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#21 |
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diyAudio Moderator
Join Date: Jan 2003
Location: Near London. UK
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The only reason for having an aerial amplifier is to overcome the losses down the cable to the receiver - and that means it should be a mast-head amplifier, probably with quite low gain (6dB). Anything else is likely to amplify muck and expose the receiver's front-end to overload and intermodulation. Nothing beats a better aerial.
Actually, one thing does. A better feeder. Most aerial cable is so leaky that it picks up as much signal as the aerial. Trouble is, unlike the aerial, it's not tuned and it's not pointed in the right direction. Use a proper low loss professional cable with a foil Rheunissen screen.
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"Scientific progress goes boink." Bill Watterson |
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#22 | |
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diyAudio Member
Join Date: Jun 2001
Location: UK
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Quote:
Feeder pickup is always a fixed, constant value (for a given installation) and always non-zero. By having excessive gain and therefore signal strength, you can attenuate at the receiver input. This brings the input signal to the correct level, whilst attenuating the fixed, unwanted, cable pickup. Andy. |
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#23 |
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diyAudio Moderator
Join Date: Jan 2003
Location: Near London. UK
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Good thought.
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"Scientific progress goes boink." Bill Watterson |
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#24 |
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diyAudio Member
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I dug out my old Heathkit switched attenuator (up to 70 db)
and plugged it into my mediocre Fisher tuner. No amount of attenuation will drive most of the local stations into the noise. Cheap plastic case and open wiring still leaks in a useable amount of signal. I'll have to try to rewire the input with proper coax; originally it was 300 ohm balanced, to which I'd added a balun and a F connector on the back panel. I was hoping to make a guess as to how much signal that jpole is pulling in; wonder if my ham neighbor has a RF microvoltmeter? My presently defunct Heathkit AJ-29 was modified for coax and that effectively eliminated leakage. At one time I had a FM band JFET preselector, designed by Bob Cooper, I think; it actually worked fairly well in combination with the attenuator (which originally came with the IM-57 TV sweep generator). There are other ham tricks I haven't tried, such as tuned cavity traps and bandpass filters, but I haven't been in a position to need such, yet. I found some general comments on RF amplifiers and preselectors with a working example of a low-noise MOSFET preamp on this page: http://www.geocities.com/toddemslie/bf981_preamp.html |
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