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RG58 and RG59 issue
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NCMau
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July 24, 2019 - 1:25 pm
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Here is my situation. I am in the process of installing a wired security CCTV 2K system. I would like to install one of the cameras by the entrance gate which is 200ft from the house. Some years ago I ran 200ft of RG58 underground along with AC wire. At the time I did not have an immediate purpose for the coax, but since I had the coax available and the ditch done, I decided to install it.

Now, RG58 being 50 ohms impedance and the DVR cameras being 75 ohms, would not be a good impedance match. Since the cable is there anyway, I will try to connect it as it is. There is an adapter available for $60  https://www.ebay.com/itm/50-to-75-OHM-Converter/183760140149, which I would get if it would work , but I wish I could find some cheaper especially if the result maybe questionable.

I would be okay if I would end up with a degraded signal as long as there is some acceptable visibility.

BTW I am aware that I will need power for the camera but that’s another issue to be resolved either with battery or AC.

What are your thoughts? Is there a practical doable solution without scrapping the idea? 

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Jose Malave
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July 24, 2019 - 2:07 pm
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You are correct with needing to match the impedance on the cables.

Impedance matching network usign resistors
The matching network shown below can be used to match two unequal impedances, provided that Z1 is grater than Z2. 

          ____
    ----|____|---+---------
             R1     |        
                    | |        
    Z1            | | R2     Z2
                   |_|        
                    |         
    -------------+----------

The resistor for this circuit can be calxulated using the following equations: 

  R1 = Z1 - Z2*R2 / (Z2+R2)
  R2 = Z2 * sqrt(Z1) / (Z1-Z2)

The table below will show some precalculated values for some most common interfacing situations: 

  Z1      Z2      R1      R2     Attenuation
 (ohm)   (ohm)   (ohm)   (ohm)   (dB)

   75      50     42,3    82,5    5,7
  150      50      121    61,9    9,9
  300      50      274    51,1   13,4
  150      75      110     110    7,6
  300      75      243    82.5   11,4

If you arent looking to build your own then you can use a UNUN Transformer with a 1-1.5 ratio which would make any 75ohm cable match to 50ohm and vice versa. Now that this is handled you have to take into consideration the fact that if you choose any AHD, CVI or TVI camera you will lose significant signal; A better solution to both power and the signal issue is the use of this device https://www.securitycameraking.com/AL-LR1000-P.html this device will use your existing coaxial cable providing not only a connection to your IP camera but to power the camera as well. Both Issues resolved. 

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