[RFI] TV RFI - an update

[Jim Brown]

On Wed, 13 Feb 2008 03:41:49 -0000, Dave Hachadorian wrote:

>Maybe you could just try some alternative hookups:

You may be right, but I doubt it. 

In other words, it's far more likely to be common mode current 
lighting up a pin 1 problem. 

If the transmit frequency is 28 MHz or 50 MHz (unlikely nowadays), 
changing between Ch 3 and 4 is more likely to do something. FWIW, 
in all of this correspondence, I have not seen transmit 
frequencies mentioned. 

BUT:I have seen individual clamp-on ferrite beads mentioned. A 
string of individual beads, even a dozen of them, provides FAR TOO 
LITTLE IMPEDANCE below 50 MHz to make a dent in RFI, even if the 
RF is being coupled on the cable that they are on (remember, we 
must add enough impedance to reduce the current), and the lower 
the frequency, the less they will do. We must use multiple turns 
around a #43 (at least six) to begin to make a dent in RFI at 
upper HP (20, 15, 10M) twice that number for 40 and 75 (and you'd 
better be using a #31 toroid on 75/80 or 160). 

Again, I remind everyone to study the part in my tutorial about 
what I call "threshold effect" and then study the measured data 
for various chokes. 

73,

Jim Brown K9YC