Any idea what the "typical" RF signal picked up by a four-foot piece of wire from a local AM radio station would be? I'm guessing someting on the order of a couple hundred microvolts, but it is exactly that, a guess. I am trying to figure out what type of input to the first stage of amplification of an AM radio would likely be, realizing first of all that it would vary greatly. Again, any help is appreciated...
Dave
Your couple of hundred microvolts sounds OK, possibly a bit high. At those frequencies a 4' piece of wire will act as a capacitive probe -- running it into the gate of a FET (or the grid of a tube if you're that sort) would be indicated.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Wow. Thank you, Tim. And yes, I am running it into the gate of an FET (the project I mentioned in another post.). Can you tell me how raising the freq might affect things? Say to 10 MHz? What about lengthening the antenna to say, 90 feet (shortwave longwire)?
Many thanks. I'll try not bother you again. :)
Dave
Hello Dave,
The voltage goes up, big time, and you'll have to begin to think about how well you amp and the stuff behind it will handle large signals. Protection diodes can begin to cause intermodultaion between signals. IOW a small signal you want to listen to might become modulated by a large local signal.
Here is some food for thought:
Regards, Joerg
At 10MHz the wire will start looking like it may have a resistive component in there someplace, you could consider driving a 50 ohm RF amplifier with it if you had the right loading coil arrangement. Also at 10MHz your FET gate won't look as much like a simple high-impedance input, so you'll have to make the amplifier with a 50 ohm input anyway.
At commercial AM frequencies that 90 feed of shortwave wire could also be profitably loaded to interface with a 50 ohm amplifier -- but the bandwidth would be extremely narrow, and at those frequencies it's easy to build a FET amplifier.
No problem -- I only answer if I feel like it.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
"Dave"
** A weakish station could be a low as that - but a few mV is more likely.The actual signal picked up will contain the **entire** local radio and TV spectrum of course - every nearby transmitter from LW to UHF and cell phones.
The potential for mutual interference ( cross modulation) in the first RF stage is enormous.
......... Phil
Newnes just took 20% off that thing.
Where'd you hear that? I just checked the Elsevier website -- it's still $60.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
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