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Author Topic: Differential preamp for loop antenna  (Read 2641 times)
4Z1UF
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Posts: 22




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« on: August 13, 2011, 08:38:08 AM »

 I'm trying to study deeper- and even try! Smiley - the receiving loop antennas for low bands. All of them requires low noise amplifiers.
All recommendations that I found on the web are for such LNA's placed close to the transceiver. It means matching balun between the loop and feed line (coax or twisted pair) running down, then struggling with common mode noise on the feed line and after that- amplification of signals +noise by LNA.
 The question: did anyone try to place the differential (operational) amplifier at the antenna side, eliminating the need of balun and not amplifying the unwanted noise? Something similar to  ADA4899-1 or AD8432?
 I received this advice from antenna pro and wonder why this solution is not popular despite it sounds very logical.
73! Ilya 4Z1UF
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G3RZP
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Posts: 7190




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« Reply #1 on: August 13, 2011, 09:05:52 AM »

I had a tuned, rotatable loop for 160. 2 turns, square, 1 metre a side. 75 feet from the vertical used for transmit, at 400 watts, it had 80 volts rms across it when I was transmitting.

I suspected that this would not be easy to reliably protect against with diodes, so I used a 6DJ8 as a differential cathode follower. When hit with the 80 volts, the grid current damped the loop Q enough to prevent damage, and a relay disconnected the rx input. Powr was by providing 6.3 volts and a small transformer in the box giving about 120 volts to be rectified for the B+.

I sed a 4:1 transformer in the cathodes: You can get a problem if the tuning capacity is too small that it oscillates: the grid to cathode and cathode to ground strays act like a Colpitts.

A ss amp at the loop needs some carefully worked out protection: an untuned loop would be a lot easier in this respect. but a tuned loop gives a lot more signal - Q times, in fact.
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4Z1UF
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« Reply #2 on: August 14, 2011, 12:42:28 AM »

 Thanks Peter.
Actually I didn't think about protecting the LNA from inducted voltage across the loop. Actually my loops a planned not to be "tuned", but anyhow...Disconnecting the the supply voltage to the preamp is initial idea but I shall check carefully what the OpAmp  (or any other LNA) can withstand even being un-powered.
73! Ilya 4Z1UF
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TANAKASAN
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Posts: 933




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« Reply #3 on: August 14, 2011, 02:29:03 AM »

I was going to point you in the direction of work done by Dallas Lankford who has done some research in this area. The URL I have in my bookmarks http://www.kongsfjord.no/dl/dl.htm now gives a 404 error so does anybody know what has happened to his website?

Tanakasan
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4Z1UF
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Posts: 22




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« Reply #4 on: August 14, 2011, 04:58:05 AM »

Tanakasan,
 I think you referring to http://groups.yahoo.com/group/thedallasfiles/
Regards,
Ilya
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