Fellow Loopers, guess "BITX20" is where it's at
a few thoughts....
Definition of a SML
1) Let's say we want to stay closer to a 1M Loop.....maybe 40" to 48" diameter which is "relatively portable".
2) Goal is 80m to 10m
Select a Band(s)
3) For 80m we probably need 2 turns separated by 10 tubing diameters (my rule of thumb...what is yours ?)
4) A single loop for 40m to 20m is fine, 17m and up is TBD
5) Loop switching is TBD
AC Loop resistance (KEY parameter).
6) Lowest reasonable tubing material for lowest resistance.....Copper vs Aluminum. That info is widely available. Cost,
weight, and availability tradeoffs. Aluminum hardline is readily available and very inexpensive used. Copper can cost
quite a bit used or not. You need TIG welding for aluminum.
7) Lowest number of joints and lowest resistance joints....bolted, soldered, silver soldered, brazed, welded. Does anyone
have some measurement data for say .....a 2" wide joint with a 1/2" overlap at 80m. We're talking milliohms and less
here......yes, it is important in order to increase efficiency. You need TIG welding for aluminum.
8) Tubing material shape, assume round tubing. The larger the better.....up to a point where you start having RF current
flow interference. A large donut shape does not work well, neither does anything flimsy due to wind effects (In my
9) Flat strap "tubing". The high current density is in the outside edges of the strap, not across the full width of the strap,
so if the strap is thin, the material conductive properties come into play.....bigtime.
10) Using several (5?, 7?) identical hardline (cheap) loops as a sturdy form, cover them with thin copper sheeting, say 10"
wide and running along the circumference of the loop. Curved (cupped) a little in the center. Do not spiral wind the
copper sheeting due to the resultant seams. The idea here is to force the RF current density to the "wide" outside side of
the copper. The other idea here is no serial seams except at the ends. A seam running the full inside diameter is OK and
minimize copper sheeting wrinkles. Attach 1ft ? copper plates at the ends for the capacitor. (just some ideas). Air
spacing or Teflon sheeting for the dielectric.
11) Many tradeoffs on the capacitor. One full range capacitor or several switched capacitors and a smaller "trimmer".....but
again all contacts are going to add to the overall loop resistance and switching implementation will not be simple.
12) Allow for BOTH rotation of the coil and movement relative to the loop circumference.
13) I would recommend that someone in your group buy a commercial loop antenna to use as a reference
73 Kees K5BCQ