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Re: Faraday cage

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, May 11, 2002 11:34 AM
Subject: Re: Faraday cage

> Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> > Was very effective in keeping high energy RF/arcs away
> > from me.  Cut off frequency is 1/2 wavelength of longest
> > opening if memory is right,
> Single longest dimension _is_ key:
> Thus a door, 2' by 6' is about as leaky as
> an 'unlaced' corner, 1" by 6".

For what it's worth, all the dimension stuff is totally irrelevant if you're
nowhere near the wavelength of the EM radiation you're trying to stop... So,
unless you're worried about 100 MHz  (3 meter wavelength) noise from your

If you're seriously worried about VHF radiated EMI, you've got bigger
problems than you're going to solve with a lash up screen room

I suspect (based on some rough analysis I did a while ago) that TC
interference comes from two sources:

1) conducted back on power lines.. mostly at the fundamental frequency, or
at multiples of the break rate (i.e. like "dimmer noise" or "vacuum cleaner
motor noise" (which is from the brushes))... The latter will be easily
recognized (for non-DC coils) by the bars on the TV (since the vertical scan
on the TV is synced to the power line, just like the TC is...)

This you get rid of with reasonable filtering, and, since it is conducted,
you need to pay attention to where the RF path is (i.e. running a 50 foot
wire to that phenomenal ground rod with the buried copper cable mesh in the
back yard next to the proverbial saltwater swamp isn't going to help a whole

2) Fairly high frequency (VHF,UHF) stuff coming from the high peak currents
when sparks start flying...   Here, the "loop" formed by topload, ground,
and spark, and the wire/conductor connecting the end of the spark back to
the bottom of the secondary is the radiator.  A fairly simple cage will help
here, just by controlling where that loop is, and providing a good path for
the "other side" of the capacitor formed by the top load and the

How to tell..

If your coil is not breaking out, and you get interference, then it's most
likely conducted (i.e. over the power line, most likely), or possibly
radiated from the spark gap forming a high frequency resonant circuit with
the tank cap... (The TC makes a terrible antenna at the typical

If you get interference when you're breaking out (only), then it's radiated
or due to the sparks.. and a faraday cage will help..

Anyone engaging in a EMI/EMC resolution process should be aware that it is a
tedious, frustrating process, characterized by a lot of trial and error to
make it right.