[RFI] Interference on TV using 2 meters
jim at audiosystemsgroup.com
Thu Oct 26 01:19:05 EDT 2006
On Wed, 25 Oct 2006 21:31:49 -0700, Gary McAdams wrote:
>145.250 is cable channel 18.
Another symptom of this leakage is carriers on repeater
frequencies. These are the carriers for the TV channels (or
sidebands of significant components of the video, like the line
rate, harmonics of the line rate, the color subcarrier, sound
carrier, etc.) within the cable.
But to clarify your response to those who may not understand how
cable TV works, the RF spectrum on the cable is different from
what is "on the air" outside the cable. That is, ordinary TV
broadcasting uses 54-88 MHz (ch 2-6), 174-216 MHz (Ch 7-13), and
470-806 MHz (Ch 14-69). The other frequencies are used for other
services. But inside the cable, those standard channels are
converted (either heterodyned or detected and re-modulated) to new
channels so that they fill the entire spectrum between HF and
about 1 GHz. In doing so, they can fit a lot more channels within
the cable, and the channels within the cable are re-numbered.
All of this works fine -- as long as the cable system is well
built. The cable channels stay within the cable, and the "over the
air" channels (and 2 meters) stays outside the cable. But the
leakage (2 meters into the cable, and the cable into 2 meters)
occurs when one or more connections on the cable system are poor.
Anything from a bad connector to a bad piece of cable to a bad
component connected to the cable system.
Jim Brown K9YC
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