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#6
July 10th 17, 03:50 AM
posted to rec.radio.amateur.antenna
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Magnetic receiving loop theory
On 7/9/2017 3:53 PM, Jeff Liebermann wrote:
On Sun, 9 Jul 2017 13:13:00 -0500, John S wrote: On 7/9/2017 12:53 PM, Jeff Liebermann wrote: On Sun, 09 Jul 2017 09:08:11 -0400, Pat wrote: I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving Loop. Since I have a lot of noise here, I am very interested in this topic. However, I have having trouble understanding the theory. My understanding regarding electromagnetic waves is you can't have one without the other. RF propogates through space my having the moving electric field create a moving magnetic field which then creates a new electric field, etc, etc. How can one exist without the other? No. No what? Please expand. Sorry. That should be "No, one component cannot exist without the other". https://en.wikipedia.org/wiki/Electromagnetic_radiation#Wave_model "The electric and magnetic parts of the field stand in a fixed ratio of strengths in order to satisfy the two Maxwell equations that specify how one is produced from the other. These E and B fields are also in phase, with both reaching maxima and minima at the same points in space (see illustrations)". I must confess that I really don't understand Maxell's equations and therefore cannot offer more detail. How can a varying electric field from a noise source not also create a corresponding magnetic field? The transmitter generates both. You can reduce the sensitivity of a receiving loop to the electric E field by shielding, leaving only the magnetic H component. Examples of shielded loop antennas: https://www.google.com/search?q=shielded+loop+antenna&tbm=isch and unshielded loop antennas: https://www.google.com/search?q=unshielded+loop+antenna&tbm=isch Is this a near-field / far-field thing? No. So near-field is not different from far field? The textbooks disagree with you. In order for RF work, there has to be both E and B fields. However, the ratio between the two components is not constant and can vary depending on the source and distances. This described the characteristics and differences far better than I could: https://en.wikipedia.org/wiki/Near_and_far_field The main characteristic is that in the far field, the signal decrease at the usual inverse square law. In the near field, it's much more rapid at what I guess could be called the inverse cubed law. I've been collecting articles on magnetic loops, tuners, theory, and such in an apparently futile attempt to find the time to design something. Maybe you'll find these articles useful. Most have references and links at the end to other magnetic loop articles: https://sidstation.loudet.org/antenna-theory-en.xhtml https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf http://www.w0btu.com/magnetic_loops.html http://owenduffy.net/blog/?cat=31 http://www.aa5tb.com/aa5tb_loop_v1.22a.xls https://frrl.wordpress.com/2009/03/21/limited-space-antennas-the-small-transmitting-loop-antenna/ There's some discussion of E and H fields he http://owenduffy.net/antenna/PA0RDT-MiniWhip/ which you might find applicable. Did you read any of the links you posted? Yes. At some point, I read most everything, but can't claim that I remember, understood, or agree with everything that I've read. You must believe that anything posted on the Internet is true. Only if I write it. Owen Duffy is an exception but you posted his radiation pattern comparing a vertical and a loop. Read his titles. STL is "Small Transmitting Loops". Note the word "loop" as in "magnetic loop". You must have a http look-up fetish. I try never to directly attack the person posting a question or answer, but you deserve an exception. Did you do anything useful here to answer the original question? If I was wrong, did you do anything to correct my errors? If you make an unsubstantiated claim, at a minimum provide a URL for where you stole your assertion. Did you write anything worth reading? Methinks not. Pot/kettle. |
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