
How Antennas Work - codesuki
http://www.antenna-theory.com/m/index.php
======
dbcurtis
How antennas work, very high level:

1\. Accelerate an electron, get a photon. A good transmitting antenna is
something that is an efficient structure for accelerating electrons.

2\. Antennas are reciprocal. They receive as well as they transmit.

3\. Resonant structures are often used because you can keep more electrons
accelerating with less energy -- the damp finger on the rim of a wine glass
effect.

4\. Power can be directed by appropriately phasing the radiating sub-
structures to create constructive and destructive interference in the radiated
energy.

The rest is modeled simply with a set of simultaneous three dimensional
second-order partial differential equations.

~~~
Enginerrrd
>2\. Antennas are reciprocal. They receive as well as they transmit.

While true, this could be really misleading in practical application. A really
good transmitting antenna doesn't always (or even usually) make a really good
receiving antenna.

~~~
dbcurtis
In terms of the properties of the antenna itself, it is reciprocal -- the "How
Antennas Work" part. You comment is more along the lines of "How to Use
Antennas", that is a different question.

The places where there is a benefit to having different antennas for receive
and transmit depends on the characteristics of the channel and on the
application. The classic example being where a receiving antenna that reduces
reception of local noise can give a better signal-to-noise ratio than an
antenna that has been optimized for the best transmitted signal footprint at
the location of the other station.

So, while I agree that in practice there are plenty of times where separate
receive and transmit antennas have a benefit, it isn't because of antenna
physics.

~~~
wglb
I think this is incorrect. See my comment at
[https://news.ycombinator.com/item?id=22790001](https://news.ycombinator.com/item?id=22790001)

~~~
dbcurtis
As far as I can tell, you are agreeing with me. It is the _channel_ that is
not reciprocal, which is why there is a benefit to separate antennas.

Do not confuse the analysis of an antenna in free space with real-world
deployment with real ground, noise sources of various kinds, and asymmetrical
propagation.

------
spapas82
One insight that may help non EE educated people understand what an antenna
does: The simplest form of an antenna would be just a simple point emmiting
electromagnetic waves. These waves would be transmitted all around that point
distributed as sphere. All points of a sphere will have the same power. Now,
an antenna has a different geometry than a point that helps somehow "focus"
the EM waves, so their power is not distributed around the sphere uniformally
but some directions get more power depending on the antenna design.

Now the thing to keep in mind is that an antenna is a passive device. It does
amplify the signal but it does not add any power to it, it just collects the
power to specific points. This may be easier to understand with a receiving
antenna (which collects the signal).

For example, consider the satellite dish which is of course an antenna. Due to
its design it should be conceptually easy to understand that the power of the
transmitted field is all gathered in a very small area in the front of the
dish. The largest the dish, the smaller the point where all the transmitted
power is pointed, so less power would be needed to cover largest distances
(and more difficult finding where the dish needs to point).

~~~
rambojazz
Great explanation! Maybe I'm asking a dumb question but I'll shoot anyway:
what is power? I mean, when I change the power of a signal what physical
variable am I playing with? Amplitude, frequency, or wavelength are intuitive
to visualize, but power?

~~~
spapas82
Power is how "strong" a signal is. I think it's the easiest one to understand,
that's why everybody's talking about dbm!

In all electric circuits to change the power a component will consume you
either change the voltage you apply to it or change its resistance, based on
the P=VI & R=V/I equations.

~~~
rambojazz
I really cannot get my head around it. What is "strong" in a signal? What
physical variable is stronger? I assume "strong" or "more power" means that
the signal attenuates less over time/distance, but what is causing this lesser
attenuation?

~~~
spapas82
No, the attenuation is exactly the same no matter the power and is analogous
to the square of the distance. So a signal that has power X in a point 10
meters from the source it will have X/100 20 meters from the source.

The analogy of the waves that another comment mentioned is a nice one (since
we talk about EM waves).

Another thing that may help you understand that more power means that the
electrical charge to the protons/electrons in the air where the EM field is
transmitted will also be more.

~~~
rambojazz
Thank you.

------
supernova87a
One thing I have always struggled to understand about antennas --

Of course I know that an antenna is most efficient at radiating power when its
length is some fraction/multiple of the emitted wavelength. But I cannot for
the life of me intuit how the electrons are being excited and behaving.

If I use the bathtub analogy of sloshing water, it cannot be (I believe) that
the electrons are sloshing in bulk up and down the antenna and "accumulating"
at one end at the speed of light.

On the other hand, if each electron along the length of the antenna is
oscillating in its own relatively stable position, what then does the length
matter to the electron at one end versus the other?

Or should I understand it as, energy is being transmitted out of the antenna,
like it is in a flute being played, and the electrons are most constructively-
interference being reinforced to resonate at the frequency desired (by
standing waves in the conductor) if the length of the material matches the
wavelength?

This has always been hard to visualize.

~~~
userbinator
Page 296 of this book has a good illustration of the field changes around a
transmitting antenna:
[https://archive.org/details/principlesunderl00unit](https://archive.org/details/principlesunderl00unit)

(It's almost 100 years old but the physics have not changed. If anything,
these early books often have far more detailed explanations because they
assumed far less about the reader.)

~~~
pfundstein
Direct link:
[https://archive.org/details/principlesunderl00unit/page/296](https://archive.org/details/principlesunderl00unit/page/296)

------
sizzzzlerz
If you're more interested in a practical approach to designing and building
antennas for radios, find a recent copy of the ARRL Handbook, the source of
information about ham radio. In it, you'll find some theory, a little math,
but mostly, how to build and deploy a wide variety of antennae to cover
frequency ranges from 2 MHz to 10 GHz. They also offer a stand-alone book on
nothing but antennas with more details and examples.

~~~
h2odragon
> and deploy

Thats one of the things that made the ARRL Antenna handbook so valuable to me;
they have some solid advice on weatherproofing and dealing with things like
wind and snow loads.

Its easy to make an antenna; its just a bent length of wire at the end of the
day. Making it be the _same_ length of bent wire today, tomorrow and beyond,
when its mounted outside and /or in harsh conditions, thats difficult.

------
205guy
This is a neat website Y an antenna engineer with lots cool info, but the big
picture is buried. From that page, click on “antenna basics” then scroll all
the way to the bottom where you can find “why do antennas radiate?”

[http://www.antenna-
theory.com/basics/whyantennasradiate.php](http://www.antenna-
theory.com/basics/whyantennasradiate.php)

That answers the fundamental question of how antennas work.

Edit: looks like kawfey’s comment answered the issue already, I didn’t scroll
down far enough:
[https://news.ycombinator.com/item?id=22787249](https://news.ycombinator.com/item?id=22787249)

------
degski
The only thing not on the web-site is how antennas work, even in theory.

~~~
kawfey
As an antenna engineer, I've referred to this site hundreds of times. To be
fair, it's almost an eli5 version of how antennas work, but it's been a good
reference for basics.

[http://www.antenna-
theory.com/basics/whyantennasradiate.php](http://www.antenna-
theory.com/basics/whyantennasradiate.php) is as deep as it gets.

For DEEP antenna theory, I can't recommend the Balanis book high enough.
[https://www.amazon.com/Antenna-Theory-Analysis-
Constantine-B...](https://www.amazon.com/Antenna-Theory-Analysis-Constantine-
Balanis/dp/1118642066)

------
thelazydogsback
I think the biggest take-away, that especially Hollywood needs to take note
of, is what everyone gets wrong -- you don't point a whip antenna _at_
something to get the highest gain, you want to be at a right angle to the
source. Remote controls often have the antenna oriented the wrong way for
distance/gain, and I've seen people orient wifi router antennas to "point" to
usage areas, etc.

~~~
katmannthree
If by remote control you mean TV remotes, those usually work via an IR LED.
The radiation pattern is pretty much what you'd get with visible light, i.e.
the highest intensity zone is directly in front of the LED.

~~~
chrisseaton
> If by remote control you mean TV remotes...

They don’t mean that.

~~~
katmannthree
Do you know what they meant? Absent any context I don't think ``remote
control'' has another well known device. I'm assuming that the person I
responded to doesn't have a background in this stuff as this is was their
first introduction to antenna radiation patterns.

~~~
chrisseaton
Not sure if you’re joking or trolling, but for example ‘remote control car’,
‘remote control helicopter’, ‘remote control boat’, ‘remote control bomb
disposal robot.’ The handset you use to control them is itself called the
‘remote control.’ That's what they mean.

~~~
katmannthree
No, I'm serious. Thank you for explaining.

I've heard people talk about tv remotes as ``remote controls'' several orders
of magnitude more times than RC gear.

I've never seen someone have to reorient their RC transmitter to get a better
signal. Modern RC airplane/car/helicopter/boat transmitters have multiple
fixed antennas (as do the receivers) and have for many years (since people
switched from 50MHz radios to 2.4GHz spread spectrum radios). The range is
long enough that it you'll generally lose sight of your vehicle before you
lose the radio connection.

------
meonkeys
Brace yourself for an onslaught of garish advertisements. If the content is
truly exhaustive, a book would be a far better presentation format. I didn't
survive long enough to find out.

~~~
iamhamm
Did we go to the same site? I didn’t get ads and the material I’ve been
clicking through is pretty good.

~~~
madengr
You mean the gal in the bikini right above the diagram of a dipole?

~~~
iamhamm
Wow! My blocking is doing great! :-)

------
peter_d_sherman
Excerpt:

"Specifically, consider this statement: _Complexity is not a sign of
intelligence; simplify. I have found this to a priceless amount of wisdom._ "

------
amai
Not complete without atomic antennas:

[https://www.technologyreview.com/s/611977/get-ready-for-
atom...](https://www.technologyreview.com/s/611977/get-ready-for-atomic-
radio/)

------
dang
A thread from 11 months ago:
[https://news.ycombinator.com/item?id=19708982](https://news.ycombinator.com/item?id=19708982)

This topic is uncommon enough that we won't call this a dupe (this came up
yesterday:
[https://news.ycombinator.com/item?id=22781498](https://news.ycombinator.com/item?id=22781498)).

------
sunstone
It's mildly irritating that the concepts of explanations like this (and almost
all others) are based on a false understanding, through the path loss equation
and the law of reciprocity, of the underlying physics of antennas.

While this kind of approach allows for the proper engineering of antenna
systems it is at least 50% wrong regarding the underlying physics.

------
brudgers
some past comments,
[https://news.ycombinator.com/item?id=19708982](https://news.ycombinator.com/item?id=19708982)

------
amelius
Can somebody explain why I can send high definition video over WiFi, but not
over an average quality USB 3 cable that is extended to a 6m length?

~~~
madengr
In a nutshell:

If you have to guide an EM wave without dispersion (e.g. TEM propagation), it
takes at least two conductors; those have loss. The smaller they get (think a
thin coax) the more lossy they become.

If you need a wide bandwidth, you need a smaller conductor arrangement to keep
it from “over-moding” (becoming non TEM). Once it’s non-TEM, you get
dispersion and corrupt your signal.

So there is a fundamental trade-off of bandwidth versus loss. Free space
propagation is always TEM, so plenty of bandwidth, but now you must direct it
with antennas as opposed to guiding it with conductors.

You can get 110 GHz of bandwidth on a 1 mm coax, but it is very lossy, so much
so that the microwave industry/research is looking into non-contact wafer
probing for mmWave and THz applications.

Fiber has similar issues. It’s extremely low-loss, and non-TEM, but that
dispersion is small enough you can multiplex in multiple channels without much
dispersion across a single channel. Over long runs, still the dispersion is
large enough that it needs to be compensated with various tricks.

A 10G Ethernet copper cable is at most 3 meters, and hard-wired to the SFP
modules. But you can buy a 10G mmWave radio and get 10 km. Now several km of
that copper cable would be hundreds dB loss.

~~~
MegaDeKay
Fiber guys are lucky that they can cheat with things like Raman amplification
that us poor RF souls have no real equivalent for.

[https://en.wikipedia.org/wiki/Raman_amplification](https://en.wikipedia.org/wiki/Raman_amplification)

~~~
madengr
I suppose parametric amplifiers would be similar in the RF realm. At least we
have distributed amplifiers, which I suppose could be an amplifying medium if
you could make them fine enough.

------
Causality1
The full-justification formatting makes for some badly mutilated paragraphs
when trying to read that site in portrait orientation.

------
humbfool2
Recommended reading - Antenna Theory Analysis and Design, by Constantine
Balanis

------
neves
wow, I thought I would get some nice tips to position my wifi routers and
repeaters antennas, but it too much information. Useful for who wants to get
deep knowlegde, but I think I still need an easier guidance for my routers.

~~~
kawfey
To be fair, it's antenna theory, not propagation theory. That by itself is
another realm of electrophysics.

[https://en.wikipedia.org/wiki/Radio_propagation](https://en.wikipedia.org/wiki/Radio_propagation)

There are some ray-tracing and wave theory solvers out there that will readily
accept a 2D floor plan, but 3D full wave solutions are intensely complicated
computational problems that probably won't even resolve to real-life just
because of the complexities of the real world. The biggest problem for AP
(router) placement is multipath interference, which either creates a null or
node at any given location, compromising of basically infinite paths between
the AP, bouncing off of walls, furniture, people, animals, and everything,

The network engineer's rule of thumb is to ignore multipathing completely, and
start at 0dB a foot from the router, and subtract 10dB for passing through
drywall, 15dB for brick walls, 8dB for glass, and 6dB for every doubling of
distance between the wireless AP and the desired client locations. As long as
you stay above around -20- -30dB, you should have good signal. I've come up
with these numbers in my own experience as an RF engineer. Typical software
usually uses n-bounce ray tracing to determine deadzones or optimum
placements, but that stuff is expensive and only as accurate as the 2D or 3D
model of your space.

An even less intense rule of thumb is place as close to common client
locations then move them by trial and error until it works best. I place a
single AP on the ground in the center of my home, and another mesh node for my
back yard, and my whole house is covered. (I use ubiquiti unifi gear, which
are far more powerful than a router/AP-in-one, and it gives you a lot of
insight on how well the clients are connected, interference, and other useful
data).

This is also why mesh wireless is getting popular, so you can dot mesh APs
around the home in just about every room, and ignore the whole problem.

------
Avamander
Had to turn the zoom to 66% to make it readable, yikes.

------
syphilis2
Is there an index of all the pages on this site?

