
Antenna Theory (2016) - dosshell
http://www.antenna-theory.com/
======
dazhbog
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..

~~~
etaioinshrdlu
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
What’s the point of applying deep learning to something for which we have an
exact mathematical theory?

~~~
badpun
I don't know about this specific case, but often the exact models are so
complex that you can't optimize them analytically, so you resort to a global
optimization best effort, where the results are not guaranteed to be any good
- so you might as well try something DL-based too, to see how it would
perform.

~~~
Gibbon1
Classic traveling salesman problem cast into a large number of analytically
incompatible and often hard to quantify dimensions.

------
amingilani
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-
eq...](https://ham.stackexchange.com/questions/12996/what-is-the-equation-for-
the-arm-lengths-of-a-half-wavelength-dipole-antenna)

~~~
dbcurtis
>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
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
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-B...](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)
but you'll need multivariable calculus to get through it.

~~~
Junk_Collector
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.

~~~
amingilani
True. I already own a Baofeng, and an SDR, but for the rest I'm honestly
thinking of using my Arduino in conjunction with some diy circuitry (if I can
wrap my head around enough of it) to build an oscilloscope, and I found an
article that used a noise generator in conjunction with an SDR as an SWR meter
[0]

It's definitely not high end, but my wife would kill me if I she found out how
much I'd have had spent a halfway decent SWR meter, if I went that route.

[0]: [https://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-
ch...](https://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-
characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/)

~~~
Junk_Collector
Not a bad idea but a few things to keep in mind. \- The sample rate on an
Arduino is pretty low, depending on which model you have it may only be a few
MHz and the absolute best you can theoretically possibly achieve with your
home o-scope bandwidth is half the Arduino clock frequency. You'll also have
to cast off the Arduino language and start moving to C/assembly to get the
full performance out of it. -The dynamic range on the Arduino ADCs is pretty
low. It's going to be limited by the number of bits in the Arduino ADC. -The
input on an Arduino ADC is going to be either very low or very high impedance.
This is also common on lower frequency scopes (low frequency in the case being
1GHz or less bandwidth) but if you try driving your input directly the
impedance mismatch from the RF is going to cause poor signal and lots of noise
on your measurement.

I don't know what SDR you have but it might be suitable for the signal capture
and you can probably gin up some filters in post processing to approximate a
Spectrum analyzer. If you have a source and 2 directional bridge couplers you
could use it to make a poor mans Scalar Network Analyzer. A noise generator or
a swept tone (Chirp) can be used in both these cases. The article you linked
basically did a 1-port Network analyzer using the bridge coupler and used to
to measure VSWR.

A lot of modern oscilloscopes, even hobby ones, now include the ability to
capture scope traces to a PC and an FFT math function. You're going to be
limited in frequency without a downconverter but it's a good way to go on a
budget when working on a bench. The Rigol's are a lot better than they were
when they first came out and make a good hobby scope for a reasonable price.
If you can be spendy, the Keysight hobby level scopes are a joy to use. Best
bet on a budget is to troll the internet for an old Tektronix, Lecroy, or
HP/Agilent/Keysight but you'll probably not get something with fast trace
capture or a built in FFT.

All of that said, making test equipment out of your Arduino is a great hobby
project that will teach you tons of useful engineering. I highly recommend it!

------
tyingq
_" 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.

------
ChuckMcM
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
> 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
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.

~~~
dbcurtis
> The range consists of basically a robot arm that can hold the antenna under
> test (AUT) in any orientation,

Which works better at 2.4GHz than at 2.4MHz :)

~~~
ChuckMcM
Absolutely, that LF and even the HF stuff needs a really really big range :-)

------
robochat
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
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
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.

------
qndreoi
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)

------
jimnotgym
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
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
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
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!

------
obviuosly
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
It would be a waveguide antenna. He shows a slotted one on the page you're
talking about.

------
sunstone
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. :)

