About coax cable

Posted: 01/06/2012 in About Coax Cable
Tags: , ,

The coax cable is a vital part of the antenna system. There are many brands and grades of coax.


The coaxial cable has basically four parts.


1) The vinyl outer jacket.


2) Just underneath the jacket is a copper braided material. This material is usually copper, and it is anywhere from 70 to 95% coverage. The higher the percent, the more surface area coverage. 95 to 100% is optimal shielding.


3) Just underneath the copper braided material is a dielectric material.

The dielectric is the insulator and insulates the center conductor from the outer shield.

There are two types of dielectric (the material that separates the center conductor from the outer shielding), polyvinyl and polyfoam.

Poly foam has more flexibility than polyvinyl. Therefore, many customers prefer the poly foam over the polyvinyl.

It’s easier to bend and installation is a little easier.

Polyvinyl is superior in its ability to withstand higher temperatures and hard compressions.


4) The very center of the coax is the center conductor. There are two types of center conductors. The solid wire and a multi-strand wire. High quality coax will have many strands making up the center conductor. The material will usually be copper.

 

The coaxial cable can be terminated with terminals or a PL-259 connector. PL-259’s of high quality are coated in either nickel or silver.

Amphenol and Astatic are two of the best brands on the market.


Coax cable for transmitters is 50 and 75 ohm impedances. A single antenna system must use 50 Ohm (RG58) coax and will normally be 18 feet long.


Dual antenna systems must use 75 Ohm (RG59) coax cable and will normally be 18 feet to each antenna.


The coax types can not be interchanged. RG58 must be used for a single and RG59 must be used in dual antenna system configurations only.


The length of the coax to be used is controversial only because most people have not educated themselves concerning this matter. At 27 MHz 18 feet of coax is the correct length and will provide for minimal loss with an optimal SWR match.


When using other lengths a phase shift will occur causing a loss in Db gain even if the SWR matches. This means that the carrier is either lost in the form of heat or the coax cable itself acts as part of the antenna lowering the Db gain. Stay with the 18 foot rule. If an 18 foot length is to short to reach the antenna use a 27 or 36 foot length. The system is in phase with 18, 27, & 36 foot lengths.



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Comments
  1. Tim Cash says:

    Clay, I understand the need for the 18 ft. rule when it comes to the length of antenna coax. But when it comes to motorcycles (Harley-touring) do you stick with the 18 ft. minimum and just coil the excess under the seat somewhere?
    Thanks

    • htmcnetwork says:

      Hello Tim, you are correct. Coil the coax cable in an 8 inch or larger diameter normally under the seat and tie strap it in place. Verify the seat does dot crimp or put pressure on an part of the cable.

      Regards,

      Clay

      • steven faircloth says:

        You just make the length of your cable 9 feet instead of 18.

      • htmcnetwork says:

        Hello Steven,
        Thats actually not correct. The 9 foot length only works about 20% of the time for auto’s and rarely ever works on motorcycles.

        Regards,

        Clay

      • Steven Faircloth says:

        Sorry did not realise it was for car or bike, thinking of home base setup, the multiple of 9 system.

        But then a lot of people say it does not matter the length of cable used as long as it tuned correctly.

        Cheers and beers.

      • Steven Faircloth says:

        Sorry did not realise it was for car or bike, thinking of home base setup

        Cheers and beers.

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