[Elecraft] Experiences using a portable HF loop

[Augie "Gus" Hansen]

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