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
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? Please
to me for consideration. Thanks.
Tracking Down a Mystery Signal
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
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
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 source.
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 amateur band.
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
Austerity measures were in place on equipment repair
and maintenance minimizing equipment upkeep.Corrective
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
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