As previously posted, I currently have my cable and antenna connected through a splitter feeding into my TV. I did this as a "see what happens" shot in the dark for a few reasons:

-it allows me to flip through the cable channels and antenna channels without switching inputs
-I do not have to use an A/B switch at the splice location
-I do not have to run a second cable to the TV from the basement

I have been told on another board that this is illegal since I am apparantly broadcasting those cable signlals (all 7 of them) through my antenna. Is there any risk to my equipment doing this? and is there any real risk of the cable company or anyone else getting PO'd over this and raising a stink? I mean, the cable company is letting the channels slip through to me for free anyways. I plan to change it, but am looking for other options that will not cause signal problems elsewhere - any ideas?

This is a bad idea all the way around. There is more than the 7 channels you can see on your TV that you are inserting into the cable system. Your antenna will pick up FM transmitters, Public Service radios, amateur repeaters, cordless telephones, weather satellites, and all kinds noise that will interfere with any and all cable channels - some consistantly, and some intermittantly
But don't take my word for it, you can ask the Cable Co. for a 2nd opinion

The 7 channels I was talking about were cable, I get close to 20 OTA. Either way I am sending everything everywhere with the splitter apparantly.

My basic question then is - is there a type of splitter or "check valve" so to speak that I can use to block any backfeed through the antenna and cable system? I would prefer to use the single "in the wall" cable to feed both systems to my TV and from what I understand diplexers won't do the job either. Sounds like my only options are an A/B switch or running a second cable to the TV and switching with the TVs inputs between the two?

What channels OTA, and what channels on the cable?

Are you getting at frequencies - and if so can I block certain ranges from each system? Because the only cable channels I want are low (3/4/6), as the rest are redundant.

OTA (plus others I have blocked - mostly analog versions of the same channels):
2.1
2.2
7.1
11 (analog)
17
23.1
23.3
29.1
29.2
34.3
34.4
39.4
39.5
43.1
43.2
43.3
49
51 (analog)

CableTV:
2
3
4
5
6
23
29
(as mentioned - I am only interested in channels 3global, 4OMNI, and 6CBC since I do not get these OTA - the rest I do)

This is easy, you need a hi-low mixer. This will pass ch.2-6 on one port (54-174 MHz) and Ch 7 (174 MHz) and above on the other. However, I am not sure where you can find one, maybe someone else can help. Make sure it also passes UHF on the high side

Paul

kb2umt,

Thanks! I will have to look around to see if I can find one of these! All the digital antenna channels are really UHF channels so that works out well.

Also, if you put any kind of amplifier on the cable signal before you mix it, the antenna signals will not be able to go back through that amp into the cable system.

As your wiring now stands, you actually have a huge theoretical problem and little real problem. While a signal going through a splitter is only reduced 3 to 4 dB, a signal usually gets reduced at least ten, and easily twenty, dB going from one input (OTA) to the other input (cable). So that means your OTA signal is already attenuated. Then, when the OTA signal hits the drop tap at the pole, chances are real good that the drop tap is directional, meaning
a) the trunk line comes into the drop tap
b) a cable output goes to you at some 12 to 20 dB lower than the trunk line
c) the trunk line continues to the next drop tap at almost the same level that it had to begin with.
d) any signal that might be coming up your line to that drop tap is automatically dropped 10 to 30 dB, and usually even more,
e) so your signal has now been dropped three, then 1 to 30, then more, dB.

Actually, the icing on the cake here is that if you can mix your OTA signal with the cable and have successful results, then if your OTA signal gets back up into the cable, there will be no problem, as there are no cable channels on those frequencies to interfere with.

"Also, if you put any kind of amplifier on the cable signal before you mix it, the antenna signals will not be able to go back through that amp into the cable system."

Could I so the same on the antenna line to block cable signals from being broadcasted over the antenna? I would worry about too strong of a signal coming into my TV's tuner though (unless I can get an amp with little gain)

How does an amplifier block the signal - are they unidirectional?

Kevin,
both answers are Yes.

You might indeed end up with too much signal. You should get the lowest gain amp that you can, assuming you have no snow on any analog channels now. Heck, you might want to get a 10 dB amp and then buy some attenuators to drop the signal level back down. This will give you splendid isolation.

Are amps directional? Yes, but not because anybody thought about that when they made them.

The simplest amp, a single transistor, is generally thought of when doing conservative thought-experiment design as having a gain of ten. That means that one milliamp of input current will result in ten milliamps of output current. If you introduced ten milliamps of other current at the output, there is nothing about a transistor that would make its input voltage change as a result, not even the one milliamp of the original input.

Actually, the only kind of amp where you are likely to have signal feed from output back to input IS an antenna or cable amp, but then only when it is turned off and high-frequency signals leak back across the components. As soon as power is applied, those signals are nicely shorted to ground.

Great - thanks for the info Ernie. I do have a couple snowy analong channels, but that is because they are not in the direction my antenna is facing (not on rotor). It sounds like a couple small amps and attenuators are the way to go for my situation.

I see by rereading this thread that I also thought the initial problem was the possibility of your injecting OTA signals into the cable, not broadcasting cable signals out of your antenna.

This stuff works the same both ways. In fact, I remember the day I had to run a satellite signal up from an equipment closet and run a modulated channel down to the equipment closet -- it was a total revelation that I could run one signal up and one signal down on the same cable. That's sort of a duh now that I have realized it, but before that time I always thought of ALL signal flow in one direction.

On a similar topic - I have a Dvico Fusion 5 USB decoder attached to the compute. Tried running a splitter attached to it and connected a silver sensor antenna & cable but it seemd to overwhelm the antenna when scanning for ota hd channels. Cable scan seemed fine though. Is there a workaround that would allow me to have both connected? (it only has one antenna port btw).

Yes, as per your comments I plan to run an amp on both feeds to block flow going back to each source.

Perhaps you could connect just the antenna and scan, then connect the cable and scan it next. In general its not a good idea to combine the two though since they share frequencies - the reason it works well for me (I think) is because the only cable channels I get are low numbers/frequencies so they don't interfere with my antenna signals.