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[Brian H]

Regular coax for TV and Cable use is 75 Ohms.
Regular coax used for many other RF uses like two way radio is 50 Ohms.
So regular is not a set 75 or 50 Ohms. Depends on where used.
RG6, RG6Quad and RG59 are 75 Ohm usually found in Cable and video use.
RG8, RG58 are two 50 Ohm coax types.

Lots of technical stuff here but near the bottom of the page is some data on what Impedance many versions of Coax Cable part numbers.
http://en.wikipedia.org/wiki/Coaxial_cable

[06M6TorridGTO]

Well crap, I wish I had known those differences before I put together that system. Do you think its fine with the 75ohm (assuming I'm using that and not 50) or is it worse than the factory antenna on the CM19A? What about on the CM15A?

[dhouston]

Bottom line - if it improved things with the CM19A (and is isolated from mains voltage) then leave it. Using 50-ohm cable/connectors would have been best but it's not supercritical, merely less than optimal. That's assuming you only changed the receiver antenna. Changes to the transmitter antenna are against FCC regulations.

However, I would suggest doing it right if you plan modifications to the CM15A antenna. If you read the Wikipedia page Brian referenced, each impedance mismatch causes reflections which are undesirable. And, the prohibition on changes to the transmitter antenna applies here, as well.

I'm not familiar with the CM19A but have used the eggbeater antenna with many other receivers (e.g. RM1SG) and the results were always 100+ feet for reception with a single, centrally located antenna, increasing to 300+ feet with a pre-amp. Even building it with insulated copper electrical wire (e.g. 14AWG) and using 75-ohm coax for the phasing line (and only for the phasing line) will give good results.

If the CM19A uses the same receiver as the CM15A and the recent TM751, the frequency mismatch I mentioned earlier is likely to be a major part of reception problems.  A better antenna will only bring marginal improvements in this case.

Finally, folks (and especially noobies) need to be extremely leery of antenna modifications to any device that plugs into the wall. Some, like the CM15A, isolate the electronics from mains voltage but others, like the TM751 & RR501, do not and adding an external antenna without isolating it can be fatal. If in doubt, don't touch it. There are schematics available for the CM15A, TM751 & RR501 so it's easy to determine which are isolated but I haven't seen schematics for the CM19A nor for security consoles so I never recommend antenna improvements for them. If I had any confidence that X-10 will continue to supply them I'd buy a CM19A and test it but I'm unconvinced by X10SupplyDepot's assurances otherwise given the number of already discontinued devices.

 

[Brian H]

CM19A uses LC components along with transistors in both the transmitter and receiver electronics. Well unless X10 redesigned it like some of the other later modules. Then who knows.

The CM19A schematic is in the FCC Database.
http://transition.fcc.gov/oet/ea/fccid/
Grantee:B4S
Product: CM19A
Details tab.

[dhouston]

Thanks, Brian. It was too early - before I had enough caffeine.  

The CM19A schematic from the FCC site shows it uses a Cypress MCU similar to the CM15A but it shows LC components for frequency control. Interestingly, the FCC entry says it uses 312MHz.

However, it shows a single antenna shared by both transmit/receive so any CM19A antenna modification is in violation of FCC regulations.

I fail to see the purpose of the CM19A. What keeps any TM751 or RR501 from responding directly to RF from various sensors? Is this just for security sensors?
 
For the OP, this thread http://forums.x10.com/index.php?topic=21000.0 covers the CM15A receiver replacement and mentions the tuning disparities found by the author. There are other threads http://www.shed.com/tutor/CM15mods/CM15mods.html showing how to add a coax connector - just use 50-ohm BNC instead of 75-ohm F-connectors.

[Brian H]

Nothing keeps a TM751, RR501 or Security Console to transceive a standard X10 RF signal from sensors and remotes.
The CM19A was X10s way to try and keep AHP going with no CM15As available. The CM19A has to send RF commands to a TM751 or RR501 just to get an X10 power line signals to other AC connected modules.

[dhouston]

To me it seems like it would cause far more trouble than it's worth. With no knowledge of what's on the powerline there would seem to be no way to prevent the CM19A from duplicating RF signals that the transceivers have already heard and transceived resulting in big jumps for Dim/Bright commands. And, of course, it doesn't come close to duplicating the functionality of the CM15A even with the dangerous and without UL approval TM751. They probably figure they'll be gone long before any lawsuits can make it to trial.

Since the CM19A has no connection to the mains, there is a way to improve reception by leaving the antenna as is for the transmitter but adding a separate one for the receiver, cutting the connection between the current antenna and receiver as I've shown here...

• http://davehouston.org/cm19a.png

[Brian H]

You are not alone in your thoughts on the CM15K {TM751 with CM19A} Kit for AHP.

I actually have seen TM751s being sold with the internal relay again. Vendors called it a TM751-C. Some also show the TM751 with Pass Through Outlets.

[06M6TorridGTO]

Wow, that's some pretty interesting information. I thought I had seen something about it being illegal to modify the transmitting receiver. Didn't realize they were the same antenna though, unlike the CM15A. Chances are I wont need to modify the CM15A. It can receive the RF signals and then send them through the power lines to devices. The power lines go around the house and I haven't seen any phase issues yet (except the weird circumstance with JUST my bedroom but I used a different TM751 on its own house code for that). So I can just figure that if there is a TM751 near any security modules, then it can receive the RF and pass the commands on through the power lines to the device or to the CM15A and then that can send it on or do what it needs to. I would assume if I dont need a macro or timer that I should let the TM751 transmit to the devices and keep those devices on a non-transceived house code to avoid repeated signals? I can also transmit to the computer from the CM15A if necessary communicating with the CM19A through RF. But the CM15A will take the bulk of the duties from the PC though now. That's basically my central backbone and a crucial part. I can't see why X10 would get rid of it without getting rid of everything except under the circumstances of leaving a computer with a CM19A connected, but that still cant do as much. But I'm good now, provided the CM15A lasts. I could probably leave the CM15A in the entertainment room and plugged directly into the PC and then it can receive PLC from the TM751 near the MS16A. For the garage, I could either leave the CM19A in place (since it work with the garage sensor right now) or I could put a TM751 in the garage and let it send signals to the CM15A. I guess preference and whether or not I want to keep the CM19A (and its network of antennas) in place. Interesting stuff. Thanks so much guys! I've got plenty of plans for different things here, especially with add-ons like PC Companion or BVC or other things. Also, like I said, considering the Wireless USB option for the CM15A. But for now, I can set up the CM15A, make sure everything is working and see how it goes from there. This should make things much easier hahaha I wish I had known about it prior to doing all that stuff with the CM19A! Would have saved a good bit of time and money wiring all that up.

On another note, the TM751 is not UL approved?!?! Like you said, they must have definitely been thinking of leaving before it was brought up in court!

[06M6TorridGTO]

x10-home-automation-shipped-free on ebay sells both the pass-through and the appliance module versions of the TM751. They sell the appliance module one for much high price though. I wish it did have that one built in. I have an appliance module in the same room (dining room) as the TM751 near the MS16A. I could have used just the TM751 on that outlet instead.

[Brian H]

There was a question on how UL would like the Pass Through Outlet model as they stuck a sticker over the hole in the front cover for the local On Off push button. Where it would be easy to remove the sticker and stick something like a metal screwdriver into it and get sparks or a shock.
I believe the relay one was UL and they left the UL logo on the Pass Through Outlet one.

The CM15A isn't noted to have great RF reception range. That is why there are many CM15A antenna mods posted here.

[06M6TorridGTO]

I noticed that sticker on there as well,. Kinda looked like a button underneath and sure enough there was. Figured that was left over from the appliance module version of it meaning they wouldnt need a new case for the new pass through version. So I guess you are saying the button could be taken out and then someone could stick something metal inside and cause a shock. Basically being cheap and not making a new casing. Although if they are making new transceivers, wouldn't it be easy to just make a case without the hole? Maybe they still have a bunch of left over cases haha if the button is in there though, wouldnt you have to physically remove it which wouldn't be a fault of X10 and more so the user?

I probably wont use the CM15A much for RF reception. If a TM751 is located near the RF device, then it could trigger the CM15A through PLC instead. However, I do have to think about the duplicate transmissions if the CM15A also receives that RF signal.

[Brian H]

I did not know they left the button on the case front and then stuck a sticker over it. Maybe the only missing part is the relay itself and the rest of the electronics are still there. I don't have one for an internal look so I can only go by what others have reported here.
The button has a large flange on its rear side. You would have to work awfully hard to get it out.

[06M6TorridGTO]

I pushed it in but it kinda stuck and didn't come back out. It's just stuck in now. And yeah you would have to work hard to get it out.

[dave w]

Well crap, I wish I had known those differences before I put together that system. Do you think its fine with the 75ohm

I am little late on the comment back. With due respect to the experts who have taken the time to provide great guidence, don't go sleepless about the mismatch. Since we are talking about the mismatch on the receiver, switching from RG-6 and "F" connectors to 50 ohm BNC's and RG-58 will cost a bunch and make vitually no detectable improvement in range. Besides RG-6 probably has less loss than RG-58 anyway.
$0.02