Home » Blog » Radio » Shortwave Radio » Loop Rotation Options Considered

Loop Rotation Options Considered

loop rotation options
Wellbrook Loop on Small Rotor

You should check out wideband loop rotation options if you plan to use the antenna for low-angle DX reception at long, medium and lower HF frequencies. What are the options? 

As I get ready to migrate my prototype wideband receiving loop into its final form, three things need consideration. First, should use a rotator? Second, how should I mount the loop? If I use rotation, mounting has some special requirements. And third, should I paint  the loop?

The painting requirement is easy. My loop is made from aluminum core PEX, which degrades in ultraviolet light. Same goes for the plastic I will use for mounting supports. So, I will cover paint selection later.

My main question is rotation. Typical loop rotation options are light duty ham rotors, or even lighter duty television antenna rotors. Some folks use the Channel Master CM-9521A which has 100 foot-pounds of rotation torque and 1 RPM speed. This unit uses a three conductor cable and should work down to -20C, an important consideration in our climate.

You can find these and other commercial rotators on Amazon for about US$150. They would do the job nicely but might be overkill for my loop, which only weighs around a pound.

In the spirit of my retirement adventures, it might be fun to build my own rotator from scratch. You can find a bunch of high-torque DC or stepper motors on e-bay for around US$10-$20. With my 3D printer I could fabricate some gears and supports easily. Control would be straightforward with an Arduino, or better yet, remote control over WIFI with NodeMCU.

It might even be possible to use the 12VDC for the loop amplifier, depending on current requirements. So, I will go exploring shortly.

Loop Rotation Options – Why Bother?

Many loop users don’t bother with rotation. You can get decent results just orienting the null towards the largest noise source and leave it at that. But, you can also gain some advantage from loop rotation options.

The wideband magnetic loop antenna has a figure eight pattern, with nulls broadside to the plane of the loop. You will find that the depth of the null varies along two factors. First is the size of the loop. As long as the circumference is less that 15-20% of wavelength, the null gets pretty sharp with high attenuation. At higher frequencies, not so much.

Second is the elevation angle of the arriving signal. Low angle signals get reduced at the null much more than high angle signals. Above 45 degrees, loops are almost isotropic.

So, if you plan to use your loop for DX on the lower frequencies (say up to 10 MHz), rotation capability can make a big different for spatial filtering.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.