I'm trying to help a friend get HD OTA to his LCD-TV. I made him an antenna that picks up the OTA Locals just fine and when connected directly to his HDTV it works great. The problem I'm having is that I'd like to utilize the existing wiring in his house that he currently is using for SD-cable to his HDTV. He has a brick house and the HDTV is in the basement, so it's kind of a pain to run a new wire to the tv.

I tried using a splitter backward as to combine the cable signal and the ota antenna, but he's got a few more splitters down stream (4 tv's total on the cable feed) and the OTA signal doesn't seem to be getting to the HDTV. For instance, I kept the OTA antenna in the same exact location and wired it straight into the HDTV and picked up the channels fine, but when I routed it through the house wiring I couldn't get the OTA channels with much if any signal.

Perhaps I'm wiring it wrong. Can someone tell me if it's possible to do what I'm trying and how I should be doing it? I haven't found where the other splitters are in the house, but obviously they seem to cut down the signal by 3.5 dB for each one. I wanted to keep it simple with not much new wiring and just mount the OTA antenna outside and combine the signal with the cable feed to the basement.

Your thoughts?

Sorry 'bout this, but I'm about to disappoint you. Hidden splitters aren't your problem; rather, cable-TV and OTA signals almost never can co-exist on the same coaxial cable. Most times, it's because local broadcasts occupy several channels that are analogous to those in use by the cable operator. Thanks to signal strengths which are often wildly different (with the cable signal being stronger by far) -- coupled with the analog signals still in use by some cable companies, as opposed to the digital-signal format of all HD broadcasts -- and you have a recipe for electronic conflict at your set-top tuner or TV set.

Not only that, your cable operator is likely to take a rather dim view of this arrangement if it causes any issues within their system. Most franchisees can and will shut off your service for a number of technical and safety reasons until you remove any and all interfering equipment. I don't know of any company that will install service without obtaining your signature on an agreement containing language to this effect.

The only workaround is a coaxial A/B switch used to select either cable or antenna signals, but please know that there's absolutely no guarantee this will work, either: The cable signals can be so strong that they overwhelm the switch's ability to isolate them from the broadcast side of the switch.

Good to know - thanks Don. I suspected such, as it really didn't work when tried. The other issue is that the HDTV needs to be told, via buttons on the set itself to switch from 'Cable' to 'Air' mode each time one would want to go to the DTV broadcast. Digital Cable with HD is expensive here, as I imagine it is in most places.

Perhaps I need to take an afternoon and see if I can run a new coax from the outside to the basement (a chore indeed), instead of running it through the basement window.

I got to thinking about this because I've heard of folks doing OTA and satellite on the same coax, but they likely aren't in the same frequency range.

You can't use just a splitter, it has to be able to combine the signals.
splitter combiner
Many of these are even labeled as Cable/Antenna. You put one in to combine and another at the other end to separate the signals.
My Dish Network install uses only 1 Cable running from the wall to the box. I have 2 of these on the back of the box because there's normally two cables coming from the dish and a third to the second TV up stairs. Thes combine the 3 signals into one and then separates them out.

A Combiner is:orginal source
A combiner is simply a splitter hooked up backwards. It combines the channels on two or more separate cables onto one cable. The only drawback to this piece of magic, is that the cables being combined cannot have any channels in common with each other. The resulting signal on that channel would be trashed.

Combiners make some neat things possible. Let's say you have cable TV that has channels 2 through 63. And you have a DSS receiver that you would like to be able to see on any TV in the house. You can hook up a modulator to the DSS receiver, set the modulator to channel 65, then combine this new channel back in your wiring closet with the cable TV coming in! Now any TV can watch DSS by simply changing to channel 65. This concept of "in-house" channel generation, together with the new cheaper and more reliable digital modulators, is opening up many new possibilities in residential video distribution.
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So I stand Corrected. If you had satellite, you could easy combine the OTA and satellite signals. In fact the ones in my house actually are labeled satellite and Antenna.
So for my Dish install does that mean that the feeds that go to receiver 1 and receiver 2 are on 2 different frequencies? I don't think so and yet the 2 signal feeds are combined and work fine with no interference.