Using a DTV antenna for EMC troubleshooting
Kenneth Wyatt- December 27, 2012Ever since over-the-air (OTA) digital television (DTV) was mandated by the U.S. FCC in June 2009 (http://www.dtv.gov) and the myriad of low-cost aftermarket DTV antennas became available, I've wanted to test one for use as an EMC antenna for troubleshooting. Most of these set-top antennas are very compact and relatively broad band.
Most North American DTV broadcast stations are assigned somewhere between the old TV channels 7 to 13 (174 to 216 MHz) and 14 to 51 (470 to 698 MHz). There are several exceptions. For example, in large metro areas, the TV channels 14 to 20 (470 to 512 MHz) are used for public service land mobile. There is also a small handful in TV channels 2 to 6 (54 to 174 MHz). Refer to http://en.wikipedia.org/wiki/North_American_broadcast_television_frequencies for more information on current TV frequency allocations.
For the purpose of this experiment, I chose a TERK Model HDTVi (HDTV Antenna Pro), marketed by Audiovox (Hauppauge, NY), which is unique in that it includes a log periodic section that is easily switched from horizontal to vertical polarization. It also includes telescoping dipole elements, which cover the lower TV channels. The antenna is compact and can sit on one end of a workbench while you perform troubleshooting at the other end on your equipment under test.
Figure 1 - The TERK model HDTVi antenna shown with dipole elements partly deployed. The telescoping elements extend to 42 inches. Note that the log-periodic elements can plug into the base in either horizontal or vertical (shown) polarization - a unique feature of this antenna.
The antenna is advertised to cover the frequency ranges 40 (sic) to 230 MHz (Channels 2 to 13) and 470 to 806 MHz (Channels 14 to 69). Note that the specification of 40 MHz should actually read "54 MHz" for TV channel 2.
The antenna is matched to 75 Ohms, of course, and the "F" connector will need to be adapted to BNC. For the purposes of troubleshooting, the 75 to 50 Ohms mismatch can safely be ignored.
Figure 2 - An "F" to BNC adapter will be required in order to connect the antenna to a spectrum analyzer.
I decided to measure the VSWR of the antenna using the Rigol DSA815TG spectrum analyzer with tracking generator in order to compare with the advertised frequency specifications of the antenna. While the Rigol can be set to 75 Ohms input impedance, I wanted to see what the match was at the default 50 Ohms, since that is the impedance of most EMI receivers.
Figure 3 - The antenna with telescoping elements stowed into the base.
Figure 4 - The VSWR plot from 10 to 1500 MHz, showing the resonant band of the log periodic antenna in horizontal polarization. The marker is at the upper band edge.
The log periodic elements are resonant from approximately 427 to 1055 MHz, with most of the frequencies better than a 2:1 VSWR. The band edges are about 3:1 VSWR. There are additional resonances from 1120 to 1500 MHz with most points better than 2:1 VSWR.
Figure 5 - The antenna with log periodic elements in the vertical polarization. The elements can be pulled straight out and turned in either polarization, due to the square base connection.
Figure 6 - The VSWR plot from 10 to 1500 MHz of just the log periodic elements in vertical polarization. The marker is at the upper band edge.
There is little difference in resonance with the log periodic elements in the vertical position. The band of resonance is about 424 to 1055 MHz with 2:1, or better VSWR and 3:1 VSWR at the band edges.
Figure 7 - The VSWR plot from 10 to 1500 MHz with the log periodic elements in horizontal polarization and the telescoping elements deployed.
With the telescoping dipole elements deployed and the log periodic in the horizontal position, we can see the resonant area contributed by the dipole ranges from about 99 to 270 MHz, with several peaks at 1.5:1 VSWR, or better. Depending on how far out the elements are extended, the primary resonance ranges from 99 to 275 MHz; not quite achieving the claimed 40 MHz or the actual 54 MHz of the actual channel 2 frequency. However, for troubleshooting purposes, the antenna should be acceptable for simple "A-B" measurements from the entire 30 to 1000 MHz band required for most electronic products.
For your reference, here are the approximate return loss (vertical scale on analyzer) and VSWR values. By examining the vertical scale on the analyzer plot, you can see at a glance what the VSWR is along the frequency axis.
Return Loss versus VSWR
Figure 8 - The antenna stowed in its box.
I found the coax cable quite stiff, due to the construction of most 75 Ohm coax cables, so I might suggest replacing it with more flexible 50 Ohm RG-58 or RG-174/U coax, or the equivalent teflon-coated cable used by the military and aerospace sectors. Because of the 75 to 50 Ohm mismatch, you'll also want to add one, or more, ferrite chokes along the cable to help block common-mode currents. The base is also a bit narrow, so the antenna is more unstable with the telescoping elements fully deployed.
As you can see, the TERK HDTVi antenna packs down very compactly and is currently priced at $27 at Amazon.com.
There is also an amplified model "HDTVa" (untested) for $40. Maybe someone could test that one for us? Finally, a future project might be to measure the antenna factor (AF) of this antenna. Any measurement ranges want to volunteer?