A System View of the Line Isolator
This is a low resolution renditions of a display panel used in our hamfest booth display. I have to say that the high definition color version looks a lot better, but I didn't think you would want to download a 4 meg file! The title should read "The RFI Quick Fix ... for over 20 years."
The illustration at the top of the page consisting of a transceiver, linear amplifier, and a transmatch. This hamshack has a full complement of T-4 and T-5 Line Isolators installed. The goal of the installed Line Isolators is to break up all RF ground loops and block conducted RF coming into the hamshack via the coaxial cable.
In the block on the bottom left is an equivalent circuit of the station with inductors representing the Line Isolators, transmatch inductance and the antenna itself. Shading represents the level of RF as a result of the floating ground system. Ground inductance includes the ground system inefficiencies, especially the ground strap, braid, or wire connecting the ground system to the station components. The longer and smaller the wire, strap, or braid, the higher the effective inductance.
The ground inductance combined with the transmatch and antenna system with their own inductances form an autotransformer. The station ground is tapped into the autotransformer at some point above ground. Thus, the station ground floats at some value of RF voltage.
Line Isolators effectively prevents RF ground loops by isolating interconnecting coax ground connections from the ground system. Remember, each piece of equipment is connected directly to the transmatch with a heavy ground strap. Doing so creates "single point" grounding. Interconnecting the station equipment with the interconnecting coaxial cables creates "loops." If the RF ground system is inadequate, RF ground loops occur and RFI problems arise. Remember that an effective d.c. ground is not necessarily a good RF ground. Check this out in my notebook.
In the diagram on the bottom right is the equivalent circuit of a T-5G installed at the same station. As you can see, RF conducted back to the shack by the outer surface of the coaxial cable's shield is routed directly to ground. Looking toward the station equipment is a very high inductance which greatly reduces RFI.
Combined, the T-4 and T-5's are capable of solving some very tricky RFI problems.
Grounded or "G" models of our Line Isolators are recommended when direct earth grounds are available. Position the ground strap end of the Line Isolator in the direction of the most likely source of RF. This improves performance by providing a direct path to ground while the RF "sees" a very high inductive reactance in the path following the ground strap.Return to Line Isolator Catalog.
Go to index of Jim's Notebook