[RFI] Interesting Case of VHF RFI caused by Chilled Water Controller

Jim Brown jim at audiosystemsgroup.com
Wed Dec 2 13:02:19 EST 2015

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 

> This case description is posted for its educational value.

And that value is considerable. Many thanks, Frank!

73, Jim K9YC

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