Ian Jackson wrote:

In message , Michael A.
Terrell writes


The 20 dB isolation means that the return loss is more than 20 dB.

No it doesn't. It means that the isolation - the loss between two
outputs (with the input terminated) - is 20dB.


Sigh. How do you think you measure that isolation?


This is to prevent the local oscillator from one TV causing interference
to another TV.

True - but, more generally, it is to minimise the effects of something
connected to one output affecting another output.


What are you connecting that doesn't have a TV tuner? FM radios are
supposed to use a single port -20 dB tap.


As well as protecting
against local oscillators, it also protects against incorrect loading
(ie if an output port is not correctly terminated in a 75 ohm load). It
should not matter (too much) if an output is left open circuit, short
circuited, has a length of cable connected to it but going nowhere, etc
etc.


Unterminated ports are not allowed in properly designed systems.
PERIOD.


I was the engineer at a CATV system with over 10,000
active drops. There are hundreds of myths and half truths about the
hardware used for CATV systems.

Was your statement above (about RLR) one of these? [Sorry. The Devil
made me say that! ;o))]

I find 2 ports with -6.5dB loss and one port with -3.5dB, which makes
more sense. (Actually, it should be -7.0dB loss).

While most 3-ways are typically 3.5/7/7, there are one or two around
with equal outputs, of appx 5.5dB (although, personally, I can't really
see much need for them).


You would, if the use of the 3.5/7/7 would require a second drop from
the line tap on the street.


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