Whats all this Coax stuff anyway? What type of Coax do I use? What do the Coax type numbers tell me? Why 50 Ohm coax.
Most of us use coax every day, We bought the antenna with coax attached or the radio instruction book said to use it. But why coax and not open line feeder cable in the 300 to 600 ohm range like they used to use? For that matter what's wrong with a wire right from the back of the radio.
A wire from the back of the radio presents some special problems. First the wire from the radio becomes part of the antenna and radiates as well as the antenna. This causes interference as well as affecting the antenna pattern. Second, a high voltage point appears in the antenna wire at, or near the connector. This is a serious fire hazard when you consider that most of us have to pass that wire through a wall of some kind and the wall is usually have wood or some other combustible in them.
Open line feeders solved some of these problems by balancing the currents between the feeder legs. The radiation field also tended to stay contained between the legs of the feeder line until the wire was split into antenna elements or legs. This minimized the interference or strong radiation fields in the shack. Tube amplifiers were also high output impedance devices, on the order of 2000 ohms. (Plate Load=Plate voltage (Typ. 1000v)/Plate current (Typ. 500ma)) so matching a tube amp with an output impedance of 2000 ohms to a 600 feed line only required 3.3 to 1 transformer.
This still leaves us with the problem of high voltage on the antenna line. Assume a power of 1000 watts, an impedance of 600 ohms and remember the formula for Voltage of E=square root(PxR). This leaves you a voltage of 774.596 volts. If the line Impedance is changed to 50 ohms this reduces the voltage on the antenna line to 223.6 Volts.
Coax came about to eliminate some of the high voltage problems and radiation problems. Coax by design is self shielding since the outside jacket is at ground potential. Because the inside conductor is insulated and shielded, hazardous voltage levels don't appear on the coax except at the antenna connection itself.
This is best answered in two parts, First, coax isn't all 50 ohm, it ranges in impedance value from 30 to 90 ohms. Second the lowest possible transmission loss occurs at about 70 to 80 ohms while the highest power handling occurs at about 30 ohms. Because of these factors a compromise between line loss and power transfer was struck at 50 ohms. This explains however why Cable TV firms who have huge distribution networks still use 75 ohm coax.
Coax is available to us by different Type Numbers. Most of us are familiar with 50 ohm cable types RG-58, RG-174, RG-213, RG-214, and 9913 and the 75 ohm cable types RG-59 and 783. Cable with a "RG" in the type number means "Registered", and all this really means is someone filled out the forms to register it, by itself this means nothing. Cables that are Mil Spec. qualified for lot certification (Per MIL-C-17D) will be listed as RG-XX/U. The numbers between the letters were assigned in numerical order, from 1. This can be used to indicate only the products registration. For example an RG-8 coax was registered before the RG-213 coax.
Coax that is marked "RG-8 TYPE" means that these cables are similar in impedance, size, power handling etc. to the Mil Spec. qualified cables, but have never gone through the actual certification. For our application these will work fine.
If you have looked at these coax types, they come in several diameters. Generally if you are going to be handling high power (200 plus Watts) or your coax runs are going to be very long (Over 100 feet) you need to be using one of the larger coax's such a RG-8, or RG-213.
If you are working the satellites and low line loss is a major concern, a coax such a 9913 or RG-214 should be used. The RG-214 coax is a double shielded RG-213 type coax and usually is a lot of trouble to find connectors for and work with.
For general use such as short runs for the car, a few jumpers for the shack or a length of coax for emergency use, RG-58 coax is a good. choice. RG-58 is the least expensive of the coax types and serves very well for low to medium power applications in the HF to UHF regions.
Coax can be found on the surplus market. But again caution should be the watchword. Much of the coax found on the surplus market are leftovers from the Cable TV industry. Avoid any coax with a Teflon dielectric. These were made for High Temperature/Hostile environment applications and usually have inferior signal handling characteristics compared to standard coax
Hard-line/Heliax type coax can be found in 50 ohm impedance, and there are coax connectors available for this type of coax. About the only application that justifies this type of coax is a repeater installation. Its expensive, hard to bend, hard to connectorize, hard to secure to a tower and its just plain heavy.
Any of the 50 ohm coax will work for our application. Choosing the best coax for you application will enhance your station operation. Using the best coax for your application can make the diffrence between being heard and being QRM.
73's and GL from WD4BIS
Copyright 2005 Gerald Crenshaw WD4BIS. All rights are reserved.
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