On 4/21/2017 9:00 PM, Ron D'Eau Claire wrote:
The very BEST magnetic loops are incredibly inefficient, yet like any "incredibly inefficient" antenna one can make amazing contacts on them when conditions are right.
If you use the suggestion that the loop perimeter (circumference for a circular loop) be 1/10 of a wavelength, then efficiency is quite low - on the order of 10%. This is the guidance given for small receiving loops to get the best nulls.
However, a transmitting loop can have a perimeter of up to 1/3 wavelength and still have a constant current throughout its length (within 1% per John Kraus). This constant loop current is what defines a magnetic loop.
The problem is with resistive losses. The day we have room-temperature superconductors, we will have efficient small transmitting loops. Until then, only a few percent of the power applied is radiated.
My loops are typically a bit short of 1/3 wavelength at the high frequency of the tuning range, and no less that 1/10 wavelength at the low end. The efficiency of a 3/10 wavelength loop, for example, is in excess of 90% provided the resistances are kept low. I typically use the outer (shield) of hard line for the loop conductor and a series vacuum variable capacitor for remote tuning.
It is also possible to use multiple turns to reduce the size of a magnetic loop even further. There is an example of a 40-80 meter mag loop on my QRZ page that uses 7/8-inch hard line in a two-turn configuration. Even though only 4.5 feet in diameter, it works quite well when compared to a typical back yard inverted vee or low dipole, and it can be hidden in an attic, the back of a garage, or tucked out of sight among trees or bushes.
73, Gus Hansen KB0YH ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
