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HOW DOES COAX WORK?

Coax cable is for the purpose of transferring RF energy from the transmitter to the antenna. I have heard remarks that coax can be used as a poor radiator. Putting a tuned circuit at the end of your coax will cause this to happen.

Actually most antennas are a tuned circuit. Other than loops, it has the characteristic of a Series-Resonant circuit. These antennas do the radiating, coax does not. However, if you use discreet components (capacitor and inductor) as the antenna, will the coax radiate?

You will have a resonance like the antenna. What will be different is the Radiation Resistance. The antenna develops a Radiation resistance anywhere from 20 ohms to 200 ohms approximately. This resistance is a result of the antenna radiating. The discreet resonant circuit has a Radiation Resistance that is very low. This is not only a mis-match to the coax, but it resembles a shorted circuit. This is great for circuits in your radio, but not for a radiating antenna. However, at the same time, the coax will not become the radiator. If it does, it needs replacing.

There is a condition called Common Mode Currents or Voltages. This is when RF energy, due to skin affect will travel on the outside shield of the coax at the feedpoint of the antenna. This small percentage of energy will radiate. However, it is only a fraction of the power you put to the antenna. It mainly produces interference problems in your own station. Any antenna can develop this situation. How is this corrected?

Normally a simple choke at the antenna. This could be made from coaxial cable, toroid cores or ferrite beads. I have had all of these used on my antennas with much success.

My first article is dealing with feedlines. Here it is again so you can see why good quality coax will not work as an antenna.

Many are familiar with Ladder Line. 2 wires in parallel from the transmitter to the antenna. If the 2 wires are spaced very carefully with little variation, one side will be of a positive value (voltage or current) and the other will have a negative value exactly equal. The 2 RF fields cancel each other. This does not take away the RF energy, but keeps it from radiating into the atmosphere and follow the wires. In affect it makes a very high resistance to the atmosphere and a low resistance along the Ladder Line.

The draw back with Ladder line is it is affected by nearby objects. Something close to the Ladder Line will unbalance it. Therefore coax was developed to reduce that problem.

Coax is doing the same thing as Ladder Line electrically. The center conductor has one polarity while the shield has the opposite. The same principal of cancellation is taking place. The shield is not really a shield in that RF is contained inside of it. There is just as much RF energy on the shield as the center conductor.

Due to the physical shape of the coax, the shield can be kept at a zero voltage in respect to your grounding. So if you had an RF volt meter you would in theory measure 0 voltage from the shield to your grounded installation such as radio chassis, tower or mast, walls of your home and so on. Now a nearby object has little affect on the balance of the coax.

At times this balance can be disrupted slightly. The result is RF feedback, interference to other electronic equipment, common mode currents or voltages, higher than normal SWR readings and more. The problems with coax is not severe, just at times a bit challenging.