I will address magnetic loops, but I will restrict myself to a subset with which I am familiar through EMI testing - namely, electrically small, electrostatically shielded loops that drive a 50 Ohm load directly. These are as opposed to (typically) unshielded tuned loops, either electrically small, tuned using an external capacitor, or electrically large loops that are themselves resonant. I have little experience with these.
One example of the type of loop to which I refer used to be used from 150 kHz to 30 MHz for VDE 0871 testing on a three or ten meter site. I believe this one was about 60 cm in diameter, a single turn, and shielded of course. The output was a bnc connector for connection to 50 Ohm coax. These could be either actively or passively matched to 50 Ohms. Another example is the MIL-STD-461 13.3 cm diameter loop. This consists of 36 turns of seven strands of AWG 41 Litz wire. One end of the wire connects to the shield and coax connector shell, while the other end connects to the coax center pin. Here there is no matching network at all; the loop is used below 100 kHz, it is just a voltage source with a frequency dependent source impedance. Matching is not an issue. >From the point of view of calibration, all that is necessary is that a homogenous magnetic field penetrate the plane of the loop at right angles. In fact, the antenna factor curve for the smaller loop which was originally presented in MIL-STD-461 basic circa late 1960s was analytically derived >from Faraday's Law. I am pretty sure that loop was built to print and no calibration as such was necessary. I have myself verified the proper operation of such a loop within a Helmholtz coil at very low frequencies, but it is clear that if you were going to calibrate the larger loop in a Helmholtz coil and at 30 MHz you would run into problems, due to coil inductance and size restrictions due to wavelength. Personally I am quite comfortable with the build-to-print approach for the small low frequency loop. An analytic approach here is sufficiently accurate. For the larger loop that works to 30 MHz, I would break the calibration into two parts. The operation of the single loop turn itself is still described by Faraday's Law. The matching network needs to be calibrated. Most such loops today use active matching, meaning an impedance matching amplifier. It is sufficient here to measure the gain of the amplifier (assuming it does not load the loop), and apply that gain to the loop antenna factor. I should note that my 60 cm Eaton loop has a flat antenna factor over most of its range, which means that either the amplifier gain decreases monotonically with increasing frequency, or the amplifier amplifies not the loop output potential, but rather the current the loop delivers into a load impedance much lower than the loop source impedance. That current would be frequency independent, since both the loop output potential and source impedance increase monotonically with increasing frequency. A final note. Since the loop is shielded, and only senses the magnetic component of the radiated electromagnetic wave, the impedance of the field impinging upon the loop is unimportant. What is important if you need to actually verify Faraday Law coupling, is that the gradient of the magnetic field across the plane of the loop is zero; you need a constant field across the loop face. > From: John Woodgate <[email protected]> > Date: Thu, 3 Aug 2006 16:45:44 +0100 > To: [email protected] > Subject: Re: Near field/Far field what is D. > > In message <c0f77bc2.40c8e%[email protected]>, dated Thu, 3 > Aug 2006, Ken Javor <[email protected]> writes > >> And the statement that antennas used for EMI measurements are >> calibrated in a 377 Ohm field is inaccurate, or misleading. > > Would you care to comment on the calibration of magnetic antennas? > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk > 2006 is YMMVI- Your mileage may vary immensely. > > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK > > - > ---------------------------------------------------------------- > 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 > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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