On Fri, 22 Jun 2007 08:57:38 -0400, Tom Rauch wrote:
>Station grounds are great for lightning and safety.
I agree with ALL of Tom's comments, including his bonding
recommendations for TV antennas (and cable TV lead-ins) and about
his comments on filters. That bonding will certainly divert any
common mode current on the coax to the power system ground and
away from the set. BUT: That common mode current CAN re-radiate
into a set that is poorly shielded. In this situation, a big choke
could raises the impedance of that coax sufficiently to kill the
current that is generating the re-radiation.
Nowadays, when we talk about TVI, that may also include RFI to an
audio system, most of which is the result of the audio cables
acting as receiving antennas and exciting pin 1 problems. Ferrite
chokes WILL fix this sort of problem, and are often the most
effective way of doing it. Proper bonding and using antennas that
are well matched ARE the right approach, but it doesn't solve
everything.
Three examples. #1 My home music system is rather simple -- even
though I'm a pro audio guy, I spend my money on jazz CD;s rather
than exotic gear. All the power wiring is exposed, so it has line
filters to prevent common mode coupling. The loudspeaker wiring is
twisted pair. But it was a 3 ft RCA jumper between my CD player
and the AV receiver that picked up enough RF from my 10M dipole
200 feet away that excited a pin 1 problem in the CD player. A
choke on that jumper reduced the antenna current enough to fix the
problem. BTW -- that's a 3-band dipole that I use a lot on 20, 15,
and 10. No problem on 20 or 15. But the combination of the shorter
wavelength and the nature of the pin 1 problem inside the CD
player made 10M a problem.
#2 K6EU, running 100 watts on 2M, was lighting up his neighbor's
active sub-woofer (that is, a loudspeaker with built-in power amp
that was part of a home theater system) enough to be annoying when
he pointed the beam in that direction. Three #43 ferrite clamp-ons
choked that current.
#3 About six months ago, I acquired an FT-1000MP and started
contesting with it. I like the radio, but had RF feedback on 15M
(that same high dipole, roughly 150 ft from the radio). In the
heat of the contest, I fixed it with a multi-turn choke on the mic
cable. A few weeks later, I pulled out the RF generator and poked
around for pin 1 problems. I quickly verified that the mic
connector has a pin 1 problem in the 4-6 MHz range, and around
15M, and the short mic cable (about 6 ft) picked up enough RF from
that dipole for the radio to detect and generate RF feedback. BTW
-- I had also gotten RF feedback reports when I checked into an
NCCC net on 3830.
As to filters: When I started running QRO in Chicago, I chased
down a good mfr of TV filters and put them in front of the
distribution for my FM and TV antennas (I didn't have cable
there). The company is Tin Lee Electronics near Toronto. I found
them with a google search.
I'll also reinforce Tom's comments about the coax, ESPECIALLY to
check the CONNECTORS. I reduced the leakage from my cable system
here in CA to my 2M radio significantly by changing out cheapie
coax jumpers for good ones that use well installed Snap N Seal
connectors.
73,
Jim Brown K9YC
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