Playing with antennas - part 2
Kenneth Wyatt- November 15, 2012Here's a simple DIY EMC measurement antenna that is adjustable from about 85 to 200 MHz (depending on the telescoping antennas used) and is small enough to throw into your EMC troubleshooting kit. It also includes a 1:1 balun and additional ferrite choke to suppress common-mode currents from flowing on the outside of the coax shield.
Of course, while troubleshooting, most any old hunk of metal will serve to pick up harmonic emissions from a product under test. I had one client set up a Wi-Fi antenna about three feet away and it worked fine while we troubleshot his product. When I use this for troubleshooting, I tape it down to one end of a non-conductive table top and set up the product under test on the other end. I can then see at a glance whether the harmonic I'm monitoring has gone up, down or stayed the same as I'm trying different fixes.
The antenna is constructed in a Bud model CU-123 cast aluminum box. I mounted three BNC connectors, as shown in Figure 1. The 1:1 balun is simply several wraps (as many as will fit) of thin coax cable wound around a ferrite toroid core (most any will work). I used some thin double-shielded coax for the internal wiring. Connect the shield to one center pin of the antenna elements and the center conductor to the center pin of the other, as shown in the figure. I clamped a standard ferrite choke around the extra coax for good measure and packed the interior with a small patch of bubble wrap to keep the two ferrites from moving around.
Figure 1 - The antenna base assembly showing the internal 1:1 balun and additional ferrite choke to suppress common-mode currents.
I used rather expensive ($30) Diamond model RH789 telescoping antennas, primarily because they happen to be marked with a frequency scale versus element length - an added feature - although lower-priced telescoping BNC antennas should work fine. These also hinge at the base, so you can collapse the whole thing a bit smaller. By hinging both sides into a slight "V"-shape, you'll achieve a better match to 50 Ohms. As a bonus, amateur radio enthusiasts can also make use of the antenna as an emergency dipole for VHF and UHF operation.
Figure 2 shows the antenna taken apart and Figure 3 shows it deployed and ready for use during troubleshooting.
Figure 2 - The antenna shown taken apart for storage.
Figure 3 - The antenna deployed and ready to use for troubleshooting.
Refer to "Playing with antennas - part 1" to learn how to characterize this antenna using a spectrum analyzer and tracking generator.