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On 12 Sep 2005 14:01:59 -0700, "K7ITM" wrote:
Actually, the thing you call a detector IS a mixer. You can probably find some references for "H-mode mixer." It's good, to be sure, but it's inaccurate to say that it's _perfectly_ linear. (I'd LOVE to find a practical sampler which had zero distortion...though then I'd need amplifiers with zero distortion, too...) As someone else pointed out, any practical antenna you have for LF is very unlikely to be a good match to 50 ohms, and is very likely to be quite reactive so that by the time you add components to tune it, the bandwidth will be pretty narrow. So if you have a tuned LF antenna, which is quite usual, the response will be quite narrow, and why would you care about a bandpass filter? I'd recommend a loop with a tuning arrangement at its feedpoint (variable capacitance), and an appropriate preamp to drive a feedline back to the receiver. With that, you won't need any filter, just a transformer going into the mixer (converter-detector-whatever). A while back, I did some work to modify a design you can find at http://www.cpinternet.com/~lyle/bal-pre/bal-pre.htm, so that the control was done as a DC current , which also fed the power to the preamp on the same line that signals come back on. It worked out well. But check out other antenna options from Lyle's website (http://www.cpinternet.com/~lyle/) or others devoted to LF, too. Thanks Tom, I've followed lowfer technical discussions for some time now and thanks to some recent input from a few of them, I have a much better idea of what I need to do to get a decent antenna up. My plan is to have a single turn or 2 turn centertapped loop, each side being 10 to 12 feet long and the turns spaced 5 inches apart. The conductor will be 200 A aluminum service entrance cable which I have laying around. The impedance of this antenna will be low and the Q should be quite high, with lots of area, so it should drive the receiver well. It has occurred to me that the antenna itself has a great deal of selectivity, yet some loop users still report front end overload from AM broadcast band and other megawatt LF rf sources. The QSD is susceptible to harmonics also, so a very high attenuation low pass filter is the minimum filter necessary to keep these signals out. Whether the tuned antenna by itself is adequate, I don't know. But, I'm considering putting in a bandpass or low pass filter designed to match the lower impedance loop antenna directly, so the filter would have input and output impedances of 5 or 10 ohms. Anyway, that's a topic for another day I suspect. Regards and thanks for the input. T |
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