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anechoic chamber or near field chamber

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rubdawg

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homemade anechoic chamber

I am looking to learn on how to construct a small near field chamber to measure antenna performance since we do not have access/budget to use/buy an anechoic chamber. I have not been able to find much about contructing this online, any help would be appreciated
 

samehham

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building rf anechoic

making an anechoic chamber is not easy thing, it is realy expensive as all the products in the RF range.
my company has one for 3 m test for precompliance testing, it is very expensive, you need to decide first what are the frequency range you are targeting.
there are few companies doing these things
you may look at this
http://www.comtestnl.com/test.shtml
 

rubdawg

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build an anechoic chamber

Yah, an anechoic chamber I have used and it is quite expensive. This is mainly needed for cell phone use so it would have to cover at least 800MHz to 3GHz(UMTS/WIFI now in phones)any one day and saw that they had a box the size of a regular office desk and used that to measure antenna perfromance, mainly it was a near-field measurement, and i know that it was homemade type, but that was about 3 years ago and not intereste in antenna, but now it is my line of work.
 

Element7k

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antenna anechoic chamber

Hi,

this might not be alot of help but I think you are talking about a reverberation chamber. Prof Kildal is leading researcher in this field. I am uncertain how much he's selling this product for.

http://www.bluetest.se/Bluetest-4posters-030810.pdf

Element7k
rubdawg said:
Yah, an anechoic chamber I have used and it is quite expensive. This is mainly needed for cell phone use so it would have to cover at least 800MHz to 3GHz(UMTS/WIFI now in phones)any one day and saw that they had a box the size of a regular office desk and used that to measure antenna perfromance, mainly it was a near-field measurement, and i know that it was homemade type, but that was about 3 years ago and not intereste in antenna, but now it is my line of work.
 

biff44

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anechoic box

Want one for almost free? Go test the antenna outside. You can set up to keep all the expensive equipment indoors, and just get a pair of antenna rotators from a local ham shack (get a digital indicator/control) to mount on a wooden pole, and put another pole or two for the destination antenna. You can make all sorts of antenna pattern tests. I did some antenna testing that way, and got very usable results.

Won't be that good for EMI testing, though.

The problems with building an anechoic chamber are two. First, usually people want them EMI tight, so all the surfaces have to be conductive. But, you do not want re-radiation from those conductive surfaces, so you then have to cover every surface with very expensive blocks of RF absorber. If you are only going to work at one frequency, you could probably get away with just a thin sheet of absorber engineered to absorb that one frequency, but usually you want some bandwidth to the room.

Most testing defines the need for a ground plane, and the outside earth serves as a good one for free.
 

vfone

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build anechoic box

rubdawg,
I know exactly what you are looking for. You want to build a small reflection free box having the dimensions of a lab desk (~2m x 1.2m) to do a quick antenna test, and not for EMI.
I can say this is possible because I helped to build a few boxes like this. For this you need first to build a wooden box with these dimensions, having an open window on the front side to allow you to manipulate things inside. After this you need some microwave absorber or urethane foam to put on the inside walls. Is not mandatory to put pyramidal absorbers, because these ones even they are the best for reflections will take a lot of space from inside volume. Usually I used only flat polyurethane foam pieces that work nice for this application.
This chamber is very useful for quick antenna tuning.
Here is some info from where you can buy the materials:
http://www.beiemc.com.cn/en/our products.htm#
http://www.ets-lindgren.com/
http://www.djmelectronics.com/rf-absorber.html
 

    rubdawg

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rubdawg

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how to build anechoic box

vfone:
You hit it on the spot, I am in need to do quick/rough testing of antenna for mobile phones(dualband/triband), and not EMI. I will read the links you sent me, any other advice would be greatful.
 

vfone

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how to measure gain in anechoic chambers

For antenna reference in this kind of chamber the best is to use a calibrated dipole. Usually the homemade dipoles are the best in terms of performances/dimension because you can make them for a dedicated band. Beware how you route the coax cables from reference dipole and from mobile antenna, that are going to your test equipment. Shall be routed underneath of the absorber for minimum radiation. Very good quality coax cable is required. From my experience if the distance between mobile and reference dipole is d > 3 x wavelength you can do even an antenna pattern test in this chamber. May be not as accurate as in a big chamber but it will give you a good idea where you are. I used a manual rotary table, but can be also an automated one, using an electric motor. Again, beware about cables inside of the room. All of them shall be under the foam for minimum radiation or coupling.
With this setup you can also test the mobile in a loop-back with your test equpment (live call) and test the radiated power, radiated sensitivity, or TX antenna pattern.
 

jallem

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homemade rf absorber

the cheapest way to measure the gain would be an open area.
if you must have a chamber then is very expensive and would
have to be justified. Asking how many antennas to test per year,
return of invetment, etc. I was in the same dilema, until I found
a lab who a chamber and they rent it for hallf day. It cost $1,600/half-day.
You can test up to 10 antennas in half day.
 

rubdawg

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anechoic foam emi

Hi Vfone, how tall should the chamber be if the length and width is 2m x 1.2m???
 

vfone

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home made near field antenna

The chamber shall be at least 1.2m tall, for a minimum of 800MHz frequency operation. The stand of the test unit and of the reference antenna shall be half of this distance, to place them somewhere in the middle.
 

mamali

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Re: anechoic box

biff44 said:
Want one for almost free? Go test the antenna outside. You can set up to keep all the expensive equipment indoors, and just get a pair of antenna rotators from a local ham shack (get a digital indicator/control) to mount on a wooden pole, and put another pole or two for the destination antenna. You can make all sorts of antenna pattern tests. I did some antenna testing that way, and got very usable results.

Won't be that good for EMI testing, though.

The problems with building an anechoic chamber are two. First, usually people want them EMI tight, so all the surfaces have to be conductive. But, you do not want re-radiation from those conductive surfaces, so you then have to cover every surface with very expensive blocks of RF absorber. If you are only going to work at one frequency, you could probably get away with just a thin sheet of absorber engineered to absorb that one frequency, but usually you want some bandwidth to the room.

Most testing defines the need for a ground plane, and the outside earth serves as a good one for free.
very nice reply. I have just two more question ( we are building our new antenna lab chamber, inside a nice old metallic room of 2 m x 2.5 m x 3 m; mostly for radiation pattern measurement in 5~40GHz range):

1. Would you please describe your antenna rotary positioner solution in more details. Your solution is interesting. How much will that cost? can I use a home satellite dish postioner instead?

2. What about the feeds? Wide band horns that can cover this range are very expensive (~3000$ each), and I need at least two of them to cover my range. Is there any low cost solution? Standard horns maybe?

Thanks in advance
 

biff44

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I use something like this AR40:

http://www.universal-radio.com/CATALOG/hamrot/hygrot.html

I have a 3' x 3' piece of plywood on the ground. I have a piece of PVC pipe mounted to it (using a plastic toilet flange). I mount the AR40 to it. Then I run another piece of PVC pipe that rotates off the top of the AR40. You can mount another flange at the top of the rotating pipe.

You have to run a rotator cable from the control unit to the rotator. I found that if you run it at right angles from the rotator out around 10', then turn 90 degrees and run to where you are set up, and turn another 90 degrees to get back to where you are sitting with a spectrum analyzer/receiving antenna.

I clamp a bunch of ferrite beads to the rotator cable, and to any RF cables used.

The higher off the ground you are with the two antennas, the better, but I have found that 4 to 5' high is pretty good.
 

mamali

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tnx, it was fast :) however, I want to use it inside our chamber, not out side. Do you know what is their rotary resolution? Can I use something like AR-35, or it's not precise enough? What about dish rotators? too many questions! last one: i prefer to use a non-metallic system, it'll add to the chamber reflections. is there any plastci one out there?
 

biff44

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I can probably get a resolution of around +/- 8 degrees worst case. The problem is that you turn a dial, and the motor moves until it thinks it is at the position you wanted. It is not a digital input. The rotating scale is calibrated in 15 degree ticks.

Not perfect, but for $85, you can not beat the price! A real antenna range rotator is in the $10,000 and up range.

If you want to get fancy, there are ham radio rotators out there that are dual axis (i.e. two motors).

There might be better ham radio units than the one I showed, maybe one with a digital input on the control unit. Worth googling.

You can call these guys up to see if they have a used professional system:
http://www.ramayes.com/Antenna_Mount_Accessories.htm
 

Kyo Kusanagi

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[/COLOR]
tnx, it was fast :) however, I want to use it inside our chamber, not out side. Do you know what is their rotary resolution? Can I use something like AR-35, or it's not precise enough? What about dish rotators? too many questions! last one: i prefer to use a non-metallic system, it'll add to the chamber reflections. is there any plastci one out there?
Hi,
We're building our own antenna chamber too and seriously thinking want to use biff44's solution for antenna rotary positioner. Just want to know, how is your progress?
 

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