Magloop

The magnetic loop (magloop, aka. small transmitting loop STL) plays an important role in NVIS communications in that it is basically the only mobile antenna that can be used while in-motion due to it’s small size relative to the NVIS operation frequencies wavelengths. Its small size also makes it a great NVIS antenna for when antenna setup area is restricted and can even operate inside a garage or attic.

Due to it’s small size it is much less efficient of an antenna on the NVIS bands (especially 80m) compared to 1/4 wave and larger antennas such as the inverted vee and must be tuned nearly every time a frequency change is made due to it’s very high Q and therefore small bandwidth. This tuning is done with an air or vacuum variable capacitor that provides a sufficient picofarad value and voltage rating as to be able to create a low swr drop on the frequency in question without creating arcing between the rotor and stator components of the capacitor.

Two main things to understand when building a magloop are:
1. the larger the diameter of the loop:
-the more efficient the antenna,
-the lower the capacitance needed
-the lower the Q (the larger the bandwidth)

2. the thicker the loop,
-the more efficient the antenna
-the higher the capacitance needed
-the higher the Q (the smaller the bandwidth)

The main loop can be made out of hard or soft copper pipe or RG8 coax with the shield and inner wire soldered together (among other possible materials). One end of the loop is terminated to the stator side of the capacitor and the other is terminated to the rotor side. The feedline connects to a smaller coupler loop (approximately 1/5 the size of the main loop) that is placed inside the loop exactly opposite where the loop connects to the capacitor. There are also other ways to successfully couple the loop but the 1/5 coupling loop is my preferred choice.