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#1
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In article ,
switcher wrote: a damped LC ??? Does it make sense to introduce a R in the LC circuit and use it as antenna ?? Does it change eg the Q ??? etc ... It depends on what you want. Maybe the bandwidth is to narrow or the tuning action is so sharp it is hard to peak it. You add resistance and the peak does go down but the bandwidth also widens. -- Telamon Ventura, California |
#2
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In article ,
switcher wrote: In article , Telamon wrote: It depends on what you want. Maybe the bandwidth is to narrow or the tuning action is so sharp it is hard to peak it. You add resistance and the peak does go down but the bandwidth also widens. I was already thinking: if DRM is 12khz wide (am I right ?), then it might be a tuned loop is too narrow ?? So an R might help ??? If the tuned loop bandwidth is to narrow then you can deliberately spoil the Q of the loop by adding resistance and broaden the bandwidth. I have not calculated it but I expect a LC loop operating in the MHz range would have a bandwidth greater than 12 KHz so I do not expect you will have a problem. -- Telamon Ventura, California |
#3
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Telamon wrote:
In article , switcher wrote: In article , Telamon wrote: It depends on what you want. Maybe the bandwidth is to narrow or the tuning action is so sharp it is hard to peak it. You add resistance and the peak does go down but the bandwidth also widens. I was already thinking: if DRM is 12khz wide (am I right ?), then it might be a tuned loop is too narrow ?? So an R might help ??? If the tuned loop bandwidth is to narrow then you can deliberately spoil the Q of the loop by adding resistance and broaden the bandwidth. . . . A really quick back-of-the envelope look at the problem indicates that adding a resistance won't make the signal any stronger when the loop is tuned away from the peak. It'll just make the signal weaker when it is tuned to the peak. So it's "wider", but no better at any tuning setting but worse at and near the peak. To check on my calculation, try it: connect and disconnect the resistor at various tuning settings, on and off peak. Does it make the signal any stronger at any setting? The only time there would be any advantage to adding an R to widen the bandwidth is if it's so narrow that it distorts a modulated signal due to uneven response across the audio bandwidth. This would mean at least several dB variation over a couple of kHz for voice SSB or over 10 kHz for broadcast quality AM. Roy Lewallen, W7EL |
#4
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In article ,
Roy Lewallen wrote: Telamon wrote: In article , switcher wrote: In article , Telamon wrote: It depends on what you want. Maybe the bandwidth is to narrow or the tuning action is so sharp it is hard to peak it. You add resistance and the peak does go down but the bandwidth also widens. I was already thinking: if DRM is 12khz wide (am I right ?), then it might be a tuned loop is too narrow ?? So an R might help ??? If the tuned loop bandwidth is to narrow then you can deliberately spoil the Q of the loop by adding resistance and broaden the bandwidth. . . . A really quick back-of-the envelope look at the problem indicates that adding a resistance won't make the signal any stronger when the loop is tuned away from the peak. It'll just make the signal weaker when it is tuned to the peak. So it's "wider", but no better at any tuning setting but worse at and near the peak. To check on my calculation, try it: connect and disconnect the resistor at various tuning settings, on and off peak. Does it make the signal any stronger at any setting? The only time there would be any advantage to adding an R to widen the bandwidth is if it's so narrow that it distorts a modulated signal due to uneven response across the audio bandwidth. This would mean at least several dB variation over a couple of kHz for voice SSB or over 10 kHz for broadcast quality AM. I read his post correctly his concern is a 12 KHz wide DRM signal and that the tuned loop response will be to sharp and narrow. Firstly he needs to build the loop and sweep it to see if has a problem to begin with. Parasitics may cause the loop response to be wide enough that 12 KHz will not be a problem. If it turns out it does he can add resistance, which will decrease the peak response and broaden it. Adding resistance will cause the loop to not work as well but that is the price you pay for greater bandwidth. -- Telamon Ventura, California |
#5
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![]() Telamon wrote: I read his post correctly his concern is a 12 KHz wide DRM signal and that the tuned loop response will be to sharp and narrow. Firstly he needs to build the loop and sweep it to see if has a problem to begin with. Parasitics may cause the loop response to be wide enough that 12 KHz will not be a problem. If it turns out it does he can add resistance, which will decrease the peak response and broaden it. Adding resistance will cause the loop to not work as well but that is the price you pay for greater bandwidth. Normally the best thing is to over-couple the loop to the receiver, rather than loading it with a resistor. The Q of the loop is not only a function of the loss resistance and L/C values, it is also a function of the loop loading caused by the feedline/receiver combination. That introduces resistance to the system. Adding a resistor is the last think I'd do. If I had a bandwidth problem in a resonant loop, I'd simply overcouple to the feedline and let the feedline/receiver resistance load the antenna. 73 Tom |
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