Topband: plasma TV RFI?

[Jim Brown]

On Sat, 02 Dec 2006 16:05:49 -0500, Jon Zaimes AA1K wrote:

>Will common mode chokes (ferrite or otherwise) help with this or is
>it transmitted directly from the big screen? 

See my response to another post on the topic of RFI. Here are the 
main points. 

1) If it IS the TV, SOMETHING connected to it is acting as a 160 
meter transmitting antenna and radiating it strongly enough for your 
receiving antennas to pick it up. 

2) There may be more than two antennas. Prime candidates are whatever 
that TV is using as a signal source (cable TV, satellite, roof 
antenna), the power line, and even connections to the audio system. 

3) Like any other 160 antenna, bigger makes a better transmitter, so 
use your noggin in figuring out which of these antennas are most 
likely to be the most effective long wire antennas. 

4) Apply BIG ferrite chokes to those antennas (not your antennas, the 
ones connected to the TV). What you are looking for is the greatest 
possible impedance from the choke on the frequencies where you are 
hearing trash. See the tutorial on my website to learn how to get 
that high impedance. It includes graphs of lots of MEASURED data 
contributed by another member of this list who did them in a very 
good lab. 

http://audiosystemsgroup.com/SAC0305Ferrites.pdf  

While the tutorial is written for pro audio people, any decent RF guy 
should be able to use it to understand, troubleshoot, and fix ham RFI 
problems. I'm working on a tutorial dedicated to ham radio 
applications. 

5) You need Z on the order of several K Ohms to make a serious dent 
in this trash. As the data shows, for 160, you are wasting your time 
(and money) with anything less than 10-14 turns around a 2.4" #31 
toroid, or 7-8 turns through one of the biggest #31 clamp-ons. 

6) Do NOT try to kill the noise by finding "a better earth 
connection" for that system. Any noise that you shove into that 
ground wire will radiate, just like any other antenna carrying RF 
current. 

7) If you are going to put a commercial AC line filter on the set, 
make sure that you have the shortest possible connection between the 
filter and the set, ESPECIALLY for the green wire. See #6 above. 

8) 12-14 turns of twisted pair like THHN stranded around that #31 
2.4" toroid makes a very good common mode choke for 160-40 meters. 
Run only "hot" and neutral through the toroid, carry the green wire 
around it. Put an RF cap of at least 0.33 uF between line and neutral 
on the power source side of the choke. This cap will form a 
differential mode filter with the imbalance in the inductance of the 
choke. It must be rated for the AC line voltage. The wire in this 
choke doesn't need to be very big -- the vast majority of standard 
IEC power cords are #18, even those that look a lot beefier. 

9) Do NOT add capacitance between hot or neutral and the green wire. 
In addition to #6 above, doing so would create more leakage current 
at power frequencies than is permitted under safety codes. 

73,

Jim Brown K9YC