2.4GHz antenna design - Page 1
Forum section: electronics / projects

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 07, 2011, 04:13:49 am
content: I'm designing a high gain (~18dBi or so) 2.4GHz antenna for use with 802.11 hardware. I decided on a Yagi design as I wanted something that takes up little space in a bag. Instead of wire elements extending out of a stick like a TV antenna, I'm thinking of taking a wooden (or plastic?) board, cutting grooves in it, and pressing copper wire into the grooves. Are the calculations different if I were to embed the elements in wood? Dry wood is an insulator but its dielectric constant is much different from that of air.As of current, my 25 element design is going to be on a board that is 3' long and 3" wide. I'm thinking of cutting it in half and adding a hinge so it folds to a more reasonable 1.5' length for carrying. Am I correct that I'll need a hinge or other mechanism made of a nonconductive material to avoid affecting antenna performance?I'm going to use homemade Litz wire to increase the efficiency, so they'll be even more fragile than solid wire.

author: kaptain_zero
author_rank: Regular Contributor
timestamp: January 07, 2011, 11:19:09 pm
content: Argh... My antenna theory is rusty at best, but do you really need 18db gain? The reason I ask is that a collapsible corner reflector antenna would meet the compact, folding and easy to make feature, but only about 12db gain to the front and 20db rejection to the rear.The most efficient design when you get up in the 2.4ghz and up range is a small parabolic dish..... Easy to get lots of gain with not much material and fairly compact.Your wooden/plastic idea could be made to work, but at those frequencies... relative humidity would become an issue and might create tuning headaches.I'd let my fingers do the walking in google, there's plenty of Ham Radio references on antenna design in the 2.4ghz range and up.... Phase 3D (a large elliptical orbit satellite built by hams) had a 2.4ghz down link and decent gain was important to hear that satellite so there was lots of home brew antennas cobbled together for that. I dropped out of the amateur satellite arena about 10 years ago.... so I'm not up on the latest. Amsat.org might have some useful information as well.RegardsChristian

author: Psi
author_rank: Super Contributor
timestamp: January 07, 2011, 11:33:15 pm
content: Most people tend to avoid RF design at frequencies that high, you can do the math and build the antenna/circuit but without very expensive test equipment it's hard to tune and fix issues. It requires a ridiculous amount of trial and error. You might be better off finding a forum that deals entirely in RF design to get a good answer to that question.There's probably only a small handful of people on here with excellent RF knowledge at frequencies that high.

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2011, 10:05:41 am
content: The easiest way to design a 2.4GHz antenna is to etch it on a PCB.

author: kaptain_zero
author_rank: Regular Contributor
timestamp: January 08, 2011, 10:52:26 pm
content: Did some slight poking around on the net, found this site......http://www.wlan.org.uk/diy-ant.htmlPlenty of ideas and information on this topic....RegardsChristian

author: sonicj
author_rank: Frequent Contributor
timestamp: January 09, 2011, 12:34:01 pm
content: i might suggest checking out the Moxon.... high gain, easy to construct, 50 ohms, tiny!

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 09, 2011, 05:03:18 pm
content: Quote from: sonicj on January 09, 2011, 12:34:01 pmThat's not enough gain for my current application, but I'll keep it in mind for some other projects.

author: sonicj
author_rank: Frequent Contributor
timestamp: January 09, 2011, 11:58:22 pm
content: like psi mentioned above, 2.4ghz is very difficult to get right without proper tuning gear! a 18db yagi at 2.4 w/o tuning gear... well... i personally wouldn't try it and I have 2.4 measurement equipment.i like hero's suggestion of printing & etching the antenna! i know the moxon doesn't give you the best numbers on paper, but it's design allows for much tighter tolerances for diy, no balun required, sma/n/whatever to be soldered directly to the board.heres a 2.4 yagi for $12usd if you have enough time for shipping from china.linktough to build one cheaper than that!patch antennas are portable and pretty easy to fabricate.16db printable patch antennalinkthe "cantenna" is another easy build with potential for high gain.my 2¢ fwiw,-sj

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 06, 2011, 10:20:55 am
content: circuit antenna...from...http://www.lelong.com.my/broadband-external-antenna-cable-connector-76380369-2011-03-Sale-P.htmmy reference for yagi...http://www.skyscan.ca/Antennas.htmGeoffS fromhttps://www.eevblog.com/forum/index.php?topic=2703.msg36602#msg36602suggesting for cabling...http://www.tigereye.net.au/vk4ion/nextg_yagi.html...i just thought i remember this threadsorry for ugly post.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: March 06, 2011, 05:26:18 pm
content: All those replicated ideas from the internet does not help any one.Its an true waste of time.Nothing compares next to an true 3Com Wifi bridge , with connector for external antenna , plus an high gain external antenna.In a cost of 250$ .Now there is some restrictions , about the total gain, but if you live in the countryside , you can bend them with out damaging any one.If some one is still in denial to get the proper equipment, he will never see an 24/24 reliable connection .

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 06, 2011, 06:14:23 pm
content: Quote from: Kiriakos-GR on March 06, 2011, 05:26:18 pm1) its science Kiri!2) $250? wow, any schematic?3) i dont need 24/24, just around 5/7 max will be ok.peace

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: March 06, 2011, 11:28:10 pm
content: Open one underground tunnel , and drop in it an UTP cable ..This will save you from all the troubles and the dog.

author: scrat
author_rank: Frequent Contributor
timestamp: March 07, 2011, 02:20:27 pm
content: Search for "can antenna" on google. An oil can or even a Pringles tube and the coax cable can be used for 2.4GHz, even if square section antennas usually work better. This kind of antennas have high directivity, someone used them for "stealing" the wi-fi connection.The simplest implementation is at 75Ohm, which could be an issue.I could provide you the math from some notes of the antennas course I attended at university.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 07, 2011, 02:33:26 pm
content: Quote from: scrat on March 07, 2011, 02:20:27 pmi'll be glad if i have that! pm or here. thanx.

author: scrat
author_rank: Frequent Contributor
timestamp: March 07, 2011, 09:35:27 pm
content: Here it is the simple calculation.The problem is that of the propagation into a waveguide, so wavelength is not the same as for a wave traveling in the open-space, and depends on the mode involved. Each mode (field conformation) has its wavelength (and propagation speed, of course). Then the ideal situation is when there is only one mode propagating, so above a certain minimum frequency and below a certain other, which are a function of the diameter of the cavity.Since the back of the can (closed side) imposes a zero to parallel electric field, positioning the pin at a distance equal to one quarter of the length lambda_g gives the best result (emitter antenna is into a point of maximum field strength).Imposing the minimum and maximum frequencies to stay in between the mode and second mode cut-off:a = d/2 > 1.83 / (2*pi*f_min*sqrt(mu0*epsilon0)) above the first mode cutoffa = d/2 < 2.405 / (2*pi*f_max*sqrt(mu0*epsilon0)) below the second mode cutoffOnce the diameter is chosen (for wi-fi, 2.4 to 2.5 GHz, it is easy to find one satisfying the above conditions)fTE_11 = 1.83 / (2*pi*a*sqrt(mu0*epsilon0)) first mode cutoff frequencylambda_g = 1 / ( f*sqrt(mu0*epsilon0)*sqrt(1-(fTE_11/f)^2) )mu0 and epsilon0 are the magnetic and electrical permeability constants of the air.For a narrow bandwidth signal, f can be the central value, for wi-fi I'd choose the geometric average f = f_avg = sqrt(f_min*f_max).In the example on my textbook it isn't mentioned, but IMO there is also an optimal can length, which should belambda_g/4 + N*lambda_g/2, with N at least 2 or 3. In that case, one could try to estimate the antenna gain considering it as a circular aperture antenna. I can't give the right formula at the moment, just can say that radiation goes as a Bessel function of the angle.The maximum in the radiation diagram is in front of the opened side of the can, of course.Since the reciprocity theorem (I think it is called so), the considerations made for the emission of radiation are true for receiving, too.Hope it's worth. If the calculation seems complex, it's just me unable to explain, so just ask.