Hi Jim, >The cable I used was armored with a spiral copper shield over 5 twisted pair lines. I did try grounding the shield at one end, and at both ends with no results. Putting caps across the twisted pair and to ground also did not eliminate the problem, but did reduce it. I used 600:600 isolation transformers in the audio input and output lines at the repeater.
>It all became a mute point when the cable got mowed in two during a grass cutting this last summer, and then we lost the public IP address and EchoLink was no longer usable. So any more trouble shooting exercises will await the return (if ever) of the public IP address. Since the other remedies haven't worked completely, you might file this away in case the situation arises again: You might be experiencing a ground loop even with coupling transformers at each end due to the capacitance to ground of the transformers. A common mode choke, if it has sufficient reactance at the noise frequency, can eliminate the noise. A common mode choke is an inductor with a single core (toroidal is good) and two identical windings connected such that each winding is in series with one of the long lines. The choke goes at the input end with the phasing dots on the same side, i.e., either toward the line or toward the equipment input. The "desired signal" current flows in opposite directions on the two lines and creates opposing magnetic fields in the choke, which cancel. The desired signal never sees the choke and its waveform is maintained. The "undesired signal" (common mode) current flows in the same direction in both lines and sees a lot of reactance in the choke because the two magnetic fields add. Much of the noise is eliminated. 73, Bob Bob Schmid, WA9FBO, Member S-COM, LLC PO Box 1546 LaPorte CO 80535-1546 970-416-6505 voice 970-419-3222 fax www.scomcontrollers.com **************************************Check out AOL's list of 2007's hottest products. (http://money.aol.com/special/hot-products-2007?NCID=aoltop00030000000001)
