What exactly is Siamese cable?
The word Siamese immediately strikes at least one of two images in the minds of those who hear it. People either think of a cat or of conjoined twins. For some it even refers to the language of central Thailand, though that is now referred to as Thai.
In the world of linked data Siamese cable refers to a cable that provides both power and a data feed. When both power and data are put together it greatly simplifies the cable installation because only a single cable actually needs to be installed.

Siamese cables have cleaner installations and tend to be more organized than other cable set ups. Because these cables provide both data and power feeds they are typically used with video, surveillance camera, closed-circuit television (CCTV), and other similar applications.
Inside a Siamese cable.
The typical Siamese cable is heavily shielded. This cable also contains a pair of 18 gauge two conductor power all within one jacket. The center conductor is accompanied by a shield and the two are kept apart by a dielectric.

There is an advantage to using RG cable. RG cable is durable, in addition to its ability to be run over very long distances before interference starts to become a problem, though it is recommended that installers cut each security camera cable run to the exact length that is needed.
Types of Siamese Cable
Siamese and coaxial cables all use RG numbers. The RG numbers (RG6, RG59, etc.) with Siamese cables can be the most confusing aspect of Siamese cabling. RG is a designation that applies to the many different types of coaxial cable. Coaxial, or coax, is a general classification as opposed to referring to any specific use. In fact if a cable has two conductors that share a single common axis then it is a coaxial cable. Nearly every common video cable is a type of coaxial cables, even s-video cable.

There is some discussion that RG-59 has a much lower bandwidth than RG-6.  Or that RG-59 is poorly shielded when compared to RG-6.  This will actually vary between different strands of cable as opposed to being true in every instance.

That has more to do with the specific manufacturer than anything else. A manufacturer that uses cheaper materials could produce an RG6 that is only as good as a high quality RG59.

But what do these numbers mean to you? Well understanding RG numbers let’s you know which cable to use. RG58 and RG8 are used primarily in radio transmissions or in computer networks. They are still coaxes but only being able to handle 50 ohms make them unusable for using as Siamese video cabling. The ideal for Siamese cabling for video is 75 ohms which are found in RG6, RG59 and RG11. In terms of greatest capacity to least capacity RG11 is the largest, RG6 is right in the middle, and RG59 is the smallest. However, like I said before, that can vary based on the quality of the cable purchased.
Even though the typical RG59 coax cable can be used over long runs there is a maximum distance, after all it is the cable that determines how far away the camera can be from the recorder. The camera doesn’t really get a say.

The furthest the security cameras can be from their recorder when using a RG59 coax is 600 feet. The RG6 coax cable, which is quickly replacing the much smaller RG59 for CCTV set ups, has an upper limit that goes up to 1,000 feet.  The RG59U Siamese coaxial cables are also perfect for covering long distances, equal to that of the RG 6 at about 1000 feet. In general RG6 is superior to RG59, however it is also more expensive.
Siamese cables versus Power over Ethernet.
In many ways Power over Ethernet (PoE) is similar to Siamese cabling. Both have the purpose of streaming power and data. Power over Ethernet is designed to transfer power over what would normally be done over wires meant for data. Siamese cables, however, have wires that are divided inside the cable. This offers insulated wiring within the cable that includes both data wires and power wires. This makes it possible to have power and data in a single cable but with little to no interference in the video transmission.
As I stated before installation is easier with Siamese cabling. There is only one cable for both power and data so only a single cable needs to be run when installing. This means that installers don’t need to wire two cables together or make multiple trips to and pulling multiple cables through a wall. An installer can pull a single length of cable form the spooled box, and run it. At a max length of 600-1000 feet a single cable can get a lot of use.