Thanks very much for this, Frank. VERY useful. See comments interspersed.
On Wed,12/2/2015 9:21 AM, Frank N. Haas KB4T wrote:
Another survey was scheduled with all sorts of county and contractor
personnel. The cabinets were opened and, once again, the actual source was
quickly found. The RFI originates from the CPU module of the ALERTON (A
Division of TRANE) chilled water pump control system. The large cabinet
housed several modules mounted in 4 rows of 3 or 4 modules. Each module
served a specific purpose and all modules were wired together by a variety
of wires and cables.
If there are possibilities for suppression, it's control of conducted
emissions on those interconnecting cables (if there is any). Given that
the problems are VHF, I'd add multiple #43 clamp-on cores to every one
of those cables, with the objective of increasing the common mode Z as
much as practical. For all practical purposes, common mode is is
proportional to total length of the cores. If there are issues at 450
MHz, I would add #61 cores to cover that range. Obviously, none of this
will help for emissions directly from the modules themselves.
This is a commercial installation. The RFI radiates from the source to a
distance of approximately 150 to 200 feet. The Alerton CPU module probably
meets FCC Class B requirements. The RFI would likely not be a problem
except for the fact that the radio tower is situated approximately 30 feet
from the NEMA cabinets. All of the antennas on the tower are within 30 to
80 feet of the RFI source.
The majority of the communications equipment at the EOC facility is 800
MHz. There is limited 150 MHz VHF and 450 UHF radio use. None of these are
affected. Only the Amateur Radio equipment is affected to a significant
degree. The county has few options for correcting the problem. The radio
tower can't be moved. The manufacturer of the control system will likely
not help especially since the contractor does not understand the problem
and the county doesn't seem overly anxious to spend much money to help
resolve the interference.
The county radio shop did offer to use existing resources to create a
remote control setup that will practically duplicate the function of at
least one affected VHF Amateur Radio.
To locate this source I used the Tecsun PL660SLV almost exclusively. My
search wasn't so much a direction finding effort as a signal
strength/proximity search. That is, I tuned the receiver to 133.325 MHz AM
that produced usable signal strength indications and useful audio. (When
close-ish to the source a distinctive noise pattern modulation could be
heard that sounded like data pulses.) As I got closer and closer to the
source, the signal strength increased and the clarity of the received audio
improved. Shortening the antenna acted as an adequate attenuator to keep
signal strength readings at midscale.
This shortening of the antenna is a really useful technique for
improvised RX tools like this. During 2M fox hunts, we've learned to
remove the rubber duck from a talkie when we get very close to the
source. Also, it MIGHT be practical to use a 440 MHz talkie with no
antenna as a probe for where the radiation is taking place within the
cabinet.
This case description is posted for its educational value.
And that value is considerable. Many thanks, Frank!
73, Jim K9YC
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