We are building a 4 ft diameter mag loop using 1" copper tube. The coupling loop will be at the top of the loop (we are using a large vacuum cap in a weatherproof box and it is too heavy to be permanently at the top of the loop in the high winds in Colorado).
Can we run the feedline within the copper tube from the top of the loop, exiting near the bottom of the loop tube? Is this a better solution than simply having the feedline drop across the nearfield?
It is a tough call.
Clearly the inside of the tube carries no RF current. This would suggest that routing the coax in the tube would be a safe approach.
But my concern is that where the coax exits the loop, there will be a strong tendancy for the uniform, high current on the outside of the loop to couple to the exterior of the coaxial shield giving rise to common mode currents on the coax. A secondary issue is due to the reduction of the loop surface area where the coax exits. This will raise the RF impedance in this area causing some reduction in efficiency and possibly localized heating.
The alternative route of letting it drop near the loop also provides the opportunity for coupling of common mode currents but at least there is a minor separation possible in this case.
In either case, a good common mode choke on the coax is warranted. You may wish to experiment with the two techniques. An RF clamp-on ammeter would help to remove ambiguity/subjectivity from the experiment..
My apologies for my tardiness in updating the bulldogtrust site.
I am too skeptical to try coax inside one side of the loop tubing, due to high voltage and asymmetrical capacitance it would introduce at the exit point.
My simple minded solution, discovered first on a Russian website, uses capacitive feed at the loop's tuning capacitor. Each side of the tuning capacitor is coupled to each side of the coax through 10-50 pF of capacitance (depending on band and loop size) per side to match 50 ohm coax. With a kilowatt there is 6 or 8 kV difference between loop and coax there, so high voltage high Q capacitors are necessary. A practical solution is a series string of 3 or 4 small inexpensive USSR surplus doorknobs on each side of the tuning capacitor. A a series string of 3-5 39, 47 or 100pF at 3 kV from Ebay will provide trial and error tapping choices for getting a match. (I use a 15 kV vacuum tuning capacitor to handle my kW amp)
Hung from a tree branch, the capacitor at the bottom of the loop should have sufficient distance to ground to be both safe and efficient. (1 diameter is usually sufficient)
Incidentally, a reasonably efficient loop will be too sharp in tuning to use a DC motor. Stepper motors (NEMA 17 is sufficient) and controllers (T-500) from pololu.com with a generic quadrature rotary encoder is one way to go.
