Ultrasound Unit For Power Line Arc Detection

This website documents my build of a 40 KHz Ultrasound parabolic dish receiver system that's based on the original work done by James Hanson (W1TRC) that was published in the April 2006 issue of QST (James is a silent key).  I also acquired a lot of knowledge from Charles (N0TT), Jeff (W4DD), and Frank (K7SFN).  I decided to create this simple website to share the numerous ideas I obtained from others and to document parts and ideas that were used by me in this project.  Here is a link to the original W1TRC QST article: https://www.arrl.org/files/file/Technology/PLN/Ultrasonic_Pinpointer.pdf 

Amplifier/Detector Circuit Board
A circuit board is available from Far Circuits.  The description of the circuit board is "HOME MADE ULTRASONIC POWER LINE ARC DETECTOR by W1TRC".  The cost of the circuit board is $5 plus shipping.  The Far Circuits website is https://www.farcircuits.net/  You can also contact Far Circuits via e-mail at mail@farcircuits.net.

There are a few capacitors and 1 resistor that were added to the circuit board design versus the original QST article, and these additional components and a lot of additional information including a schematic showing the additional parts are available in a Far Circuits .pdf document and here is a link to that document: https://www.farcircuits.net/w1rtc_notes1.pdf.   Frank (K7SFN) also has a webpage that documents his build which also includes a link to the Far Circuits document, and Franks website is https://www.k7sfn.com/projects/7-ultrasonic-powerline-arc-detector and is well worth reading.

Amplifier/Detector Circuit Board Construction

I used metal thin film resistors versus other types of resistors to minimize the generation of noise from the resistors themself.  The use of metal thin film resistors was originally recommended to me by Charles (N0TT) and confirmed to be wise based on many online sources.  

The polyester capacitors specified in the QST article were not available at Mouser so I used polyester (Mylar) capacitors that I had on hand at all locations similar to the QST article except at C4 where a temperature stable capacitor is required and I therefore used Mouser Part number 80-C315C181J1G  which is a C0G (NPO) MLCC capcitor and this is the same part number used in the QST article.  

Polyester (aka: mylar, polyethylene, polyethylene terephthalate (PET), etc.) capacitors appear to be preferred versus MLCC capacitors in regards to noise and distortion at audio frequencies.

Note: I did add a 1N5819 Schottky diode between the battery and the on/off switch for reverse polarity protection.

Enclouse
I used a Hammond Manufacturing part number 1590S for the amplifier/detector circuit board enclosure.  I chose this all-metal enclosure to shield my electronics from nearby electric fields to minimize picking up stray electrical noise.

I attached the amplifier/detector enclosure to my 1 x 2 wood handle using double stick tape after cleaning the back of the enclosure with isopropyl alcohol.

Microphone (called a ultrasonic receiver)
The original 40 KHz receiver (microphone element) used in the QST article was not available so I used the following microphone element.
Pro-Wave Electronics ULTRASONIC RECEIVER model number 400SR12M (Mouser Part number 114-400SR12M ).

I used a piece of 1/2" L type copper pipe around my microphone that's the same height (8.8mm) as the microphone to provide some shielding from stray electric fields.  My copper pipe ID = 0.540 inches versus the microphone OD = 0.500".

Neither of the legs on the microphone are connected to the aluminum shell of the microphone and I measured 3 pf of capacitance between the shell and either leg.  My microphone shell is floating (not tied to circuit common/ground) in my build but I also tried grounding it and noticed no difference.

Parabolic Reflector

Back in 2021 Charles (N0TT) provided some excellent information on the RFI reflector about a 20 inch diameter Squirrel Baffle he used that had a true parabolic geometry which is critical for this project and here is a link to that post. http://lists.contesting.com/archives//html/RFI/2021-04/msg00165.html  He mentioned that Duncraft.com sold a dish that appeared identical to what he used so I created my own spreadsheet so I could generate a true parabolic profile based on depth and diameter and I overlayed my profile on a picture of the baffle being sold by Duncraft.com and I concurred that it looked to be a true parabola so I purchased one and that's what I used in my build.  When I received the baffle from Duncraft.com I noticed the packaging said songbirdessentials.com so I went to their website and saw they were selling the same baffle but $15 less than Duncraft.com so I purchased a spare baffle from songbirdessentials.com.

Microphone (Ultrasonic Receiver) Mounting Assembly

I designed my microphone mounting assembly to provide the least amount of alteration to the reflector (squirlle baffle), and to minimize signal blockage between the reflector and the microphone.  I based the general concept of using 2 parallel all thread rods similar to the original QST article but I used a Meteric M6 hollow through bolt to hold everything together (1 x 2 wood handle (actual size 0.7" x 1.5"), reflector and microphone mounting assembly) which provided a less complicated overall final assembly.  I used 3 mm diameter all thread rods to keep weight to a minimum and to minimize signal blockage.  The M6 hollow through bolt (overall length = 44 mm) has an inside diameter just slightly larger than the RG-174 A/U I was using to connect the microphone to the amplifier and this allowed the RG-174 A/U to pass through the center of the dish without needing to drill an additional hole.  The original baffle center hole had a diameter of 3/16" and I enlarged it to 15/64".  I used double sided copper clad circuit board that was 0.058" thick to make a "top plate" that the microphone would mount to as well as a place to secure one end of the all thread rods, and a "bottom "plate" that would secure the opposite end of the all thread rods located near the inside center of the reflector.  I used M6 flat washers that had an outside diameter of 18mm as normal flat washers and to provide some spacing between the bottom plate of the microphone assembly and the reflector so the M3 nuts on the 3mm all thread rods would not come into contact with the reflector strengthening ribs.  I also used blue loctite on the M3 nuts located on the bottom side of the bottom plate.

I used a dremel tool with sanding wheel to remove copper from the center section of the top plate on the side where the microphone is located to prevent any inadvertant shorting out of the microphone pins.  I also used the dremel tool with a cut off wheel on the opposite side of the top plate to isolate the microphone positive pin solder pad from circuit common/ground as shown in a picture of the top plate where the microphone pins are soldered.

Note: my all thread rods are longer than they need to be but I'm keeping them long for now in case I decide to use them for an optical sighting system in adition to the simple sighting system I'm currently using.  I consider the 3 mm all thread rods somewhat fragile so I will need to be careful when I handle the system but they might be more stout than I think.

Note: The original design in QST used a rubber mounting plate for mounting of the microphone and I thought this might have been done to provide some acoustic isolation between the microphone and the rest of the mechanical details but I have no evidence that it's necessary based on my build.  

Locating The Microphone

Charles (N0TT) suggested pointing the reflector at a distant light source such as a light bulb and then a piece of paper can be moved in and out from the reflector to locate the focal point and this works really well.  I then moved the paper in closer to the reflector so a diameter slightly larger than the microphone element was illuminated and this is where I located the face of the microphone (per Charles recommendation).  I also confirmed this location works well using the 40 KHz transmitter.  Also after mounting the microphone I'm able to point the system at a distant light bulb to assure the microphone is still being properly illuminated.

PVC Stand

I made a simple stand to hold my parabolic dish during transport in my car and for storage.  I used 1 1/2" schedule 40 PVC pipe and a 5 way PVC elbow.

Testing The System

Charles mentioned that I should buy a 40 KHz ultrasonic transmitter transducer to help during my build so I purchased a Pro-Wave Electronics ULTRASONIC XMTR 40KHz part number 400ST12M , Mouser part number 114-400ST12M.  I used this transmitter on my electronics test bench using a function generator set to minimum amplitue output and set at 40 KHz to to directly drive the transmitter (no capacitance coupling) and at times I even used a 20 dB attenuator between my function generator and the transmitter in order to generatre a very low level tone.  Having the transmitter was very helpful during the original development/build phase of my project.

Jeff (W4DD) told me it would be very helpful to have a arc generator for testing my build and especially when working on an optical sighting system for it.  Jeff uses an inexpensive spark plug tester available on Amazon.com.  Here is a link to the one I purchased and it works very well https://www.amazon.com/dp/B0BLNN3PDJ?ref=ppx_yo2ov_dt_b_fed_asin_title   It generates a spark at a frequency of 120 Hz when its control potentiometer is set almost all the way counter clockwise which is ideal for mimicing power line noise.  This unit comes with a SMPS but I often use a 12 Volt SLA battery to power the unit directly which allows for easy operation of it out in the field (backyard testing).

I have now tested my finished ultrasound unit on a local utility pole that I know generates RFI which I use for testing various pieces of equipment and for training local utility company employees on their Radar Engineers equipment and I can easily hear arcing from 100 feet away with my near ultrasound unit.  Based on the dimensions of my reflector the half power beam width is 1.2 degrees which is very narrow and very helpful in pinpointing where the arc is being generated.

Website created and maintained by Don Kirk (wd8dsb) March 25, 2026