Usually, the shield of the coaxial feedline is grounded somewhere near the input. Preferably, it will be grounded before the coax enters the shack such as at an antenna switch. The velocity factor of the coax is not involved in determining the length in wavelengths. If the feed line is not grounded outside the shack but is brought to the transceiver and the transceiver is grounded, the length of the ground wire must be added to the length of the coax plus some allowance for the equivalent length of the transceiver case.
In this case, one is bringing the radiating coax into the shack which is not good. Some Hams have also experienced computer problems because of a high amount of RF.
To cure this one needs to install an additional current balun at the transceiver end of the coaxial cable which presents a high impedance to currents flowing on the outside of the shield. All orders made by midnight EST on November 15th will be fulfilled.
The terms Current Balun and Voltage Balun are often used. This article explains the key characteristics that determine which best describes a particular balun. The approach taken is to charaterise a balun because of how it behaves externally rather than the internal implementation. Balun loads are often though of as a simple two terminal device. Baluns may appear ideal if they are driving an isolated two terminal load, and there would be no need for a balun if that was the case. An ideal current balun delivers currents that are equal in magnitude and opposite in phase.
A good current balun will approach the ideal condition. It will deliver approximately equal currents with approximately opposite phase , irrespective of the load impedance including symmetry. If the load impedance is not symmetric, then the voltages at each output terminal will not be equal in magnitude and opposite in phase.
A parameter often used to quantify the effect of a current balun is its common mode impedance or choking impedance. Finally, open-wire line doesn't like to be bent at severe angles - gentle, long-radius curves are all it can tolerate.
So it is obvious why coaxial cable came into existence. We run cables alongside plenty of metal objects, often burying them in hostile environments. We put all kinds of twists and turns into cable and not forgetting connections all of which diminish signal quality that kind of path is a nightmare to negotiate with ladder line.
But there's a problem. The AC voltages in coaxial cable want to travel as if they are along open-wire transmission lines. Electrically, that's not a big problem one leg flows in the center conductor, and the other flows in the outer shield. The problem occurs when these AC signals arrive at their "load", which could be a speaker, antenna, resistor, splitter, or an amplifier, and one side is grounded. Most AC loads are designed to function as balanced components - most antennas use a dipole radiator as the driven element.
Connecting that unbalanced, grounded transmission line coaxial cable directly to a balanced load can result in all kinds of problems, such as currents flowing back down the shield to the source. These stray currents create standing-wave problems, resulting in a mismatch from the cable to the load. The result can be ringing or ghosting, not to mention unwanted radiation of the signal from the coaxial cable which is now working as part of the load.
The solution to these issues is to use a small transformer to convert from an unbalanced to a balanced signal, and vice-versa. This transformer is known as a balun, and they are in operation in anything from telephone lines to transmitters. Thus, the core must have minimal loss, since any loss would degrade antenna efficiency and contribute to heating the core.
Moreover, the core must not saturate at the current expected at maximum operating power. You'll note that many current balun designs use ferrite materials and a closed toroid core. This closed magnetic circuit and the material properties of ferrite maximize impedance, but also offer a low saturation current. The balun you picture uses a rod core, which affords much less impedance but a much higher saturation current.
It may also use a powdered iron rather than ferrite material. While these design choices allow the core to handle more power as a voltage balun, they would also make it ineffective as a current balun. I suggest you use the balun as-is.
Voltage baluns aren't necessarily bad, and provided the dipole, feedline, and surroundings are reasonably symmetrical, a voltage balun could work just fine. You can give it a try and see how well it works:. If it doesn't work so well then you can try adding or replacing with a current balun, or perhaps you can make some adjustments to your installation to make it more symmetrical, giving the voltage balun a better chance to work. Yes, it does appear that re-winding that balun would be a good idea, as the primary purpose of a choke balun is to cut off common-mode currents on the transmission line.
Peter Buxton, you are right. The ferrite rod used here is most probably mix 43, the one commonly found in the BC Radio receivers. The one with the 3 windings is an voltage BalUn, the center blue winding being the magnetising winding. If you removed this Blue winding you get current balun or CMC choke that is what you require to suppress the CMC that tends to flow on the outer surface of the coax braid. Phil Frost has mistaken the core material as iron carbonyl mix 2 but whatever he said about the 2 is correct; and it is used mainly as high Q inductor as in harmonics filter below 10 MHz.
SWR arises due to mismatch of impedance at the feed point of the dipole and the coax cable. There is an electrical discontinuity at the center point of the dipole; where the dipole is "balanced", and the feed coax is "unbalanced". This discontinuity causes some disruption in the flow of energy, a reflection of some amount of it.
Specifically in the center-fed resonant dipole, this engineer recommends a voltage balun aka transformer. It will overcome the discontinuity while also greatly reducing any "feedback" RF energy from traveling back down the coax.
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