Dipole shape variations..The wire of a dipole doesn’t have to be run in a straight line. A dipole doesn’t have to be perfectly horizontal. That’s the way it is usually depicted in books and magazin
es, but you can bend the legs of the antenna up, down or sideways. If you make either wire one-half wavelength long and carefully prune it to resonance, you can use it without a tuner on and near its resonant frequency. Both antennas have the current part at the top where most of the radiation takes place. The vertical parts of these antennas radiate a weak vertically polarized wave. The only reason these dipoles are contorted this way is to make them full-sized and to fit in the available space. Other shapes are possible, and you can be creative at your location. There are many more dipoles than the ones just described.
Calculating the length of a ½-wave Resonant Dipole..The
approximate length in meters of a ½-wave resonant dipole is found by:
142.5 / freq MHz = length in meters The approximate length in feet of a ½-wave resonant dipole is found by:
468 / freq MHz = length in feet
The actual length of it will be determined by several factors. Using larger diameter wire will make the dipole resonate lower in frequency. Therefore, to make it resonant at the higher desired frequency, it must be shortened. Raising a dipole higher above ground will make it resonate higher in frequency. An insulated wire will make the dipole resonate lower in frequency than a bare wire. Using the above formulas, cut the antenna a little longer than the calculations say. If the SWR is best at a lower frequency than you desire, the antenna will have to be made shorter by pulling the excess wire through the end insulators, folding the ends of the extra wire back on itself. Then wrap the ends of the overlapped wire on itself so it wont come loose. This causes the excess wire to "short" itself to the rest of the antenna. If you are using insulated wire, you will need to cut off the excess wire. The reverse is true if the antenna resonates too high in frequency. The extra wire can be let out to make it resonate on a lower frequency. This is why you originally cut the wire a little longer.
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.
Dit soort antenne dat reeds vele jaren zijn toepassing kent in talrijke ontvangers, bestaat meestal uit een gewone staafantenne met daaraan gekoppeld een breedbandversterker. Het is bewezen dat een staafantenne van 0,7m tot 2,5m, bijna evenveel signaal opvangt als een halve golf antenne voor die frequentie. In de praktijk komt men niet tot die resultaten, omdat de energie aan die korte antenne moeilijk kan worden onttrokken. Dit wordt dan anders opgelost.
Men verbindt de korte antenne (0,7m tot 2m) met de GATE van een FET. Hoe korter de verbinding hoe beter. Hoe kleiner de ingangscapaciteit, hoe beter de signaal-ruisverhouding.
Wat zijn nu de resultaten?
Een actieve antenne geeft minstens evengoede resultaten als een antenne van normale grote. De signaal-ruisverhouding kan op de laagste banden tot 40db beter zijn dan een langedraad antenne of zelfs een afgestemde dipool. Waarom dit verschil? Door de kleine afmeting van de actieve antenne, kan men deze in een storingvrije zone opstellen. Neem zelf de proef op de som. Neem een gewone actieve autoantenne voor de lange, midden, korte en de FM-band. Nu verlengen we de antenne tot ongeveer 1m. Stel die antenne op een storingvrije, zo hoog mogelijke plaats op. Verbind de antenne met een 50Ω coax naar een hf-ontvanger. Heeft U te weinig selektiviteit of ondervindt U storingen van andere stations, dan kan U een PI-filter plaatsen tussen antenne en ontvanger. Dan gaan we al in de richting van de afstembare voorversterker. De voordelen van een goede actieve antenne zijn: -klein van formaat, -goed rendement, -gebruik van coax vermindert de storingen, -extra versterking van het signaal mogelijk, -zeer geschikt op frequenties beneden de 4Mhz. Nadelen zijn kruismodulatie, kans tot oversturing van lokale stations en er is een voedingsspanning nodig. Actieve antennes zijn bruikbaar van 150Khz tot 30Mhz. Ikzelf gebruik de MFJ-1024 actieve antenne al enkele jaren met goede DX-resultaten op 80 en 160m.
es, but you can bend the legs of the antenna up, down or sideways. If you make either wire one-half wavelength long and carefully prune it to resonance, you can use it without a tuner on and near its resonant frequency. Both antennas have the current part at the top where most of the radiation takes place. The vertical parts of these antennas radiate a weak vertically polarized wave. The only reason these dipoles are contorted this way is to make them full-sized and to fit in the available space. Other shapes are possible, and you can be creative at your location. There are many more dipoles than the ones just described.
