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#29
July 11th 12, 09:30 PM
posted to rec.radio.amateur.antenna
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common mode current
Ian Jackson wrote:
In message , writes Szczepan Bialek wrote: Simply same of you do not know that "dipole" is a radiator and counterpoise. A dipole does NOT have a counterpoise. As I keep saying, I'm convinced that Szczepan is thinking of a situation where a dipole fed directly with coax. He may have seen diagrams which show this, and they have become fixed in his mind. I think he is totally unable to understand that an antenna and a transmission line are two different things. Then, because the coax is (usually) grounded at the TX end, he believes that the leg of the dipole, which is connected to the coax screen, is also grounded - and therefore it serves the function of a counterpoise - like you might use in a situation where the antenna is a directly-fed end-fed wire situations, and it is not possible to get a good ground connection. In a way, Szczepan is sort of right. Consider the situation where a dipole (no balun) is connected to the TX via essentially zero length coax, and the TX chassis is not well grounded. [For example, possibly there is a physical safety ground connection, but it is too long to be effective at RF.] The 'ground' side of the of the dipole would indeed function as a counterpoise, which might - or might not - radiate effectively (depending on its height, its physical relationship relative to the 'live' leg etc). That does not change a dipole into anything other than a dipole. Feeding an antenna with any transmission line ill suited for the antenna will of course lead to unwanted currents which will radiate to some degree. However, the antenna is defined by the geometry of the antenna and the transmission line does not change the radiation of the antenna unless you are doing something really stupid like taping the transmission line to one of the elements of the antenna, which changes the antenna geometry. Feeding an unbalanced load such as a ground plane with balanced line will also lead to unwanted currents which will radiate to some degree, but the ground plane antenna is still a ground plane antenna. |
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