Exactly. Diplexers specifically do not allow any power to go to the antenna leg of the diplexer. Antennas can look like a direct short circuit at DC, so this would be a bad idea.

Do you really need an amplifier? This is the problem; it requires a separate lead from the antenna down to the amp. After that, you can combine through the 3x4. A look at antennaweb.org shows that you should be able to get eleven digital stations with the simplest kind of local antenna, which strongly implies that an amplifier is not necessary. I can see the broadcasting towers thirty miles from my home on the mountain where Los Angeles stations broadcast, and antennaweb says I can also get eleven digital stations with the simplest antenna. To prove it, I have a $30 five year old Radio Shack antenna split four ways and I get those stations.

If you have two leads coming down from the dish now and you don't need an antenna amplifer, there is a way to use one of the existing cables for antenna.

First, one of the dish leads just stays the same and runs to the multiswitch.

Second, the other dish lead gets a diplexer near the dish and the antenna signal is mixed in with the dish signal. It is important to get a diplexer that will not allow low-frequency garbage from the LNB to muck up your OTA signals; any diplexer that is about the size of a two-way splitter will NOT work for combining antenna and satellite, although they are great for separating. You want a diplexer that is about two inches by almost four inches. Look up ChannelMaster satellite products and read about their diplexers. They specifically mention one big one that is for combining, for this very reason. Other brands also have larger diplexers for combining.

So now you have one LNB signal and antenna going down to the multiswitch. There you have to place another diplexer to separate the antenna signal from the LNB signal. You take the antenna signal there and insert it into the third input on your multiswitch.

The data says you will have a darn strong antenna signal with an ordinary antenna. Try this, maybe just disconnecting the dish from its cable and using that to feed a tuner directly, to see how things work without an amp. If you feel you MUST have an amplifier, you could put the whole thing between the separating diplexer and the multiswitch. The kind of amp that goes up near the antenna is only for real low-signal areas. You could use a 10 dB amp near the multiswitch. I bet you won't need it, though.