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Loop Antenna at ~60 kHz
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. Anyone have an idea of how to estimate the radiation resistance of a tuned, shielded loop antenna? The other factor I don't understand how to factor in is the distributed capacitance of the coax. Is that a significant influence on an antenna or is it in the noise compared to the tuning capacitor. The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or would the resistance be a round trip measurement of both inner conductor and shield? I assume the shield has a much lower resistance than the inner conductor but I don't know that for sure. -- Rick |
Loop Antenna at ~60 kHz
rickman wrote in :
I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. MSF time signals? Just a thought... If you're interfacing an analog signal to digital, one trick I used (for audio but it ought to help here too) is a CA3140 with a bit of positive feedback through a few Mohms for hysteresis to clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or 6V, could be sensitive to take a lot of strain off your antenna. Whether this alone gives you enough gain I don't know, but it is cheap to try. |
Loop Antenna at ~60 kHz
In rec.radio.amateur.antenna rickman wrote:
I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. Anyone have an idea of how to estimate the radiation resistance of a tuned, shielded loop antenna? The other factor I don't understand how to factor in is the distributed capacitance of the coax. Is that a significant influence on an antenna or is it in the noise compared to the tuning capacitor. The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or would the resistance be a round trip measurement of both inner conductor and shield? I assume the shield has a much lower resistance than the inner conductor but I don't know that for sure. Google DIY WWVB antenna 16,900 results. As for the output voltage, you do know FET input opamps work quite well at 60 Khz and are dirt cheap? FYI for those on the other side of the pond, WWVB is a US 60 kHz time and frequency station. -- Jim Pennino |
Loop Antenna at ~60 kHz
"rickman" wrote in message ... I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. I don't think I would try and reinvent that type of antenna. There are several designs on the web that use a loop about 3 feet in diameter and several turns of wire inside the shield. In most cases a low noise preamp is needed, but that shold be simpleand inexpensive to build. Go to this page and go toward the bottom for some loop antenna ideas. http://www.w4dex.com/lf.htm I have known Dexter for around 40 years. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Loop Antenna at ~60 kHz
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Loop Antenna at ~60 kHz
On 10/28/2014 5:24 PM, Lostgallifreyan wrote:
rickman wrote in : I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. MSF time signals? Just a thought... If you're interfacing an analog signal to digital, one trick I used (for audio but it ought to help here too) is a CA3140 with a bit of positive feedback through a few Mohms for hysteresis to clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or 6V, could be sensitive to take a lot of strain off your antenna. Whether this alone gives you enough gain I don't know, but it is cheap to try. Thanks for the suggestion. I'm not sure this would be any better than feeding it directly into my digital input. That is a differential input and I expect to use feedback to overcome the residual input offset. So the input will be pretty sensitive, the question is whether I need mV level signals or maybe just uV signals which might not require an amp. By using positive feedback the threshold would be shifting and the amount of level shift would set the floor for the signal level from the antenna I think. -- Rick |
Loop Antenna at ~60 kHz
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Loop Antenna at ~60 kHz
On 28/10/14 20:33, rickman wrote:
I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. To my mind you seem to be over-thinking, and perhaps over-engineering, this project. I'm a string-and-sealing-wax UK-based Amateur, and my solution to a similar problem was to take a simple approach: I put a one-turn loop round the outside of a wardrobe and linked that straight into the 600-ohm balanced input to my receiver. That was enough to drop the local noise levels by a dramatic amount, and was easily sufficient for my purposes. Using an electric aerial, the signal was unreadable. My suggestion is to start simple and find out if that is enough, and make improvements one at a time. There could well be no real need to have a computer-generated solution requiring high-grade components to function. Whatever route you choose, good luck! -- Spike "The greatest dangers to liberty lurk in the insidious encroachment by men of zeal, well meaning but without understanding" Louis D. Brandeis |
Loop Antenna at ~60 kHz
On 10/28/2014 6:14 PM, Ralph Mowery wrote:
"rickman" wrote in message ... I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. I don't think I would try and reinvent that type of antenna. There are several designs on the web that use a loop about 3 feet in diameter and several turns of wire inside the shield. In most cases a low noise preamp is needed, but that shold be simpleand inexpensive to build. Go to this page and go toward the bottom for some loop antenna ideas. http://www.w4dex.com/lf.htm I have known Dexter for around 40 years. I am not sure what you mean by "reinvent" that type of antenna. Every antenna can be optimized for a given design. My requirements are very unique. I need as much voltage from the antenna as possible. My receiver input impedance can be very high (~1 Mohm) which is very different from a typical receiver. I have already gone down the road of looking extensively at loop antenna designs. I have not found a significant difference other than the ease of construction. That is one reason why I chose to use coax rather than wire within a shield like pipe or a bicycle rim (as I found in one project). My current design is 100 feet (the 50 feet I said originally was due to my poor recollection) wound on a 2 foot diameter spoke arrangement of wood which turned out pretty well for a first pass. I have yet to characterize the antenna which may be the easier path than trying to construct a good model from theory and the known details. Several people have suggested that a preamp will be required. That may be possible. But this is not an analog receiver and don't need a lot of SNR for it to work. The time code signal is modulated at 1 bps using both phase and amplitude modulation and pulse width bit encoding. I will need a resolution of no worse than 100 milliseconds to decode the bits. So I figure a bandwidth of 10 Hz should be plenty enough. This means I can vastly over sample the signal and get lots of gain digitally. So the tricky part is to overcome the poor analog characteristics of the differential digital input. I only need it to turn the input signal into a one or a zero, but it needs to be sensitive to a very small signal. With the various imperfections of input offset, hysteresis, etc., I will be lucky if it works with very low voltage signals at all. I could rig up a test circuit and see just what signal levels are needed. The other part is that the purpose of this design is to receive the signal digitally on as low a power level as possible. The entire power budget is a couple hundred microwatts. I have yet to find an amplifier that will fit this power budget. Oddly enough some folks in s.e.d told me that transistors don't work well with low bias currents, but that may only apply to bipolar amps. They make time code receiver chips to do this on a few hundred microwatts and have an internal amplifier. So obviously it can be done. I just can't find a low enough power opamp for a 60 kHz signal. Also this a learning exercise for me. So reinventing something would be ideal! -- Rick |
Loop Antenna at ~60 kHz
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Loop Antenna at ~60 kHz
rickman wrote:
On 10/28/2014 8:18 PM, wrote: snip Not sure what you are saying. There are tons of examples. But when I did my search there was very little info on the design of loop antennas. At least not much in depth enough to let me figure out how much signal I might get from a given circuit. I am asking about specific details of loop antenna design. I'm not sure why people keep suggesting I look at "examples". WWVB is a US time and frequency standard station and the Internet is full of articles on DIY antennas and receivers for WWVB. Many of those articles go into great detail about their design. As you asked about 60 kHz, it would seem to me to be the place to start to look for words of wisdom on the subject, no matter what particular detail you are looking for. -- Jim Pennino |
Loop Antenna at ~60 kHz
rickman wrote:
On 10/28/2014 6:14 PM, Ralph Mowery wrote: "rickman" wrote in message ... I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. I don't think I would try and reinvent that type of antenna. There are several designs on the web that use a loop about 3 feet in diameter and several turns of wire inside the shield. In most cases a low noise preamp is needed, but that shold be simpleand inexpensive to build. Go to this page and go toward the bottom for some loop antenna ideas. http://www.w4dex.com/lf.htm I have known Dexter for around 40 years. I am not sure what you mean by "reinvent" that type of antenna. Every antenna can be optimized for a given design. My requirements are very unique. I need as much voltage from the antenna as possible. My receiver input impedance can be very high (~1 Mohm) which is very different from a typical receiver. I have already gone down the road of looking extensively at loop antenna designs. I have not found a significant difference other than the ease of construction. That is one reason why I chose to use coax rather than wire within a shield like pipe or a bicycle rim (as I found in one project). My current design is 100 feet (the 50 feet I said originally was due to my poor recollection) wound on a 2 foot diameter spoke arrangement of wood which turned out pretty well for a first pass. I have yet to characterize the antenna which may be the easier path than trying to construct a good model from theory and the known details. Several people have suggested that a preamp will be required. That may be possible. But this is not an analog receiver and don't need a lot of SNR for it to work. The time code signal is modulated at 1 bps using both phase and amplitude modulation and pulse width bit encoding. I will need a resolution of no worse than 100 milliseconds to decode the bits. So I figure a bandwidth of 10 Hz should be plenty enough. This means I can vastly over sample the signal and get lots of gain digitally. So the tricky part is to overcome the poor analog characteristics of the differential digital input. I only need it to turn the input signal into a one or a zero, but it needs to be sensitive to a very small signal. With the various imperfections of input offset, hysteresis, etc., I will be lucky if it works with very low voltage signals at all. I could rig up a test circuit and see just what signal levels are needed. The other part is that the purpose of this design is to receive the signal digitally on as low a power level as possible. The entire power budget is a couple hundred microwatts. I have yet to find an amplifier that will fit this power budget. Oddly enough some folks in s.e.d told me that transistors don't work well with low bias currents, but that may only apply to bipolar amps. They make time code receiver chips to do this on a few hundred microwatts and have an internal amplifier. So obviously it can be done. I just can't find a low enough power opamp for a 60 kHz signal. Also this a learning exercise for me. So reinventing something would be ideal! For commercial designs, I keep seeing references to a ferrite core with a winding on it, as an antenna. The article here, describes two kinds of receivers. One is sensitive to AC pickup, so would only be a candidate in special physical circumstances. The other uses the high impedance input. http://home.pon.net/785/equipment/build_your_own.htm It suggests to me at least, you want plenty of gain on the input stage, plus enough filtering to reject louder noise sources. Your digital processing section can provide the selectivity. But if spurious out of band signals saturate your gain stage, you might not get the desired result. It would all depend on the tradeoffs you want to make. You'll always require a gain stage. Perhaps the antenna of your choice (not your final design) and a spectrum analyser that works in that range of frequencies, you can do a survey to see what is possible. What noise sources are immediately evident, and so on. No big antenna here. The antenna is one of these. http://www.maplin.co.uk/p/ferrite-rod-aerial-lb12n http://www.burningimage.net/clock/20...0khz-receiver/ I think by "sensitive" what they meant was "it picked up the signal I wanted". The circuit diagram would have been labeled "insensitive" if no signal was found. Or if it didn't oscillate at 60KHz on its own (like a couple amplifiers to drive speakers have done here) :-) I think some audio circuit I built, checking with a scope later on, indicated a nice fat signal at 500KHz. Great. Perhaps using your big loop of wire, you get to remove one of the op-amps. ******* The circuit above uses TL-081, with gain bandwidth product of 3MHz. So I guess that's why there is still a bit of gain at 72KHz. In school, were were shown an example of a filter that used only resistors. An example is seen on Fig 2.27(c) on PDF page 70. The neat thing about this topology, is it was working at 50KHz on a pair of $0.25 opamps. It uses the pole of the output stage of the opamp, as a filter element. We had some afternoon lab to do, with this circuit as part of the work. http://www.springer.com/cda/content/...022-p174507347 9780817683573-c1.pdf 3,791,230 bytes The book table of contents is here. It's by Mohan, P.V.A. With ISBN 978-0-8176-8357-3. I was hoping the topology had a name, but I don't see one. http://www.springer.com/cda/content/...069-p174507347 So the circuit could be in range of some opamps. And then you might not need a huge antenna. HTH, Paul |
Loop Antenna at ~60 kHz
On 10/28/2014 9:27 PM, wrote:
rickman wrote: On 10/28/2014 8:18 PM, wrote: snip Not sure what you are saying. There are tons of examples. But when I did my search there was very little info on the design of loop antennas. At least not much in depth enough to let me figure out how much signal I might get from a given circuit. I am asking about specific details of loop antenna design. I'm not sure why people keep suggesting I look at "examples". WWVB is a US time and frequency standard station and the Internet is full of articles on DIY antennas and receivers for WWVB. Many of those articles go into great detail about their design. As you asked about 60 kHz, it would seem to me to be the place to start to look for words of wisdom on the subject, no matter what particular detail you are looking for. I believe I said I have read much of that info. I did this a couple of years ago and picked an approach. I was not able to convince myself it would work properly. So now, before I build anything more, I would like to fill in some of the details. Of all the design info I found, not one discussed optimizing the antenna for maximum voltage. When I was discussing this in another group, specifically about a spice simulation of the circuit, someone pointed out that I needed to include the effect of the radiation resistance. Again, I have not found any other discussions of the radiation resistance of a receiving antenna, specifically a tuned, shielded loop antenna. Is this a red herring? When designing an antenna with a very high Q, can the radiation resistance of a shielded loop antenna be ignored? You say I should "start" with the many words of wisdom on the subject. I am not "starting" and I have found many words of wisdom on loop antennas in general, but not much on the specific questions I am asking. It's a little bit funny, but when the one who shall not be named asked about short antennas the discussions were full of info on radiation resistance and details. Now that I am asking about my design, no one wants to discuss the technical issues and just recommend some site where they tell you how to build the antenna that suited their purpose. -- Rick |
Loop Antenna at ~60 kHz
rickman wrote:
On 10/28/2014 9:27 PM, wrote: rickman wrote: On 10/28/2014 8:18 PM, wrote: snip Not sure what you are saying. There are tons of examples. But when I did my search there was very little info on the design of loop antennas. At least not much in depth enough to let me figure out how much signal I might get from a given circuit. I am asking about specific details of loop antenna design. I'm not sure why people keep suggesting I look at "examples". WWVB is a US time and frequency standard station and the Internet is full of articles on DIY antennas and receivers for WWVB. Many of those articles go into great detail about their design. As you asked about 60 kHz, it would seem to me to be the place to start to look for words of wisdom on the subject, no matter what particular detail you are looking for. I believe I said I have read much of that info. I did this a couple of Yes you did, after you made the post I responded to. years ago and picked an approach. I was not able to convince myself it would work properly. So now, before I build anything more, I would like to fill in some of the details. Of all the design info I found, not one discussed optimizing the antenna for maximum voltage. When I was discussing this in another group, specifically about a spice simulation of the circuit, someone pointed out that I needed to include the effect of the radiation resistance. Again, I have not found any other discussions of the radiation resistance of a receiving antenna, specifically a tuned, shielded loop antenna. Is this a red herring? When designing an antenna with a very high Q, can the radiation resistance of a shielded loop antenna be ignored? The radiation resistance is a reciprocal property, i.e. it is the same for transmitting and receiving. I will assume you already know the relationship of resistance to Q. You say I should "start" with the many words of wisdom on the subject. I am not "starting" and I have found many words of wisdom on loop antennas in general, but not much on the specific questions I am asking. That's all well and good but not evident until way into the postings. It's a little bit funny, but when the one who shall not be named asked about short antennas the discussions were full of info on radiation resistance and details. Now that I am asking about my design, no one wants to discuss the technical issues and just recommend some site where they tell you how to build the antenna that suited their purpose. Again, radiation resistance is a reciprocal property. To determine such things, you need to use an antenna analysis tool and plug the resultant numbers into Spice which will tell you whether or not it can be ignored. -- Jim Pennino |
Loop Antenna at ~60 kHz
On 10/28/2014 9:53 PM, Paul wrote:
rickman wrote: On 10/28/2014 6:14 PM, Ralph Mowery wrote: "rickman" wrote in message ... I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. I don't think I would try and reinvent that type of antenna. There are several designs on the web that use a loop about 3 feet in diameter and several turns of wire inside the shield. In most cases a low noise preamp is needed, but that shold be simpleand inexpensive to build. Go to this page and go toward the bottom for some loop antenna ideas. http://www.w4dex.com/lf.htm I have known Dexter for around 40 years. I am not sure what you mean by "reinvent" that type of antenna. Every antenna can be optimized for a given design. My requirements are very unique. I need as much voltage from the antenna as possible. My receiver input impedance can be very high (~1 Mohm) which is very different from a typical receiver. I have already gone down the road of looking extensively at loop antenna designs. I have not found a significant difference other than the ease of construction. That is one reason why I chose to use coax rather than wire within a shield like pipe or a bicycle rim (as I found in one project). My current design is 100 feet (the 50 feet I said originally was due to my poor recollection) wound on a 2 foot diameter spoke arrangement of wood which turned out pretty well for a first pass. I have yet to characterize the antenna which may be the easier path than trying to construct a good model from theory and the known details. Several people have suggested that a preamp will be required. That may be possible. But this is not an analog receiver and don't need a lot of SNR for it to work. The time code signal is modulated at 1 bps using both phase and amplitude modulation and pulse width bit encoding. I will need a resolution of no worse than 100 milliseconds to decode the bits. So I figure a bandwidth of 10 Hz should be plenty enough. This means I can vastly over sample the signal and get lots of gain digitally. So the tricky part is to overcome the poor analog characteristics of the differential digital input. I only need it to turn the input signal into a one or a zero, but it needs to be sensitive to a very small signal. With the various imperfections of input offset, hysteresis, etc., I will be lucky if it works with very low voltage signals at all. I could rig up a test circuit and see just what signal levels are needed. The other part is that the purpose of this design is to receive the signal digitally on as low a power level as possible. The entire power budget is a couple hundred microwatts. I have yet to find an amplifier that will fit this power budget. Oddly enough some folks in s.e.d told me that transistors don't work well with low bias currents, but that may only apply to bipolar amps. They make time code receiver chips to do this on a few hundred microwatts and have an internal amplifier. So obviously it can be done. I just can't find a low enough power opamp for a 60 kHz signal. Also this a learning exercise for me. So reinventing something would be ideal! For commercial designs, I keep seeing references to a ferrite core with a winding on it, as an antenna. Yes, a ferrite antenna is commonly used because of it's small size. But when I crunched the numbers a larger loop produces a larger output voltage than did the small loop of a ferrite antenna. The ferrite only increases the output by the relative permeability, a constant of the ferrite material that is relatively small compared to the gain of a larger loop which goes by the the area of the loop proportional to the square of the radius/circumference or for a constant length of wire is inversely proportional to the number of turns. In other words you can do more by making your loop larger than you can by using a ferrite core... assuming you are not restricted to your loop size. The length of the antenna wire is important because it determines much of your losses and so the Q. The Q of the antenna is the ratio of the total loss resistance to the inductive reactance. Since the Q depends on the inductance things get complex. L ∝ N^2 * A where N is the number of turns and A is the loop area The output voltage of the tuned antenna circuit is the product of the effective height, Q and field strength or V ∝ he * Q since the field strength is constant. Effective height is the number of turns times the area divided by the wavelength. he ∝ N * A since the wavelength is constant. This gives V ∝ N^3 * A^2 / Rloss Looses are from wire resistance with skin effect and radiation resistance. Assuming Rloss is mostly from the resistance of the wire with skin effect which will be related to the wire length we can hold that constant and look at V as a function of the tradeoff between A and N. N ∝ 1/r and A ∝ r^2. So replacing both N and A we have V ∝ (1/r)^3 * r^4 or r, so a larger radius gives the strongest signal everything else being equal. While the permittivity may affect the signal from the antenna, the typical ferrite antenna is many small if not tiny loops while fewer, larger loops without a ferrite should give a stronger signal. I think this is the first time I have done this all as one line of thought, so I may have made a mistake somewhere. But I'm pretty sure the result is correct. It may be mitigated by the small gauge of the wire normally used for ferrite coils allowing more turns to be used. But again, that same wire can be used with a larger loop size even if it does lower the Q. More interesting is the impact of wire diameter on the whole thing. The RG-6 wire I chose is about optimal regarding the conductor diameter with the skin affect making anything larger not of much value. Of course the fact that it is coax makes it a lot larger when using lots of turns. This page has a very good drawing of the circuit showing all the elements about a quarter of the way down the page. http://sidstation.loudet.org/antenna-theory-en.xhtml The article here, describes two kinds of receivers. One is sensitive to AC pickup, so would only be a candidate in special physical circumstances. The other uses the high impedance input. http://home.pon.net/785/equipment/build_your_own.htm It suggests to me at least, you want plenty of gain on the input stage, plus enough filtering to reject louder noise sources. Your digital processing section can provide the selectivity. But if spurious out of band signals saturate your gain stage, you might not get the desired result. It would all depend on the tradeoffs you want to make. You'll always require a gain stage. I'm not sure what you mean by AC pickup, I guess you mean stray power line signal? The E field receiver is pretty much what I don't want. The antenna picks up very little signal because of the small physical size while being very large. The E field is allegedly the source of a lot of near field interference from appliances. The (again alleged) advantage of the magnetic antenna is that the shield blocks the E field and reduces many interference sources. I say alleged because I have not seen much verifiable info on this and at least one source I found (and have since lost) disputed the claim of reduced interference by the shield. The only thing I found of value from this link was the emphasis on low pass filters, which in my case will be band pass filters, first in the antenna itself and then in the receiver. Perhaps the antenna of your choice (not your final design) and a spectrum analyser that works in that range of frequencies, you can do a survey to see what is possible. What noise sources are immediately evident, and so on. No big antenna here. The antenna is one of these. http://www.maplin.co.uk/p/ferrite-rod-aerial-lb12n http://www.burningimage.net/clock/20...0khz-receiver/ I think by "sensitive" what they meant was "it picked up the signal I wanted". The circuit diagram would have been labeled "insensitive" if no signal was found. Or if it didn't oscillate at 60KHz on its own (like a couple amplifiers to drive speakers have done here) :-) I think some audio circuit I built, checking with a scope later on, indicated a nice fat signal at 500KHz. Great. Perhaps using your big loop of wire, you get to remove one of the op-amps. ******* The circuit above uses TL-081, with gain bandwidth product of 3MHz. So I guess that's why there is still a bit of gain at 72KHz. In school, were were shown an example of a filter that used only resistors. An example is seen on Fig 2.27(c) on PDF page 70. The neat thing about this topology, is it was working at 50KHz on a pair of $0.25 opamps. It uses the pole of the output stage of the opamp, as a filter element. We had some afternoon lab to do, with this circuit as part of the work. http://www.springer.com/cda/content/...022-p174507347 9780817683573-c1.pdf 3,791,230 bytes The book table of contents is here. It's by Mohan, P.V.A. With ISBN 978-0-8176-8357-3. I was hoping the topology had a name, but I don't see one. http://www.springer.com/cda/content/...069-p174507347 So the circuit could be in range of some opamps. And then you might not need a huge antenna. Thanks for your suggestions. My purpose in building this is not to receive the WWVB signal. If it were I would just buy one of the small kits that do it with two chips and a ferrite antenna. My purpose is to receive the WWVB signal with a digital receiver that is close to the power consumption of the analog receiver. -- Rick |
Loop Antenna at ~60 kHz
On 10/29/2014 12:10 AM, wrote:
rickman wrote: On 10/28/2014 9:27 PM, wrote: rickman wrote: On 10/28/2014 8:18 PM, wrote: snip Not sure what you are saying. There are tons of examples. But when I did my search there was very little info on the design of loop antennas. At least not much in depth enough to let me figure out how much signal I might get from a given circuit. I am asking about specific details of loop antenna design. I'm not sure why people keep suggesting I look at "examples". WWVB is a US time and frequency standard station and the Internet is full of articles on DIY antennas and receivers for WWVB. Many of those articles go into great detail about their design. As you asked about 60 kHz, it would seem to me to be the place to start to look for words of wisdom on the subject, no matter what particular detail you are looking for. I believe I said I have read much of that info. I did this a couple of Yes you did, after you made the post I responded to. years ago and picked an approach. I was not able to convince myself it would work properly. So now, before I build anything more, I would like to fill in some of the details. Of all the design info I found, not one discussed optimizing the antenna for maximum voltage. When I was discussing this in another group, specifically about a spice simulation of the circuit, someone pointed out that I needed to include the effect of the radiation resistance. Again, I have not found any other discussions of the radiation resistance of a receiving antenna, specifically a tuned, shielded loop antenna. Is this a red herring? When designing an antenna with a very high Q, can the radiation resistance of a shielded loop antenna be ignored? The radiation resistance is a reciprocal property, i.e. it is the same for transmitting and receiving. Yes, but I've never calculated it for either. I have been working with effective height. That is one thing I'm not clear on, how the two effects can be separated. I guess the radiation resistance has different impact depending on the circuit used. It will be more apparent in a high Q circuit than a low Q one. I will assume you already know the relationship of resistance to Q. You say I should "start" with the many words of wisdom on the subject. I am not "starting" and I have found many words of wisdom on loop antennas in general, but not much on the specific questions I am asking. That's all well and good but not evident until way into the postings. It's a little bit funny, but when the one who shall not be named asked about short antennas the discussions were full of info on radiation resistance and details. Now that I am asking about my design, no one wants to discuss the technical issues and just recommend some site where they tell you how to build the antenna that suited their purpose. Again, radiation resistance is a reciprocal property. To determine such things, you need to use an antenna analysis tool and plug the resultant numbers into Spice which will tell you whether or not it can be ignored. Actually, I was researching to verify my conclusions made the last time I took a stab at this and found a page that gives a formula for radiation resistance proportional to (μr N A/λ^2)^2. I hope the Greek letters show properly. Holding μr and λ constant that makes the radiation resistance proportional to r^2. I will need to check this to see if it is significant compared to the resistive losses. -- Rick |
Loop Antenna at ~60 kHz
rickman wrote in :
MSF time signals? Just a thought... If you're interfacing an analog signal to digital, one trick I used (for audio but it ought to help here too) is a CA3140 with a bit of positive feedback through a few Mohms for hysteresis to clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or 6V, could be sensitive to take a lot of strain off your antenna. Whether this alone gives you enough gain I don't know, but it is cheap to try. Thanks for the suggestion. I'm not sure this would be any better than feeding it directly into my digital input. That is a differential input and I expect to use feedback to overcome the residual input offset. So the input will be pretty sensitive Well, try it. :) If it works then inputs are better these days. Or at least, more sensitive to small changes. As far as I know, digital inputs are usually specified with a wide dead band for levels, amounting to HUGE hysteresis and a need for a lot of gain first sp you already ned an op-amp stage no matter what unless your digital inputs have hair triggers at exactly the threshold you wanr. The thing about the CA3140 is that with just three passive parts: M-ohmage of positive feedback, input series capacitance, and input ground resistor after the cap, you can empirically set some very nice signal preconditioning as well as raw gain, all on a very convenient single rail supply at 5V. |
Loop Antenna at ~60 kHz
rickman wrote in :
By using positive feedback the threshold would be shifting and the amount of level shift would set the floor for the signal level from the antenna I think. Yes, basically like a noise gate. The op-amp trick is nice though, it gives you fine control of it. |
Loop Antenna at ~60 kHz
rickman wrote in :
I only need it to turn the input signal into a one or a zero, but it needs to be sensitive to a very small signal. My widget was aimed at exactly this need. :) That's why I recognised a new need. My original need was for an electret mic. but it had to be so sensitive I could whistle gently with barely pitched sound on the other side of a quiet room and have it track like a fighter jet's navigation. It was a lovely combination of sensitivity and clean reliability too, intended as the front end control of an electronic musical instrument. I'd used a bit of gain and bandpassing before the CA3140 Schmitt trigger, but in the case of time signals I doubt it would need this extra preprocessing. Just add a cap and resistor on output to integrate the 60KHz into clean slow pulses. |
Loop Antenna at ~60 kHz
rickman wrote in :
The entire power budget is a couple hundred microwatts. There's a tiny Texas Instruments one that might do it, very cheap too. TLV2341, uses as little as 17A single rail supply at up to 8V. I didn't use it because it wasn't fast enough for what I bought it for, but it might be worth trying for MSF signals. |
Loop Antenna at ~60 kHz
rickman wrote in :
The entire power budget is a couple hundred microwatts. I have yet to find an amplifier that will fit this power budget. That TLV2341 will stretch to do this drawing just 17A, UGB is only 27KHz, but if you set it for medium bias, consuming 250A, you'll get 300KHz. Not sure how much gain it will let you have for 60KHz, but I think it's one to try. |
Loop Antenna at ~60 kHz
El 28-10-14 21:33, rickman escribi:
I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. Anyone have an idea of how to estimate the radiation resistance of a tuned, shielded loop antenna? The other factor I don't understand how to factor in is the distributed capacitance of the coax. Is that a significant influence on an antenna or is it in the noise compared to the tuning capacitor. The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or would the resistance be a round trip measurement of both inner conductor and shield? I assume the shield has a much lower resistance than the inner conductor but I don't know that for sure. To get some idea of the output voltage of a loop you need to know: The fieldstrength of the desired signal at your area. This is probably given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to convert this to H-field [A/m]. EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number of turns, w = radian frequency, u0 = magn. permeability for air. The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q is the difficult part. You can't just use resistive loss (even when corrected for skin effect). As you have a multi-turn loop there is an eddy current loss due to proximity of the turns (the so-called proximity loss). At these frequencies loss due to radiation is negligible, unless you make very large coils. Practically spoken you can't model the proximity loss in spice. In my opinion you should measure the Q of your loop, or do some search on Q-factor of VLF/MF coils for your coil geometry. That result you can put into spice together with the induced EMF. At these frequencies, external (induced) noise is the dominant factor, think of man made noise. Only the resistive loss part of the capacitor generates thermal noise. Using a coaxial cable as tuning capacitance will not give the highest Q as you have a long/thin conductor. A parallel plate capacitor has less resistive loss. Are you able to use good quality RG58? As far as I know RG6 for consumer CATV has low copper content and may have a CCS center conductor. -- Wim PA3DJS Please remove abc first in case of PM |
Loop Antenna at ~60 kHz
On 10/29/2014 6:45 AM, Wimpie wrote:
El 28-10-14 21:33, rickman escribi: I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. Anyone have an idea of how to estimate the radiation resistance of a tuned, shielded loop antenna? The other factor I don't understand how to factor in is the distributed capacitance of the coax. Is that a significant influence on an antenna or is it in the noise compared to the tuning capacitor. The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or would the resistance be a round trip measurement of both inner conductor and shield? I assume the shield has a much lower resistance than the inner conductor but I don't know that for sure. To get some idea of the output voltage of a loop you need to know: The fieldstrength of the desired signal at your area. This is probably given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to convert this to H-field [A/m]. EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number of turns, w = radian frequency, u0 = magn. permeability for air. The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q is the difficult part. You can't just use resistive loss (even when corrected for skin effect). As you have a multi-turn loop there is an eddy current loss due to proximity of the turns (the so-called proximity loss). At these frequencies loss due to radiation is negligible, unless you make very large coils. Practically spoken you can't model the proximity loss in spice. In my opinion you should measure the Q of your loop, or do some search on Q-factor of VLF/MF coils for your coil geometry. That result you can put into spice together with the induced EMF. At these frequencies, external (induced) noise is the dominant factor, think of man made noise. Only the resistive loss part of the capacitor generates thermal noise. Using a coaxial cable as tuning capacitance will not give the highest Q as you have a long/thin conductor. A parallel plate capacitor has less resistive loss. Are you able to use good quality RG58? As far as I know RG6 for consumer CATV has low copper content and may have a CCS center conductor. It is good to hear from you again, Wim. I have missed your very knowledgeable posts. John KD5YI |
Loop Antenna at ~60 kHz
On 10/29/2014 6:53 AM, Lostgallifreyan wrote:
rickman wrote in : MSF time signals? Just a thought... If you're interfacing an analog signal to digital, one trick I used (for audio but it ought to help here too) is a CA3140 with a bit of positive feedback through a few Mohms for hysteresis to clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or 6V, could be sensitive to take a lot of strain off your antenna. Whether this alone gives you enough gain I don't know, but it is cheap to try. Thanks for the suggestion. I'm not sure this would be any better than feeding it directly into my digital input. That is a differential input and I expect to use feedback to overcome the residual input offset. So the input will be pretty sensitive Well, try it. :) Yes, easier said than done. The receiver isn't built yet, I am currently looking at the antenna design again and wish to improve my simulation by adding the radiation resistance. If the antenna will only put out microvolts even after tuning I will need to figure out how to add the amp without having to double or quadruple the power budget. If it works then inputs are better these days. Or at least, more sensitive to small changes. As far as I know, digital inputs are usually specified with a wide dead band for levels, amounting to HUGE hysteresis and a need for a lot of gain first sp you already ned an op-amp stage no matter what unless your digital inputs have hair triggers at exactly the threshold you wanr. This is a differential input which is not far from an analog input. Actually even single ended digital inputs don't have much hysteresis unless they are designed for that. But there is always some because of the parasitic capacitance between the input and output of the buffer. The thing about the CA3140 is that with just three passive parts: M-ohmage of positive feedback, input series capacitance, and input ground resistor after the cap, you can empirically set some very nice signal preconditioning as well as raw gain, all on a very convenient single rail supply at 5V. This design won't have a 5 volt rail. Most of the design will run on 1.2~1.8 volts with some I/O at 3.3 volts to drive an LCD. It's very low power, remember? -- Rick |
Loop Antenna at ~60 kHz
On 10/29/2014 7:05 AM, Lostgallifreyan wrote:
rickman wrote in : The entire power budget is a couple hundred microwatts. There's a tiny Texas Instruments one that might do it, very cheap too. TLV2341, uses as little as 17A single rail supply at up to 8V. I didn't use it because it wasn't fast enough for what I bought it for, but it might be worth trying for MSF signals. GBW is only 0.79 MHz @ 3V Vdd, so I could only get a gain of... well not much at 60 kHz. For an opamp to work as an opamp it needs to have significant gain over the BW in use. I suppose I could use it open loop, but then it would act as a low pass filter with a high gain and a very low corner frequency. -- Rick |
Loop Antenna at ~60 kHz
rickman wrote in :
Actually even single ended digital inputs don't have much hysteresis unless they are designed for that. Well, as a proportion if they only go high above soem fairly close approach to V+, then low when close to 0V, then the dead band could be wide, the aim was to eliminate false states so they ARE usually designed for it. :) I take your point on very low volt systems, if the actual difference is small even though proportionally it may not be. Anyway, now I know that the supply is so small, your suggestion of discrete transistors is almost certainly the way to go, unless there is enough similar demand out there to have cause an off-shelf part to be made. Normally I'd just look at how others are solving similar problems, so I guess the question I can ask is: what is the signficant difference in this case that prevents the nearest off-shelf answer from working? |
Loop Antenna at ~60 kHz
rickman wrote in :
GBW is only 0.79 MHz @ 3V Vdd, so I could only get a gain of... well not much at 60 kHz. True, I looked at it more earlier this evening, at 3V supply you'd be lucky to get much more than a gain of 40 I think, so some specific and discrete transistor fix might be best. |
Loop Antenna at ~60 kHz
rickman wrote in :
For an opamp to work as an opamp it needs to have significant gain over the BW in use. Ok, how about just enough gsain to get a buffered output of some oomph to survive integration to slow clean pulses? That might not take so much to do, and if it works, it really takes the strain off the real gain stage which follows it because that will be operating pretty much at DC capability. :) |
Loop Antenna at ~60 kHz
On 10/29/2014 7:45 AM, Wimpie wrote:
El 28-10-14 21:33, rickman escribi: I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. Anyone have an idea of how to estimate the radiation resistance of a tuned, shielded loop antenna? The other factor I don't understand how to factor in is the distributed capacitance of the coax. Is that a significant influence on an antenna or is it in the noise compared to the tuning capacitor. The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or would the resistance be a round trip measurement of both inner conductor and shield? I assume the shield has a much lower resistance than the inner conductor but I don't know that for sure. To get some idea of the output voltage of a loop you need to know: The fieldstrength of the desired signal at your area. This is probably given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to convert this to H-field [A/m]. EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number of turns, w = radian frequency, u0 = magn. permeability for air. This is new to me. I guess I have been mistakenly using the E field formula. The field strength at optimum times is estimated at 100 uV/m at my location which is at the weak end of the CONUS map. I will plug the numbers into your H field version of the equation. The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q is the difficult part. You can't just use resistive loss (even when corrected for skin effect). As you have a multi-turn loop there is an eddy current loss due to proximity of the turns (the so-called proximity loss). At these frequencies loss due to radiation is negligible, unless you make very large coils. I have not seen the proximity effect taken into account in any calculations for similar antenna, so I assumed it was also not appreciable at this frequency. I'm not at all sure about the radiation resistance. I will be plugging the numbers into the equation I have. I assume this resistance would be in parallel with the inductor so a high value is better. Or would it appear in series with the inductor and a low value is better? Practically spoken you can't model the proximity loss in spice. In my opinion you should measure the Q of your loop, or do some search on Q-factor of VLF/MF coils for your coil geometry. That result you can put into spice together with the induced EMF. I'm surprised you feel the Q can't be calculated. When originally digging into this I found that the calculation of inductance is an amazingly complex thing. There are lots of equations out there each of which simplifies some aspect of the phenomenon and have different applications. I would not expect the proximity effect to be any more complex. At these frequencies, external (induced) noise is the dominant factor, think of man made noise. Only the resistive loss part of the capacitor generates thermal noise. Using a coaxial cable as tuning capacitance will not give the highest Q as you have a long/thin conductor. A parallel plate capacitor has less resistive loss. Q is important, but not the only factor. The coax was chosen to be inexpensive and easy to work with. RG-6 with an 18 ga solid center conductor is just slightly bigger than the skin effect and so is about as usefully large a conductor without it being hollow. So I'm not sure what might be better. I suppose Litz wire could improve the Q, but I'm already looking at a Q of ball park 100 or more. Once you get a very high Q it become hard to use the device without ruining the Q. Are you able to use good quality RG58? As far as I know RG6 for consumer CATV has low copper content and may have a CCS center conductor. I picked an RG-6 with a solid center conductor. The specified resistance is 6.5 mohm per foot. Funny, I'm sure most RG-6 is used for cable TV where the center conductor is steel for strength with copper plating for conductivity at high frequencies. One vendor argued with me that solid copper cores were not available in RG-6. lol BTW, I measured the resistance of my 50 foot of cable and it is in the right ball park for 6.5 mohm/foot. The shield measured in the same range as well. I thought the shield might have had a lower resistance because it would amount to a larger cross section, but I guess not. I don't think the shield resistance factors into the Q, but I'm not certain of that. -- Rick |
Loop Antenna at ~60 kHz
On 10/29/2014 3:06 PM, Lostgallifreyan wrote:
rickman wrote in : Actually even single ended digital inputs don't have much hysteresis unless they are designed for that. Well, as a proportion if they only go high above soem fairly close approach to V+, then low when close to 0V, then the dead band could be wide, the aim was to eliminate false states so they ARE usually designed for it. :) I take your point on very low volt systems, if the actual difference is small even though proportionally it may not be. Anyway, now I know that the supply is so small, your suggestion of discrete transistors is almost certainly the way to go, unless there is enough similar demand out there to have cause an off-shelf part to be made. Normally I'd just look at how others are solving similar problems, so I guess the question I can ask is: what is the signficant difference in this case that prevents the nearest off-shelf answer from working? What off the shelf answer? I have not seen any all digital receivers for any frequency. I think it may only be practical for this case and I"m not sure of that. lol This signal is very unique in that it has a very low data rate. This allows integration in the digital domain over a large number of samples. Theoretically the signal would be detectable with a negative SNR. There are actually a number of issues I need to solve to get a prototype working. The big one is being able to get a large enough signal that even statistically it is noticeable at the receiver input. -- Rick |
Loop Antenna at ~60 kHz
rickman wrote in :
What off the shelf answer? I just meant in terms of interfacing. :) Never mind, one of my other replies might be far more useful. While you can integrate digitally, why do so? It seems to me (if I haven't missed something I shouldn't) that you might get away with much less gain before analog integration, then you can boost the resulting slow signals with much less struggle with gand bandwidth products and slew rates for low power and such. If you can do it this way, the resulting slow pulses can be boosted with CMOS which at those speeds will be pretty much nanopower. |
Loop Antenna at ~60 kHz
El 29-10-14 21:03, rickman escribi:
On 10/29/2014 7:45 AM, Wimpie wrote: El 28-10-14 21:33, rickman escribi: I have a project in mind that would need a very good antenna in the frequency range of 60 kHz. Originally I looked at loop antennas and liked the idea of a large shielded loop made of coax tuned with a capacitor. My goal is to get as large a signal as possible from the antenna and matching circuit to allow the use of a receiver with very low sensitivity... in fact an all digital receiver. I spent some time simulating antennas in spice and was able to get a bit of a feel for the circuit, but I'm not convinced it would work the way I want. Just before I set the project aside I was told I needed to model the radiation resistance. That has the potential of wrecking the Q of the circuit. I am counting on the high Q to boost the output voltage. If the radiation resistance is at all appreciable I would lose the high Q and need to start over. Anyone have an idea of how to estimate the radiation resistance of a tuned, shielded loop antenna? The other factor I don't understand how to factor in is the distributed capacitance of the coax. Is that a significant influence on an antenna or is it in the noise compared to the tuning capacitor. The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or would the resistance be a round trip measurement of both inner conductor and shield? I assume the shield has a much lower resistance than the inner conductor but I don't know that for sure. To get some idea of the output voltage of a loop you need to know: The fieldstrength of the desired signal at your area. This is probably given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to convert this to H-field [A/m]. EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number of turns, w = radian frequency, u0 = magn. permeability for air. This is new to me. I guess I have been mistakenly using the E field formula. The field strength at optimum times is estimated at 100 uV/m at my location which is at the weak end of the CONUS map. I will plug the numbers into your H field version of the equation. Based on your 100 uV/m, H = 0.27 uA/m Using a coil with 2 ft diameter, this would result in EMF = 35 nV for a single turn. The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q is the difficult part. You can't just use resistive loss (even when corrected for skin effect). As you have a multi-turn loop there is an eddy current loss due to proximity of the turns (the so-called proximity loss). At these frequencies loss due to radiation is negligible, unless you make very large coils. I have not seen the proximity effect taken into account in any calculations for similar antenna, so I assumed it was also not appreciable at this frequency. I'm not at all sure about the radiation resistance. I will be plugging the numbers into the equation I have. I assume this resistance would be in parallel with the inductor so a high value is better. Or would it appear in series with the inductor and a low value is better? What are you going to make (a link to a drawing may be helpful)? What equations do you have for the Q factor for your geometry? Practically spoken you can't model the proximity loss in spice. In my opinion you should measure the Q of your loop, or do some search on Q-factor of VLF/MF coils for your coil geometry. That result you can put into spice together with the induced EMF. I'm surprised you feel the Q can't be calculated. When originally digging into this I found that the calculation of inductance is an amazingly complex thing. There are lots of equations out there each of which simplifies some aspect of the phenomenon and have different applications. I would not expect the proximity effect to be any more complex. If calculation of L is very difficult, Q will be also, as they are related. Many formulas for Q factor for certain geometry are (partly) empirical. Formulas for Q for real coils take proximity into account. You may know that Q-factor heavily depends on frequency. At these frequencies, external (induced) noise is the dominant factor, think of man made noise. Only the resistive loss part of the capacitor generates thermal noise. Using a coaxial cable as tuning capacitance will not give the highest Q as you have a long/thin conductor. A parallel plate capacitor has less resistive loss. Q is important, but not the only factor. The coax was chosen to be inexpensive and easy to work with. RG-6 with an 18 ga solid center conductor is just slightly bigger than the skin effect and so is about as usefully large a conductor without it being hollow. So I'm not sure what might be better. I suppose Litz wire could improve the Q, but I'm already looking at a Q of ball park 100 or more. Once you get a very high Q it become hard to use the device without ruining the Q. Are you able to use good quality RG58? As far as I know RG6 for consumer CATV has low copper content and may have a CCS center conductor. I picked an RG-6 with a solid center conductor. The specified resistance is 6.5 mohm per foot. Funny, I'm sure most RG-6 is used for cable TV where the center conductor is steel for strength with copper plating for conductivity at high frequencies. One vendor argued with me that solid copper cores were not available in RG-6. lol BTW, I measured the resistance of my 50 foot of cable and it is in the right ball park for 6.5 mohm/foot. The shield measured in the same range as well. I thought the shield might have had a lower resistance because it would amount to a larger cross section, but I guess not. I don't think the shield resistance factors into the Q, but I'm not certain of that. If you use the cable dielectric as part of the tuning, it is good that you have cable with solid copper instead of CCS, otherwise lots of the current would be into steel instead of copper. Your DC resistance value is correct for copper (assuming about 1 mm diameter). Your probably found that turns should not touch (increases proximity loss and loss due to the jacket) to get highest Q factor. A high Q factor helps you rejecting out of band signals. What values of inductance do you expect? In parallel equivalent circuit, the loss resistance (Rp) equals: Rp = XL*Q = w*L*Q. When the output goes directly to the input circuitry, Zin Rp to avoid reduction of Q. -- Wim PA3DJS Please remove abc first in case of PM |
Loop Antenna at ~60 kHz
On 10/29/2014 4:41 PM, Lostgallifreyan wrote:
rickman wrote in : What off the shelf answer? I just meant in terms of interfacing. :) Never mind, one of my other replies might be far more useful. While you can integrate digitally, why do so? It seems to me (if I haven't missed something I shouldn't) that you might get away with much less gain before analog integration, then you can boost the resulting slow signals with much less struggle with gand bandwidth products and slew rates for low power and such. If you can do it this way, the resulting slow pulses can be boosted with CMOS which at those speeds will be pretty much nanopower. Before integration comes demodulation. How would you demodulate and integrate in the analog domain on a 100 uW power budget? The signal is PSK. But that is not the real reason. My goal is to show it is possible to do this entirely in the digital domain. The devices I have available are not 100% optimized for low power at low clock rates, but they are pretty good. If I can find devices that have lower quiescent current the digital design has potential of being lower power than the analog approach. -- Rick |
Loop Antenna at ~60 kHz
rickman wrote in :
Before integration comes demodulation. How would you demodulate and integrate in the analog domain on a 100 uW power budget? The signal is PSK. But that is not the real reason. My goal is to show it is possible to do this entirely in the digital domain. Low Vf diode in feedback loop of op-amp? I'm curious though, it's an interesting thought, doing it all in digital equipment, but why? The main drive behind me 'off-shelf' remark is that I suspect the best answer already exists in many forms. I'm curious about what makes a need to keep searching. :) I'm not denying it, far from it, there's usually more than one good way to do something, I'm just not sure what the differentiating factor is in this case. |
Loop Antenna at ~60 kHz
rickman wrote in :
The signal is PSK. I missed that bit. :) I thought it would be simple AM.. If the integrated signal (after feedback diode demod) differ enough in amplitude (or AC content) with frequency, threshold detection might be enough. I'm just pondering it though, I have no idea if it can be done with less power than you can give it. |
Loop Antenna at ~60 kHz
rickman wrote in :
The signal is PSK. My sight isn't very good. That's Psk, not Fsk... Phase? What did I miss. :) I've been hung up on the notion that this is an MSF time signal thing, and I just looked at the spec for the UK one which is a simple switch on/off of a carrier, so easy to detect efficiently. Yours is something else entirely, but what? You may need to lay a lot more cards down before you find an answer you can use, unless you hunt in the dark. (No reason not to, I usually do, on most things I do, as the net usually makes some light at greatest need). |
Loop Antenna at ~60 kHz
On 10/30/2014 1:02 PM, Lostgallifreyan wrote:
rickman wrote in : Before integration comes demodulation. How would you demodulate and integrate in the analog domain on a 100 uW power budget? The signal is PSK. But that is not the real reason. My goal is to show it is possible to do this entirely in the digital domain. Low Vf diode in feedback loop of op-amp? I'm curious though, it's an interesting thought, doing it all in digital equipment, but why? The main drive behind me 'off-shelf' remark is that I suspect the best answer already exists in many forms. I'm curious about what makes a need to keep searching. :) I'm not denying it, far from it, there's usually more than one good way to do something, I'm just not sure what the differentiating factor is in this case. I don't know about "best" but you can buy a time code receiver chip that spits out a demodulated signal to be decoded by an MCU. At that point the data rate is pretty low so an MCU can run at very low power levels, likely dominated by the quiescent current. When you suggest an op amp, we already covered that ground and they aren't low power enough. I'm curious how they amplify the signal in the receiver chip with the whole circuit drawing a very low power level. -- Rick |
Loop Antenna at ~60 kHz
On 10/30/2014 1:23 PM, Lostgallifreyan wrote:
rickman wrote in : The signal is PSK. I missed that bit. :) I thought it would be simple AM.. If the integrated signal (after feedback diode demod) differ enough in amplitude (or AC content) with frequency, threshold detection might be enough. I'm just pondering it though, I have no idea if it can be done with less power than you can give it. The signal is also AM, but the PSK is supposed to be detectable at lower signal levels. -- Rick |
Loop Antenna at ~60 kHz
On 10/30/2014 2:01 PM, Lostgallifreyan wrote:
rickman wrote in : The signal is PSK. My sight isn't very good. That's Psk, not Fsk... Phase? What did I miss. :) I've been hung up on the notion that this is an MSF time signal thing, and I just looked at the spec for the UK one which is a simple switch on/off of a carrier, so easy to detect efficiently. Yours is something else entirely, but what? You may need to lay a lot more cards down before you find an answer you can use, unless you hunt in the dark. (No reason not to, I usually do, on most things I do, as the net usually makes some light at greatest need). I have not studied the international time signals extensively, but I believe they all use AM. The US located beacon added PSK a few years back to make the signal easier to receive. The US is large enough that reception is poor in some of the east coast areas. I am east coast and would like to see just how much I can do to optimize the antenna to make this work well. -- Rick |
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