EMC antennas for troubleshooting and pre-compliance testing

Article By : Kenneth Wyatt

EMC antennas are an important component for capturing radiated harmonic emissions from products being tested.

EMC antennas are an important component for capturing radiated harmonic emissions from products being tested. There are two considerations: simple troubleshooting or calibrated pre-compliance testing. For troubleshooting purposes, most any uncalibrated antenna may be used. I’ve even had clients use Wi-Fi antennas for detecting emissions in the 50 to 200 MHz range. Basically, if your antenna can sense the emissions, it can be used for general troubleshooting. Pre-compliance testing, on the other hand, is where you’re trying to duplicate the emissions testing as performed by an EMC test lab and that will require a more expensive calibrated antenna.

Remember, EMC troubleshooting relies more on relative changes, rather than measuring absolute levels. For example, if you know your product is failing by 4 dB, reducing the problem harmonic by 10 dB at your own facility, as measured with a nearby antenna, should provide a reasonable assurance of passing.

Figure 1 shows how I normally set up a product for troubleshooting radiated emissions.

typical setup used to troubleshoot radiated emissions

Figure 1 The typical setup used to troubleshoot radiated emissions. Position the antenna and spectrum analyzer (left) about 1 m away from the product under test (right) so you can observe progress in real time.

If your intent is pre-compliance testing, where you’ll be setting up a 3-m test area in order to duplicate the measurements from a third-party test lab, then you’ll need a more sophisticated and calibrated EMI antenna. Full-sized EMI antennas can be very expensive (e.g., $4k to $6k) and I’ll discuss more affordable options later in this chapter.

Using a DTV antenna for EMC troubleshooting

There are myriad low-cost aftermarket “Digital” TV (DTV) antennas. Most of these set-top antennas are very compact and relatively broadband.

For example, RCA makes a low-cost DTV antenna (Figure 2) that includes telescoping dipole elements for the VHF channels and a simple loop for the UHF 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. You’ll want to avoid purchasing DTV antennas with built-in amplifiers because these may limit reception of harmonics outside the normal DTV bands.

I recently used an inexpensive DTV telescoping dipole antenna to perform some quick troubleshooting for a client last year. I was on vacation and got the call for help. Since I had not thought to bring any gear with me, I stopped at a hardware store and picked up an antenna and a few other bits. We set up the antenna on his workbench and proceeded to apply some fixes to his noisy DC brush motor and within a couple of hours had reduced the emissions by 20 dB. I let him keep the antenna for future tests.

The RCA model ANT121E DTV antenna shown with dipole elements partly deployed

Figure 2 The RCA model ANT121E DTV antenna shown with dipole elements partly deployed.

The antenna is advertised to cover the frequency ranges 54 to 230 MHz (channels 2 to 13) and 470 to 806 MHz (channels 14 to 69). The antenna is matched to 75 Ω, of course, and the “F” connector will need to be adapted to BNC (Figure 3). Most RF measurement equipment is designed for 50 Ω, but some may be switched to 75-Ω inputs. For the purposes of troubleshooting, the 75 to 50 Ω mismatch can safely be ignored.

An “F”-to-BNC adapter will be required in order to connect the antenna to a spectrum analyzer.

Figure 3 An “F”-to-BNC adapter will be required in order to connect the antenna to a spectrum analyzer.

I found the coax cable quite stiff, due to the construction of most 75 Ω coax cables, so you may wish to replace it with more flexible 50 Ω RG-58 or RG-174/U coax, or the equivalent Teflon-coated cable commonly used by the military and aerospace sectors. Because of the 75 to 50 Ω mismatch between the antenna cable and 50-Ω input to the spectrum analyzer, we’ll tend to get reflection and corresponding common-mode currents reflected back on the cable. To alleviate any measurement errors, you can add one, or more, ferrite chokes along the cable to help block these common-mode currents.

The base is also a bit narrow, so the antenna is more unstable with the telescoping elements fully deployed. Taping it down to a solid surface can help.

Kent Electronics PC board log-periodic antennas

One of my favorite antennas for troubleshooting is the PC board design by Kent Britain, of Kent Electronics. It is easily packed into the troubleshooting kit, along with the attached coax.

Kent Electronics sells a number of off-the-shelf and custom PC board antenna designs. I decided to order three types of log periodic (LP) antennas designed for the frequency range of 400 MHz to 11 GHz (Figure 4). Together, they cost me $62.

three models of log-periodic antennas

Figure 4 The three models of log-periodic antennas. Prices range from $9 to $33.

After receiving them I installed suitable PC-mount SMA connectors at the tips—mounting each on the “ground” side of the array and running the coax cable straight down the center.

Construction

I wanted to be able to disassemble and store the antennas and a small table-top tripod in my troubleshooting kit, so I fashioned a mounting scheme based on standard 3/4-inch PVC pipe. After a bit of experimentation at the local hardware store, I ended up with the components shown in Figure 5. By drilling and tapping an end cap with a 1/4-20 thread, it will then screw onto a standard photographic tripod. Then, the 90-degree adapter is screwed on to the cap, hand-tightening the two together.

various components used for the antenna mount

Figure 5 Here are the various components used for the antenna mount. The end cap was drilled and tapped with a standard 1/4-20 thread size, which is a standard photographic tripod mount. The drill and tap is sometimes sold as a set at hardware stores.

Using a hack saw, I carefully cut a slot partway into the horizontal pipe and pressed it (or glued it, if required) onto the rear of the LP antenna as shown in Figure 6. The other end of the horizontal pipe was simply pressed into the 90-degree adapter, so that I could easily rotate the antenna for horizontal or vertical polarization.

completed antenna mounted to the table-top tripod

Figure 6 The completed antenna mounted to the table-top tripod.

The tripod used was a table-top model “T-Pod” from Trek-Tech ($49.95). It collapses and is stored in a small pouch. The sections connect together with strong neodymium magnets, so it’s structurally strong and holds the antenna without any trouble. Other small-sized tripods may be used, as well.

I measured the voltage standing wave ratio (VSWR) of the antenna from 200 to 1,500 MHz and it looked very good over the advertised range of 400 to 1000 MHz—mostly staying better than 2:1 across the band. Interestingly, there was also good resonance from 1,200 to 1,500 MHz—better than 2:1, or so.

I wish the 400 to 1000-MHz LP would go lower in frequency, but then it wouldn’t fit so nicely (barely) in the Pelican 1510 roller case I use for my troubleshooting kit. Also, in discussing the issue with Kent, he said the size was at least partly limited due to the maximum panel size used by his PC board fabricator. However, I’ve discovered the antenna works well enough down to 100 MHz for troubleshooting purposes. Basically, if you can “see it,” you can “fix it.”

Calibrated reduced-size EMI antennas

Standard EMI antennas are usually large and difficult to store. The advantage in using the following smaller-sized antennas is that they are calibrated. However, they also aren’t very efficient antennas, especially below 200 MHz; therefore, their antenna factors (AFs), expressed in dB/m, will be rather high in that frequency range. For best results, a full-sized EMI antenna will work better for pre-compliance testing, but will carry a higher price tag. We’ll discuss full-sized EMI antennas below.

One of the more important specifications of an EMI antenna is the chart of antenna factors provided by the manufacturer. A resonant dipole antenna at 30 MHz is about 16 feet long end-to-end. Even “full-size” EMI antennas are only about four feet wide at most, so will not be nearly as efficient as a dipole antenna. These reduced-size “table top” antennas are even worse.

Because the antenna factor needs to be added to each measured harmonic, high antenna factors are a distinct penalty when used for pre-compliance testing. For general troubleshooting, where absolute accuracy is not that important, they should work fine. I’d still try to choose one with the lowest AF possible at the lower and upper frequencies.

One excellent choice might be the York ARA-01 active antenna. Its AF is particularly low, but being a “receive-only” antenna, it can’t be used for transmitting. It should be perfectly OK for both troubleshooting and in particular, pre-compliance testing.

Most of these antennas are small enough that they can be mounted on a table top or floor standing camera tripod.

Tekbox TBMA1 antenna

Tekbox Digital Solutions recently introduced a scaled down table-top EMI antenna that is calibrated and should be usable for both troubleshooting radiated emissions and pre-compliance testing (Figure 7). It covers 30 to 1000 MHz, can transmit up to 2 W, and comes in a nice wooden box with table-top stand. Because of its small size, the antenna factor ranges from 40 to 15 to 30 dB/m as frequency is increased. The resonance point is 200 MHz (16 dB/m antenna factor). See Figure 8.

The Tekbox TBMA1 tabletop antenna

Figure 7 The Tekbox TBMA1 tabletop antenna.

antenna factor versus frequency chart for the Tekbox TBMA1 antenna

Figure 8 The antenna factor versus frequency chart for the Tekbox TBMA1 antenna.

Eurofins-York Miniature active antenna

The Eurofins-York ARA01 active receive antenna, while small, has better antenna factors than most full-sized EMI antennas due to its low-noise preamplifier (Figure 9). This should be completely adequate for general radiated emissions troubleshooting, as well as 3- or 10-m pre-compliance testing and is highly rated by my colleague and EMC consultant in the UK, Keith Armstrong of Cherry Clough Consultants.

While both dipole sets are calibrated from 30 to 1000 MHz, there are two configurations; a base unit that comes with shorter dipole elements and covers 200 to 1000 MHz (optimum) and a model with longer dipole elements that covers 30 to 300 MHz (optimum). See the AF curves in Figure 10. Both configurations are more sensitive (lower antenna factor) than a full-sized EMI antenna in the range 30 to 300 MHz where most emissions failures tend to occur. Personally, I’d purchase both sets of dipole elements, because most emissions troubleshooting seems to occur from 30 to 300 MHz, so the longer elements are well worth the lower AF. York antennas are distributed in the U.S. by Reliant EMC.

York ARA01 active receive antenna

Figure 9 The York ARA01 active receive antenna has a particularly low antenna factor.

antenna factor chart for the ARA01’s two configurations

Figure 10 The antenna factor chart for the ARA01’s two configurations (red and blue lines) of dipole elements. The upper line (green) offers a comparison to the full-size Chase CBL6111C EMI antenna.

Calibrated full-size EMI antennas

For serious pre-compliance testing there’s simply no substitute for a full-sized calibrated EMI antenna and a quality tripod to hold it. Several companies manufacture these: Amplifier Research, ETS-Lindgren, A. H. Systems, Com-Power, Rhode & Schwarz, Teseq, York and many others. Some companies like Com-Power and A. H. Systems make fold-up antennas that can be easily stored away when not needed.

Sometimes used antennas are available from eBay and other used equipment dealers. Be sure used antennas are not damaged and include a calibrated antenna factor chart that matches the antenna serial number.

Com-Power antennas

Com-Power makes a variety of full-sized EMI antennas (Figures 11 and 12 show a couple of examples), as well as a “starter” emissions kit that includes two antennas, a preamplifier, comb generator, near-field probe kit, and coax cables (Figure 13). The case is an option.

Com-Power model ABF-900A folding biconical antenna

Figure 11 Com-Power makes their model ABF-900A folding biconical antenna that is resonant from 25 to 300 MHz. The price is $2,025.

Com-Power ALC-100 log-periodic design

Figure 12 The Com-Power ALC-100 log-periodic design is resonant from 300 to 1000 MHz and costs $1,450.

Com-Power model ANK-310 "starter" kit for radiated emissions

Figure 13 A nice “starter” kit for radiated emissions from Com-Power is their model ANK-310 and costs $8,210. It includes antennas for 30 to 1000 MHz, a broadband preamplifier, harmonic comb generator, near-field probes, and cables.

A.H. Systems antennas

A.H. Systems also makes a range of affordable full-sized EMI antennas. An example of one of their kits is shown in Figure 14.

radiated emissions antenna kit from A. H. Systems

Figure 14 A similar radiated emissions antenna kit from A. H. Systems. It covers 25 to 2000 MHz.

Teseq/Chase Bilog antenna

The Chase (now Teseq) CBL6111 bilog antenna is one I use myself (Figure 15 and 16). The biconical elements can be removed easily so the antenna may be stored relatively flat.

The Teseq/Chase CBL6111 biconical EMI antenna

Figure 15 The Teseq/Chase CBL6111 biconical EMI antenna has a range of 30 to 1000 MHz without the need to change antennas partway through the band.

antenna factor for the Teseq/Chase CBL6111D

Figure 16 The antenna factor for the Teseq/Chase CBL6111D. This chart is very typical of full-sized EMI biconical antennas.

Antenna Tripods

Some full-size EMI antennas can be heavy—especially those that combine a biconical element with a log-periodic design—and will need a firm support. Some companies elect to make their own supports from 4×4 and 2×4 lumber. Be sure to use non-conducting materials. Camera tripods will not work, due to their conductive metal or partly conductive carbon-fiber construction. I’m currently using one of the Com-Power tripods and am happy with it. Most companies that make EMI antennas also make tripods to go with them.

Com-Power tripods

Com-Power makes a series of custom tripod supports (Figure 17) that are designed to fit their own antennas. I had to modify their adapter mounts to fit my Chase CBL6111 antenna. Their tripods have an optional wheeled transit case, which I would recommend. Prices range from $1,000 to $1,500.

Com-Power makes a range of tripod antenna mounts

Figure 17 Com-Power makes a range of tripod antenna mounts. I’m currently using the one in the middle and find it sturdy enough for full-sized EMI antennas.

A.H. Systems tripods

A.H. Systems makes a couple types of more conventional wooden tripods with mounting adapters that fit their antenna models (Figure 18).

conventional tripod design from A. H. Systems

Figure 18 A more conventional tripod design from A. H. Systems.

Absolute EMC low-cost tripod

For lower budgets, the Absolute EMC Tripod is constructed of 1-inch wooden dowels and custom 3D-printed attachments and tripod heads, along with several other accessories. The cost is less than $350 for the tripod, storage bag and accessories. I purchased one and it seems steady enough, but is not designed for the full-size bilog-style EMI antennas. However, it should work well for antennas from 10 to 20 pounds, if center-mounted. If you use an end-mounted antenna, you’ll need the included weight bag hooked to the tripod.

Absolute EMC’s low-cost tripod

Figure 19 Absolute EMC’s new low-cost tripod is constructed from 1-inch wooden dowels with custom 3D-printed assembly hardware. It is sturdy enough for lightweight center-mounted antennas.

The combination of a set of Kent Electronics antennas for troubleshooting (or similar) and some sort of used or new calibrated EMI antenna should prove valuable for both troubleshooting radiated emissions issues as well as performing pre-compliance testing in-house. Use of these tools should save a lot of money and time in getting products to comply without having to repeatedly make trips to the third-party compliance test lab. You’ll find in-house testing a huge advantage!

For casual pre-compliance testing, I really like the York ARA01 antenna in combination with the Absolute EMC tripod for best performance for the money. For more serious pre-compliance testing, you’ll want to choose the larger calibrated EMI antennas along with a more robust tripod.

For general radiated emissions troubleshooting, the Kent Electronics PC board antennas are ideal.

This article was originally published on EDN.

Kenneth Wyatt is principal consultant of Wyatt Technical Services LLC and served three years as the senior technical editor for Interference Technology Magazine from 2016 through 2018. He has worked in the field of EMC engineering for over 30 years with a specialty in EMI troubleshooting and pre-compliance testing. He is a co-author of the popular EMC Pocket Guide and RFI Radio Frequency Interference Pocket Guide. He also co-authored with Patrick André the book EMI Troubleshooting Cookbook for Product Designers, with forward by Henry Ott. He is widely published and authors The EMC Blog hosted by EDN.com and continues to blog for Interference Technology. Ken is a senior member of the IEEE and a longtime member of the EMC Society. To contact Ken or for more information on technical articles, training schedules and links, check out his web site: http://www.emc-seminars.com

Note: This article is a partial excerpt (with some additions) from “Create Your Own EMC Troubleshooting Kit (Volume 1)” by the same author. The full book may be ordered from Amazon.

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