If you missed the SWLing Post articles about SULA, you should check them out. An easy to build small unidirectional loop antenna.
Earlier this year, SWLing Post ran a series of articles about a small unidirectional loop antenna. As shown above left, SULA is a 30″ per side loop antenna mounted 10′ above ground. You will see this antenna is a loop variation featuring a 9:1 balun at the feedpoint, and a 530Ω terminating resistor opposite the feed. A picture of the project is shown above left.
As described, SULA covers Medium and Short Waves. Most important, its radiation pattern is cardioid, which means a beam on one direction only and a null at the back. You can find the original article here, featuring many videos of loop performance. And, here are the instructions on how to build this neat antenna. I think I will build one and get it up before winter.
We all know that a loop antenna has a figure-8 pattern, with lobes in the antenna plane. So, how do you get a cardioid pattern from a single loop?
Turns out the technique was discovered back in the 1920’s by adding a vertical sensing antenna to the loop. By combining the omni-directional pattern from the sensing antenna with the figure-8 of the loop, you could derive a cardioid or heart-shaped pattern.
Another way to achieve the same thing is to place a terminating resistor, typically 200Ω to 2000Ω at the loop’s midpoint. You have probably heard about many terminated loops used for shortwave reception such as the EWE, Flag, Pennant and K9AY configurations, as shown above right.
SULA – How Terminated Loops Work
SULA and its cousins combine both E and H field reception. The resistor effectively creates a small sense dipole antenna, which receives E field and generates a voltage at the feedpoint from received signals. At the same time, the magnetic H field generates currents in the loop which results in out of phase voltages across the terminating resistor.
Voltages mix in the loop in-phase from one direction, and 180º out of phase from the other direction. As a result, your reception from one direction is cancelled, and the figure-8 is changed to a cardioid pattern. Your terminating resistor effectively sets the feedpoint impedance, which remains relatively constant across different loop sizes and frequencies.
When you scale a terminated loop down to SULA size, area decreases and voltages generated are much smaller. So, a low noise amplifier may be needed to get enough performance. Anyway, check these articles out and build one.
You may also want to read the article Portable Flag Antenna for MF/HF Direction Finding in March 2021 QST. You can watch a video of this antenna in action, which is also sold by DX Engineering.