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Author Topic: Antennas for the CM15A  (Read 40697 times)

dbemowsk

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Antennas for the CM15A
« on: February 02, 2010, 03:19:55 AM »

I have read some posts here about antenna mods to extend the range of the CM15A.  The ones I looked at used 1/4 wavelength as their antenna length which is 9.058 inches.   A 1/2 wavelength antenna arial would be 18.116 inches at 310MHz which would make a 3/4 wavelength 27.174 inches and a full wavelength 36.232 inches.  I do realize that it would make the antenna a bit larger, but would you get a more useful increase in gain using a 1/2, 3/4, or even a full wavelength antenna?

Also, I am thinking of extending my antenna quite a distance from the CM15A.  What type of cable would be better for the run, 50 ohm or 75 ohm?  I have a fair amount of 50 ohm that I could throw some BNC connectors onto and use, but I also have access to some quad shield RG6U 75 ohm that I can crimp some F connectors onto if that will work better.

The next question is, do I extend both the internal and external antennas of the CM15A?

I thought of first trying a full wavelength of wire coming out of the unit as an initial test to see what that might get me since that will be easy.  If that doesn't get me what I want, I may take one of the approaches above and build a nice antenna.

Dan B.
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Brian H

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Re: Antennas for the CM15A
« Reply #1 on: February 02, 2010, 05:59:47 AM »

The Gray internal antenna is the transmit boards antenna and I would leave it alone.

I believe the original modification on AccessHA said the PC Board was a ground plane and their F connector mod brought out the boards ground and the center wire to an F connector. Some used a BNC as a substitute.

The early CM15A had the extra receiver antenna wire balled up in a large random bundle [I have a few of them]. The later ones have the extra receiver antenna wire glued around the case. So technically the antenna from the factory is longer but X10 glued it inside the case ::)
I just took one of the early ones of mine apart. There was about 4.5" of antenna wire balled up in a random bundle between the plastic antenna base and the RF Receiver Boards Antenna connection. That almost doubled the total lenght of the wire from the board to the antenna tip.

Do you have the CM15A schematics? They are available on the FCC Grantee web site.

https://fjallfoss.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=476706&fcc_id='B4SCM15A'
« Last Edit: February 02, 2010, 06:47:35 AM by Brian H »
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eagle

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Re: Antennas for the CM15A
« Reply #2 on: February 02, 2010, 08:37:35 AM »

Hi Brian,

Please forgive the stupid question about the ground wire ...

I've seen the pictures about the F connector mod.  It shows that the ground wire goes to one of the ANT holes on the board shown here:  http://www.shed.com/tutor/CM15mods/CM15%20mods-Pages/Image9.html

Looking at the photo of the underside of the board at the webpage below, the ANT holes appear to be soldered closed.
https://fjallfoss.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachment_id=347602&native_or_pdf=pdf

So, since the ANT holes appear to be blocked from the underside, is the ground wire in the mod simply pushed in and "hanging" there?  It doesn't appear to be soldered.  Perhaps glued in place?

Would appreciate your help.

Thanks,
eagle
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JMac

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Re: Antennas for the CM15A
« Reply #3 on: February 02, 2010, 09:32:55 AM »

It's been awhile since I did one of these, but don't you just heat up the hole (with the iron) and shove the wire in place to solder?  Glue is not the answer, I wouldn't think.
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eagle

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Re: Antennas for the CM15A
« Reply #4 on: February 02, 2010, 09:52:04 AM »

Thanks Jmac.

I actually found the answer.

It's on the archived page for accessX10.  It says to solder it.  :)

-------
http://web.archive.org/web/20050420014752/http://www.accessx10.com/howto_fconnector_page3.html

Insert the black wire into the VSS hole and turn the PCB over. You should now be viewing the green side of the circuit board.

CAUTION: Only add a small amount of solder to the ground wire and the PCB, just enough to secure the wire to the PCB. Be extremely careful when making this connection to ensure solder doesn’t get anywhere else on the PCB. Tug lightly on the black ground wire to ensure a solid connection. I went one step further by adding a little hot glue around the top side of the black wire for added safety.
-------

r,
eagle

« Last Edit: February 02, 2010, 09:54:58 AM by eagle »
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Brian H

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Re: Antennas for the CM15A
« Reply #5 on: February 02, 2010, 10:03:40 AM »

I believe the original CM15A was designed to have a different RF daughter card and the silk screen is confusing.
The white wire on the receiver daughter card is the Antenna and VSS is the common. The pad you are showing may actually be a VSS pad originally for the daughter card and the ANT hole may have been the original Receivers Antenna connection.
In the original AccessHA project. The VSS hole slightly closer to the edge was used and not the one with solder flow over it.
« Last Edit: February 02, 2010, 10:21:46 AM by Brian H »
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eagle

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Re: Antennas for the CM15A
« Reply #6 on: February 02, 2010, 10:25:49 AM »

You're absolutely right Brian.

I looked at the pictures of the two boards closer and they are different indeed. 

Thanks.

r,
eagle
« Last Edit: February 02, 2010, 12:07:01 PM by eagle »
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Brian H

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Re: Antennas for the CM15A
« Reply #7 on: February 02, 2010, 10:44:39 AM »

The author in the Shed Article. Said they chose the ANT pad to isolate the whole system. Since the power transformer in the CM15A isolates most of the board from AC. Not sure if that was needed.

http://www.shed.com/tutor/CM15mods/CM15%20mods-Pages/Image2.html
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dave w

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Re: Antennas for the CM15A
« Reply #8 on: February 02, 2010, 12:08:25 PM »

I have read some posts here about antenna mods to extend the range of the CM15A.  The ones I looked at used 1/4 wavelength as their antenna length which is 9.058 inches.   A 1/2 wavelength antenna arial would be 18.116 inches at 310MHz which would make a 3/4 wavelength 27.174 inches and a full wavelength 36.232 inches.  I do realize that it would make the antenna a bit larger, but would you get a more useful increase in gain using a 1/2, 3/4, or even a full wavelength antenna?


Dan,
FWIW
Good RF transmission on a 1/4 wave relies on a ground plane at least the equivalent of the radiating element. Rule of thumb is:the more GP the better. However 1/2 wave does not rely on GP like the 1/4 wave. 

When I did my CM15A mod I compared a 1/4 wave with ground plane to a 1/2 wave with no ground plane ( for the 1/2 wave I just stripped and cut 18 inch of coax shield away from RG6, leaving 18 inch of center conductor which I tied to the roof beam in the attic). Using a highly scientific and empirical method of judging effective reception distance ("uh, how many steps can I get from house before my Palm Pad don't turn front porch light no more") I found the 1/2 wave to have slightly better results. I never tried a 3/4 wave as the 1/2 wave satisfied my needs. Some RF guru out there correct me if I am wrong, but I thought a full wave antenna was lossy due to phase cancellation on the element(?). I know I am talking transmit, but receive should work the same.

As far as 50 ohm and BNC vs 75 ohm and F connector, with out knowing what the receiver input impedance is, it is a coin flip. I know Brian H has ferreted all sort of FCC information on the CM15A and the receiver board so he might be able to shed some light, but the general design of antenna and receiver board tells me it ain't that critical. I went for ultimate cheap, resulting in F connector and RG6, and with the wind to my back, standing on one foot and wildly waving the Palm Pad, I can get about 100 feet range
« Last Edit: February 02, 2010, 12:11:17 PM by dave w »
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Brian H

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Re: Antennas for the CM15A
« Reply #9 on: February 02, 2010, 12:32:51 PM »

First I have to say. X10 makes changes and doesn't give any details on what they did. My extra antenna wire found in the older date codes starting with 04 are long and wrapped in a random ball of wire. A date code with a 08 seems to have a shorter wire gently looped. I think I have some in between 04 and 08 that where long but hot glued to the case. Also the newer RF receiver board looks like it has different parts on it and the adjustable coil is now a preset one that looks like they bend to tune then glue in place.

Receiver schematic isn't too clear on its input impedance and my math is rusty.
The schematic says the antenna is 240 mm [9.45"]. Stretching the internal random ball of excess antenna with the plastic parts length. Does measure about 9.45" by my ruler. Also the receiver schematic shows R4 [from antenna to common] has a * next to it. Yes folks a * is a component deemed not needed and is missing.


A WGL V572A antenna is about 9.5" long on a chassis mount F connector. It mounts on a 2 7/8" X 2 7/8" L bracket and the shell of the antenna F connector is connected to the bracket. The cable is RG6 with a male F connector on each end.
« Last Edit: February 02, 2010, 01:51:53 PM by Brian H »
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Duck69

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Re: Antennas for the CM15A
« Reply #10 on: February 03, 2010, 08:21:12 PM »

Here are photos of the CM15A mod that I used.
Found the photos but not the guide.
Maybe someone could add the instructions ?

Sorry ! I have to post 3 times so that the photos will load.

Here is photo 1
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Duck69

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Re: Antennas for the CM15A
« Reply #11 on: February 03, 2010, 08:22:09 PM »

Antenna mod photo 2
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Duck69

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Re: Antennas for the CM15A
« Reply #12 on: February 03, 2010, 08:22:47 PM »

Antenna mod photo 3
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Puck

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Re: Antennas for the CM15A
« Reply #13 on: February 04, 2010, 09:14:04 AM »

As far as 50 ohm and BNC vs 75 ohm and F connector, with out knowing what the receiver input impedance is, it is a coin flip.

Re: Project To Increase Range Of CM15A
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dave w

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Re: Antennas for the CM15A
« Reply #14 on: February 04, 2010, 12:05:18 PM »

As far as 50 ohm and BNC vs 75 ohm and F connector, with out knowing what the receiver input impedance is, it is a coin flip.

Re: Project To Increase Range Of CM15A

Forgot about that Puck, thanks!

I wonder if BNCs and RG58 would have a significant difference compared to "F" connectors and RG6 (other than cost).
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