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

https://www.marines.mil/Portals/1/MCRP%203-40.3C%20With%20Erratum%20z.pdf

by joos3 • 3 years ago

310 87 3 years 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?

Frummy2• 3 years 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

eternauta3k2• 3 years ago

No, the "photons" here are not created by that phenomenon but rather by accelerating electrons. https://en.wikipedia.org/wiki/Larmor_formula

max_• 3 years 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Its abit outdated.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

amadsen• 3 years 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• 3 years ago

The EM field is always there

Did you mean to say "the E field is always there"?

Loved this explanation otherwise.

rusk3• 3 years ago

The EM field is always there

Hmm sounds dangerously close to aether theory. I thought we had moved past this … that EM waves exist in their own right without need for a medium. Unlike sound say.

jcims• 3 years 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.

amelius• 3 years ago

Circuit theory breaks down when it comes to antennas.

rusk3• 3 years ago

Circuit theory breaks down at high frequencies and the appropriate generalisation of Kirchoffs is embodied in Maxwell’s equations.

H8crilA6• 3 years 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://www.youtube.com/watch?v=bHIhgxav9LY

It is a very useful tool in practica.

jacquesm18• 3 years 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.

ajsnigrutin2• 3 years 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.

jacquesm18• 3 years 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!

NovemberWhiskey• 3 years 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• 3 years 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. 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

[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.

eternauta3k2• 3 years 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• 3 years 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

squarefoot• 3 years 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

andromaton• 3 years ago

NEETS is a treasure trove of practically.

8bitsrule• 3 years 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.

jacquesm18• 3 years 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• 3 years 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.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• 3 years 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.

jacquesm18• 3 years 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.

nunez2• 3 years ago

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

throwanem• 3 years 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• 3 years 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/se...

sdfgsrget• 3 years 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.

ethbr02• 3 years ago

I desperately wish there was more awareness of military writing styles in the civilian world.

- They're organizations that have existed for as long as their countries.

- They're incredibly large.

- They have to communicate critical information to a very diverse set of readers.

- They iteratively update their approaches, over decades, often in direct reaction to the most stringent real-world tests.

... how could we not learn something from the systems they're currently using?

My favorite nuggets are (1) the glory of BLUF (bottom-line, up front; or an executive summary of everything to follow), (2) including a formal intent preface to any document (to guide writers and readers in what to include/exclude), (3) thoroughly defining terms (to avoid "we're using the same word with different meanings" problem), and (4) rigorously structuring information into separate sections.

Which isn't to say militaries get everything right all of the time. They do tons of stupid things. But dismissing or ignoring them as a source to cherry-pick best practices is shortsighted.

p_j_w2• 3 years ago

Antennas and communications systems are two very different disciplines, by the way. The two ARE linked together in a single larger system quite often, so you’ll have people who have got their hands in both areas, but aside from the baseline knowledge of calculus involved, having ability in one area won’t make it easier to understand the other.

leroy_masochist• 3 years ago

Pretty surreal to see this trending on HN!

"The NVIS loop: so easy, a marine could do it"

antegamisou• 3 years 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-B...

teeray• 3 years ago

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

stall842• 3 years ago

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

stall842• 3 years 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)

kepler12• 3 years 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• 3 years 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.

kepler12• 3 years 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• 3 years 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• 3 years ago

where?

diracs_stache• 3 years 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• 3 years 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-6...

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

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