Re: RF field strength units
Bill Flanigan asked: >> I have purchased a field-strength meter which displays - over a range of frequencies - field strength in units dBmV, dBuV or dBm. It uses a whip antenna. Is there any way to convert any of these figures to V/m (I am doing RF immunity testing and I need to ensure 10 V/m)? Or is this << It may not be useful for _this_ purpose. >From the description, it is an RF voltmeter with a whip. If accurately calibrated, it could be quite helpful comparing "before and after" fields. However, in order to tell what the field IS, you need the whip's antenna factor (installed on the instrument) and it is more than possible the manufacturer doesn't know that. If you can connect an antenna whose factors are known, you need only sum that with the reading you get. 0 dBmV is 1 millivolt. 1 volt is 1000 mV, 60 dBmV; 10 volts is 10,000 mV, 80 dBmV. A biconical dipole might from 30 to 200 MHz have an antenna factor varying between 6dB and 30dB (or higher). In a field of 10 V/m, that antenna would deliver (not counting feedline loss) anywhere between 316 mV and 5 volts. The corresponding numbers in dBmV are 50 dBmV to 74 dBmV. And if your instrument has a flat response across the frequencies you want to measure, it'll work. That was the last of the good news; using a bicon or other full-sized antenna affects the field one is measuring. For checking radiated fields for immunity testing, you need really small antennas (and instruments). Since a 10V/m field demands no great sensitivity, commercial sensors for immunity work usually have 3-axis (polarization irrelevant) short, resistive dipoles with detectors at the antenna feeding rectified DC to a high impedance measuring circuit, and also, with some way to get the readings out of the chamber without affecting the field with long wires; fiber optic, usually. You pay for this; precision and suitability for the job don't come cheap. Good luck! Cortland KA5S - 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 emc-p...@ieee.org 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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
RE: RF field strength units
Bill: The "antenna factor" is a conversion factor that converts the field strength of an E-Field that the antenna sees to the voltage (Volts or micro-volts present on the output terminals of the antenna into a specified load (Normally 50 ohms). The AF for an antenna will vary over frequency. Therefore, the manufacturer should generally provide you with a graph showing frequency vs. AF, or a suitable table. The other option is to have the antenna calibrated by a lab that performs that service such as Liberty Labs. If you are going that route, be sure to specify how you are going to use the antenna. (Immunity vs. radiated emissions measurements). John Shinn, P.E. Manager, Laboratory Operations Sanmina-SCI _ From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Bill Flanigan Sent: Monday, March 20, 2006 11:17 AM To: 'Emc-Pstc' Subject: RF field strength units Associates, I have purchased a field-strength meter which displays - over a range of frequencies - field strength in units dBmV, dBuV or dBm. It uses a whip antenna. Is there any way to convert any of these figures to V/m (I am doing RF immunity testing and I need to ensure 10 V/m)? Or is this conversion dependent on knowing the antenna factors over the frequency ranges? I am in the middle of unproductive communications with the manufacturer (Korea) and the vendor (Arizona). WmFlanigan _ Scanned by Sanmina-SCI eShield - 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 emc-p...@ieee.org 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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc _ Scanned by Sanmina-SCI eShield - 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 emc-p...@ieee.org 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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
