On Wed, 04 Nov 2009 16:19:00 -0800, les wrote:

some trimmed


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

One thing that you are defiantly missing is the fact that hooking up an
air antenna brings into your system all kinds of random air signals from
everything including garage door openers, some wireless security devices,
industrial wans, powerline interference, and external signal hetrodynes
that the TV is unaware of because it rejects them. They are showing up
when you add the non air signals to the mix.

To properly isolate your system from stray signals you would have to do
what was done years ago in MATV (building antenna) systems and that is to
include filters for passing each active channel on to the DA.

You might want to check the modulator that you are using to see if the
output frequency is tweakable since it could be off frequency enough to
force the TV set's aft into the guard band thus making the signal appear
noisy when another close by signal is present.

As an alternative you can place a tunable filter in line with the antenna
and adjust it for minimum interference to you cameras and hopefully the
antenna signal.


Gnack