On Fri, 23 Dec 2005 09:15:42 -0500, "Eric Fairbank"
wrote:

These are "active" antenna's. Power is provided via the coax and there is
a PCB in there with a pre-amplifier circuit. Some units have notch filters
and/or bandpass filters in them.

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Absolutely right. If anyone really wants to hack around with this
stuff I have a couple of Northstar LORAN-C units sitting around
without antennas or couplers. They were first class units in their
day and have SO-239 coax connectors on the back.