Problem with results in an anechoic chamber

hello

i have a problem at my work with the anechoic chamber ,
at some frequencies the result are quit good and at other freq the result are worse (different then at open space test) .
the chamber supposed to work from 0.8GHz to 18GHz .
but at 2.4, 5.5 the result are not reasonable.
the chamber is 10x5x5 meter
does anyone have any suggestion?

thanks

Re: anechoic chamber

Dear bbvv

can you upload the jpg of this freq.

I can give some idea.
If it noisy you have problem with the generator try freq. step of 0.01 GHz near to above freq. 5≈2.4x2 first and second harmonic

If it not smooth maybe its reflection from somewhere in the chamber or problem in Tx. antenna
Did you check another antenna in these frequencies?

pl

Re: anechoic chamber

Dear plasma

I think the problem is reflection from the walls but Im not sure, is there any way to check it? (and not by VSWR check).

Because the problems occur when i test a sector (60,90,120), omni and small low gain antenna (one patch ).

At the sector an the omni the patterns are not correct compare to the open field test and the simulation result.

At the small antenna, I got a lot of noise and some ripple of about 1 dB at zenith.

Do you think the reason can be :

1. Tx power should be high or low? (between -10 to 10 dBm)
2. IF freq should be 10-3000?
3. Maybe i should narrow the center of the chamber at the middle where the reflection are max ?

Thank in advance

bbvv

Re: anechoic chamber

80 percent of the range problems are connectors. The remaining 20 % of the problems are connectors too. Start there.

You don't say if it is a compact range or a far field range. If it is a compact range you may have to review the focus alignment.

As a quick guess if the absorber is on the order of two feet thick everything should be fine at L and S band.

Make sure your fixturing is not creating a problem.

Re: anechoic chamber

Hi
I read any where that the feed cable influence the radiating pattern, so u have to use a material (ferrites in french don't know it in english sorry)that isolate it to avoid its influence on ur work refer to :

C. ICHELN, J. OLLIKAINEN, P. VAINIKAINEN
Reducing the influence of feed cables on small antenna measurements
Electronics Letters, Vol. 35, n°15, pp. 1212-1214, July 1999

C. ICHELN, P. VAINIKAINEN
Dual-frequency balun to decrease influence of RF feed cables in small antenna
measurements
Electronics Letters, Vol. 36, n°21, pp. 1760-1761, October 2000

C. ICHELN, J. KROGERUS, P. VAINIKAINEN
Use of balun chokes in small-antenna radiation measurements
IEEE Transactions on Instrumentation and Measurement, Vol. 53, n°2, pp. 498-506,
April 2004

Re: anechoic chamber

1. Tx power should be high or low? (between -10 to 10 dBm)

Re: anechoic chamber

The more power you put, give you cleaner pattern
If you use network analyzer you can see the response level on display and see if it clean or noisy.

pl

Re: anechoic chamber

plasma said:

The more power you put, give you cleaner pattern
If you use network analyzer you can see the response level on display and see if it clean or noisy.

pl

Click to expand...

But the more power you put, give you higher reflections.

The chamber has -80dBm isolation that why the more power you put the reflection from the walls are higher.
To get clean patterns the IF freq should be as low as possible,
But the lower the IF gets the longer its take to complete a single pattern.
what is the best configuration get correct result ? (power/IF)

Re: anechoic chamber

You might want to try moving your source and test antennas closer together. Since it sounds like you are testing low gain antennas you will be able to get closer without near field problems and get more direct signal and less relative scattered signal. That may improve your test result.

anechoic chamber

you should use LNA to amplified the received level in your receiver. i last month test a L1 GPS antenna in our university chamber, when we use a LNA the results of our mesurement are qualified so much!