Coax Coil Choke
With Altoid's Mint SO-239 to Binding Post Interface
Used in the Dec. 2002 ARRL 10-Meter Contest
COAX CONNECTION TO ANTENNA (same for all set-ups):
I was going from coax to a wire connection with all the makeshift antennas I played with during this contest. I was concerned about extraneous energy flowing down the outside of the coax shielding. To control this possibility I rigged up the coil choke described below.
I ran a 60 foot length of RG-58 coax out my shack window and onto the roof, wrapping 5 times around a plastic coffee jar and then going into my home-made Altoids mint box.
The mint box allows me to go from coax to a binding post. This allows me to attach two wires to the coax via the SO-239 / Binding Post Interface, be they ladder line, or just lengths of wire as I used in the contest. However, my mint box makes no allowance for any kind of balun. This is why I wrapped the coax around the plastic coffer jar five times.
I didn't take the time to look up the formula in the "ARRL Antenna Book" I just did it (4 feet, 6 to 8 turns, for anyone who cares, is actually what the text calls for).
I then took two short lengths of #12 insulated, stranded, wire with about 1/2 inch wire exposed on each end. These ran from the binding posts to a hose clamp. I placed the hose clamp around what seemed to be an assessable portion of the "antenna". Placing an exposed end of the wire between the side of the hose clamp (opposite the screw) and the antenna, I then tightened the hose clamp until the connection was pretty snug. No sanding of the antenna surface, just a "pretty tight" pressure connection. Some of the items were painted, such as the dolly and others, such as the easels were not, so some items may have had a little extra capacitive resistance created at their feed point.
To get the hose clamp snug on the tape measures I placed a plastic "dog bone" insulator inside the clamp. I should have done this with the ground plane chicken wire too, but was in a hurry and didn't bother. Needless to say that connection was not very tight, but there was not an air gap either.
For the tape measure di-pole, the wires running from the binding posts were about a foot long. For the other antennas, they were about two-foot long. I wanted to strike a balance between these connecting wires becoming effective radiators (undesirable) and being able to easily hook to a wide variety of antenna objects (desirable). Right or wrong, the conclusion I came to was to use either the one, or two,-foot lengths of wire. Is this cheating, do you think? How much additional RF radiation is being obtained from these short lengths of wire? If you have an informed idea, please let me know!