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#11
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Loop antenna matching question
"Owen Duffy" wrote in message ... Michael Coslo wrote in news:gvhhkj$do4m$1 @tr22n12.aset.psu.edu: I'm starting up an old project that I never quite finished - my loop antenna. When I first put it together, I tried a simple loop of coax to couple the radio to the loop. Was never really satisfied with that though. I'm wanting to try a gamma match, possibly something a little like what Owen Duffy posted a nice version of. The Gamma match is fairly understandable to me, and I expect it to work well. What I am wondering about is in the coax loop coupling system, how is the best match obtained? Loop size? orientation? Luck? Mike, you haven't given much information about the loop (size, frequency). Is loop balance / symmetry important? If it was a small loop, I would not be thinking about a gamma match because of the impedance ratios for just one reason. Owen Owen - I've had this nagging idea for a couple of years: What if the (single-turn?) loop passes through a toroid which already has, say, 10 or so turns on it? That should give an impedance step-up of 100 or so. Would that not be a good way to at least get the matching closer? In fact, the tuning capacitor could be connected to the high impedance side to reduce the capacitor's value. Your comments will be appreciated. John |
#12
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Loop antenna matching question
John KD5YI wrote:
"Owen Duffy" wrote in message ... Michael Coslo wrote in news:gvhhkj$do4m$1 @tr22n12.aset.psu.edu: I'm starting up an old project that I never quite finished - my loop antenna. When I first put it together, I tried a simple loop of coax to couple the radio to the loop. Was never really satisfied with that though. I'm wanting to try a gamma match, possibly something a little like what Owen Duffy posted a nice version of. The Gamma match is fairly understandable to me, and I expect it to work well. What I am wondering about is in the coax loop coupling system, how is the best match obtained? Loop size? orientation? Luck? Mike, you haven't given much information about the loop (size, frequency). Is loop balance / symmetry important? If it was a small loop, I would not be thinking about a gamma match because of the impedance ratios for just one reason. Owen Owen - I've had this nagging idea for a couple of years: What if the (single-turn?) loop passes through a toroid which already has, say, 10 or so turns on it? That should give an impedance step-up of 100 or so. Would that not be a good way to at least get the matching closer? In fact, the tuning capacitor could be connected to the high impedance side to reduce the capacitor's value. This has been done in a variety of ways (notably some papers from 20 years ago where they used this scheme to couple to the windshield window frame on a jeep. For Rx only, there's no real issues For Tx, though, a small loop will tend to have high currents. There's a LOT of energy stored in the magnetic field (and in the E field of the capacitor that tunes the system). All that reactive energy will be flowing back and forth through the transformer, so it will need a high VA rating. Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. |
#13
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Loop antenna matching question
Jim Lux wrote:
Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Works on zero point energy? - 73 de Mike N3LI - |
#14
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Loop antenna matching question
Ian Jackson wrote:
The question is, "Do you use a quarterwave of 75 ohm cable to get something like a 50 ohm impedance at the TX end - even if this means using more coax than you really need or, to minimise losses, do you simply use the shortest length of coax - either 50 or 75 ohm?" Usually not a problem since 1/4WL of RG-11 at 4 MHz is only ~40 feet - hopefully less than the height of the antenna. 1/4WL at any higher HF frequency would be shorter than that. Excess coax can usually be coiled into a choke. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#15
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Loop antenna matching question
"Jim Lux" wrote in message
... John KD5YI wrote: "Owen Duffy" wrote in message ... Michael Coslo wrote in news:gvhhkj$do4m$1 @tr22n12.aset.psu.edu: I'm starting up an old project that I never quite finished - my loop antenna. When I first put it together, I tried a simple loop of coax to couple the radio to the loop. Was never really satisfied with that though. I'm wanting to try a gamma match, possibly something a little like what Owen Duffy posted a nice version of. The Gamma match is fairly understandable to me, and I expect it to work well. What I am wondering about is in the coax loop coupling system, how is the best match obtained? Loop size? orientation? Luck? Mike, you haven't given much information about the loop (size, frequency). Is loop balance / symmetry important? If it was a small loop, I would not be thinking about a gamma match because of the impedance ratios for just one reason. Owen Owen - I've had this nagging idea for a couple of years: What if the (single-turn?) loop passes through a toroid which already has, say, 10 or so turns on it? That should give an impedance step-up of 100 or so. Would that not be a good way to at least get the matching closer? In fact, the tuning capacitor could be connected to the high impedance side to reduce the capacitor's value. This has been done in a variety of ways (notably some papers from 20 years ago where they used this scheme to couple to the windshield window frame on a jeep. For Rx only, there's no real issues For Tx, though, a small loop will tend to have high currents. There's a LOT of energy stored in the magnetic field (and in the E field of the capacitor that tunes the system). All that reactive energy will be flowing back and forth through the transformer, so it will need a high VA rating. Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Adding the transformer changes the circulating energy in the system how? (Excluding losses) |
#16
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Loop antenna matching question
Michael Coslo wrote:
Jim Lux wrote: Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Works on zero point energy? Nope, the 100k joules/sec was stored in the system during the initial transient key-down state and was not radiated. It won't be radiated until the transient key-up state. This is a characteristic of standing- wave antennas. In a 1/2WL dipole, only about 20% of the energy stored in the antenna is radiated. In the small loop above with its astronomically high SWR, only about 0.1% of the energy stored in the antenna is radiated during steady-state. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#17
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Loop antenna matching question
Michael Coslo wrote:
Jim Lux wrote: Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Works on zero point energy? - 73 de Mike N3LI - not at all.. that's the definition of Q.. it's the ratio of stored energy: energy lost in 1 cycle. |
#18
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Loop antenna matching question
John KD5YI wrote:
"Jim Lux" wrote in message ... John KD5YI wrote: "Owen Duffy" wrote in message ... Michael Coslo wrote in news:gvhhkj$do4m$1 @tr22n12.aset.psu.edu: I'm starting up an old project that I never quite finished - my loop antenna. When I first put it together, I tried a simple loop of coax to couple the radio to the loop. Was never really satisfied with that though. I'm wanting to try a gamma match, possibly something a little like what Owen Duffy posted a nice version of. The Gamma match is fairly understandable to me, and I expect it to work well. What I am wondering about is in the coax loop coupling system, how is the best match obtained? Loop size? orientation? Luck? Mike, you haven't given much information about the loop (size, frequency). Is loop balance / symmetry important? If it was a small loop, I would not be thinking about a gamma match because of the impedance ratios for just one reason. Owen Owen - I've had this nagging idea for a couple of years: What if the (single-turn?) loop passes through a toroid which already has, say, 10 or so turns on it? That should give an impedance step-up of 100 or so. Would that not be a good way to at least get the matching closer? In fact, the tuning capacitor could be connected to the high impedance side to reduce the capacitor's value. This has been done in a variety of ways (notably some papers from 20 years ago where they used this scheme to couple to the windshield window frame on a jeep. For Rx only, there's no real issues For Tx, though, a small loop will tend to have high currents. There's a LOT of energy stored in the magnetic field (and in the E field of the capacitor that tunes the system). All that reactive energy will be flowing back and forth through the transformer, so it will need a high VA rating. Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Adding the transformer changes the circulating energy in the system how? (Excluding losses) It doesn't.. it's that the circulating energy in a high Q system is quite large, so you need to size the transformer (core) appropriately. The transformer size needed to handle a 100W Tx in a resonant dipole is MUCH smaller than the transformer needed to handle a 100W Tx in a compact loop with a Q of 1000. The transformer has to handle not only the input(and radiated) power, but also the circulating power. Actually, ALL the components have to deal with the circulating power, but most builders of compact loops are aware of the need for low resistance in the loop and high voltage ratings in the capacitor. |
#19
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Loop antenna matching question
On May 29, 10:49*am, Jim Lux wrote:
John KD5YI wrote: "Owen Duffy" wrote in message ... Michael Coslo wrote in news:gvhhkj$do4m$1 @tr22n12.aset.psu.edu: I'm starting up an old project that I never quite finished - my loop antenna. When I first put it together, I tried a simple loop of coax to couple the radio to the loop. Was never really satisfied with that though. I'm wanting to try a gamma match, possibly something a little like what Owen Duffy posted a nice version of. The Gamma match is fairly understandable to me, and I expect it to work well. What I am wondering about is in the coax loop coupling system, how is the best match obtained? Loop size? orientation? Luck? Mike, you haven't given much information about the loop (size, frequency). Is loop balance / symmetry important? If it was a small loop, I would not be thinking about a gamma match because of the impedance ratios for just one reason. Owen Owen - I've had this nagging idea for a couple of years: What if the (single-turn?) loop passes through a toroid which already has, say, 10 or so turns on it? That should give an impedance step-up of 100 or so. Would that not be a good way to at least get the matching closer? In fact, the tuning capacitor could be connected to the high impedance side to reduce the capacitor's value. This has been done in a variety of ways (notably some papers from 20 years ago where they used this scheme to couple to the windshield window frame on a jeep. For Rx only, there's no real issues For Tx, though, a small loop will tend to have high currents. *There's a LOT of energy stored in the magnetic field (and in the E field of the capacitor that tunes the system). *All that reactive energy will be flowing back and forth through the transformer, so it will need a high VA rating. Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Cecil had something to say like that( different figure) and I find it quite interesting! The idea of a loop antenna acting as a battery I find odd. The loop is a tank circuit where the power supplied replaces the power lost in the resistance of the components Without the resistance it would be continual motion! So where is this "radiation" ratio coming from as the power supplied is purely a replacement of resistance losses which includes the energy supplied for radiation? The voltage is in the kilo volts but with current extremely low I fail to see where the 100kW comes from which makes it a battery Art |
#20
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Loop antenna matching question
"Jim Lux" wrote in message
... John KD5YI wrote: "Jim Lux" wrote in message ... John KD5YI wrote: "Owen Duffy" wrote in message ... Michael Coslo wrote in news:gvhhkj$do4m$1 @tr22n12.aset.psu.edu: I'm starting up an old project that I never quite finished - my loop antenna. When I first put it together, I tried a simple loop of coax to couple the radio to the loop. Was never really satisfied with that though. I'm wanting to try a gamma match, possibly something a little like what Owen Duffy posted a nice version of. The Gamma match is fairly understandable to me, and I expect it to work well. What I am wondering about is in the coax loop coupling system, how is the best match obtained? Loop size? orientation? Luck? Mike, you haven't given much information about the loop (size, frequency). Is loop balance / symmetry important? If it was a small loop, I would not be thinking about a gamma match because of the impedance ratios for just one reason. Owen Owen - I've had this nagging idea for a couple of years: What if the (single-turn?) loop passes through a toroid which already has, say, 10 or so turns on it? That should give an impedance step-up of 100 or so. Would that not be a good way to at least get the matching closer? In fact, the tuning capacitor could be connected to the high impedance side to reduce the capacitor's value. This has been done in a variety of ways (notably some papers from 20 years ago where they used this scheme to couple to the windshield window frame on a jeep. For Rx only, there's no real issues For Tx, though, a small loop will tend to have high currents. There's a LOT of energy stored in the magnetic field (and in the E field of the capacitor that tunes the system). All that reactive energy will be flowing back and forth through the transformer, so it will need a high VA rating. Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't unusual), and you're radiating 100W, then there's 100kW of circulating power in the system. Adding the transformer changes the circulating energy in the system how? (Excluding losses) It doesn't.. it's that the circulating energy in a high Q system is quite large, so you need to size the transformer (core) appropriately. The transformer size needed to handle a 100W Tx in a resonant dipole is MUCH smaller than the transformer needed to handle a 100W Tx in a compact loop with a Q of 1000. The transformer has to handle not only the input(and radiated) power, but also the circulating power. Actually, ALL the components have to deal with the circulating power, but most builders of compact loops are aware of the need for low resistance in the loop and high voltage ratings in the capacitor. I must have missed something. I did not realize that Michael intended to use the loop with a 100 watt transmitter. My assumption was that it was primarily a receiving antenna. However, it doesn't matter because your comments are equally applicable to a short wire antenna. Therefore, I would urge readers not to consider the small loop any worse than a short wire antenna in these regards. |
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