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RF noise signal for antenna tuning?	Reply
by KD5OPP on June 4, 2001	Mail this to a friend!
I have been considering making a simple zener diode rf noise source (either modulated or just noise) for antenna tuning. I have three (related) questions. What is the proper place (and way) to inject the noise among the antenna line, tuner, and xcvr? Then I set the rcvr to the desired frequency and twiddle the ATU for a null, right? Anyone have a schematic? I am thinking of using the noise source from a noise bridge ckt that was in the '94 ARRL handbook. I have seen some references for a 2020N (?) kit but the web links i find are dead. -73 de KD5OPP

RE: RF noise signal for antenna tuning?	Reply
by WB6BYU on June 5, 2001	Mail this to a friend!
A typical bridge circuit has 4 components: input, output, standard, and unknown. When the unknown port (the coax going to the antenna through the antenna tuner) matches the standard (usually an internal 50 ohm resistor) then the signal applied to the input port is nulled at the output port. For common noise bridges, the input is the noise source, and both it and the 50 ohm standard are inside the box, so the only external connections are the unknown and the receiver (output). The noise bridge goes in the line between your rig and the antenna tuner. The noise output at any frequency is dependent on the load presented by the antenna (and antenna tuner) at that frequency. So, yes, you tune your receiver to the desired frequency and adjust the tuner for minimum noise level. (You may have to watch the S meter, as this can be difficult to do by ear with the AGC enabled.) Be sure to remove the noise bridge before transmiting! The bridge circuit doesn't really care what you use for the signal source (input) or the detector (output). You can drive the bridge with a tuned oscillator and use an RF voltmeter for the detector. You can also use a 100 kHz crystal calibrator in place of the noise generator and use the receiver as a detector (of course, only on 100 kHz increments.) Good luck! - Dale WB6BYU

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