The Half-Wave Flat-Top Dipole..
Most dipoles consist of two pieces of wire of equal lengths with one of the two ends connected together through an insulator. The far ends of the wires are also connected to insulators. The two conductors of a feed-line are separated and connected across the gap at the center insulator. The antenna is held up by rope that connects the insulated ends of the antenna to two supports. It is a "balanced" antenna, because equal currents flow on both halves of the antenna. Coax is an unbalanced feed-line. The dipole that is stretched between two high supports is called a flattop dipole. The simplest antenna system of all is the ½-wave resonant dipole fed with coax and no tuner. The only reason for using a ½-wave resonant dipole antenna is to eliminate the need for a matching device such as a tuner. The feed-point impedance will be near 50Ω at ordinary heights and they can be fed directly with 50Ω coax from the output of the tranceiver. The two halves of a dipole are fed 180 degrees o
ut of phase, meaning when one side is fed positively, the other side is fed negatively. That is why a feed-line has two conductors. Of course, the sides swap polarity on each half cycle. If you could visualize the current flowing on the ½-wave dipole, the current will appear to be standing still. The maximum current will be seen at the center of the wire and no current will be at the ends. This occurs because the electrons flowing out to the ends reflect back toward the center where they meet the next wave and the current is reinforced there. The minimum voltage occurs at the center and the maximum voltage occurs at the ends of the half-wave resonant dipole. If you were to measure the voltage and the current at any point on the dipole wire, the voltage times the current will equal the power in Watts.
Inverted-V Dipole..
Another configuration for the ½-wave resonant dipole is one having one support in the center and the ends stretched down toward the ground. The single support can be a tree, mast, or tower. The ends of a dipole have high RF voltages on them, and need to be at least 3m (10feet) above ground. This antenna is called an "inverted-V", because the shape of the dipole looks like a "V" turned upside down. This configuration works well because the current is concentrated on the middle two-thirds of the antenna at the apex. The current in an antenna is what is responsible for the radiation. The ends of the antenna have very little current in them and it doesn’t matter if the ends are close to the ground. The middle of the antenna is up high where the radiation is taking place and that is the place you want the radiation to be. An inverted-V has an advantage that the horizontal space required for it is less than what is needed for a flattop dipole. The angle between the wires on an inverted-V needs to be greater than 90 degrees. The gain of an inverted-V is 0.2 dBd and it has a radiation pattern nearly omni-directional. Since it is easy to construct and works so well, the inverted-V is the most commonly used dipole. It is a myth that a horizontal antenna orientation makes a difference on 80m at heights used by most amateurs. I have heard many amateurs say on 80m, "The reason my signal is weak to you is because you are off the end of my dipole". The radiation pattern from a dipole is essentially non-directional until the dipole is elevated more than a half wave, that is about 40m (125feet) on 80m, and it is 20m (65feet) on 40m. The main reason it makes no difference regarding orientation is because propagation for signals closer than 800km (500miles), the distance of most 80m contacts, is essentially by high angle radiation nearly straight up and down. Only signals radiated and received at low angles make a difference in antenna orientation even at low heights above ground. At low heights, there are nulls about 3 to 4 dB off the dipole ends.
Most dipoles consist of two pieces of wire of equal lengths with one of the two ends connected together through an insulator. The far ends of the wires are also connected to insulators. The two conductors of a feed-line are separated and connected across the gap at the center insulator. The antenna is held up by rope that connects the insulated ends of the antenna to two supports. It is a "balanced" antenna, because equal currents flow on both halves of the antenna. Coax is an unbalanced feed-line. The dipole that is stretched between two high supports is called a flattop dipole. The simplest antenna system of all is the ½-wave resonant dipole fed with coax and no tuner. The only reason for using a ½-wave resonant dipole antenna is to eliminate the need for a matching device such as a tuner. The feed-point impedance will be near 50Ω at ordinary heights and they can be fed directly with 50Ω coax from the output of the tranceiver. The two halves of a dipole are fed 180 degrees o
ut of phase, meaning when one side is fed positively, the other side is fed negatively. That is why a feed-line has two conductors. Of course, the sides swap polarity on each half cycle. If you could visualize the current flowing on the ½-wave dipole, the current will appear to be standing still. The maximum current will be seen at the center of the wire and no current will be at the ends. This occurs because the electrons flowing out to the ends reflect back toward the center where they meet the next wave and the current is reinforced there. The minimum voltage occurs at the center and the maximum voltage occurs at the ends of the half-wave resonant dipole. If you were to measure the voltage and the current at any point on the dipole wire, the voltage times the current will equal the power in Watts.Inverted-V Dipole..
Another configuration for the ½-wave resonant dipole is one having one support in the center and the ends stretched down toward the ground. The single support can be a tree, mast, or tower. The ends of a dipole have high RF voltages on them, and need to be at least 3m (10feet) above ground. This antenna is called an "inverted-V", because the shape of the dipole looks like a "V" turned upside down. This configuration works well because the current is concentrated on the middle two-thirds of the antenna at the apex. The current in an antenna is what is responsible for the radiation. The ends of the antenna have very little current in them and it doesn’t matter if the ends are close to the ground. The middle of the antenna is up high where the radiation is taking place and that is the place you want the radiation to be. An inverted-V has an advantage that the horizontal space required for it is less than what is needed for a flattop dipole. The angle between the wires on an inverted-V needs to be greater than 90 degrees. The gain of an inverted-V is 0.2 dBd and it has a radiation pattern nearly omni-directional. Since it is easy to construct and works so well, the inverted-V is the most commonly used dipole. It is a myth that a horizontal antenna orientation makes a difference on 80m at heights used by most amateurs. I have heard many amateurs say on 80m, "The reason my signal is weak to you is because you are off the end of my dipole". The radiation pattern from a dipole is essentially non-directional until the dipole is elevated more than a half wave, that is about 40m (125feet) on 80m, and it is 20m (65feet) on 40m. The main reason it makes no difference regarding orientation is because propagation for signals closer than 800km (500miles), the distance of most 80m contacts, is essentially by high angle radiation nearly straight up and down. Only signals radiated and received at low angles make a difference in antenna orientation even at low heights above ground. At low heights, there are nulls about 3 to 4 dB off the dipole ends.
0 reacties:
Een reactie posten