hi folks, we are testing radiation patterns for different types of antennas in our lab . let me explain you the setup : we have a antenna test kit from signet , basically a tx source at 800 Mhz using folded dipole and at the rx we have a stepper motor turntable on which we fix Rx antenna under test . the problem is that lot of interference occurs and even if some one walks from the test region the readings are affected. please suggest some ways to isolate our test bench . if i use a metallic mesh i think reflections will occur and cant use expensive obsorbers . want to make something effective enough from scratch - wood , foam , aluminum foilsetc .. !!!!
thanks and cheers ,, always good to be on eda
How about 'Air'?
As in an OATS (Open Air Test System) range - out in the parking lot maybe?
Jim
Testing for EMC compliance: approaches and techniques - Mark I. Montrose, Edward M. Nakauchi - Google Books
Cellphones work around the 800MHz frequency, don't they? You'd have much interference and you can't take away the multipath effects (it still reflects of the concrete floor or w/e).
Best setup would be something like this: View attachment 69596
Although I have no idea what material was used.
OATS testing is an industry accepted test method, although attention must be paid to nearby potential reflectors.
Recall, Johnny, the RADAR equation and the 1/x^4 power as it relates to returned power from 'targets' (or any nearby reflector). Remember, many times these tests are performed on the 'cheap' or very quickly; if something more permanent needs to be set up, that is a different issue.
And suitable selection of an RF test frequency and us of the narrow BW on the test receiver (with generator set for CW) can mitigate potential interfering terrestrial RF sources. It would also help to use a directional antenna on the source, to minimize 'stray' RF that might reflect off walls and such.
For antenna performance, it is usually necessary to find the angular 3 dB points; this can even be done indoors, ignore the 'reflections' that may only be down 10 or 15 dB and find the 3 dB beamwidth points for that measurement, and realize this is a first-order approximation (some data, even rough data, is better than no data).
Before indoor ranges and so-called compact ranges OATS ranges were the only option. I have have had access and use of all three.
Did you make reference to the cited text by any chance, where some of these issues are addressed?
Jim
You are 20 guys doing these things without thinking? And your prof., is he just smiling?
Almost everything is wrong set up in your antenna measurement chamber, if you intend to measure antenna radiation pattern.
1. An antenna test chamber without absorbers is not usable to measure antenna radiation pattern at all. You will get reflections from all directions. If you understand ground principles for how radio-waves behaves when radiated against metallic walls, must that be pretty obvious?
2. A dipole antenna as measurement antenna in a chamber, fine if you want to measure everything that comes from ALL directions, but for measuring radiation pattern is it almost forbidden.
3. Antenna under measurement as RX antenna in an anechoic chamber for measuring radiation pattern is a general error. Try google to find basic information how and why to use a anechoic chamber for antenna measurement.
Why not to use a dipole as measurement antenna => you will reduce quite zone to nothing. A common preferred antenna selection is a horn antenna with correct directive radiation pattern and well defined polarization.
Functional absorbers are not easy to do by yourself as absorber geometry is vital for total chamber function including were the quiet zone will be localized, were the turntable should be placed.
See this which shows an example of a simple anechoic chamber. It can as well be a rectangular chamber, but it cost more absorbers. Even if the chamber have absorbers all around are they not total absorbing all RF energy, so do not use a measurement antenna that have a radiation patter suited to receive 1:st reflections (as a dipole antenna would do).
It is possible to simplify the chamber further then what is showed in the picture, by only having absorbers on the wall behind the turntable, at cost of reduced dynamic as even good absorbers not attenuate 100% and a horn antenna also will see some of the reflection coming from surrounding outside of the wall, but it is better then nothing.
If we in this later case assumes opposite signal direction, DUT is used for RX will you measure signal from all directions which would result in reduced dynamic in your measurement results for radiation pattern and phase&polarization calibration for the chamber would be depending on DUT antenna properties. Full chamber calibration for each new DUT measurement, is a vast of energy.
There are a lot of other things, not covered here, that are more complex and that also must be handled if you should be able to have a chamber with repeatable results and low measurement error. Keeping measurement result reliable and repeatable do more require that you know what you are doing, then quality of chamber as that part is static.
In many ways is it much less complex to measure in free field. However do you still need to think a bit about what you are doing. Assume for example that you want to measure a cell phone antenna radiation pattern and you can not use a rural enough measurement location, free from interfering transmitters.
Then can you at least reduce interfering levels compared to signal level from DUT a lot by also here use a directive measurement antenna for RX as you then can select quietest direction and better reduce ground reflections if DUT is placed high enough. Another common measurement error is due to measurement coaxial cables that is connected to DUT and extends DUT ground-plane, adds standing waves and reflections. For a cell phone, if you can control it via a communication tester, can it be set transmitting without external cables, which reduces cable errors and simplifies that measurement part as no external RF generator is needed.
Drawback with using a directive measurement antenna, is that a such antenna needs to be placed at a longer distance, compared to a dipole, to still be in fare field, and if it not is a static measurement range, must antenna orientation be more exact verified then if a omnidirectional antenna is used.
thanks a lot for your replies . what i can interpret from this discussion is that instead of dipole we should use a directive antenna for tx . Dut is a t rx side and far feild region is greator than normal dipole . anecoid chamber is unavoidable . please correct if i am wrong some where . thanks once again
Yes anechoic chamber is unavoidable. If you have to do the measurement elsewhere, it pretty much useless and inaccurate your data would b.
Directive antenna such as a horn would be good. Remember and understand why you measuring the radiation patterns, whether co-polar or cross-polar.
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