Magnetic receiving loop theory
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? Here's a quote from the ad, "The MFJ-1886 drastically reduces noise and interference by receiving the magnetic field and rejecting the electric field". How can a varying electric field from a noise source not also create a corresponding magnetic field? Is this a near-field / far-field thing? Pat |
Magnetic receiving loop theory
On 7/9/2017 8:08 AM, 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? You are correct, they can't. Here's a quote from the ad, "The MFJ-1886 drastically reduces noise and interference by receiving the magnetic field and rejecting the electric field". How can a varying electric field from a noise source not also create a corresponding magnetic field? Is this a near-field / far-field thing? Pat I seem to recall that within a couple of wavelengths, there is a difference in the magnetic and electric fields and the magnetic field diminishes rapidly beyond that but remains associated with the electric field as you say. I have read trusted authors who say that the real value in a small loop is the ability to null the incoming interference. MFJ is lacking in technical knowledge. They said that my MFJ analyzer would measure impedance (Z). It does not measure the imaginary part. It measures absolute value of impedance (|Z|). Cheers, John |
Magnetic receiving loop theory
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. Here's a quote from the ad, "The MFJ-1886 drastically reduces noise and interference by receiving the magnetic field and rejecting the electric field". 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. 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. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Magnetic receiving loop theory
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. Here's a quote from the ad, "The MFJ-1886 drastically reduces noise and interference by receiving the magnetic field and rejecting the electric field". 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. 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? You must believe that anything posted on the Internet is true. Owen Duffy is an exception but you posted his radiation pattern comparing a vertical and a loop. You must have a http look-up fetish. |
Magnetic receiving loop theory
On Sun, 09 Jul 2017 10:53:33 -0700, Jeff Liebermann
wrote: 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. I understand making antennas that are sensitive to only the H field. My question is why would I want to? If the noise has both components, how does an H field only antenna reduce unwanted noise? Pat |
Magnetic receiving loop theory
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. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Magnetic receiving loop theory
On Sun, 09 Jul 2017 14:16:20 -0400, Pat wrote:
On Sun, 09 Jul 2017 10:53:33 -0700, Jeff Liebermann wrote: 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. I understand making antennas that are sensitive to only the H field. My question is why would I want to? If the noise has both components, how does an H field only antenna reduce unwanted noise? Pat That should be the E and B field, not H field. My mistake. I wish that I had a supportable answer to this question. There are quite a few opinions on the topic. Here's one that says that all the shield does is make it easier to build a balanced antenna: https://electronics.stackexchange.com/questions/70262/what-if-anything-makes-shielded-loop-antennas-so-great-at-rejecting-local-nois It also states that it is impossible to block either the E or B fields, which contradicts what I wrote. To be uncharacteristically honest, I don't know exactly what the shield does and how it works. I do know that in building LF (30-300KHz) loop type direction finders, the noise levels with a shielded loop were far lower than with an unshielded loop. How much? I don't recall as it was a long time ago, but it was quite noticeable. Whether this also applies at HF frequencies is also unknown. I have my own simplistic understanding of how a magnetic loop operates. It works because the Q of the loop is very high. In some cases, so high that the operating bandwidth of the loop is narrower than modulation bandwidth. For example, if I use the default numbers in the AA5TB loop calculator spreadsheet: http://www.aa5tb.com/loop.html http://www.aa5tb.com/aa5tb_loop_v1.22a.xls it shows a Q=1746 at 7MHz. BW = freq/Q = 7MHz/1746 = 4.1KHz That's the width of about 2 SSB signals at 2KHz modulation bandwidth each, which is barely acceptable. I think you can see that if I play with the dimensions, which will increase the Q, it won't take much to end up with an antenna that's narrower than the signal it's trying to receive. Another problem with a high Q antenna is that it has to be constantly tuned to compensate for changes in tuning caused by mechanical vibrations, changes in nearby metal objects, rotation, etc, as well a slight changes in operating frequency. That begs the question, what does such high Q do for you? Well, it dramatically reduces interference from other stations on nearby frequencies. It produces a very efficient antenna. I improves receiver sensitivity by removing quite a bit of noise, EMI, and RFI that might sneak in through the receiver bandpass, through various possible mixes (usually with stations on adjacent frequencies), through receiver images, and through static buildup on the antenna. The price you pay is having to use VERY rigid construction, expensive (vacuum or butterfly) tuning capacitors, silver solder, a potentially complicated automatic antenna tuner, and having to retune every time you change ANYTHING while operating. Is it worth it? I think so. I may soon see how well a magnetic loop really works. A friend recently installed a 55ft tower and a collection of HF yagi antennas. I bet him that I could build a magnetic loop antenna that would hear the same stations as his monster yagi, but near ground level and much smaller size and cost. The bet is for lunch at the local coffee shop. This is going out on a limb, but I believe that it can be done receive. Unfortunately, because of the narrow antenna bandwidth, I can't use WSPR and PSK Reporter to compare gain and coverage. Suggestion: Use the AA5TB spreadsheet, 4NEC2, etc to design something. Or, just follow someone's construction instructions. Go cheap initially so that you can see how it should be done. Improve the design as you go along. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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. |
Magnetic receiving loop theory
"Pat" wrote in message ... Is this a near-field / far-field thing? Hi Not really But most statements about magnétic loops are false because the ground is much too close to the antenna in terms of wavelenght. And average ground has little magnetic effect and big electric one. Making a VHF/FM magnetic loop is a good idea to start understanding how it really works. It is very cheap and small too :-) |
Magnetic receiving loop theory
On Sun, 09 Jul 2017 14:29:18 -0700, Jeff Liebermann
wrote: On Sun, 09 Jul 2017 14:16:20 -0400, Pat wrote: On Sun, 09 Jul 2017 10:53:33 -0700, Jeff Liebermann wrote: 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. I understand making antennas that are sensitive to only the H field. My question is why would I want to? If the noise has both components, how does an H field only antenna reduce unwanted noise? Pat That should be the E and B field, not H field. My mistake. E and H are fine. I think it depends on which books you are reading or maybe how old you are? I remember E and H from school (a long time ago). snip I may soon see how well a magnetic loop really works. A friend recently installed a 55ft tower and a collection of HF yagi antennas. I bet him that I could build a magnetic loop antenna that would hear the same stations as his monster yagi, but near ground level and much smaller size and cost. The bet is for lunch at the local coffee shop. This is going out on a limb, but I believe that it can be done receive. Unfortunately, because of the narrow antenna bandwidth, I can't use WSPR and PSK Reporter to compare gain and coverage. I look forward to hearing the results. Sounds like a great experiment. Thanks for your responses. Pat |
Magnetic receiving loop theory
On Mon, 10 Jul 2017 10:21:42 +0200, "bilou" wrote:
"Pat" wrote in message .. . Is this a near-field / far-field thing? Hi Not really But most statements about magnétic loops are false because the ground is much too close to the antenna in terms of wavelenght. And average ground has little magnetic effect and big electric one. Making a VHF/FM magnetic loop is a good idea to start understanding how it really works. It is very cheap and small too :-) Interesting. I hadn't thought of ground effects. I may do some experimenting. I would love to hear the HF bands without the constant S9 noise I have now. (The noise is from multiple sources - power lines, my electronic gadgets, neighbor's electronic gadgets, TVs, etc.) |
Magnetic receiving loop theory
On 2017-07-09 o 15:08, Pat writes:
understanding regarding electromagnetic waves is you can't have one without the other. Yes, every transmitting antenna creates perturbance that spreads around as electromagnetic (EM) wave with electric and magnetic component but... In the _near field_, magnetic antenna creates mainly the Magnetic component of the field and Electric antenna (in ex. dipole) creates electric field. The same goes to receiving. Shielded magnetic loop is quite deaf to M component of the EM field. (Un)fortunately most of the local noise sources create perturbance in E field. Therefore chances are that magnetic loop will be in some degree immune to them. The other noise sources may or may not be in the near field range but the furher away they are the less they add to the total noise level. Of course you've read this? https://en.wikipedia.org/wiki/Near_and_far_field 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? Here's a quote from the ad, "The MFJ-1886 drastically reduces noise and interference by receiving the magnetic field and rejecting the electric field". How can a varying electric field from a noise source not also create a corresponding magnetic field? Is this a near-field / far-field thing? Pat |
Magnetic receiving loop theory
On Mon, 10 Jul 2017 06:28:08 -0400, Pat wrote:
On Sun, 09 Jul 2017 14:29:18 -0700, Jeff Liebermann wrote: That should be the E and B field, not H field. My mistake. E and H are fine. I think it depends on which books you are reading or maybe how old you are? I remember E and H from school (a long time ago). At this time, I'm 25,384 days old[1]. That's long enough to have forgotten or confused most everything which I had pretended to learn in skool. I'm perpetually mangling the various fields. So, I decided to search for some clarification. This is least confusing explanation I could find: https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/#post-2537765 I think I understand most of it, maybe, or at least some of it: https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/ There are 114 articles in the thread, most of which disagree with each other. That suggests that not everyone understands the various fields in quite the same manner. E and B are the total electric and magnetic fields. D and H are the free electric and magnetic fields. P and M are the bound electric and magnetic fields.? E = D + P (except that for historical reasons E is defined differently, so we need to multiply it by the permittivity, and for some reason P is multiplied by minus-one). B = H + M (except that for the same historical reasons B is defined like E, so we need to divide it by the permeability). At this point, I usually say "I hope this help". However, I think that "I hope this doesn't hurt too much" might be more appropriate. I look forward to hearing the results. Sounds like a great experiment. I'll post something. Right now, I don't see it happening until after I design and build the one, true, ultimate, and best magnetic loop antenna. Probably next year. [1] http://www.calculator.net/age-calculator.html?today=01%2F10%2F1948&ageat=07%2F10 %2F2017&x=54&y=14 -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Magnetic receiving loop theory
On Mon, 10 Jul 2017 18:36:07 -0700, Jeff Liebermann
wrote: On Mon, 10 Jul 2017 06:28:08 -0400, Pat wrote: On Sun, 09 Jul 2017 14:29:18 -0700, Jeff Liebermann wrote: That should be the E and B field, not H field. My mistake. E and H are fine. I think it depends on which books you are reading or maybe how old you are? I remember E and H from school (a long time ago). At this time, I'm 25,384 days old[1]. That's long enough to have forgotten or confused most everything which I had pretended to learn in skool. I am 1.088 kilodays younger than you. Not much in the overall scheme of things. (Thats only 78 fortnights. I had a professor in school who would measure velocity in furlongs per fortnight.) I'm perpetually mangling the various fields. So, I decided to search for some clarification. This is least confusing explanation I could find: https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/#post-2537765 I think I understand most of it, maybe, or at least some of it: https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/ There are 114 articles in the thread, most of which disagree with each other. That suggests that not everyone understands the various fields in quite the same manner. E and B are the total electric and magnetic fields. D and H are the free electric and magnetic fields. P and M are the bound electric and magnetic fields.? E = D + P (except that for historical reasons E is defined differently, so we need to multiply it by the permittivity, and for some reason P is multiplied by minus-one). B = H + M (except that for the same historical reasons B is defined like E, so we need to divide it by the permeability). At this point, I usually say "I hope this help". However, I think that "I hope this doesn't hurt too much" might be more appropriate. True, but I appeciate your responses anyway. I look forward to hearing the results. Sounds like a great experiment. I'll post something. Right now, I don't see it happening until after I design and build the one, true, ultimate, and best magnetic loop antenna. Probably next year. Sounds good. As an aside, I just watched a youtube video of someone trying out one of these magnetic loop antennas. With his particular set of circumstances, it reduced the noise floor on 80 meters significantly. [1] http://www.calculator.net/age-calculator.html?today=01%2F10%2F1948&ageat=07%2F10 %2F2017&x=54&y=14 |
Magnetic receiving loop theory
Bartolomeo wrote on 7/10/2017 7:31 PM:
On 2017-07-09 o 15:08, Pat writes: understanding regarding electromagnetic waves is you can't have one without the other. Yes, every transmitting antenna creates perturbance that spreads around as electromagnetic (EM) wave with electric and magnetic component but... In the _near field_, magnetic antenna creates mainly the Magnetic component of the field and Electric antenna (in ex. dipole) creates electric field. The same goes to receiving. Shielded magnetic loop is quite deaf to M component of the EM field. I think you mean the shielded loop is "deaf" to the electric field. (Un)fortunately most of the local noise sources create perturbance in E field. Therefore chances are that magnetic loop will be in some degree immune to them. The other noise sources may or may not be in the near field range but the furher away they are the less they add to the total noise level. Of course you've read this? https://en.wikipedia.org/wiki/Near_and_far_field 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? In the near field the antenna itself will generate a field (either magnetic or electric depending on the design). This field falls off rapidly with distance. The antenna also generates an EM wave which radiates and is dominant at distance (far field). Here's a quote from the ad, "The MFJ-1886 drastically reduces noise and interference by receiving the magnetic field and rejecting the electric field". How can a varying electric field from a noise source not also create a corresponding magnetic field? Is this a near-field / far-field thing? The ad copy isn't saying there is no magnetic field from the noise source, but most noise sources are close enough to be near field with a much stronger E field than the magnetic component. This is *very* much a near/far field thing. -- Rick C |
Magnetic receiving loop theory
On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote:
Sounds good. As an aside, I just watched a youtube video of someone trying out one of these magnetic loop antennas. With his particular set of circumstances, it reduced the noise floor on 80 meters significantly. Compared to what other antenna? He probably reduced the received signal strength by the same amount leaving the SNR unchanged. That's why I included a link to the PA0RDT mini-antenna, which explains why such a small antenna works: http://dl1dbc.net/SAQ/miniwhip.html A loop works much the same way. One way to benefit from a small antenna is to do something to improve the SNR, which the loop does by narrowing the RX bandwidth, as I explained in a previous rant. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Magnetic receiving loop theory
On Tue, 11 Jul 2017 10:11:11 -0700, Jeff Liebermann
wrote: On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote: Sounds good. As an aside, I just watched a youtube video of someone trying out one of these magnetic loop antennas. With his particular set of circumstances, it reduced the noise floor on 80 meters significantly. Compared to what other antenna? He probably reduced the received signal strength by the same amount leaving the SNR unchanged. Not really. Both signal and noise are reduced, but the SNR must be better because you can hear an SSB conversation with the loop that is not there with his sloper. Search youtube for MFJ-1886 and N9BC. Of course, there is no mention of antenna patterns. Maybe the QSO he was listening to is in a null or his other antenna. Not enough info to really tell. |
Magnetic receiving loop theory
On Tue, 11 Jul 2017 17:39:37 -0400, Pat wrote:
On Tue, 11 Jul 2017 10:11:11 -0700, Jeff Liebermann wrote: On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote: Sounds good. As an aside, I just watched a youtube video of someone trying out one of these magnetic loop antennas. With his particular set of circumstances, it reduced the noise floor on 80 meters significantly. Compared to what other antenna? He probably reduced the received signal strength by the same amount leaving the SNR unchanged. Not really. Both signal and noise are reduced, but the SNR must be better because you can hear an SSB conversation with the loop that is not there with his sloper. Search youtube for MFJ-1886 and N9BC. https://www.youtube.com/watch?v=ECDklLp2FOk (2:56) Nice of him to crop off most of the spectrum analyzer display at the top of the screen so that I couldn't see the SNR changes. Also, he didn't indicate which antenna he was testing in the first video. He mostly fixed those problems in the 2nd video. https://www.youtube.com/watch?v=_-GPS1Kqfec (4:13) Of course, there is no mention of antenna patterns. Maybe the QSO he was listening to is in a null or his other antenna. Not enough info to really tell. Nice of him to compress the signal strength in the spectrum display so that it's difficult to compare SNR between antennas. There might be a difference between antennas, but my guess(tm) is that he has the AGC turned off in his SDR receiver. If AGC were on, the base line noise level would be about the same for both receivers, which would raise some questions as to whether there really was a difference. With the AGC turned off, the higher gain of the sloper antenna will show more baseline noise, which is what your seeing on the spectrum display. However, if I freeze the 2nd video for each antenna, and just look at the RELATIVE levels of the signals to the base line noise levels for each antenna, I think you'll see that they're fairly close[1]. That's the SNR which is what's important, and not the absolute levels of the noise and signal. Please note that the MJF-1886 is an amplified broadband untuned loop which means it has a rather low Q. It obtains no benefits from the narrowing the receive bandwidth as would be found in a high-Q transmit loop. Strong signals anywhere in the 1-30MHz amplifier bandwidth will create intermod products which might land where you're listening. Incidentally, if you disconnect the MJF-1886 amplifier, and use it like the PA0RDT miniwhip, my guess(tm) is that the loop and the miniwhip will work almost identically. Ok, Methinks I see the problem. The MFJ-1886 looks too good. With antennas, the uglier it looks, the better it works. Nice looking antennas just don't seem to work well. [1] I have a customer on the phone who wants my attention so this will need to wait. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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