these can be reflected from the end of the tube, you'd have to model it as a tube full of marbles and springs to account for that) but it serves for some discussion purposes.
NovemberWhiskey 3y ago
>regular monopole antennas There is no real monopole antenna - there's always another "half", like a ground plane, a counterpoise system or even just the body of the person holding a handheld radio.
myth2018 3y ago
> how can the conductor conduct when one of the ends is just hanging in the air? AC magic I suppose I struggled with these matters too. I wouldn't call it "AC magic", but "RF magic". Notice that you need a different mental model to understand those circuits. In DC and low-freq AC, it's enough to think that voltages and currents manifest themselves instantaneously in a conductor. But that is a (useful) simplification. Roughly speaking, the energy supplied by the power source takes time to propagate over the wire [0]. Having that in mind, take a look at the animated gif in this page: https://en.wikipedia.org/wiki/Dipole_antenna https://en.wikipedia.org/wiki/Dipole_antenna. I believe it's gonna make more sense now. This mental model also helps to understand why a magnetic loop isn't simply a piece of short-circuited wire -- for a DC or low freq AC circuit, it is indeed a short-circuit; for an RF circuit, it is not. [1] [0] https://en.wikipedia.org/wiki/Velocity_factor https://en.wikipedia.org/wiki/Velocity_factor [1] this mental model is not enough to fully understand any antenna. Many designs also depend on other phenomena, like the electromagnetic interactions between its components, and interactions with its surroundings. But I think that, equipped with this mental model, you'll be able to research further in case you feel interested.
eternauta3k 3y ago
> how can the conductor conduct when one of the ends is just hanging in the air? At frequencies where the wire radiates, you can't model a wire as an open resistor. You need to model it as a bunch of Rs, Ls and Cs, and there you will see the current flow.
jerry1979 3y ago
Quick answer: The antenna is like a tube, and the energy stuff is like liquid that sloshes back and forth in the tube generating a kind of wiggle. This Canadian military video should help: https://www.youtube.com/watch?v=-F7KYLO4Bkg https://www.youtube.com/watch?v=-F7KYLO4Bkg

squarefoot 3y ago

Also worth mentioning, the US Navy Electricity and Electronics Training Series (NEETS), plus other interesting documentation one can find from the top menu here. https://maritime.org/doc/#neets https://maritime.org/doc/#neets

andromaton 3y ago
NEETS is a treasure trove of practically.

8bitsrule 3y ago

I liked to play with longwires (sect. 4-22) back in the day ... the longer, the more directional they get. Fun to experiment with (back when copperweld was cheap). But when I did eventually wind up living on a farmstead for a couple of years, I didn't have the time (or the nearby trees) to try out really really long wires.

jacquesm 3y ago
You can try barbed wire fences! On a dry day they can work very well, obviously you won't have much say in their direction but if you're near a fence it can't hurt to hook up a sensitive receiver to see what you get.

aaron695 3y ago

There is a MCRP 8-10B.11 (Formerly MCRP 3-40.3C) (2016) but running a diff the text is exactly the same except the first title pages - https://w5sc.org/wp-content/uploads/2020/06/MCRP-8-10B.11.pdf https://w5sc.org/wp-content/uploads/2020/06/MCRP-8-10B.11.pd... It's hard to believe nothing has changed in 20 years, we've move on a lot passed 1999 aka pre-internet society.

ianburrell 3y ago
The basics or antenna design have been known for a long time. Any advances have been in the small antennas used in devices, not the large antennas strung up by Marines.
- jacquesm 3y ago
  That's not really true though, phased arrays are a fairly recent development and now that we have the computational power to use them more effectively are being used in many more practical applications.

nunez 3y ago

This is really well written and surprisingly accessible! I know nothing about communication systems but was able to read this pretty easily.

throwanem 3y ago
Well, it is meant for Marines. Hence also the wide margins, so that even after perfect binding and trimming there's still plenty of space for making notes with a half-eaten crayon. Joking aside, lots of military manuals share the trait of introducing their subject matter well, in order to ensure that whoever's using the material has at least a reasonably strong basis in the "what" and "why" as well as the "how" - I suppose ideally that's something that'd be covered in training, but also that manual authors don't assume they are writing for an audience that is guaranteed to have been trained. The result is often a supremely useful resource in whatever subject it treats. Frankly, I can think of worse models for the technical documentation we as software engineers produce...
- jabroni_salad 3y ago
  I started using their template for writing a mini ops manual as a read-only-friday project, and it's still in use and the format became the business's standard for other documentation. feels nice when I see a new hire reading it. Also, I like the cookies. https://quartermaster.army.mil/jccoe/publications/recipes/section_h/H02000.pdf https://quartermaster.army.mil/jccoe/publications/recipes/se...
sdfgsrget 3y ago
Military training stuff has to be accessible. Here's Frank Wilczek: WHEN I WAS ABOUT TO BEGIN TEACHING at Princeton, my friend and mentor Sam Treiman called me into his office. He had some wisdom to share. Sam pulled a well-worn paperback manual from his desk and told me, "During World War Il the Navy had to train recruits to set up and operate radio communications in a hurry. Many of those recruits were right off the farm, so bringing them up to speed was a big challenge. With the help of this great book, the Navy succeeded. It's a masterpiece of pedagogy. Especially the first chapter. Take a look." He handed me the book, opened to the first chapter. That chapter was titled "Ohm's Three Laws." I was familiar with one Ohm's law, the famous relation V = IR that connects voltage (V), current (I), and resistance (R) in an electric circuit. That turned out to be Ohm's first law. I was very curious to find out what Ohm's other two laws were. Turning the fragile, yellowed pages, I soon discovered that Ohm's second law is I = V/R. I conjectured that Ohm's third law might be R = V/I, which turned out to be correct.

leroy_masochist 3y ago

Pretty surreal to see this trending on HN! "The NVIS loop: so easy, a marine could do it"

antegamisou 3y ago

For those with some fundamental undergrad math background, check out the gold standard book on antennas Antenna Theory: Analysis and Design, Constantine Balanis https://www.amazon.com/Antenna-Theory-Analysis-Constantine-Balanis/dp/1118642066/ https://www.amazon.com/Antenna-Theory-Analysis-Constantine-B...

teeray 3y ago

The ARRL antenna book is also worth your while: https://www.arrl.org/arrl-antenna-book https://www.arrl.org/arrl-antenna-book

stall84 3y ago

radio is probably one of my all time fav concepts. thanks for this

stall84 3y ago

Say what you will about American militarism... but the branches put out some exceptionally thorough, clear, and practical training documents... in most cases. (Same goes for the FAA from personal experience)

kepler1 3y ago

Two observations, one mundane, one serious. Funny how even in reference textbooks on radio/EM/etc, for some reason people cannot help but use half circles on a plot about a sine function, instead of the proper function shape. Don't know why that is! See Figure 1-2. And more seriously, I think that all of our useful military education texts date from pre-1990s, when the military was one part an able research and development (and educational) and contracting institution. Now today most all of that capability has been outsourced to the military contractors and anyone who could write such a text (or properly design an aircraft on the military side) has long left.

zrail 3y ago
At least in terms of aircraft I think you have some camp tinted glasses on. Aircraft, vehicles, ships, and equipment of all sorts has always been developed by private industry. Often in very close partnership with the military, of course. Any manufacturing capacity directly owned by the US Government has always been extremely specialized. Think Los Alamos and Oakridge vs Boeing.
- kepler1 3y ago
  Of course they were developed largely by industry in putting designs into execution and production, but decades ago, the in-house knowledge to spec and actively participate in the detailed technical/engineering design stages still existed in large measure within the military. Think of most aircraft types up to the 70s -- people in the military (engineers, designers) worked side by side with Grumman, Northrop, etc. in the designs and saying what kind of technical envelope was possible. Actually drawing up designs. In large part (generalization of course, except for some pockets of expertise) the Pentagon is a bunch of project managers. Skilled I'm sure, but the depth of technical knowledge that used to exist has largely moved to the contractors. And this migration/dispersal of responsibility -- which notably caused a loss of single-minded direction in creating aircraft/ships that meet a specific desired need, driven by deep expertise -- has led to the hugely expensive but bland/mediocre/too many chefs in the kitchen projects that you often think of today. Just my opinion of course.

entropicgravity 3y ago

Ya but what's that 'f' (or lamba) term doing in the path loss equation? I'm pretty sure Einstein or Feynman would cast a jaundiced eye at that.

shotnothing 3y ago
where?

diracs_stache 3y ago

During my time in the Navy doing radio comms and intelligence, I quickly learned a mastery of Maxwells equations, information theory, Fourier series etc. was much less important than learning and applying practical knowledge from manuals like this. For the folks discussing transmission lines I think the telegraphers equation is a useful tool.

nier 3y ago

Reminds me, the other day I came upon this article that was written about military planes using 5 mile long antennas to communicate with submarines. https://www.thedrive.com/the-war-zone/31477/heres-why-an-e-6b-doomsday-plane-was-flying-tight-circles-off-the-jersey-shore-today https://www.thedrive.com/the-war-zone/31477/heres-why-an-e-6...