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. |
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