Peter....
Sounds similar, so that's a good start.
I didn't mention this before, but I considered that there might be an
"emission" from the antenna
through the coupling, so I also had tried an amp on the antenna (which
should isolate any
back radiation.......but,.without any luck) So, the antenna had some phathom
effect
on the remaning system..........which is still this mystery..
I had considered using a notch filter on the antenna side, but didn't build
it as I wasn't
convinced that it would be worth the effort.
I'd be interested to try the "black box" you mentioned, if you can
give me some more info about it.

Les


"PeterD" wrote in message
...
On Wed, 4 Nov 2009 16:19:00 -0800, "les" wrote:

Hello....
I have a peculiar problem that I can't understand, so I can't reslove it
I'm using a few CCTV cams into a 8 channnel DVR, and have tried to
then piggyback the auxiliary composite signal using a Ch. 4 modulator.
This RF then mixes into the regular antenna cable (OTA) that feed four
digital TV's throughout the house.
I figured this simplistic RF mixing would avoid redundant cabling, but
have
been
surprised that I havea poor signal issue. I assumed that with one signal
being
around 70 MHZ, and the remainder in the UHF bands, any heterodyning
would be unlikely. At this point I'm not sure if this is THE issue, or
something else.

My arrangement is this:
l------l
Antenna--------l l
l l-----combined coax-----To
distribution Amp----
local modulator-- -l l
l------l signal splitter/mixer

The symptoms are that if I run the above arrangement shown above WITHOUT
the
antenna attached to the splitter, the local signal looks great. When I
reattach, the signal
drops dramatically, becomes noisy. I attached a 75 ohm load to the open
port
to see if
a termination would simulate the antenna load, but the picture looked
great
as usual.
I tried an 12dB amp prior to the mixer, but it had no beneficial result.
It
seems no matter
what I try, the external antenna somehow interferes with the modulator.

Help! Any ideas?? What am I missing?

Les


I do virtually the same thing here (but channel 3) and use a small
'black box' to do the connection. It has a filter for channel 3 to
block signal from going back out the antenna (cable connection, if you
are cable) and only allows the signal to go into the house system. It
also (of course) attenuates the channel 3 signal if there is one so it
won't interfer with the injected signal.