Also...be careful of the feed cable orientation. The loop impedance doesn’t match the coax impedance so reflections will be considerable, along with currents on the outside of the cable shield and their resulting magnetic field. Ferrite beads can help.
Best Regards, Mike On Oct 12, 2018, at 5:14 PM, Ken Javor <[email protected]<mailto:[email protected]>> wrote: Based on Ed’s post, I maybe should have qualified my response by saying it is applicable only when the loop is electrically short, meaning its circumference is short wrt a wavelength. At 100 MHz, that means the circumference is less than 30 cm, or one foot. Based on the OP loop description, I pictured something may be an inch or two in diameter, which meets the criteria, but if it is much bigger than that, it is not a quasi-static source. Ken Javor Phone: (256) 650-5261 ________________________________ From: Edward Price <[email protected]<mailto:[email protected]>> Reply-To: Edward Price <[email protected]<mailto:[email protected]>> Date: Fri, 12 Oct 2018 20:00:27 +0000 To: <[email protected]<mailto:[email protected]>> Conversation: [PSES] Loop antenna theory Subject: Re: [PSES] Loop antenna theory We have always thought that a single-turn, small loop antenna was one of those “pure and simple” models that you could always count on, like Ohm’s Law in a DC circuit. And then along comes some recent research, published in QEX in the last couple of months by Kai Siwiak(?), which talks about vertical plane loops actually having non-linear current distribution around the loop! (He gets very math intensive, too many nested integrals for me to follow, and he references some IEEE Antenna Transactions articles.) It’s a small factor, but still it’s a real thing. I don’t recall if it also affected horizontal plane loops, so maybe it is the result of interaction with the Earth’s magnetic field or maybe the Earth acting as a counterpoise. Ed Price WB6WSN Chula Vista, CA USA From: Ken Javor [mailto:[email protected]] Sent: Friday, October 12, 2018 8:03 AM To: [email protected]<mailto:[email protected]> Subject: Re: [PSES] Loop antenna theory There is a simple equation for the axial field at the center of the loop as a function of distance from the loop, the loop radius, and of course, current. Current will depend on the impedance, and that in turn depends on loop inductance, which will depend on radius. The radius isn’t listed below, but you could use the MIL-STD-461 CS114, RTCA/DO-160 section 20 or automotive BCI spec (can’t recall the number now) fixture to hold a current probe that measures the current induced in the fixture when driven by a 50 ohm source. Then you would have all the info necessary to calculate the axial field at a distance the loop center. Ken Javor Phone: (256) 650-5261 ________________________________ From: Amund Westin <[email protected]<mailto:[email protected]>> Reply-To: Amund Westin <[email protected]<mailto:[email protected]>> Date: Fri, 12 Oct 2018 06:15:01 +0200 To: <[email protected]<mailto:[email protected]>> Subject: [PSES] Loop antenna theory I’m looking for some good and practical loop antenna theory. Have a RF signal of +40dBm @ 100MHz and a self-made 1 turn semi-rigid coax loop with inner lead soldered to the shield. Theory (or rule of thumb) about the generated fields from such antenna are welcome. Will use this loop antenna under RF immunity troubleshooting. Thanks. BR Amund - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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