Antonio, I've seen problems the other way around, i.e. the probes that have been calibrated in far-field conditions, with the probe, the leads, and the readout unit exposed to far field measuring erroneously in near-field. Measurements obtained by a probe calibrated in far-field conditions or in a TEM-cell, when taken in a near field of a source can have huge errors, depending on the combination of the following:
type of the EM field (E or H) type of the field probe (E or H) orientation of the probe with its leads and readout unit (interaction of the whole measuring unit with the field). I have also seen large differences in probe's responses to calibration (and measurement errors connected to it) when only the probe tip was exposed to the field, as opposed to the whole body. That would be a typical case of calibration using small TEM-cells or similar striplines vs. calibration in a chamber using transmitting antennas. In your case (depending on the design of the probe), you will probably be okay. I would suggest you to calibrate it in front of a horn antenna, at some distance, so that you are in far field and the leads are exposed. If you have enough distance from the reflecting objects, calculate how much free-space attenuation the reflections will go through, and make your judgment if they matter. You can check the influence of the cable by trying to orient the leads (cable) parallel to the E, H, and Pd field vectors. You may also shield the probe only and see of there is any pickup from the leads. One concern I have is the sensitivity of your probe, and if you will pick enough signal to measure at 5m-100 away from the source. There are many issues, but I hope this may give you some directions. Cheers, Neven PS Pozdrav ekipi s 12-og kata, posebno Juri. ----- Original Message ----- From: Antonio Sarolic <[email protected]> To: <[email protected]> Cc: Antonio <[email protected]> Sent: Wednesday, November 24, 1999 3:07 AM Subject: Near field magnetic probes > > Hello group > > This discussion list is worth reading and very helpful. I thought I might > put a question that's been bothering me. > > I want to measure EM field (H field) originating from GSM base station > antenna (distances 5m-100m), using near field probes for magnetic field. > They are commonly used for locating EM radiation sources on PCBs or leakage > sources. > > I see a few problems concerning this measurement. > > 1. These probes are constructed for sources (PCB, enclosure leakage) that > are very close to the probe tip. Power density of the source falls rapidly > with distance, so the rest of the probe, including connection cable, is > placed in the field that is much lower than close to the probe tip. That is > not the case in the far field. Is the calibration still valid for the > measurements in far field? > > 2. Also, measurements should be done outside and inside a room. That means > lot of reflections. There must be some cable pick-up. Does this usually add > more field or not? > > After all, I think the measurement will not be too accurate, but can provide > the order of magnitude at least. I know I would be better off with the > isotropic probe with optic link, but right now I am interested in near field > probe. > > I would like some opinions please. Am I right or am I wrong? Am I missing > something? > > Thanks a lot. > Antonio > > Antonio Sarolic > Faculty of Electrical Engineering and Computing > Dept. of Radiocommunications and Microwave Engineering > Unska 3, HR-10000 Zagreb > CROATIA > tel. +385 1 61 29 789, fax. +385 1 61 29 717 > E-mail: [email protected] > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > > > > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
