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Magnetic Loops
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. -- Rick |
Magnetic Loops
On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. Indeed it is and why do you worship Wikipedia. w. |
Magnetic Loops
On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. The shielded loop reduces local noise pickup by eliminating much of the electric component of that noise in the near field. Since the ability of a small loop antenna to hear properly is primarily an exercise in improving the SNR, any reduction in noise levle, without a corresponding reduction in signal level, is a very good thing. More detail: http://electronics.stackexchange.com/questions/70262/what-if-anything-makes-shielded-loop-antennas-so-great-at-rejecting-local-nois I've built small loops that were not shielded and measure the SNR of some stable signal, such as WWV. I then wrapped the loop in aluminum duct tape, leaving a gap to prevent a shorted turn problem, retuned, and found that the baseline noise level had decreased and the SNR had improved. It works. -- 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 Loops
rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. I have read that the electric field sensitivity is non-directional, and therefore interferes with directivity even though the sensitivity is low. I have no idea if this makes sense when worked out quantitatively. -- Roger Hayter |
Magnetic Loops
rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. The single-turn tuned magnetic loop as used for transmitting is a different animal than the aperiodic loop of usually a couple of turns that is used for receive-only applications. The tuned loop cannot be shielded because of the parasitic capacitance that would add, it would limit the high end of the tuning range. Of course a shielded loop also will resonate at some frequency due to parasitic capacitance. |
Magnetic Loops
On 10/14/2015 1:34 PM, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. I bought a "Pixel" shielded magnetic loop from Pixel. It included a 30db LNA. It works better than my dipoles for receive on the 40 meter band on up. I guess I should be clear. I don't have 6 meters, so I am talking about 40, 20, 17, 15, and 10. The SNR is better than my dipoles on all these bands. It is significantly worse on 75 and 160. It was well worth the money. It is probably the best 400 bucks I have ever spent on ham radio. I just bought a used FTDX-3000. It has a special coax connector just for a receiving antenna. I can switch receive antennas on the front of the radio. A nice feature. |
Magnetic Loops
On 10/14/2015 3:23 PM, Jeff Liebermann wrote:
On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote: I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. The shielded loop reduces local noise pickup by eliminating much of the electric component of that noise in the near field. Since the ability of a small loop antenna to hear properly is primarily an exercise in improving the SNR, any reduction in noise levle, without a corresponding reduction in signal level, is a very good thing. More detail: http://electronics.stackexchange.com/questions/70262/what-if-anything-makes-shielded-loop-antennas-so-great-at-rejecting-local-nois I've built small loops that were not shielded and measure the SNR of some stable signal, such as WWV. I then wrapped the loop in aluminum duct tape, leaving a gap to prevent a shorted turn problem, retuned, and found that the baseline noise level had decreased and the SNR had improved. It works. I hope you realize that your experiment is not at all conclusive since wrapping the duct tape around your loop changes many things other than just adding a shield. Those other effects may or may not improve any given loop antenna. Do you understand the details of how such a shield should work? The link you provided gives several conflicting opinions on this including one very detailed post which claims there is little or no suppression of the E-field, rather it is only the nulls that are useful. It was finding posts like this that have made me doubt the suppression of the E-field by the shield. -- Rick |
Magnetic Loops
On Wed, 14 Oct 2015 20:38:23 -0400, rickman wrote:
I hope you realize that your experiment is not at all conclusive since wrapping the duct tape around your loop changes many things other than just adding a shield. Those other effects may or may not improve any given loop antenna. Yep. However, wrapping did improve the SNR a few dB, which is a sure sign that I must have done something right. Do you understand the details of how such a shield should work? The link you provided gives several conflicting opinions on this including one very detailed post which claims there is little or no suppression of the E-field, rather it is only the nulls that are useful. It gets worse, I just found this link, which says my explanation doesn't work: http://www.w8ji.com/magnetic_receiving_loops.htm What little is mentioned about shielded loops claims that it does not suppress the E-field and details how skin effect makes it work. I gotta work through this again to make sure I understand it. It was finding posts like this that have made me doubt the suppression of the E-field by the shield. Yep. The author of the above article definitely agrees with that. It may take me a while before I agree, but only after I understand how a shielded loop really works. -- 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 Loops
In message , Jeff Liebermann
writes On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote: I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. The shielded loop reduces local noise pickup by eliminating much of the electric component of that noise in the near field. Since the ability of a small loop antenna to hear properly is primarily an exercise in improving the SNR, any reduction in noise levle, without a corresponding reduction in signal level, is a very good thing. More detail: http://electronics.stackexchange.com...-if-anything-m akes-shielded-loop-antennas-so-great-at-rejecting-local-nois I've built small loops that were not shielded and measure the SNR of some stable signal, such as WWV. I then wrapped the loop in aluminum duct tape, leaving a gap to prevent a shorted turn problem, retuned, and found that the baseline noise level had decreased and the SNR had improved. It works. I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie |
Magnetic Loops
On 10/15/2015 6:13 AM, Brian Howie wrote:
In message , Jeff Liebermann writes On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote: I just read the wikipedia article on small loop antennas and it seems I was laboring under a misapprehension. I thought receiving loops were "magnetic" because they were shielded (this is often stated in various web pages about constructing such loops). But the wikipedia article on small loop antennas says the nature of a small loop is to not be very sensitive to the E field in near field. So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A lot of antenna designs make a big deal of the shield. So I assume it must be a useful addition to the small loop antenna for some purpose. The shielded loop reduces local noise pickup by eliminating much of the electric component of that noise in the near field. Since the ability of a small loop antenna to hear properly is primarily an exercise in improving the SNR, any reduction in noise levle, without a corresponding reduction in signal level, is a very good thing. More detail: http://electronics.stackexchange.com...-if-anything-m akes-shielded-loop-antennas-so-great-at-rejecting-local-nois I've built small loops that were not shielded and measure the SNR of some stable signal, such as WWV. I then wrapped the loop in aluminum duct tape, leaving a gap to prevent a shorted turn problem, retuned, and found that the baseline noise level had decreased and the SNR had improved. It works. I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. I assume there is nothing to space the wires from the pipe other than the insulation. Maybe you could use wire with thicker insulation? Or if you are using straight pipe, could you use a fabricated spacer at the corners? I guess that might be hard to assemble with soldering the joints. -- Rick |
Magnetic Loops
On Wednesday, October 14, 2015 at 1:34:16 PM UTC-5, rickman wrote:
So if the shield has little to do with rejecting near field electrical noise, what does the shield do? A shield with the usual gap promotes balance. And a small loop with a gapped shield is no quieter than a regular solenoid or pancake wound loop. Much of the usual "magnetic loop" theory that is on the web is malarkey. A small loop is a small loop is a small loop as long as all are properly balanced. |
Magnetic Loops
In message , rickman
writes I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. I assume there is nothing to space the wires from the pipe other than the insulation. Maybe you could use wire with thicker insulation? Or if you are using straight pipe, could you use a fabricated spacer at the corners? I guess that might be hard to assemble with soldering the joints. No just the insulation. It was hard enough to thread it without spacers .. I should have stuck to the original design that used plastic pipe with aluminium foil stuck to the outside Brian GM4DIJ -- Brian Howie |
Magnetic Loops
On 10/16/2015 6:53 AM, Brian Howie wrote:
In message , rickman writes I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. I assume there is nothing to space the wires from the pipe other than the insulation. Maybe you could use wire with thicker insulation? Or if you are using straight pipe, could you use a fabricated spacer at the corners? I guess that might be hard to assemble with soldering the joints. No just the insulation. It was hard enough to thread it without spacers . I should have stuck to the original design that used plastic pipe with aluminium foil stuck to the outside I saw one receiving antenna made from a bicycle rim. Easy to thread. I assume you only use this for receiving? -- Rick |
Magnetic Loops
On 16/10/15 17:07, rickman wrote:
On 10/16/2015 6:53 AM, Brian Howie wrote: In message , rickman writes I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. I assume there is nothing to space the wires from the pipe other than the insulation. Maybe you could use wire with thicker insulation? Or if you are using straight pipe, could you use a fabricated spacer at the corners? I guess that might be hard to assemble with soldering the joints. No just the insulation. It was hard enough to thread it without spacers . I should have stuck to the original design that used plastic pipe with aluminium foil stuck to the outside I saw one receiving antenna made from a bicycle rim. Easy to thread. I assume you only use this for receiving? At an AR convention in the Netherlands ,last year , there was a 14 MHz bicycle rim loop (aluminium) with motorised variable capacitor ,very well made. Recently I got a bicycle rim , made of stainless steel ,hence probably not very effective as tx antenna . But the rim can also serve as a guide for bending a copper loop .....shall try that after testing the stainless steel rim. Frank GM0CSZ / KN6WH |
Magnetic Loops
In message , rickman
writes On 10/16/2015 6:53 AM, Brian Howie wrote: In message , rickman writes I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. I assume there is nothing to space the wires from the pipe other than the insulation. Maybe you could use wire with thicker insulation? Or if you are using straight pipe, could you use a fabricated spacer at the corners? I guess that might be hard to assemble with soldering the joints. No just the insulation. It was hard enough to thread it without spacers . I should have stuck to the original design that used plastic pipe with aluminium foil stuck to the outside I saw one receiving antenna made from a bicycle rim. Easy to thread. I assume you only use this for receiving? Neat idea . I used a hula hoop for a previous version. Yes receive only. I wanted to cover 136kHz and 472Khz . In theory it should have done it , but for the capacitance. I had to take a lot of turns off, which also meant the coupling winding loaded the loop a lot more, reducing the Q factor. Brian -- Brian Howie |
Magnetic Loops
"Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. |
Magnetic Loops
"J.B. Wood" wrote in message ... Hello, and that seems to be ham radio jargon. Hams seem to think the adjectives "magnetic" and "electric" are needed when referring to loop and dipole antennas, respectively. Textbooks on electromagnetics and antennas don't use those terms except in the case when discussing theoretically small radiators, i.e. "magnetic dipoles" and "electric dipoles". My hypothesis on the ham terminology is that a loop is viewed as an inductor. That's OK for close-in (non-radiative) mutual coupling to some source but when you're several wavelengths away (in the far field) then the loop (or dipole antenna for that matter) responds to the electromagnetic field (the electric and magnetic far fields can't be considered separately). The fact that an axis of either antenna lines up with the electric or magnetic field vector in the far field is moot. Does this mean that the loop doesn't have inductance? Of course not and it plays a role in establishing the feedpoint impedance of the loop at the operating frequency. Now if folks would just stop using that word "literally" so damn much... Sincerely, and 73s from N4GG0, Hi I totally agree with you. You only get a feeling of an antenna behaviour a few wavelength from it. This is very hard to do at HF for amateurs. Specially in the vertical plane. I made a few tests of small loops in the broadcast FM band. What surprised me was their ,almost perfect,omnidirectional behaviour in horizontal polarisation. A too small vertical dipole needs to be loaded by a coil. The loop ,for me, is a too small slot aerial and it needs to be loaded by a capacitor. On receive both have a small efficiency due to their small size On HF this is hiden by the high level of noise. |
Magnetic Loops
In message , bilou
writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. Brian -- Brian Howie |
Magnetic Loops
On 10/19/2015 3:34 AM, Brian Howie wrote:
In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. I can't imagine why more turns won't help a receiving loop. I guess it depends on what is limiting reception. Adding a coil may improve the Q or it make make it worse depending on the Q of the coil. More turns won't help the Q of a receiving loop, other than reducing the significance of the resistance of connections and other components. More turns *will* increase the signal strength. How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. -- Rick |
Magnetic Loops
Brian Howie wrote:
In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. Brian To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. The number of turns greatly effects the inductance. Multiturn loops are used at VLF frequencies to get the inductance large enough so the loop resonants with a practical capacitor. Unless you are trying to operate on the 2200 meter band, forget multiple turn loops. -- Jim Pennino |
Magnetic Loops
On 10/19/2015 2:28 PM, wrote:
Brian Howie wrote: In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. Brian To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. If you don't like this reference, I know I have seen this info in other places too. The number of turns greatly effects the inductance. I believe it is N squared. Twice as many loops *and* twice as much interaction between the loops. Picture a triangle formed by the sum of the progression 1, 2, 3. That area is the inductance as you add loops. 1 1,2 1,2,3 1,2,3,4 1,2,3,4,5 I spent a great deal of time once trying to understand the formulas for inductance. Seems the problem is the non-idealities of coils significantly affect the results and vary a lot for different form factors, etc. So it is *very* hard to produce an equation that is good for all. The result is a number of different equations for different shapes and many different equations over the years as better approaches are found. I think the Lundin formula was the best one I found, even if a bit complex. The Wheeler formula is not as general or accurate, but simpler. Every formula I found used an N^2 term for the number of turns. Wheeler formulae http://home.earthlink.net/~jimlux/hv/wheeler.htm I can't find a good reference for Lundin's formula, but if you want I will copy my spread sheet data here or email a copy. :) -- Rick |
Magnetic Loops
"rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. |
Magnetic Loops
On 10/19/2015 2:14 PM, rickman wrote:
To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. Correct me if I'm wrong, A 1 meter square loop with 5 turns would equal 5 square meters. A = 5 sq. meters. A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters. A = 5 sq. meters. A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25 sq. meters. A = 25 Sq. meters. A 5 times increase in A (S) means about a 7db increase in signal strength. (minus losses caused by series inductor) Does that all seem right? Mikek |
Magnetic Loops
On 10/19/2015 3:50 PM, bilou wrote:
"rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... -- Rick |
Magnetic Loops
On 10/19/2015 7:55 PM, amdx wrote:
On 10/19/2015 2:14 PM, rickman wrote: To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. Correct me if I'm wrong, A 1 meter square loop with 5 turns would equal 5 square meters. A = 5 sq. meters. A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters. A = 5 sq. meters. A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25 sq. meters. A = 25 Sq. meters. A 5 times increase in A (S) means about a 7db increase in signal strength. (minus losses caused by series inductor) Does that all seem right? I forgot to include the following definitions. Е – is the voltage between antenna terminals in uV; е – is the intensity of electromagnetic wave in uV/m. Not sure where you are going with this. For the purpose of calculating the received signal strength of an antenna without factoring in resonance, the area is just the area of one loop (S = pi r^2), not the loop area times the number of turns. The number of turns (w) is multiplied by the loop area in the formula along with the relative permeability of the core material to get the effective area. Is that what you mean? The post that Jim made explicitly stated, "the number of turns has little to no effect on that", with "that" meaning "the amount of signal captured", or E in the above formula. That is the point I was correcting. So why do you feel the need to include a series inductor with the 25 m^2 1 turn loop? If you want to exercise some of the math for this, try the page here and tell me if the example about half way down the page was done correctly. I get a different value for the radiation resistance and I'm pretty sure the skin effect was not done correctly for the AC resistance. http://sidstation.loudet.org/antenna-theory-en.xhtml -- Rick |
Magnetic Loops
On 10/20/2015 3:03 AM, rickman wrote:
On 10/19/2015 7:55 PM, amdx wrote: On 10/19/2015 2:14 PM, rickman wrote: To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. Correct me if I'm wrong, A 1 meter square loop with 5 turns would equal 5 square meters. A = 5 sq. meters. A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters. A = 5 sq. meters. A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25 sq. meters. A = 25 Sq. meters. A 5 times increase in A (S) means about a 7db increase in signal strength. (minus losses caused by series inductor) Does that all seem right? I forgot to include the following definitions. Е – is the voltage between antenna terminals in uV; е – is the intensity of electromagnetic wave in uV/m. Not sure where you are going with this. For the purpose of calculating the received signal strength of an antenna without factoring in resonance, the area is just the area of one loop (S = pi r^2), not the loop area times the number of turns. The number of turns (w) is multiplied by the loop area in the formula along with the relative permeability of the core material to get the effective area. Is that what you mean? Yes. I was getting at the point, a loop single turn loop of 2.23 meters square will have the same E as a 1 meter square loop with 5 turns. Just some idea to consider when it comes to construction. The post that Jim made explicitly stated, "the number of turns has little to no effect on that", with "that" meaning "the amount of signal captured", or E in the above formula. That is the point I was correcting. For equal capture area, a single turn loop uses less than 1/2 the wire of a 5 turn loop. However you do lose inductance. So why do you feel the need to include a series inductor with the 25 m^2 1 turn loop? My thoughts are for a AMBCB loop, generally a 240uH loop and a 365pf cap. So I need the extra inductance to resonate it in the AM broadcast Band. If you want to exercise some of the math for this, try the page here and tell me if the example about half way down the page was done correctly. I get a different value for the radiation resistance and I'm pretty sure the skin effect was not done correctly for the AC resistance. http://sidstation.loudet.org/antenna-theory-en.xhtml I'm a good constructor, but as much as I'd like to, I can't help you with the math. Mikek |
Magnetic Loops
On 10/19/2015 10:53 PM, rickman wrote:
On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. Mikek |
Magnetic Loops
On 10/20/2015 10:44 AM, amdx wrote:
On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? -- Rick |
Magnetic Loops
On 10/20/2015 10:41 AM, amdx wrote:
On 10/20/2015 3:03 AM, rickman wrote: On 10/19/2015 7:55 PM, amdx wrote: On 10/19/2015 2:14 PM, rickman wrote: To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. Correct me if I'm wrong, A 1 meter square loop with 5 turns would equal 5 square meters. A = 5 sq. meters. A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters. A = 5 sq. meters. A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25 sq. meters. A = 25 Sq. meters. A 5 times increase in A (S) means about a 7db increase in signal strength. (minus losses caused by series inductor) Does that all seem right? I forgot to include the following definitions. Е – is the voltage between antenna terminals in uV; е – is the intensity of electromagnetic wave in uV/m. Not sure where you are going with this. For the purpose of calculating the received signal strength of an antenna without factoring in resonance, the area is just the area of one loop (S = pi r^2), not the loop area times the number of turns. The number of turns (w) is multiplied by the loop area in the formula along with the relative permeability of the core material to get the effective area. Is that what you mean? Yes. I was getting at the point, a loop single turn loop of 2.23 meters square will have the same E as a 1 meter square loop with 5 turns. Just some idea to consider when it comes to construction. Un-resonated E is not the only issue and often the size of the loop is limited because of the application. There are many tradeoffs involved in a receiving loop. Here are some shorthands that may help in seeing the issues. The starting point of L being proportional to r rather than rln(r) or the complex details of the inductance formula, which is an approximation I don't believe affects the results too much. L ∝ r * N² (if you see a funny symbol after the N, it's N squared) l ∝ r * N (that's wire length, not inductance) R ∝ l (resistance rather than radius) Q ∝ N (this is important to the result) E ∝ r² * N * Q E ∝ r² * N² E ∝ l² Once you take Q into account, the voltage from an antenna is primarily a function of the length of wire used rather than the other details. Of course the initial approximation has some impact on the results, but this points out that most of the issues involved in trading off size for turns is icing on the cake rather than the steak and potatoes. How do you like that metaphor? If you are Q limited (too much Q can narrow the bandwidth too much) then the above relations don't apply and E ∝ the total area or r² * N as you wrote. Making the inductance more accurate using L ∝ r * ln(r) * N² gives Q ∝ ln(r) * N E ∝ r² * N * Q E ∝ r² * ln(r) * N² E ∝ l² * ln(r) So a larger loop will give some better performance than more turns, but not hugely so. In the end convenience and practicality will have to limit the size of the loop with little degradation to performance. I just added this and have not reviewed it extensively, so please correct me if I've made an error. The post that Jim made explicitly stated, "the number of turns has little to no effect on that", with "that" meaning "the amount of signal captured", or E in the above formula. That is the point I was correcting. For equal capture area, a single turn loop uses less than 1/2 the wire of a 5 turn loop. However you do lose inductance. That is a *key* factor since Q is usually involved. So why do you feel the need to include a series inductor with the 25 m^2 1 turn loop? My thoughts are for a AMBCB loop, generally a 240uH loop and a 365pf cap. So I need the extra inductance to resonate it in the AM broadcast Band. You added the inductor for the 25 Sq. meters loop, but not the 5 sq. meters loops. That is my point. They would all need the inductor I think, no? Why not more turns to raise the inductance? If you want to exercise some of the math for this, try the page here and tell me if the example about half way down the page was done correctly. I get a different value for the radiation resistance and I'm pretty sure the skin effect was not done correctly for the AC resistance. http://sidstation.loudet.org/antenna-theory-en.xhtml I'm a good constructor, but as much as I'd like to, I can't help you with the math. I'm not looking for help, I'm pointing out an error in a web page. I don't like trusting any one resource. Heck, I've seen errors propagated across many web sites before as one borrows from another without checking. That's largely why I'm here and in a number of Yahoo groups. I want to get the straight skinny on things before I build mine. I'm in no hurry to get things built. Measure twice (or twenty times) and cut once. The Yahoo groups are more oriented to transmitting loops which is also very interesting. Seems to be a lot of experience, but sometimes lacking in true understanding. Not sure which is more important, I'm still short on both, lol. -- Rick |
Magnetic Loops
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Magnetic Loops
On 10/20/2015 3:03 AM, rickman wrote:
On 10/19/2015 7:55 PM, amdx wrote: On 10/19/2015 2:14 PM, rickman wrote: To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. Correct me if I'm wrong, A 1 meter square loop with 5 turns would equal 5 square meters. A = 5 sq. meters. A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters. A = 5 sq. meters. A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25 sq. meters. A = 25 Sq. meters. A 5 times increase in A (S) means about a 7db increase in signal strength. (minus losses caused by series inductor) Does that all seem right? I forgot to include the following definitions. Е – is the voltage between antenna terminals in uV; е – is the intensity of electromagnetic wave in uV/m. Not sure where you are going with this. For the purpose of calculating the received signal strength of an antenna without factoring in resonance, the area is just the area of one loop (S = pi r^2), not the loop area times the number of turns. The number of turns (w) is multiplied by the loop area in the formula along with the relative permeability of the core material to get the effective area. Is that what you mean? The post that Jim made explicitly stated, "the number of turns has little to no effect on that", with "that" meaning "the amount of signal captured", or E in the above formula. That is the point I was correcting. So why do you feel the need to include a series inductor with the 25 m^2 1 turn loop? I don't know what the inductance of a 1 turn 25 m^2 loop is, but I think it would need a very large variable capacitor to tune it. (Gut feeling) Just want to keep it under 1200pf. Because I have that size variable inductor. Mikek |
Magnetic Loops
On 10/20/2015 1:56 PM, rickman wrote:
On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. Mikek |
Magnetic Loops
On 10/20/2015 9:21 PM, amdx wrote:
On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. -- Rick |
Magnetic Loops
On 10/20/2015 9:17 PM, amdx wrote:
On 10/20/2015 3:03 AM, rickman wrote: On 10/19/2015 7:55 PM, amdx wrote: On 10/19/2015 2:14 PM, rickman wrote: To be a bit simplistic, the amount of signal captured is proportional to the loop area; the number of turns has little to no effect on that. I'm pretty sure that is not correct. The signal strength is proportional to the number of turns *and* the loop area. I will have to dig out my notes on this, but some factors (like Q) even out with various changes in antenna parameters such as number of turns, loop size, etc. But signal strength is proportional to the area of the loop and the number of turns. From http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm E = 2pi w S µR e / λ λ is the wavelength in meters w - the number of ML turns; S – is the area of the windings in m2; μR is the effective magnetic permeability of the ferrite rod SML. μR is always less than the permeability of the material used and depends from the size, geometry and the way the windings are constructed. μR = 1 for aerial loops. The product: А = w μR S (3) is called effective area of the SML. Correct me if I'm wrong, A 1 meter square loop with 5 turns would equal 5 square meters. A = 5 sq. meters. A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters. A = 5 sq. meters. A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25 sq. meters. A = 25 Sq. meters. A 5 times increase in A (S) means about a 7db increase in signal strength. (minus losses caused by series inductor) Does that all seem right? I forgot to include the following definitions. Е – is the voltage between antenna terminals in uV; е – is the intensity of electromagnetic wave in uV/m. Not sure where you are going with this. For the purpose of calculating the received signal strength of an antenna without factoring in resonance, the area is just the area of one loop (S = pi r^2), not the loop area times the number of turns. The number of turns (w) is multiplied by the loop area in the formula along with the relative permeability of the core material to get the effective area. Is that what you mean? The post that Jim made explicitly stated, "the number of turns has little to no effect on that", with "that" meaning "the amount of signal captured", or E in the above formula. That is the point I was correcting. So why do you feel the need to include a series inductor with the 25 m^2 1 turn loop? I don't know what the inductance of a 1 turn 25 m^2 loop is, but I think it would need a very large variable capacitor to tune it. (Gut feeling) Just want to keep it under 1200pf. Because I have that size variable inductor. That's not the question. I'm asking why you think this antenna needs an inductor and the other two don't. I'm guessing this is the only configuration you are considering. I'm not sure how practical a 5 meter tall loop will be if you are really serious about building it. If you make it from copper pipe it will be not only large, but heavy and require a lot of support to be used outside in winds. The capacitance needed will depend on the frequency you wish to tune. A round 5 meter single loop will be 29.5 uH. At 1 MHz it will require somewhat less than 1 nF if I've done the math right. I've got this in a spread sheet, but I've never verified it is set up correctly. If you want to work at lower frequencies you can use a smaller antenna radius and more turns which will increase the inductance letting you use a smaller cap to tune it. L ∝ r * N² Cut the radius by X, increase the number of turns by X and the inductance increases by X. Signal strength will only go down by a small amount related to the ln(). -- Rick |
Magnetic Loops
In message , rickman
writes On 10/19/2015 3:34 AM, Brian Howie wrote: In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. I can't imagine why more turns won't help a receiving loop. I guess it depends on what is limiting reception. Adding a coil may improve the Q or it make make it worse depending on the Q of the coil. More turns won't help the Q of a receiving loop, other than reducing the significance of the resistance of connections and other components. More turns *will* increase the signal strength. How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. The capacitance of the loop to the screen meant that at the minimum variable C setting ,I couldn't get the maximum frequency of about 500KHz I wanted, so I had to take turns off. I now need more parallel C to tune the look down to 136KHz. Brian -- Brian Howie |
Magnetic Loops
On 10/21/2015 2:18 AM, Brian Howie wrote:
In message , rickman writes On 10/19/2015 3:34 AM, Brian Howie wrote: In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. I can't imagine why more turns won't help a receiving loop. I guess it depends on what is limiting reception. Adding a coil may improve the Q or it make make it worse depending on the Q of the coil. More turns won't help the Q of a receiving loop, other than reducing the significance of the resistance of connections and other components. More turns *will* increase the signal strength. How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. The capacitance of the loop to the screen meant that at the minimum variable C setting ,I couldn't get the maximum frequency of about 500KHz I wanted, so I had to take turns off. I now need more parallel C to tune the look down to 136KHz. Wow, that loop must have a *lot* of capacitance. Is there a way to space the conductors away from the copper tubing in the run? I'm curious why you would use copper pipe for the shield. Because it provides both shield and support? I guess there are a million ways to build a shielded loop. I like the idea of using coax, but I don't know if that also has serious limitations from the capacitance between loop conductor and shield. -- Rick |
Magnetic Loops
In message , rickman
writes The capacitance of the loop to the screen meant that at the minimum variable C setting ,I couldn't get the maximum frequency of about 500KHz I wanted, so I had to take turns off. I now need more parallel C to tune the look down to 136KHz. Wow, that loop must have a *lot* of capacitance. Is there a way to space the conductors away from the copper tubing in the run? Not easy I'm curious why you would use copper pipe for the shield. Because it provides both shield and support? I guess there are a million ways to build a shielded loop. I like the idea of using coax, but I don't know if that also has serious limitations from the capacitance between loop conductor and shield. It seemed a good idea at the time. The original design used plastic pipe covered with tin-foil ,but I wanted something that would survive a Scottish winter outdoors. PVC 4-7 Loop Antenna Al Burzynski KA5JGV ( it's on the NDB yahoo group) it used 12 turns. I think the use of plastic pipe and external tinfoil reduces the C. My loop does work quite well, and has survived outdoors but I think it could be improved Brian -- Brian Howie |
Magnetic Loops
On 10/20/2015 10:35 PM, rickman wrote:
On 10/20/2015 9:21 PM, amdx wrote: On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. It obviously works. It is not ideal because it would have a lot of interwinding capacitance. Also the interwinding capacitance is not a quality capacitance thus the Q is lowered. It could be built with space between wire and layers, and 256 solder connections is not a great idea when trying to insure high Q. As far as "bad idea", all it has to do is receive enough signal to keep the clock accurate, more than that is interesting, but useless. Mikek |
Magnetic Loops
On 10/21/2015 2:06 AM, rickman wrote:
On 10/21/2015 2:18 AM, Brian Howie wrote: In message , rickman writes On 10/19/2015 3:34 AM, Brian Howie wrote: In message , bilou writes "Brian Howie" wrote in message ... I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with a gap , 7 turns inside plus a coupling winding. It does a good job eliminating local noise (mostly ASDL hash from the phone lines) compared with a vertical. However the capacitance between the shield and turns seems to load it quite a bit meaning I can't get the tuning range I'd like. Brian GM4DIJ -- Brian Howie Hi My own experience is that ,at least for receive, multi turn loops are useless. Instead you can use a single turn one with a good coil in serial. The tuning range for a given variable capacitor is much greater especially if ,at low frequency, the coil is using ferrite . Switching the coil can increase the tuning range easily. The coil, with a secondary winding,is also very useful to adjust the coupling to the receiver. I'd have thought I'd get a better signal from more turns, but maybe better coupling and a higher Q from your suggestion would do the same. I can't imagine why more turns won't help a receiving loop. I guess it depends on what is limiting reception. Adding a coil may improve the Q or it make make it worse depending on the Q of the coil. More turns won't help the Q of a receiving loop, other than reducing the significance of the resistance of connections and other components. More turns *will* increase the signal strength. How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. The capacitance of the loop to the screen meant that at the minimum variable C setting ,I couldn't get the maximum frequency of about 500KHz I wanted, so I had to take turns off. I now need more parallel C to tune the look down to 136KHz. Wow, that loop must have a *lot* of capacitance. Is there a way to space the conductors away from the copper tubing in the run? I'm curious why you would use copper pipe for the shield. Because it provides both shield and support? I guess there are a million ways to build a shielded loop. I like the idea of using coax, but I don't know if that also has serious limitations from the capacitance between loop conductor and shield. 30pf per ft is a general number for capacitance of coax, but you know it varies with type. I have some coax for automobile radio antennas (AM/FM) that has 8pf per foot. Mikek |
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