For many years I mostly built and repaired home audio and studio recording equipment, but lately I too found that I needed to find a way to "see" the EMI/RFI that was causing me so much grief with my audio equipment. This had to be done economically (luckily, I'm mainly interested in relative measurements. I just need to see if my added shielding, capacitors, ferrite beads, etc were helping squash the unwelcome high frequency interferance).
The URLs listed below are where I found most of the info I used to build some "sniffer probes" and also two high frequency current probes to use mainly with my low cost RF Explorer-3G Combo handheld spectrum analyzer (15MHz - 2.7GHz BW). The included photo is of my "poor man's EMC troubleshooting kit". It containing an assortment of home-brewed E-Field and H-Field probes, and the two high frequency current probes. To my surprise, I put together this complete kit of probes for a grand total of about $100 USD (inexpensive, interesting, but also fairly tedious). I later included a couple of attenuators and sma to BNC adapters in the kit too for good measure.
The materials needed to build all of these devices was pretty reasonably priced. I bought a few used semi-rigid coax cables from ebay. These already had the male sma connectors needed for my handheld analyzer installed. I just cut each coax in half and made two different diameter sniffers from them. This saved me the trouble of soldering the sma connectors on.
The black square shaped probe on the left side is not made from semi-rigid coax. This one is "The Paperclip Probe" from Doug Smith's site, where he states that "a paper clip (any stiff wire will do) is bent into a square shape and covered with insulation, forming an unshielded magnetic loop probe".
The high frequency current probes are made with the components specified in Ken Wyatt's multi-part article listed below. One current probe is made from a toroid core and the other one (directly below it in my kit) is made from a clamp-on type RF choke. That one allows me to conveniently snap it onto the wire or coax that I'm investigating for noise issues.
With all the info, so generously provided at Mr Smith's and Mr Wyatt's sites, I was able to make what I needed for my home testbench.
Douglas C. Smith's site (www.emcesd.com
) is loaded with terrific EMC related info. He also has some interesting videos on youtube.
Also see the multi-part article by Kenneth Wyatt at the following URL... www.tmworld.com/electronics-blogs/emc-emi-rfi-esd/4378152/An-EMC-Troubleshooting-Kit--Part-1a-Emissions-
Mr Wyatt's own site (http://design-4-emc.com
) has lots of good EMC info too.
Mr Henry Ott's site is also very much worth visiting (www.hottconsultants.com/tips.html
The late, great Jim Williams has an app-note that shows a very slender tipped diy sniffer probe which has a stated spacial resolution of about 1 millimeter. http://cds.linear.com/docs/en/application-note/an118fa.pdf
See Appendix E of this application-note. That section shows how to build the probe and also shows some situations where the probe can be used to great advantage.
I made one of these probes (the one with the "plump" body and BNC connector in the photo) and I'm impressed with its ability to pinpoint a high frequency noise source on a circuit board, etc. I use this probe with my scope and my RF Explorer-3G (I really like this recently purchased little analyzer).
I finally have a fairly low cost way to "see", and hopefully tame, some of the EMI/RFI that is affecting my audio gear. I hope the above URLs help you out too.
EDIT 4/1/13: If anyone is interested in making the two High Frequency Current Probes shown in my DIY Probe kit photo below, see the following pdf.www.interferencetechnology.com/wp-content/uploads/2012/04/Wyatt_NA_DDG12.pdf
This pdf had all the info that I needed to construct mine (see figures 4 and 5 on page 3 of the pdf), plus Mr Wyatt has a section at the end of the document on how you can easily characterize the performance of the probes after you build them.
These are the various DIY probes in my "EMC Kit" that I fabricated for use with my scope and RF Explorer-3G spectrum analyzer (my scroll saw came in hand to cut out the enclosure's foam inserts to form the pockets which hold the probes neatly in place).