Frankly, you can make you own couplers for just a few cents each from copper
tubing that has an inside diameter of 1/4 inch. Cut off a length about 1 inch
long and then drill a hole near each end. Then "tap" that hole for a machine
Now you will need a tubing cutter which is just a few dollars at any home
improvement center, a drill and bit, and a tap. However, these tools are
already in the possession of just about any amateur radio operator who does any
building at all.
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