Actually the reactance of a single turn loop with diameter between 1 2 inches is going to be fairly close to 50 ohms. If the object is to test only at 100 MHz, the loop diameter could be chosen to make the reactance 50 ohms (inductive). Alternatively, if again the goal is single frequency operation, a suitable capacitor could be placed in series with the loop, tuning out the reactance totally and then John Woodgate¹s 50 ohm series resistor could function as a perfect match, with no reflections.
Ken Javor Phone: (256) 650-5261 From: John Woodgate <[email protected]> Reply-To: John Woodgate <[email protected]> Date: Sat, 13 Oct 2018 07:58:24 +0100 To: <[email protected]> Subject: Re: [PSES] Loop antenna theory Put 50 ohms in series where the cable joins the loop conductor. John Woodgate OOO-Own Opinions Only J M Woodgate and Associates www.woodjohn.uk <http://www.woodjohn.uk> Rayleigh, Essex UK On 2018-10-13 01:52, Heckrotte, Michael wrote: > > > > > > > > > > > > > Also...be careful of the feed cable orientation. The loop impedance doesn¹t > 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]> wrote: > > > > Re: [PSES] Loop antenna theory Based on Ed¹s 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]> > Reply-To: Edward Price <[email protected]> > Date: Fri, 12 Oct 2018 20:00:27 +0000 > To: <[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 ³pure and simple² models that you could always count on, like Ohm¹s 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.) It¹s a small factor, > but still it¹s a real thing. I don¹t recall if it also affected horizontal > plane loops, so maybe it is the result of interaction with the Earth¹s > 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] > 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 isn¹t listed below, but > you could use the MIL-STD-461 CS114, RTCA/DO-160 section 20 or automotive BCI > spec (can¹t 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]> > Reply-To: Amund Westin <[email protected]> > Date: Fri, 12 Oct 2018 06:15:01 +0200 > To: <[email protected]> > Subject: [PSES] Loop antenna theory > > I¹m 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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