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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
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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
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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
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