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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