WØIVJ's Receive Loop Project
(Click on pictures to enlarge)
Email: w0ivj@arrl.net

About a year ago I noticed a strong interfering signal on the 40 meter band.  Since the interference was coincident with the work day and week, I suspected it was coming from the industrial building just behind our house.  But, in order to be sure, I decided to build a receive loop to track the noise down.  The loop is made of 1/4 inch copper tubing and a piece if 1 X 2 lumber.  The large loop is 24 inches in diameter and the small loop is 6 inches in diameter.  The large loop is connected in series with a 220 pF silver mica capacitor which is in parallel with a 26 to 170 pF air variable capacitor.  This is a nominal capacitance of 318 pF to resonate the loop.  The 6 inch pick up loop is fastened to the other side of the 1 X 2 and connected to a short feed line of RG-58 coax.  The loop is connected to a receiver and the variable capacitor is adjusted for maximum noise on the 40 meter band.  Pictures A, B, and C show the construction of the loop.

A..Hi Res..B..Hi Res..C..Hi Res

By the way, the noise turned out to be a switching power supply in a halogen track light, but that is another story.

Since the loop described above, I have unfortunately had many more opportunities to track down RFI, mainly grow light ballasts.  Consequently I have built a few more loop antennas. Picture D shows a schematic of the receiving loop.  Picture E shows the three loops that I have built so far.  The loop on the left is the 24 inch loop described above and is the most sensitive.  The loop in the middle in Picture E is the 20 inch loop I use most of the time because it is the most versatile.  It will tune 160m through 10m. Details of this loop are shown in picture F. The 20 inch loop has a SPDT Center Off switch to switch in extra capacitance.  The main loop is made of 0.5 inch aluminum and the pick up loop is made from #12 wire inserted in 0.25 inch plastic tubing.  The pick up loop is about 5 inches in diameter.  Finally the loop on the right is the lightest loop but the least sensitive.  It is 16 inches in diameter with a 4.5 inch pick up loop.  This loop is made from #22 wire inserted in 0.25 plastic tubing.

Check the loop for resonance by tuning for an increase in noise.  You should see a peak in signal when you tune the variable capacitor.  If not, change the fixed padder capacitor.

D...Hi Res..E...Hi Res..F...Hi Res..

The antenna factor of the loop is important.  If you can hear RFI on your home antenna but cannot hear it on your loop antenna, searching for the RFI becomes much more difficult.  You will have to walk or drive around until you can hear it.  Larger, high Q loops have better antenna factors and are therefore more sensitive.  The larger the diameter of the main loop conductor, the higher the Q.  The conductor material is also important.  I did an EZNEC analysis of various loops in order to compare them.  The results are in the following table : DF Loop Values.pdf   You can see that the loop construction is important.

In order to use the loop, you need a receiver and preferably an attenuator.  Picture G shows the RFI tracking system that I use.  Most commercial receivers that I have did not have the attenuator and S-Meter features that I wanted so I built a small Direct Conversion receiver with plug-in coils for 80m, 40m, and 20m.  The schematic is shown in Picture H and the receiver is shown in Pictures I and J.  Of course you can use the receiver you have on hand.

Once you have your RFI tracking system assembled, it is a good idea to practice using it.  I constructed the test oscillator shown in Picture K  whose schematic is shown in Picture L.  This little oscillator has the advantage that you can orient it so it radiates with vertical or horizontal polarization.  With a little practice you can become quite proficient tracking down RFI.  If your RFI is predominant on a band other than 40m, you may want to build the test transmitter whose schematic is shown in Picture M.  Larry, W0QE, also has a test oscillator that you can easily couple into your house wiring to confirm that the signal polarization is indeed vertical.  It can be found by clicking HERE.

G...Hi Res..H...Hi Res..I Hi Res..J...Hi Res..K...Hi Res..L...Hi Res..M...Hi Res

More details of the receiver are shown in Pictures N, O, P, and Q.  Picture N and O show details of a coil module that mates with a 10-pin header shown in the upper left of Picture P.  All coils are tapped 1/4 of the turns from the ground end.  Picture Q shows the attenuator in the background.  The attenuator is encased in a brass box.