[RFI] Low pass filter opinions

[Jim Brown]

On 1/13/2011 11:58 AM, John Geiger wrote:
> Hi Jim,
> Even with the digital TV switchover, aren't most people still using 
> cable and getting signals the old fashion way? I know we are.  We 
> still have the same analog TV connected to the cable.  I don't think 
> the fact that the over the air TV signals are now digital has affected 
> my house at all.

That's right.

> If it is a pin 1 problem then it is interesting in that it only 
> affects certain TV channels.

Pin 1 problems take many forms. One is at the point where the antenna 
(or CATV coax) comes into the TV set or cable box. Another is a poor 
shield connection anywhere in that path.. These errors couple RF into 
the TV, or into the CATV equipment. Most of these have nothing to do 
with the cleanliness of your transmitter. Now, if your transmitter IS 
putting out trash, these errors can couple that trash into the TV system.

> I will check on other TVs in the house that have the VCR/DVD unit 
> input through the coax jack.  The one in the living room uses audio 
> cables intstead.

The equipment where those cables connect nearly always have pin 1 
problems on both ends. Pin 1 problems occur on ALL cables, not only 
audio or RF cables. Most serial ports have pin 1 problems.

> Also found that for the shack TV, it will shut off if I do 100 watts 
> on 6m with the beam pointed east.  Rotate it west and the TV shows no 
> interference.  That is interesting since the beam sits almost directly 
> above where the TV is.

I'd look for one of those wiring problems or CATV equipment in the east 
direction from your beam.  For example, if there's a bad F-connector, 
the adjacent piece of coax will couple it into the coax, and from there 
to CATV system and/or the TV.  If the set goes blank, I'd guess that 
it's front end is simply being overloaded.  Lots of F-connectors that 
LOOK good are actually BAD.  When I moved to CA about 5 years ago, I was 
hearing CATV leakage around 145.25. I reduced it a lot by replacing one 
of my own cables that I'd crimped at least 10 years before with a new 
cable using much higher quality Snap N Seal connectors. The fact that 
some leakage remains simply indicates that there are almost certainly 
more poorly crimped connectors in that system.  Looking for that leakage 
with a 2M talkie is a great way to find and fix this part of the 
problem.  That's how I did it -- the noise around 145.25 will be loudest 
near each bad connector.  And don't quit when you find one bad one -- 
keep looking. If there's one there can easily be more.

Another suggestion. All recent editions of the ARRL Handbook include a 
chart showing the harmonic relationship between ham bands and TV 
channels, and between the translated channels used on CATV systems.  
Study that chart for 6M and the cable channels where you have interference.

> I received plenty of other emails from hams who run legal limit with 
> no affects on their own TVs.  I would assume that many of them would 
> still have the crappy equipment (TVwise) that the rest of us do with 
> their pin 1 problems.  Wonder what I am doing wrong that they aren't.

It probably is not what you are doing wrong, but what various equipment 
mfrs and CATV installers are doing wrong. :)  Most RFI is coupled by 
various cables acting as antennas. The locations of those cables with 
respect to your antennas, the quality of those cables, and how those 
cables are connected to equipment, has a lot to do with whether there 
will be RFI in any given situation.

Study my RFI tutorial.    http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim Brown K9YC