This blog was created to share my projects, ideas, and experiences in amateur radio with other operators around the world who also enjoy this wonderful hobby.

Wednesday, August 1, 2012

75 Ohm Coaxial Cable as an Alternative to 50 Ohms

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Coaxial cables with 75 ohm impedance can be obtained at reasonable prices and sometimes free. They are most of the time used for Satellite, Cable TV and CCTV ( Closed Circuit TV) video applications. In the picture above and from top to bottom, RG-11U, RG-6 and RG-59. RG-11U and RG-6 are used by satellite and cable companies in signal distribution and home installations. The main problem using this type of cable in 50 ohm applications is not the impedance factor. Most of these coaxials utilize an aluminum braid (shield) with a metalized foil, most of the time the ratio between braid and foil is 40/60. That is , an aluminum braid with 40% shielding, and the foil completing the remaining 60% for a total of 100% shielding. Ordinary PL-259 type connectors can not be used with RG-11U or RG-6 because of the aluminum shield. Aluminum can't be soldered as easy as copper, for example. All 50 ohm coaxial cables used in amateur radio applications are 50 ohm impedance and the braid (shield) is either copper or tin plated wire mesh. I will show you how to install the regular F connectors to the different types of 75 ohm coax cable and utilize an F to PL-259 adapter to build a good quality but cheap transmission line.


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There are a variety of "F" type connectors for 75 ohm coax cable. From left to right, Ideal, snap ring type, for RG-6 and RG-6 Quad Shield, available at Home Depot. Augat, Snap and Seal ,by Thomas and Betts and the ordinary crimp type for RG-59 and RG-6 (F-59 and F-56) . The snap and seal type, are superior to the ordinary hex crimp type "F' connectors, because they  offer a very tight connection and are virtually impossible to strip from the cable.Snap and seal connectors need a special crimping tool to install them on the cable, the regular F-59 and F-56 are crimped with an "hex" crimping tool like the one pictured above. RG-59 and some varieties of RG-6/U can be found with copper shield and in that case ordinary PL-259's with the appropriate reducer adapter can be soldered directly to the cable. Use the UG-175/U reducer adapter with RG-59 and the UG-176/U reducer with RG-6/U. You have to make sure that the shield is copper or tin plated. As a general rule , if regular rosin core 60/40 solder does not adhere to the braid, most probably, it is not copper or tin plated. In that case you need to use the "F" connector approach.


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The above photo shows the different types of crimping and wire stripping tools available to work on 75 and 50 ohm coaxial cables. The top one (blue handles) is the regular "hex" type crimping tool for F-59 and F-56 connectors. Use the F-59 with RG-59/U and the F-56 with RG-6/U. If the coaxial you are using is the quad shield type ( the one with two aluminum and foil shields), make sure the F-56 connector is suitable for both RG-6 and RG-6 Quad, otherwise the connector won't fit on the quad shielded coax. The crimping tool in the middle is the Augat crimping, tool for the Thomas and Betts Snap and Seal connectors. The red crimping tool is from a different manufacturer and will crimp most of the snap type connectors from different manufacturers.The other two remaining tools are coaxial wire stripping tools for RG-11/ RG-8 type coax and RG-58/ RG-59/ RG-8X or RG-6 coaxial cables

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The first step is to remove about 3/8" from the outer cover, shield and dielectric at the cable end. The coaxial wire stripping tool shown above is from Radio Shack and is a two step stripping tool. Use the outermost position to strip the cable tip and the inner position to strip the outer cover (second step) This tool will work on RG-58, RG-59, RG-8X and RG-6 coax cables. If a coaxial wire stripping tool is not available, use a sharp knife to remove the outer cover, braid and dielectric material (foam or polyethylene), be careful not to cut the center conductor. Other types of stripping tools have two blades, and will strip the coax in just a single step. The cable an the top is RG-6/U and the bottom  is RG-59/U.


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In this photo, both coaxial cables after removing the outer cover, using the innermost position on the two step Radio Shack type tool (second step). About 3/8" of the outer cover is removed, if using another method, be careful not to cut the shield wires.


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After the cables are properly stripped, fold the braid down on top of the outer coaxial cover. Top coaxial cable is RG-6/U and bottom is RG-59/U


photo 7

This is how the cables look with the F-59 and F-56 type connectors installed and ready for crimping. Remember that they are crimped using the "hex" type crimping tool.


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If using the snap and seal type of connector insert the plastic ring first into the cable , then insert the connector. When the center conductor is visible at the top of the connector, the cable and connector are ready for the crimping tool.


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Insert the coaxial cable end with the connector inside the crimping tool and press the tool lever. The plastic ring will compress inside the bottom of the connector, the top part of the connector will move to the left up to the compress line. Once the ring is fully inserted, remove the connector from the tool and verify  the center conductor is centered inside the threads. Sometimes, if the center conductor is not centered prior to the crimping process, the crimping tool piston can bend the center conductor. If this happens ,use  long nose pliers to re-shape the center conductor.


photo 10


In this photo, notice the piston of the crimping tool as the lever is manipulated. Thomas and Betts snap and seal connectors use a band of different color on the plastic ring to identify the type of cable the connector will fit. Orange band is for RG-59, blue band is for RG-6 and purple band is for RG-6 Quad.. Please make sure you are installing the correct type on your coaxial cable, otherwise the connector is not going to install properly or won't fit.

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The other crimping tool I have for the snap and seal type "F" connectors. The process is the same as with the Augat tool explained above.  There are RCA and BNC  type connectors for the different coaxial cables available utilizing the snap and seal principle. The greatest advantage is that you don't have to solder and it will save time when installing multiple connectors. If you don't have or don't want to invest in the crimping and stripping tools there are other alternatives in those cases. One is , using the cheaper F-59 or F-56 "F" type connectors, the other is the poor mans approach to installing the snap and seal types. Place the connector inside the open jaws of a large plier or , drill a small hole into a piece of wood to insert the tip of the center conductor and the connector facing with the threads down. Hit the ring with the open jaws of a second plier , until it is fully inserted into the body. Be careful not to strip or damage the coaxial's outer cover as you hit the ring.


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In this photo, observe the different colors of the Thomas and Betts connectors, the type of cable they will fit is indicated by the color band. In this example, the one to the left (blue) is for RG-6/U , the one to the right (purple) is for RG-6 Quad Shield.

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In this photo, notice the difference in diameter between the F-59( (left) and the F-56 (right) "hex" type "F" connectors. They can be found at very low prices and offer a cheap alternative to terminate your coaxial cables. During an emergency ,when no electric power is available, while operating from remote locations or just to experiment with a new antenna , 75 ohm coaxial cable represent a good alternative prior to investing in higher cost ,expensive 50 ohm coaxial cable. Most hybrid transceivers with a pi- network tuned circuit on the final amplifier stage will easily load into 75 ohm coax. Most modern transceivers have built in tuners capable of tuning with 75 ohm coax. A half wave dipole on free space or installed high above ground will exhibit and impedance close to 70 ohms, thus making the 75 ohm coaxial cable a very good option as a transmission line. For the purists, a matching transformer can be used to transform the 75 ohm line to 50 ohms. On the other hand RG-6/U coax has a power handling capability very close to RG-213 type coax and very little loss. It can handle more power than RG-58, RG-59, RG-8X and even some types of RG-8's. The following table from VK1OD ,will show the power handling capability of the different types of popular coaxial cables.


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Once the "F" type connectors are installed , PL-259 to female "F" connector adapters can be used to easily convert the cable assembly. The adapters are also available in other configurations, BNC to "F", SO-239 to "F" , etc. If the cable assembly is going to be used outside, remember to protect the "F" connector and the adapter with heat shrink tubing or good quality electrical tape. I have used this type of coaxial assemblies with adapters with 500 watts of RF power in the HF bands without problems.


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Remember to tighten the "F" connector to the adapter firmly before protecting the connector , in case an outdoor application is needed. 75 ohm coaxial assemblies are also a very good option with receiving antennas and for the higher frequencies on scanner antennas.


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In this photo , a closer view of the completed "F" connector and adapter termination on RG-6/U coaxial cable. Another advantage is that a single length of 75 ohm coaxial cable, with "F" connectors on both ends, is very easy to convert to another type of cable assembly , just by changing the adapters.


photo 17

In this photo, the RG-11/U, 75 ohm  coaxial cable used in CATV distribution in some countries and the Augat cable stripping tool. This coax stripping tool will prepare the end of RG-11/U and RG-8/U in just one step for the connector installation. Two blades are utilized, one will strip the center conductor and the other will remove the bottom  outer coaxial cover.


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The RG-11/U as it looks after preparing the cable for the installation of the "F" connector. In this case the Tru-Spec crimping "F" type connector is used. Remember , most RG-11's like RG-6's use aluminum braid and foil ,thus soldering a PL-259 connector to them is not possible.This type of connector is crimped using a "hex" type crimping tool with a larger diameter. The center pin is pre assembled into the connector, there is no need to solder or crimp a center pin , like on other types of "F" connectors for RG-11.


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The RG-11/U coaxial cable after the "F" type connector is installed. It is a very fast and good quality termination for RG-11/U coaxial cable.



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The RG-11/U cable end with the installed "F" type connector and PL-259 to "F" female adapter. Remember , tighten the connector to the adapter firmly, and protect the assembly with heat shrink tubing or good quality electrical tape if going to be used outside.



photo 21

Comparison of the two types of "hex" crimping tools.The upper tool is for RG-11/U or RG-8/U coax. The one at the bottom is for RG-6 or RG-59 type cables. I have used the Tru-Spec RG-11/U "F" connector in RG-8/U and RG-213/U with good results, when PL-259's are not at hand, or just to experiment with a new antenna.


photo 22
A closer view of the jaw on the "hex" crimping tool for RG-59 / RG-6 coax cable.



photo 23

A closer view of the jaw on the "hex" crimping tool for the RG-11/U coax cable.


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Another alternative to the 'F' type connectors with PL-259 adapters are the crimp type PL-259 connectors available for RG-58 and RG-59. They are installed in the same way as the "F" type connectors, but require a different type of crimping tool. The tip of the connector to the center conductor can be crimped also, but I prefer to solder the center conductor and crimp the shield.


photo 25

The Amphenol ,ratcheting, crimping tool for RG-58, RG-59, RG-214 and other types of coaxial cables. The tool is expensive, but if you happen to have one available, it is a very good alternative to assemble jumper cables or coaxial cable assemblies on the fly.


photo 26

The Amphenol, ratcheting, crimping tool in action  with a crimp type PL-259 on RG-59 coaxial cable. The RG-6U connector will also work on RG8-X. In order to install an "F" connector on stranded center conductor coaxial cables like RG-8X , the center conductor wires need to be soldered to prevent strands from separating when installing the adapter or inserting the connector. Be careful not to use excessive solder, just the right amount to keep the strands as a solid conductor.There are also crimp type connectors available for RG-8/U type coaxial, but they are hard to find especially good quality ones. I hope this information , cable preparation and connector installation will help others  make good use of available 75 ohm coaxial cable in their respective countries. Here in Puerto Rico, satellite and cable TV installers ,throw away coaxial cable reels ,when the length of the remaining coax is not long enough for a new installation. I have found discarded reels ,with sometimes more than 70- 80 feet of coax, perfect for a new antenna project or experiment, and best of all, free. Any comments or contributions to this information are welcome.Until the next blog entry, 73's

7 comments:

  1. Quad-Ring Seals are vulcanized as a continuous ring

    ReplyDelete
    Replies
    1. Vulcanized? As in the actual process of adding sulfur to rubber? Or just weather sealed?

      Delete
  2. Wow! Very informative blog.Complete knowledge about Coaxial Cables.To know more you can visit Coaxial Cables

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  4. Glad to know there are others using the RG-6 and RG-11 coax for amateur radio antennas. I have been using RG-6 coax for years on some of my HF antennas and on all my scanners. 73 de Ernie AJ4BP (ex-KP4EIH)

    ReplyDelete
  5. Crimp style termination of coax should be avoided. It only offers 6 (a finite) points of contact to the outer Shield. Compression style offers infinite points of contact 360° around the conductor.
    The same theory as to why we use wire wraps in Telecom Central Office main distribution frames.

    Good to know I can use my nearly endless supply of readily available CATV/IPTV coaxial cable for my antenna!

    ReplyDelete
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