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[pvignola]

TM751 is not polite and technically can step on other power line signals. If the CM15A and TM751 don't transmit together. It maybe OK.
The RR501 is polite and can also be controlled by power line signals. Many online dealers have them fairly reasonable in price.

The TM751 not being polite was also noted in an earlier post.  It was said that it waits 1.5 seconds before sending a command.  Does it actually poll the lines to see if there is a signal currently being sent before it sends?  If not what good would waiting do?  Isn't it possible that in 1.5 seconds the CM15A might be sending a command?

[Brian H]

The TM751 has no power line receiver in it and has no way to know if a signal is on the power lines. It just merrily blasts a signal on the power line. Stepping on anything.

The RR501 if it see power line traffic waits until the line is clear. I believe by a random generated time. So if you had more than one they both would not time out together. The CM15A is also polite.

[pvignola]

Thanks Brian for the explanation. 

I have been reading as often as I can and the more I read the more I realize there is so much to learn.  I really love X10 automation so far.  For all of the pros and cons I feel it's a great system if you can put the time into doing things right.  This forum is a huge help in that area. Keep up the great work guys.

[pvignola]

I mentioned that I had made the CM15A antenna mod with a 9.5 inch piece of wire.  Well after reading more on these forums I saw that others used 18.5 inches instead.  So I added another piece of 9 inch wire to my mod and now I don't have any dead spots in the house that I can find.  Man life is good.

All I did was snip the very end of the CM15A antenna exposing a bit of the wire from the antenna and connect a piece of 9.5 inch stranded wire (from and old parallel printer cord).  That worked pretty well but after I added the additional piece of 9 inch wire (the same as before) I got rid of any dead zones that I previously had.

[Noam]

I mentioned that I had made the CM15A antenna mod with a 9.5 inch piece of wire.  Well after reading more on these forums I saw that others used 18.5 inches instead.  So I added another piece of 9 inch wire to my mod and now I don't have any dead spots in the house that I can find.  Man life is good.

All I did was snip the very end of the CM15A antenna exposing a bit of the wire from the antenna and connect a piece of 9.5 inch stranded wire (from and old parallel printer cord).  That worked pretty well but after I added the additional piece of 9 inch wire (the same as before) I got rid of any dead zones that I previously had.

The "Correct" 18.5-inch CM15A antenna mod is actually to add a "reflector," not to extend the antenna itself.
This is actually a lit simpler, and doesn't void the warranty.
Simply take an 18.5-inch length of stiff wire (coat-hanger works, as does #12 or #14 electrical wiring), and place it ALONGSIDE the CM15A's antenna. Line the bottom of the wire up with the "elbow" at the bottom of the CM15A's antenna. This mod doesn't require an electrical connection between the new wire and the antenna. Most people use a bit of tape to hold the two together.
I was skeptical at first, but this has gotten rid of all of the dead spots in my house. It really does work.

[pvignola]

Thanks Noam.   

I figured that the CM15A can be had for relativity short money if I messed it up and I believe I have a spare packed in my garage somewhere.  I read about the 18.5" coat hanger mod but since I didn't have one and for some other reason which escapes me right now I decided to dive in and clip the antenna.

I'm not sure of all of the technical details (reflector vs. added antenna length) I did read something about a 1/2 wave antenna.  I just figured I would post my results not as a "How to" but more as a "This is what I did" for a reference point. 

You guys have been so much help and this forum has a wealth of info to be uncovered.  I'm glad I found it!