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 can't tell what your diagram means due to font differences (I tried
both fixed and proportional fonts), but I can think of two
possibilities...
1. There is an OTA signal near the modulator's output frequency. (You
say near 70MHz. Is it 70 MHz through 76 MHz or something else?
Aviation markers use 75 MHz.
2. Your antenna has some bizarre impedance at that frequency. Trying
the 75 ohm terminator was a good idea, but the antenna might not be
anywhere near 75 ohms at that particular frequency.

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

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.

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.

les wrote in message ...
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 had the same problem. The cure was to put a pad between the antenna and
distribution amp. I'm not sure if the antenna was over driving the input or
there was feedback from the amp output back into the antenna. Whatever the
case, try padding the antenna level down.
bg

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