Answering 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 interference source. Do you have a good work-related anecdote to share?
it to me for consideration. Thanks.
Tracking Down a Mystery Signal
By Denny Condron - K0LGI
An 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.
Although 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 signal origin.
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
After several 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 was valid.
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.
In 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
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
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
Maintenance efforts were reestablished after acknowledging the RF leakage existence
was substantial and causing interference.
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 leakage.
Posted February 14, 2014