Out of Order: Tracking Down a Mystery Signal
my call for entries in the new Out of Order feature on RF Cafe, Mr. Denny Condron responded
with a great saga of his use of finely tuned (pun intended) use of
fox hunting skills to track down
and ultimately resolve a bad case of interference in the 2-meter amateur radio band. Although
names are omitted to protect the innocent (and the guilty) parties, it is good to know that
the offending source was remedied willingly by the owner of the equipment: a manufacturing operation.
This is a fine example of the term "unintentional radiation," and why ignorance is usually only
bliss, as the saying goes, for unaffected and usually oblivious parties. Denny noted separately
that this tale is a greatly abbreviated version of the effort required to find and remedy the
Tracking Down a Mystery Signal
By Denny Condron - K0LGI
interfering signal had been perplexing amateur repeater operators in a Midwest city. Several
that were operating above 147 MHZ in the amateur 2 meter band often experienced an intermittent,
raucous sounding, drifting signal modulated by 360 Hz AM and FM components that came and
went for several months. Interference was primarily in the FM band segment.
there had been discussions about what it could be and what might cause such an interference,
little confident interference source directional information had been determined in reference
to its location.
After confirming the typical daily frequency drift pattern and on-off
timing characteristics, the mystery signal was exhibiting from the home location over a period
of several weeks. A heading roughly towards the city center had been established and with this
information in hand, a determination was made to locate it using by now well-established methods
from a mobile unit.
A portable HP spectrum analyzer, a broadband HP unit preamp and heavy
duty 12 volt to AC inverter was obtained and connected with the car portable DF (direction finding)
antenna. On a late evening of a cold, snowy December day we set out to locate the offending
Because of the characteristics of the signal, the mobile Doppler DF unit
could not be used without constant frequency changes. The spectrum analyzer was essential as
a time saver not only because of the highly variable frequency of operation and drift characteristics,
but also the sporadic on-off times of this unknown interference source.
miles of driving - with some minor off-nominal signal arrival direction headings due to
the usual city environment multipath - finally at the top of a hill it was noted that the signal
had a dramatic increase in strength. That was in contrast to any previous DF location observed
en route when heading in a similar direction or on the back side of the hill. The DF equipment
continued indicating a straight-ahead bearing, assuring a good clean, non multipath heading
As the mobile unit continued on this street, the signal levels were soon
of such high levels that the preamp was no longer needed and in fact, several 10 dB steps
needed to be added to the spectrum analyzer RF input to reduce the signal power to more usable
levels for DFing.
At last during on this very cold, inclimate evening, after seeing the
DF indicating a hard right turn was obviously needed, I arrived in the front lot of a facility
from which the signal was being broadcast. A DF read was made from the front of the building
and also 90 degrees offset from the original heading at the south end of the building, confirming
finally this spurious signal emission orgin.
The only obvious accessible entrance to
this facility at that time of night was in the rear side of the suspect building. A contact
was made with an employee after ringing a kind of alarm bell to alert staffers to activity.
The employee that responded as if he had no idea or interest in what I was referring to, and
suggested I contact the front office staff during the daytime hours for help with the issue.
Doing so provided, eventually, an invitation by an official at the company to bring
whatever was needed to show a company representative exactly what the issue was and to demonstrate
it to him.
Gladly accepting this opportunity, the same test equipment was used and I
was greeted the next Saturday morning by the company representative that secured the equipment.
In addition, I was taken on a tour of the manufacturing area when employees were on break, so
they were not present when I was in the area.
Later as employees again entered the production
area and began processing the products of a variety of types and designs, it was noticed that
the RF equipment that had been seen during the initial guided tour was in now in use. It was
used mainly to preheat powdered plastic which was then formed in a mold shaped and pressed into
the product sizes required.
The RF equipment that this company was using consisted of
over 20 manually operated RF power oscillators generating approximately 0.5 kW of RF power
and one semi-automated oscillator generating approximately 2 kW of RF power.
watching the ones that were in use, it soon became obvious that the high power semi-automated
unit was showing considerable visible arcing around the chamber access doors where the products
were being transported in and out. Upon seeing the arcing, it was immediately evident from where
and by what means the RF was radiating.
After the employees once again left the production
area, I proceeded to set up the equipment I had brought with me, including a small probe connected
to the spectrum analyzer to sample each of the manual RF ovens, including the major radiator:
the higher power semi-automatic unit.
It was then realized that in fact these long-sought-out
interfering sources were operating on a 'grand fathered' ISM (Industrial, Scientific, Medical)
band at about 74 MHz! Also verified was the same 360 Hz modulation components noticed
previously in measurements that had been made in both fixed and mobile environments.
Although it was never witnessed that the RF devices were ever operating near to or at the
airport marker beacon frequency of 75.00 MHz, it could have been a far more serious issue
if they had drifted much higher in frequency. Previous measurements made in the mobile unit
and in the field had been done on the second harmonic of these erratic signals, occurring in
the amateur 147-148 MHz, 2 meter amateur band.
After several lower power RF
oven units were also verified as having substantial leakage radiation, the semi-automated unit
was approached with caution as it was loaded with the required amount of product plastic powder
and molds in use at the time.
Although I don't recall the level of the RF that was measured
emitting from this massive beast, I do remember that the previously used probe did not
need to be located anywhere near as close to the oven to see major radiation from it - even
with significant attenuation set on the spectrum analyzer RF input. It was this source that
had the 360 Hz modulation component which had been plaguing our radio operations, and now
was confirmed by the discovery that it used 3 phase AC power input for a supply.
Manual and automatic oven door RF gasket finger stock was
either missing or broken on both types of RF heaters.
Austerity measures were in place
on equipment repair and maintenance minimizing equipment upkeep.
Maintenance efforts were reestablished after acknowledging the RF leakage existence was
substantial and causing interference.
was gone within days in the 147 MHz region that had been noted before the discovery visit.
Awareness was made to the company regarding interference caused to other services, including
concern of the nearby airport operating Marker Beacon frequency.
Awareness was made
of the concern of RF exposure to employees who were operating deficient units due to excess
Posted February 14, 2014