You guys are really super! I appreciate your advice and support. But, I am ready to tell the owner to hire someone who knows what he is doing. I just spent another two hours researching active repeaters, passive couplers, plug-in amplifiers.. I need to rent a meter (no way I am buying one for just one or two jobs). And, there is little hope that I will be able to read the meter once I get it! I'm just an old wire-yanking electrician.
I put the system back together. I am using an old WS467 switch that doesn't blow the neutral on the gfi breaker for the pool light. The owner has said that we can just put everything on X10 "toggle" switches, but he prefers that we find "Decora" styled ones. So, question #1: Is there an X10 wall switch in Decora style that does not use a neutral? If so, what is it? WS12A? WS13A?
Next problem... The switches only work occasionally. Obviously, the signal is too weak, or there is too much electronic noise, to work reliably. Question #2: What is going to be my best "bang for the buck" option?
These switches are in a cabana, which is fed by a sub-panel that has lots of pool equipment loads. The Leviton HCC4D controller is in the kitchen, which is fed through another sub-panel.
I have a passive phase coupler in my home that i am going to yank out of my panel and try out at the client's house (I guess I'll put it in the main panel). I had some problems at my home decades ago that I solved with a coupler. Everything at my place works.