WD8DSB tuned shielded loop antenna for RFI direction finding

Below is one of my favorite HF direction finding antennas that I often use when tracking down RFI on 160 meters, especially if I want to go stealth. This antenna is very simple, and it's based on a design in the July 1977 issue of QST by W1FB (the article was called Beat the noise with a "scoop Loop").

I actually just hold the antenna down by my side with one hand and rotate it as I walk toward the source of RFI, and I often use the null of the antenna to pinpoint the source versus peaking on the signal. Here is a link to a video showing the immunity of this antenna to the effects of my hand touching it : http://www.youtube.com/watch?v=oMFMrOrAY3w

Here is a link to a video showing the tuned loop in action tracking down plasma TV RFI and it shows me rotating the antenna (held in my hand) as I get right on top of the signal as the signal exits the homes utility feed : http://www.youtube.com/watch?v=mhJYCxJtFv0

Before using this antenna I use my pennant array to determine what direction the noise is located from my home. Then I head out with my portable system that consists of this loop connected to a DX440 portable receiver. I often use a preamp (normally use a W1FB adjustable gain preamp) when first heading out with this antenna and as I get close to the source I reduce the gain of the preamp and when I get very close I typically remove the preamp from the system and when very very close I even have to back the gain of the receiver down to be able to detect the null. I often do not use a matching transformer with this antenna, but for maximum output signal a matching transformer does help. (Note : to put things into perspective this system when used with the preamp pretty much allows me to hear anything my 68 foot base loaded TX vertical can hear on 160 meters.)

Details :

14" Diameter

4 turns of RG58U coax

1 turn pickup loop (also approximately 14" in diameter just taped to the 4 turns of coax)

Variable capacitor (for tuning to resonance)

(click on images below to enlarge)

Picture of the tuned loop without preamp or matching transformer

Picture showing tuning capacitor, 1inch break in the shield at the mid point of the 4 turns, and one turn pickup coil (red wire)

Picture showing portable system ready to go

(using Binocular core matching transformer and no preamp)

(matching transformer connected between the pickup coil and 33 inch section of feedline, but I normally don't use the matching transformer)

Impedance measurement of the loop shown below using a matching transformer (BN-73-202: Primary = 3 Turns, Secondary = 12 Turns)

Website created and maintained by Don Kirk (wd8dsb), Oct. 1, 2014