TVI Tips

[W8JITom at aol.com]

A few more comments on TVI.

In a message dated 96-04-24 02:35:53 EDT, you write:
>Cheapest source of toroids I've found is from junked old TV sets; there's a
>big one around the picture tube yoke; about 4-5 inches diameter on a
>fullsize TV and smaller on more modern, smaller sets. Remove all the wire
>and any bracket holding the two halves of the toroid, then tape them
>together (i use black electrical tape). Wind the line cord to the TV around
>the core as many turns as possible. 

Low frequency ferrites become completely non-magnetic at high frequencies.
Randomly "selected" cores may work anywhere in the range of great to actually
reducing the impedance of the winding!

The most difficult TVI situation I was involved in involved a 50 kW
transmitter less than 100 feet from an apartment complex. The RF levels were
severe. In some apartments arcs could be drawn touching CATV lines with a
screwdriver.

The single best cure was ground the outlet box (the safety ground, not the
power line neutral!) to the CATV ground and running the CATV line and power
line together to the TV. When a group of equipment like a VCR, TV, Stereo,
and CATV descrambler were involved, the CATV, speaker, and power cords were
joined at one common outlet strip and the speaker and CATV cables were all
brought to the wall outlet together...where the lines were all bypasssed or
grounded to the electrical safety ground.

I found this technique cured almost all the TVI-RFI. Some sets required a
common mode TVI choke in the antenna lead and power leads, as well as line
bypass caps at the outlet to the safety ground. In all cases the CATV cable
shield was grounded to the outlet safety ground.

Watch out for cable type high pass filters. Most of them allow common mode
signals through without any attenuation, and common mode is the usual source
of overload. And don't worry about a filter with good IF frequency rejection,
the days of unsheiled hand wired long IF wiring are long gone, get a low pass
with maximum attenuation for the channels you use. Some low pass and high
pass filters are real junk, even when they look great and cost a whole bunch.

73 Tom 

>From Douglas Zwiebel <KR2Q at worldnet.att.net>  Wed Apr 24 14:49:54 1996
From: Douglas Zwiebel <KR2Q at worldnet.att.net> (Douglas Zwiebel)
Date: Wed, 24 Apr 1996 13:49:54 GMT
Subject: first rigs
Message-ID: <199604241349.NAA16390 at mailhost.worldnet.att.net>

256BC: Discovered that shouting was 3-6db better than talking.
255BC: Found that cupping my hands around my mouth clipped sound..+2db.
1754:  Read an article in Popular Cheerleading and built my own 24"
       megaphone.  Gained over 10db!
1799:  Got help from friend with big thorax and oral cavity.  Got
       reclassified as multi-shouter.
1801:  Ran a series of Tin cans connected by waxed strings to others in
       in "the contest" to overcome path loss of atmosphere.  Got DQ'ed
       for use of "interstring" links.
Later: Everyone starting to use "spark."  Decided technology had gone too
       far and the individual's talents no longer mattered.  Quit contesting.

;-)


Sorry...couldn't resist.  de Doug  KR2Q