Hi Grace Some more answers,
In addition to previous comments: _____ From: Grace Lin <[email protected]> List-Post: [email protected] Date: Thu, 20 Dec 2007 07:36:29 -0500 To: <[email protected]> Subject: Antenna Pattern Dear Members, Could you please share your knowledge about antenna pattern measurement? I am asked to look into the capability to perfom this measurement. My questions are: 1. Is there any particular site/chamber requriement, such as a full anechoic chamber, etc.? The chamber should be fully lined. The reflections from the walls and floor (even when lined) will cause the pattern to change. If (for example) there is a null in the antenna pattern of 60dB down on main lobe, you will not see it if the reflections from the wall are 40dB down. So the reflectivity of the chamber determines the accuracy of the measurement and the depth of the nulls that you can measure. Since nothing is perfect, there is always a limitation. Our antenna chambers can measure 200MHz to high microwave. At 200MHz the reflectivity off any surface is better than 20dB and is better than 40dB above 1GHz. 2. Is any particular instrument required in addtion to a spectrum analyzer/receiver and antennas for radiated emission measurement per FCC Part 15? You will also need a signal generator to provide a stimulus signal, usually into the antenna under test. This can usually be a simple CW source, and 0 dBm is usually quite adequate. This assumes a relatively simple antenna. If the antenna is active (like a phased array or synthetic aperture), then you will have to test the complete device with the antenna. While the 0dbm might be OK for some measurements, there are some other factors to consider. If the source antenna is, say, an isotropic (or has very little gain) the 1mW will be radiated in all directions. At 3m, the power density will be 0.88uW/sq.cm The power delivered to the spectrum analyser will depend on the capture area (or effective area) of the receive antenna - and this is given by wavelength squared times numeric gain all divided by 4pi. If the capture area is small, then the receive power is small on the main lobe. The depth of null that can be measured is given by the signal to noise ratio that you have on the analyser when the main main lobe is aligned. 3. Is software/firmware available for antenna pattern measurement? Take a look on radio ham sites, there are some simple polar diagram programs. If you want accurate measurements, then you need to drive the turntable and spec. an. with the same software. Accuracy is improved by only turning the turntable in one direction during the test. I would expect that an EMC turntable might only give you a real accuracy of 2 or 3 degrees. For real accuracy (points of a degree) you will need a substantial antenna positioner and a big chamber (at least 6m cube with some very large absorber (1.5 to 2m). It will still be limited at the low frequency end. 4. How to look for the labs that provide services for this measurement? Further definition of the antenna and desired test frequencies will help to narrow your selection of labs. I hope everybody has an enjoyable time during the forthcoming Christmas and New Year. Regards Tim SELEX Sensors and Airborne Systems Limited Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14 3EL A company registered in England & Wales. Company no. 02426132 ******************************************************************** This email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ******************************************************************** - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
