Quoting P.L.Mason-at-bath.ac.uk:

 PL> I built a small coil some time ago which produces 3" sparks. 
 PL> There is no shielding of any description around the unit.    
 PL> Now I find that when I operate it near any 'consumer'        
 PL> electronic equipment, it messes around with their functions 
 PL> Example - My Hitachi TV will switch on to channel 3 when the 
 PL> coil is running several feet away.

 PL> I have read many times about how small coils shouldn't be an 
 PL> emission problem so I can't understand what is going on. The 
 PL> coil runs on a ~200W supply, and is an old design which is   
 PL> an absolutely basic circuit - xformer, cap, gap and coils.

 PL> Comments?

First check your RF ground. Is it connected in some fashion to
the same ground you are using for your consumer solid state
electronics? A Tesla coil produces substantial RF current at the
base of the coil. This current is such that it "powers up" an RF
ground to the point that low pressure tubes glow, pickup coils
resonate, and sometimes spark can be drawn directly from the
ground strap or ground itself. I strongly advocate building a
heavy dedicated RF ground for Tesla work, even with small coils.
By using a dedicated RF ground connected to the step-up xfmr
core, spark gap center post, and secondary coil base wire, a
coiler can reduce or eliminate 90% of interference problems.
Proper RF grounding also increases coil performance and safety. 

Secondly, are you using reversed line filters in your low
frequency (50-60) cycle feed lines to the coil transformer? Even
with proper RF grounding, bleedover from the control circuits and
transformer supply lines into the house wiring can be a problem.
A reversed line filter will prevent RF "hash" generated in the
equipment from entering into the house wiring.

Are your spark gaps visually shielded? Some remote controls, 
fire sensors, etc.. operate with infrared detectors. An exposed
arc at the spark gap can not only damage the eyes with hard UV,
they will certainly trigger infrared sensors.

Additional measures to reduce RF interference would include
placing bypass capacitors and RF chokes in the HV supply lines to
the Tesla tank circuit, placing RF shielding around the spark
gaps, and using a properly grounded Faraday cage around the

This advice is given with the assumption that the coil is
efficiently processing energy to spark, i.e., that the coil 
is in good tune, properly coupled, and sparking well.

As I have mentioned, following these guidelines will cure most of
your problems. Any remaining interference is a result of the
inductive field surrounding the coil, this inductive interference
is very limited in range.

Richard Quick

... If all else fails... Throw another megavolt across it!