Modifying the MFJ-5008 parabolic ultrasonic receiver for better sensitivity and wider frequency response

Figure 1:
Front view MFJ-5008 parabolic dish with
integrated microphone and receiver
(located on the back side).
Click on the image for a larger version

The MFJ-5008 Parabolic Ultrasonic receiver

Note:

Since the MFJ-5008 is no longer being sold it can be found only on the "used" market.  A future posting in this blog will (hopefully) show how to construct a similar unit using readily-available kits and parts.

The MFJ-5008 was marketed primarily for detecting arcing on failing power line hardware, but there are other reasons why you might use such a device:

• Listen to Bats' echolocation.  The "clicks" emitted by bats are well above human hearing.
• Listen to other animals and insects.  Other animals and insects also emit ultrasonic sounds - both for echolocation and communication.
• Find high pressure leaks.  Leaks in high-pressure systems (water, gas, engines, compressors) often make a lot of noise at these frequencies.
• Locate switching power supplies.  These devices often make noise due to magnetostriction of devices (transformers, coils.)

As I find this topic to be interesting, I've written about the detection of ultrasonic signals on two previous occasions in this blog:

• Improving my ultrasonic sniffer for finding power line arcing by using MEMs microphones - Link
• An ultrasonic superheterodyne receive converter (e.g. "Bat Listener") - Link
  

* * * * *

While there are several devices out there that you can buy to enable listening at these frequencies, the landscape has changed in the past few years when it comes to how one might do this on a budget:

• In years past, the MFJ-5008 was available - its primary purpose being to locate and identify arcing on power lines and related infrastructure.  As MFJ is no longer in business, this device is available only on the used market.
• Some "bat listeners" have used electret microphones.  These inexpensive capsule microphones - while having good response across the human hearing range - lose sensitivity rapidly above this, limiting their usefulness above 20-30 kHz.  In doing A/B testing with a MEMS and an Electret ("capsule") microphone, the MEMS appears to be superior in every way when it comes to ultrasonic response.
  
• Many ultrasonic detectors - including "bat listeners" - have used ceramic transducers.  Most often found for the 40 kHz range (and some were made at lower frequencies) these can be fairly sensitive.  Their frequency range is quite limited and they are only usable within a few kHz above and below their design frequency at best.  As different types of ultrasonic noise sources tend to occur at various frequencies, being able to detect such energy at various points across the spectrum can improve the usability of the device.
  
• MEMs-based microphones have become cheap and available.  These devices - based on microscopic elements - can operate over a frequency range from a few 10s of Hz to over 100 kHz making the excellent replacements for the (increasingly hard-to-find) ceramic transducers.  Having a wide frequency range allows the user to tune to the peak frequency of the noise source rather than being limited to the immediate vicinity of 40 kHz.

The problem

Both the MFJ-5008 and the device described in the April 2006 QST article (noted below) utilize the then-available 40 kHz ceramic transducers.  These devices seemed to be a reasonable choice as they were relatively inexpensive and sensitive - but they are quite narrowbanded, unable to detect much once you get more than a kHz or two away from their 40 kHz center frequency.  (Note:  Similar devices were made for other frequencies - including 25 kHz - but these were not as readily availble.)

From my experience, the strongest peak of ultrasonic energy from arcing power line hardware is in the area of 25-32 kHz - wholly outside the frequency range of not only the passband of the tuning of the onboard potentiometer, but also well below the 40 kHz peak frequency at which the 40 kHz transducer has usable sensitivity.  What this means is that for these types of noises, the MFJ-5008 is quite "deaf":  Some of the arc's energy is around 40 kHz, but little enough that the sensitivity of the unit is really quite poor.  As noted above, an electric microphone would have been a better choice at this frequency range, despite its response typically dropping off rapidly above the audible range.

I can't help but think that the designers of the MFJ-5008 and the advice used to inform the design of the ultrasonic receiver described in the April 2006 QST article was misplaced:  For the reason most amateurs would have purchased or built a device - to find noisy power lines - the 40 kHz transducer is a very poor choice!

* * * * *

How the MFJ-5008 works

Made by (the now defunct) MFJ Enterprises, this includes a 18" (46cm) diameter vacuum-formed plastic parabolic dish with a 40 kHz ceramic transducer at its focus.  Mounted on the back of the dish is a direct-conversion receiver centered at about 40 kHz that converts energy around this frequency to the audible range.   As can be seen in Figure 1 there is a bar across the front in which the ceramic transducer is mounted - but it also has holes that - along with one located behind it in the plastic dish - form a crude sighting system that works quite well to determine from where detected noises might be emanating.

If one disassembles the electronics of the MFJ-5008 they will discover a small circuit board with rather common components - namely a 555 timer used as the oscillator, an LM386 audio amplifier to drive the headphones and a few common transistors to amplify and convert the ultrasonic signals to audible.  There is a "tuning" control on board consisting of a 10k trimmer potentiometer, but it is not accessible from the outside - and it has a range of about 38-48 kHz:  A slight modification will be necessary to allow us to take advantage of the wider frequency response of the MEMS microphone.

Consider the (annotated) schematic of the MFJ-5008, below: 

Figure 2:
Schematic of the MFJ-5008 ultrasonic receiver.  The circuitry is straightforward - a simple, run-of-the-mill direct-conversion design that is very similar to the one described in the April, 2006 QST article.  Changes to C2/C8 and the added inductor are noted on the diagram.
Click on the image for a larger version.

 

If we compare the above schematic with that from the April, 2006 QST article, A Home-made Ultrasonic Power Line Arc Detector - link) we see some very striking similarities:  Both use a 555 timer for the local oscillator, both use a series of bipolar transistors for signal amplification, and both use a single JFET for the frequency conversion mixer.  There are some differences, but these are pretty much superficial when you consider that the same goal is accomplished with the same types of components.

A cursory analysis of the above diagram shows that the first two amplifier stages are coupled with 1uF capacitors allowing the full audio frequency range to pass:  This mystified me at first, but in looking at the circuit board and noting some unpopulated parts locations I realized that there may have been plans to allow this circuit to be used at audio frequencies - and, perhaps, have a switch to select audible or ultrasonic ranges as well.

For the original 40 kHz ceramic transducer, this wide frequency range isn't a problem, but for a MEMS microphone - which can hear equally well over a 100Hz through at least 60 kHz, this would be:  As the mixer (Q3) is just single-ended, it will happily amplify the original input as well as do a frequency conversion meaning that you are likely to hear audio-frequency "bleedthrough" on the audio output - and indeed, when I retrofitted it with a MEMS microphone (to be described shortly) I did.

Figure 3:
Picture of the MFJ-5008 with location of the various
various components and board locations involved in the
modifications annotated.
Click on the image for a larger version.

Adding "proper" high-pass filtering to the MFJ-5008

The only sort of "high-pass" filtering present are capacitors C10 and C11 which are conspicuous by their being in series:  Why use two capacitors (1000pF and 220pF) rather than just a single 180pF capacitor?  The answer lies on the circuit board where there are unpopulated locations marked "L1" and "L2" (see Figure 2) which correspond with an (uninstalled) pair of inductors between the junction of C10 and C11 and ground.

To make the unit much less sensitive to audio frequencies - and to make it more compatible with a MEMs microphone, several changes should be made:

• Change C2 and C8 to 0.01uF (e.g. 10nF) capacitors.  This will prevent the first two amplifier stages from being overloaded by audio frequencies and go a long ways in prevent "bleedthrough".
• Install inductance at the positions of L1 and L2.  I suspect that two inductors were in mind when they designed the board as high-inductance, surface-mount devices are comparatively rare and expensive, so they could use a pair of lower-value coils in series to get the desired value.  See the footnotes on the bottom of this blog for suggested inductors.
  

• 

  Figure 4:
  Apparently designed to be used in several ways, the MFJ-
  5008's board has several unused parts locations, including
  positions for inductors that could be used for improved
  high-pass filtering as shown here.
  Click on the image for a larger version.
  

  Connect a 4.7k resistor between the center pin of the RCA connector (to the microphone) and the "V+" pad near the un-populated switch.  This inserts a current-limited 9 volt supply on the microphone lead.
  

The amount of inductance to install at L1 and L2 isn't too critical, but finding such components may be awkward - but the total amount of inductance to use may be anything between 27mH (that's milliHenries!) and 68mH with 47mH being optimal - a relatively huge amount for an SMD device.  In perusing my collection of inductors, I found a through-hole 27mH inductor that I tacked into place, securing it with glue:  Note that it gets soldered across the two pads of L1 and L2 closest to the socketed 555 IC as Figure 4 depicts.

When modifying the MFJ-5008, the MEMS microphone was fitted first and it became clear that audio-frequency energy sailed right through the system, significantly reducing its efficacy at the detection of ultrasonic energy.  It is my opinion that both the changing of C2 and C8 to 0.01uF capacitors and the addition of the inductor are necessary modifications for good performance. 

Note:

If you don't have a suitable inductor for the above modification, the receiver will still work, but you will hear a bit of audible frequency bleedthrough:  In a location with high ambient noise, this may be a problem, but in an otherwise quiet location, it probably won't be an issue:  Changing C2 and C8 do a reasonable job of reducing audio-frequency response and should be considered to be mandatory if you use a MEMS microphone.

In other words, if you don't install the inductor, don't let that stop you from making the modification to the MFJ-5008 and using it with a MEMS microphone - just be aware of the audio frequency "bleedthrough" issue.

Extending the tuning range of the MFJ-5008

Figure 5:
Potentiometer R10 - originally 10k - was replaced with a
50k miniature potentiometer to allow tuning.  A 4.7k resistor
paralleling R22 can be seen in this fuzzy photo.
Click on the image for a larger version.

As the tuning control isn't readily accessible - unless you drill a hole in the box and use a screwdriver - a modification is required to both make the tuning accessible and increase the range.  To do this, I found a small 50k potentiometer and soldered it into place where the original 10k trimmer (R20) was:  Note that two of the potentiometer's leads are connected together, so the "new" device would go between Pin 7 of the 555 in the schematic and resistor R22.  While doing this, R22 should be changed from its original value of 10k to 3.3k (or you could tack a 4.7 or 5.1k resistor in parallel with it).  Increasing the value of R20 from 10k to 50k allows the frequency to be tuned down to 20-22 kHz while lowering the value of R22 allows it to be tuned above 50kHz, all of this encompassing the frequency range where noisy, arcing connections (and bats!) are likely to be found.

Figure 6:
The modified MFJ-5008 with the (barely visible)
tuning knob sticking out on the left.  The blue
label indicates the approximate tuning frequency.
Click on the image for a larger version.

While I was able to cram the (very small) potentiometer onto the board (Figure 5), you may need to be creative - possibly mounting the potentiometer on the cover or side of the box using (very short!) flying leads:  If you use a metal potentiometer, I suggest connecting is body to the "ground" of the circuit (e.g. the outside shell of the microphone's phono plug) to prevent pick-up of nearby electric fields that might affect tuning.

The final result of the modification can be seen in Figure 6:  The cut-off shaft of the potentiometer protrudes slightly out of the left side of the enclosure and there is a label depicting the approximate frequency of the oscillator (and the center of the converted range) with respect to the adjustment of the potentiometer and its white paint mark.

What potentiometer to use?

To fit in the location of the original 10k trimmer, one needs to use a small potentiometer:  A suitably small potentiometer is the Bourns 3310C-001-503L which is available from DigiKey HERE and from Mouser Electronics HERE.  With a bit of care, it can be mounted to the board and the case modified to allow the shaft to protrude out the side - but it would be a good idea to use something (e.g. "hot melt" glue) to make it more rigid and prevent fatiguing/breaking the potentiometer's leads.  If you are creative, a larger potentiometer might be usable, attached with flying leads, but if it's metal, be sure to connect its body to the V- (battery negative - the shell of the phono plug will work) to minimize noise pick-up.

Note:

If you don't make the (highly recommended!) "tuning" modification, the MEMS microphone is still useful in that its sensitivity extends over a wide frequency range:  You may be able to adjust the original potentiometer (which can be adjusted between 35 and 48 kHz) to a frequency that is more suited for the types of noises that you are seeking - which would be around 28-32 kHz for power-line hardware arcing.

Using a MEMS microphone

Note:  

In this section, I refer to a "homebrew" MEMS microphone carrier board - but there are "breakout" boards available that are already assembled:  This next section describes how either a "breakout" board or a homebrew board like this may be mounted in the focus of the dish.

Figure 7:
The original 40 kHz ceramic transducer and
carrier board (top) and the homebrew version with
the MEMS microphone (bottom) both mounted using
the pairs of screws on stand-offs in the front bracket.
Click on the image for a larger version.

Farther down this page you will find a description of a commercially-available MEMS break-out board (from SparkFun) and how it may be used, should you be unwilling to assemble your own!

                    * * * * *

For the specific MFJ-5008 depicted in this article I used an already-prepared MEMS microphone module:  This was described in a previous article linked HERE.  This circuit was designed to accept a wide range of voltages (3.5-10) to be imposed onto the same conductor as the audio, making it easy to interface on a single cable as we did here.

In the MFJ-5008, there is an aluminum "U" channel across the front in which the ceramic transducer is mounted and its location places it at the focus of the parabolic dish.  What this means is that when we replace this device with something else - a MEMS microphone in this case - it must not only be located at the same axial position (left, right, up, down) as the original, but the sensing element must also be at the same distance from the surface of the dish.

Behind the nesting cover (accessible via the removal of four screws - two at each end) there is a circuit board mounted on two stand-offs and the focus of this dish is precisely midway between the two.  Removing this and peering inside the original ceramic transducer, you can see the element located inside, recessed slightly from the front grille:  The distance of that element from the circuit board is that which should be replicated with the replacement microphone.

Figure 8:
Homebrew carrier board with MEMS
microphone installed, facing the surface of the
dish.  The microphone's "sound hole" - facing
from the camera in this photo - is located
precisely between the two mounting screws.
Click on the image for a larger version.

As can be seen in Figures 7 and 8, I mounted the homebrew MEMS modules on the "front" side of a piece of PCB prototype board, taking care of placing the center of the microphone (not visible in the photo) on the center line between the two screws and equidistant between them.  Once this was done, the "new" microphone was mounted back in the "U" channel and the wires soldered as seen in Figure 8.  As it turned out, the thickness of the homebrew board placed the MEMS element at the same distance from the dish as the original element - a fact later verified by noting that the "sharpness" and accuracy of the pointing with the new element seemed to be the same as before.

Using a Sparkfun MEMS microphone "breakout" board

Soldering a tiny microphone module successfully to a circuit board requires a bit of skill - but there are "breakout" boards that already have the microphone and some of the needed components already on them - and one of these is available from SparkFun (the "BOB-19389") for about US$9.00 at the time of writing. While it is possible to order from SparkFun directly, I ordered it via Amazon for the same price - plus shipping was "free".  Detailed information on this board may be found here:

https://www.sparkfun.com/sparkfun-analog-mems-microphone-breakout-sph8878lr5h-1.html

This breakout board contains both a microphone and an operational amplifier and here are their respective data sheets:

• Microphone element data sheet - LINK
• Op Amp data sheet - LINK
  

As originally designed, the SparkFun board "sort of" works for ultrasonic detection, but there are a few circuit elements that require attention before we use it.  Consider the schematic, below:

Figure 9:
Diagram of the SparkFun BOB-19389 MEMS breakout board.  As can be seen,
there's nothing special about this design:  A microphone coupled to a single op-amp section - but
but there's a problem with this circuit in our application:  The gain set by R4 is unnecessarily high
for our needs and this - along with C3 - reduce the useful frequency response to less than 30-sh kHz.
Click on the image for a larger version.

The implementation of this breakout board is nothing special - and it's worth noting that even without the gain of the op-amp, the MEMS microphone itself would have a suitable amount of drive for the MFJ-5008.

As part of our circuit analysis, I will call the reader's attention to R4 and C3 (300k and 27pF, respectively) which form a simple low-pass filter - but these components, along with the unity-gain bandwidth product of this op amp being 1 MHz - conspire to cause the frequency response to roll off rather dramatically above 15-20 kHz or so:  It will still detect lower-frequency ultrasonic signals, but sensitivity is reduced at higher frequencies while the signals that we don't want (e.g. audio-range frequencies) are not attenuated - and even if the frequency response was flat into the ultrasonic range, it would have way too much gain for our application, anyway:  Unless the SparkFun board were modified, the gain would be so high that the unit would be completely unusable!

Figure 10:
A close-up of the SparkFun BOB-19389 MEMS microphone
break-out board.  The location of C3 - now replaced by a
resistor.  Not also that the "sound hole" of the microphone
is on the bottom of the board, facing down in this photo.
Click on the image for a larger version.

The "fix" is to replace C3 with a resistor.  For the MFJ-5008 I would suggest using a 10k 4.7k resistor in this location and by lowering the gain, the op amp's bandwidth product isn't going to get in the way of the needed frequency response.  While it doesn't really matter if one removes the capacitor or not when using a 4.7k resistor (the -3dB point for a 4.7k resistor and 27pF capacitor is somewhere north of 700 kHz) it's pretty easy to remove just the capacitor and replace it with the resistor if you have SMD parts on hand.  If you have only through-hole parts, it should be possible to tack a 1/4 or 1/8 watt 4.7k resistor across them.  (Note:  I used the MEMS board in Figure 10 for a different project which is why there's a 47k resistor at the position of C3:  A 4.7k resistor is appropriate for the MFJ-5008.)

The other issue is that of the voltage range of the breakout board's components.  In testing, the board worked "OK" at just 1.8 volts - below the "official" specifications of the the Op Amp - but it worked "better" in the specified 2.3-3.6 volt range.  In the modification for the MFJ-5008 described above, the addition of the 4.7k resistor across the "audio in" phono plug put the full 9 volts battery voltage (minus resistive drop) on this line so we need to do two things to make this work:

• Limit the voltage to the 2.3-3.6 volt range.
• Combine split the audio signal from the voltage at the microphone breakout board.

Fortunately, this is quite easy, requiring just a small number of components and the following diagram shows:

Figure 11:
Powering the SparkFun MEMS break-out board from the audio cable with DC bias on it as depicted in the MFJ-5008 modifications, above.  Capacitor C1 blocks the DC from the Op Amp,
resistor R1 isolates the audio and DC lines while LED1 is used as a voltage shunt to limit the
voltage to somewhere between 2.3 and 3.6 volts:  An ordinary white or blue LED is perfect for this
as they are readily available and provide a voltage in the middle of this range.
Click on the image for a larger version.

Note:  I could have simply run a separate DC line from the circuit board to the detector, but this would have still required regulating the voltage down to the voltage needed for the MEMS device:  Putting DC on the signal line is easy to do and it requires only a few, inexpensive components.

Capacitor C1 has two functions:  Block the DC from the "Audio Out" terminal and to offer a bit of a high-pass frequency response to filter audio-range energy.  Resistor R1 extracts the voltage from the "DC + Audio" line and sends it to the "VCC" terminal on the breakout board and across this, the LED acts as a voltage limiter.  As noted in the diagram above, one can use a blue or white LED as the voltage limiter:  These will "turn on" at between 2.8 and 3.2 volts which is right in the range that we need.  Alternatively, if you have some "old fashioned" red LEDs that operate from about 1.7-1.8 volts, two of these in series will do the job.

Figure 12:
The SparkFun MEMS microphone break-out board with
the circuitry in Figure 11.  These components could be
"dead bug" mounted like shown in the photo or they
could be incorporated on the "carrier" board used to hold
it at the focus of the dish - either method works!  The
"sound hole" can be seen in the lower-right portion of the
board, just above the letter "H".  Note that it is not
centered on the board - something to note when mounting.
Click on the image for a larger version.

It is recommended that you use the "diode test" function of an volt-ohm meter to verify the turn-on voltage of your LEDs and to make sure that they are connected correctly.  If you have a variable-voltage bench power supply, connect it across the two leads and, starting out at less than 2 volts, slowly increase it while measuring the voltage across the "GND" and "VCC" connections:  The voltage should limit in the 2.3-3.6 volt range and you should see the LED(s) dimly illuminate.  In testing I haven't found that light falling on the LED causes any effects in the audio, but if you are, for some reason, worried about that, feel free to cover the LED with black paint, put it in some black heat-shrink tube or shield it from light in some other way.  (Note that in the MFJ-5008, the carrier board is contained within the "C" channel aluminum pieces and mostly shielded from light, anyway.)

These three components may be mounted either as shown in Figure 12 with the components' "flying leads" holding things together, or on a piece of prototype board to function as the "carrier" board of the same type shown in Figures 7 and 8.  Note that the "sound hole" on the breakout board is on the "back" (non-component) side of the circuit board (visible in Figure 12) and that it is NOT in the center of the board and take this into account when you are mounting it to the "carrier" board.

Mounting the SparkFun MEMS microphone in the MFJ-5008 ultrasonic dish

 As with the homebrew MEMS module shown in Figure 7, a small "carrier" board - made from a small piece of prototype "perf" board - was used to mount the SparkFun MEMS module at the focus of the dish.  As the "sound hole" of the microphone is on the bottom side of the circuit board, the MEMS boar's "component" side is against the carrier board as shown in Figure 13.

Figure 13:
The SparkFun MEMS microphone board attached to the
"carrier" board using UV-cured epoxy.  The notch near the
GND connection was filed prior to mounting.
Click on the image for a larger version.

The same external components - the LED, resistor and 0.01uF capacitor - were used, but mounted to the carrier board - but there is one minor complication:  The SparkFun MEMS board itself.

For whatever reason, the designers of the SparkFun MEMS board chose to locate the "sound hole" for the microphone NOT on either the "X" or "Y" axis, but at some random location in the lower-right of the the PC board as can be seen.  For the focusing and sighting/aiming of the MFJ-5008 dish to be correct with its existing view holes, the "sound hole" of the microphone must be located at exactly the center point between the two mounting posts and in line with them - but if that is attempted, the SparkFun MEMS board gets in the way.

One option is to simply move one of the mounting posts slightly, but I chose another option:  Make slight modifications.  As can be seen in Figure 13, a small "notch" was filed in the SparkFun MEMS board using a needle file in line with the "GND" connection:  As there are no components near this connection, it was fairly safe to do so - but be absolutely sure to cover the "sound hole" with a piece of tape when filing to prevent dust from getting inside the microphone and, possibly, ruining it - and remembering to remove this tape just before installation.  As seen in Figure 13, a bit of the red coating on the SparkFun MEMS board was scraped off to provide a new GND connection to it.

Figure 14:
The left-hand mounting post, the top filed away
to provide clearance to the SparkFun MEMS
circuit board.  An alternative solution would
be to have moved the post 1/4" (5mm) farther
apart.
Click on the image for a lager version.

A bit of additional work is required to make the board fit and the mounting post itself was attacked with a file as seen in Figure 14 to gain additional room to provide clearance to the SparkFun MEMS board.

The SparkFun MEMS board itself was mounted to the homebrew carrier board using some UV-cured epoxy - although some "hot melt" glue would probably have worked just as well.  As seen in Figure 13, the board is rotated slightly counter-clockwise so that the "sound hole" would alight with the center line between the two screw mounts and also so that the divot that was filed out at the GND connection would line up with where the mounting post would be.

When all was finished, the result looked very similar to that in Figure 8:  The MEMS board was facing toward the surface of the dish and the original cable carrying audio - and now power - was attached to the connections shown in Figure 11.

Final words on the MFJ-5008 modifications

The above modifications should allow the MFJ-5008 to work over a wider variety of frequencies to allow optimum detection of energy from electric arcs, high-pressure gas leaks, bats, insects, switch-mode power supplies and many other things.

Prior to modification of the first unit, a "test range" was set up in my back yard:  A 40 kHz transducer was driven with a sweep/function generator (an old Wavetek Model 180) and the output level at its lowest-possible setting.  From about 33 feet (10 meters) away the "warble" from the swept output was easily audible - but not particularly strong.

After the modification, the subjective impression was that the sensitivity was equal or better than the original 40 kHz ceramic transducer - but a quick walk around the house revealed the ringing presence of several switch-mode power supplies, each producing low-level noises of their own due to magnetostriction of components within - something that was totally inaudible prior to the modification, made possible only by the broad-range response of the MEMS microphone and the added ability to tune the center frequency.

* * * * *

In-field comparisons

Recently, a modified MFJ-5008 was put to test in the field alongside an unmodified MFJ-5008 to locate a noisy power pole.  As it turns out, the pole itself had been identified - from a distance on HF and a small magnetic loop and then up-close using a small Moxon on 2 meters - both using AM on a Yaesu FTX-1.  The role of the ultrasonic dish in this case was to try to identify the specific hardware on the pole.

With the modified MFJ-5008, with the center receive frequency tuned in the vicinity of 28 kHz or so, one could easily hear the arcing from several 10s of feet (5-10 meters) away and as such, it was possible, using the "iron sights" (holes) in the MFJ-5008 zero in specific hardware on the pole.  When the modified MFJ-5008 was tuned above 35 kHz or so - outside the majority of the noise energy of the arcing - it became nearly inaudible as this shifted the noise peak in the headphones from a few kHz to over 10 kHz - likely too high for the ears of anyone but a young person:  This further reinforces the need for a tunable, broad-band detection system.

With the unmodified MFJ-5008, one had to stand as close as possible to the noise source - right under the pole - to even hear the ultrasonic noise from the arcing hardware and this vantage point did not lend itself to trying to identify the failing hardware since it was all straight above one's head.  Unlike the MEMS microphone - which had a broad response - the narrow response of the 40 kHz transducer meant that there really wasn't any hope of even hearing the peak around 28 kHz, even if one were able to retune the receiver.

Shortly after this comparison, the owner of the unmodified MFJ-5008 came over to my house with the parts necessary for modification (e.g. SparkFun MEMs board, 50k potentiometer, inductor for the high-pass filter modification, etc.) and it, too, was retrofitted.  In comparing the two modified MFJ-5008 units side-by-side, they performed identically with the modification opening up an entire new "world" in ultrasonic sounds from power supplies, insects, birds, vehicles, etc. that had previously been all but inaudible with the original fix-tuned 40kHz-only MFJ-5008!

 * * * * *

Footnote:

• Here are a few suggested parts for the inductor in the modification of the MFJ-5008 - all 47mH:
• https://www.mouser.com/ProductDetail/Murata-Power-Solutions/17476C?qs=5CKLVr1iF0nvNdEM16T%2F2A%3D%3D
• https://www.mouser.com/ProductDetail/EPCOS-TDK/B82144A2476J?qs=v4Mlc8l4PHmthTExsnwGmg%3D%3D
• https://www.digikey.com/en/products/detail/bourns-inc/RLB1014-473KL/2561378
• https://www.digikey.com/en/products/detail/murata-power-solutions-inc/22R476C/1924732
• https://www.digikey.com/en/products/detail/central-technologies/CTS4HTF-473J/16048522

 * * * * *

This page stolen from ka7oei.blogspot.com

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Pine needle what? (Keeping your coniferous antenna supports alive!)

The afflicted tree:  The needles are more normal at
the top, but looking rather "thin" farther down.
Because of the number of pine cones, the tree
looks "browner" in this picture than it really
is - but it is definitely under stress!
Click on the image for a larger version.

Late last year I noticed something amiss with a fairly large (50 foot/15 meter tall) Scots Pine tree in my front yard:  The needles looked a bit "thin" and short.  In the past this tree would tend to shed needles and pine cones all over the place on a seasonal basis, so I kind of ignored it over the winter - but this spring, when I started yard work again, I became concerned.

Something was definitely wrong with this tree.  While the needles themselves seemed to be green and flexible, I noticed that at the top of the tree they looked normal-ish, but the bottom 2/3rds of the tree looked "skimpy", so I decided to investigate more closely.

If you've followed this web page you've probably figured out that while I'm probably not a yard and garden person, I do what I need to do to keep the yard in reasonable shape, asking my Dad or friends for advice when something wasn't quite right.   A couple of weeks ago I finally did what I should have done months ago:  Take a very close look at the tree.

On the lowest branches - where the problem seemed worst - I could tell that the limbs were very green and flexible - but the needles themselves, while green, were thinner than they should have been - and were covered with small spots.  Doing what many people do these days I resorted to Google and it "told" me about all sorts of possible fungal infections and other things - but I wasn't satisfied that it was describing what I was seeing, so I asked a friend of mine who'd had tree work done in the past year or so.  He referred me to the guy that did his tree work - a "semi-professional" who did this as a combination of a hobby (I gathered that he really likes trees!) and a second job.

After a couple weeks of phone tag, I was finally able to talk to him and I described what I was seeing.  Almost the first thing he asked was "Do the spots scrape off?"

"What?" I thought.  "Those are probably small bugs" he continued.  I must admit that it never once occurred to me to try scraping them off - and it then I realized that I should have looked at the needles with a magnifier.  He continued to explain that he suspected that the tree was suffering from "Pine Needle Scale" - an infestation of small insects that feed on the sugars in the needles - and this would probably kill the tree if left un-checked.  He then explained that he could probably drive out to my house and charge me a chunk of change to tell me this same thing in person - and then charge me more to treat it, or he could just save himself some time and me some money and have me treat it myself.

A close up of a bough on the affected tree.  As you can see, the needles are a bit shorter and thinner ("thin" like paper rather
than in number) and paler than they should be - and there are lots of spots!
Click on the image for a larger version.

After a few more minutes detailing the common treatments, we got off the phone and this time, armed with more definitive information, I did a bit of online research and what I saw in the pictures looked very much like what I'd seen.  It wasn't until I got home and plucked a few needles off the tree and looked through a magnifier that I saw that these spots were, in fact, small insects - looking exactly as he described and much like the pictures on the web.

A close-up of one of the needles showing the infestation
of what are probably Chionaspis pinifoia - little, hard-shelled
insects that literally suck the life out of the tree!  These will scrape off
with a fingernail revealing more-or-less normal looking needle
underneath - except that it's wet with weeping liquid from the tree.
Click on the image for a larger version.

I have two other pine trees in my yard that look healthy - so I inspected them, but the results were inconclusive:  I saw nothing obvious, but I didn't check everywhere within reach. If they were infested, it wasn't bad... yet...  A more thorough inspection has revealed that both of the trees in the back yard are infested - but it appears to be mild as I had to look a bit for them.


Why did the tree in my front yard get infested but the others not as bad?  I have a suspicion:

Two years ago, during the installation of a solar power system, it turned out to be necessary to upgrade the utility power connection to my house, so a narrow, 48" (122cm) deep trench was dug through my front yard - but the path took it very close to this same tree.  When the trench was open I could see that there were several rather large roots that had been cut - and this concerned me a bit.  What I suspect happened was that this weakened the tree a bit, making it more susceptible to an infestation - but then again, it could have just been bad luck!

Between talking to the tree guy and going online, I read about three common ways to control these critters - typically Chionaspis pinifoia (read about this insect at the "Tree Geek" web site).  These methods include:

1. Spraying with an insecticide.  This is the "kill them right now!" approach - but it may not get the entire tree (particularly if it is a tall tree that is difficult to spray in its entirety) and this is most detrimental to beneficial insects like bees, ladybugs and other things if they happen to be on the tree, downwind, or very nearby.
2. Ground soak.  A solution of insecticide and water is poured (usually in a small "moat" to confine it) around the base of the tree so that it is quickly absorbed into the root system.  This systemic treatment is slower to take effect, but is longer lasting and will protect the entire tree - and it is somewhat less harmful to beneficial insects since it is more or less confined to the tree.
3. By injection.  If the tree is in really bad shape, insecticide is injected directly into the tree where it can more-quickly be taken up.  The "tree guy" with whom I was speaking seemed to think that since I didn't (yet!) have large sections of die-off that this wouldn't be necessary.

Comment:

There are more ways to deal with these things that are less harmful to beneficial insects - but the general opinion seemed to be that these were best for preventing infestations, controlling those infestations that were minor, or in those situations where there was a need to minimize the effect on the "good" bugs (e.g. to protect pollinators, etc.)  For trees that were under significant stress, arborists seem to recommend the "strong" approach where it can be safely done.

Based on what I read, my tree wasn't in "really bad shape" since the needles - while getting a bit pale - weren't dying off in large quantities... yet - but it is under a fair amount of stress.  Apparently, it is about this time of year (April, May) around here that these bugs start to reproduce and become more active - so this is the time to do the treatment.  I decided on a combined approach:  Spraying where I could reach and doing a ground soak around the base.

To this end, I sprayed the tree on a wind-less day as far as I could reach with a solution of "Sevin" (a carbaryl inseciticide):  I was able to get the bottom 1/4-1/3 of the tree, which encompasses about 1/3-1/2 of the pine needles.  Because of the height of the tree, I couldn't really go much higher than I could reach via the spray while working from a free-standing ladder - but this would, at least, have an immediate effect on a significant part of the tree.

The second treatment is a ground soak (an imidacloprid-based insecticide) as described by the "tree guy", the guy at the local farm supplies distributor from whom I bought the stuff, and the online descriptions:  This latter application will take a couple of weeks to work its way through the entire tree - but it is, by all accounts, considered to be very effective.  While I'm at it, I'll also proactively treat the other two pine trees in my yard with the ground soak - just in case those critters managed to get around - however they do that.  Based on what I have been told and what I have read, this treatment will become an annual, spring ritual.  Since pine trees are less attractive to bees than other plants, I'm hoping that the effect on them will be minimal - although they may collect some components of propolis from them.

So, the next few months will be telling.  I really do like my pine trees:  I think that they look nice, they offer a bit of cooling shade to the house - and they provide nice anchor points for my ham radio antenna!

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Update - 6 June, 2018:

It's been about six weeks since treating the tree - and it is no worse off, but the bugs are still hanging on, so I went a step further as recommended by an expert:  Treating the tree with "Safari 20SG" - a Dinotefuran-based insecticide.  According to available information, this insecticide is taken up by the tree more quickly and is more effective at eradicating parasitic insects.  Hopefully, once these bugs are knocked down, the tree will recover and will be able to better-resist them in the future.

The instructions indicated that 1.0-2.2 oz were recommended for every 10 feet of tree height, so for a tree of this size (about 55 feet tall) the entire 12 oz container (which cost about US$110) was dissolved into several gallons of water and poured around the base using "soil drench" techniques.

Update - 27 June, 2018 - Success, I hope:

 About a week ago, I examined the tree again and saw that the bugs were rapidly dying off:  In fact, it was difficult to find a live one.  This die-off must have started happening a week or so before this as the needles are already looking "plumper" and slightly more green - a sign that fewer of these things are still trying to suck the life out of the tree!


There are still a lot of these things covering the needles even though they are dead, but at least some of them should weather off - and all of their carcasses will eventually fall with the needles as the tree replaces them in its normal cycle.

Update - August, 2019:

The bugs have remained dead and the tree is looking much more healthy, once again in the process of producing pine cones.  From what I have read, it takes about 3 years for this type of tree to completely replace its pine needles, but new needles are easy to spot - partly because they don't have the carcasses of bugs on them!

It looks as though a simple ground soak of an imidacloprid-based insecticide has halted the (mild) infestation of the trees in the back yard - but from what I have read and what I have been told it will be a constant vigilance going forward as one can never really get rid of them completely, mostly likely due to the fact that there is always the potential for re-infestation from wherever these things came from in the first place!

Update - October 2024:

All three trees are healthy and produce copious quantities of pine cones every year.  I did notice on a walk around my extended neighborhood a few weeks ago that my pine trees are some of the very few that are still alive - let alone healthy:  A few of them were still up, but dead - no doubt the owners dreading shelling out $$$$ to have them removed.

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This page stolen from ka7oei.blogspot.com