For watching the 10m, 6m and 4m bands ES openings this magnetic loop antenna was built. A juvenile bicycle rim is used as the loop.
The Butterfly-C is made of some laser-cut aluminum of 3mm.
The antenna in C max position = 27,99 Mhz
It is quite difficult to reach mechanically exact parallel fixing of the butterfly plates to have maximum C.
Distance between the plates ~ 1mm
Antenna tuned for 6m
highest frequency is 72 MHz
Here some details:
The feedpoint is a 50 Ohm coax connector fixed with two brass screws M3.
! Avoid to use magnetic materials as steel iron etc. for building magnetic loop antennas!
Use only aluminum, brass, stainles steel, copper...
Iron etc. will make losses.
The "gamma" match.
This kind of antennas work fine with SDR receivers. Antenna's small bandwidth gives good preselection for the SDR.
Loop diameter is 45cm.