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4. Antenna Coupling Techniques: Part 1.
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>>>> Work in progress Introduction: It is not the purpose of this chapter to provide a comprehensive coverage of the subject of radio antennas. A vast literature already exists, and interested parties will sensibly begin by acquainting themselves with the ARRL Antenna Book [1] and various other excellent publications. The intention here, instead, is to look at some of the various ways in which power can be delivered from a transmitter to a radiating system; the objective being to address certain myths and misconceptions which prevail and are even incorporated into some examination sylabuses. The first point which must be understood is that an antenna either is, or must be reduced to, a two-terminal network before it can be connected to a radio transmitter. That is one of the reasons why we use the term 'antenna' rather than 'aerial'; because an antenna may have an aerial (up-in-the-air) part, but the aerial is not necessarily the whole of the antenna. The common usage of the word 'aerial' is also frequently ridiculous; such as in describing the loop-stick antenna of a portable radio receiver as a "ferrite-rod aerial". It might be considered to be an aerial if the receiver is being used by a builder while he points a chimney-stack, but down in the kitchen, it's definitely an antenna. In Biology, an antenna is a protruding sensory organ. In Radio, by analogy (coined in 1902), an antenna is a structure used for sensing electromagnetic (EM) radiation; and, by another ill-understood principle, that of reciprocity; something that receives EM radiation can also radiate it. An old-fashioned Marconi installation has an aerial and an earth; but it is the combination of the aerial and the earth that constitutes the antenna in that case. Because there is a certain ambiguity in the concept 'antenna' (is it the antenna, or is it just an aerial?) we often use the somewhat pedantic term 'antenna system'. The redundancy is useful, because it conveys the idea that an antenna is a collection of conductors (and also dielectric materials) that do not have to be connected in the DC sense. In this way we guard against the first misconception; which is that a structure designated as an antenna is somehow isolated from all of the other objects in its vicinity. This might be a reasonable simplification in the case of a VHF antenna, when all of the nearby objects that are orientated in such a way as to scatter its radiation are many wavelengths away; but it is usually a bad assumption when interpreting the behaviour of antennas used for short-wave and lower-frequency communications. The presumption that a designated antenna is isolated from its environment can sometimes lead to severe errors in the evaluation of performance. The classic example of a bad antenna test is that of making A to B comparisons between two 'antennas' erected on the same site. It might be found, by this method, that an electrically-short antenna is only marginally inferior to a half-wave dipole antenna; and this is the basis on which certain small antenna designs purport to have exploited some loophole in the theory governing efficiency. What the test really shows is that it is slightly better to connect directly to the half-wave wire than to send energy to it by other means. Take away the large antenna, and it will be found that the small one conforms to the theory after all. The two alleged 'antennas' are in fact a single antenna system, with options in respect of where to connect the wires from the transmitter. In truth however, many electrically small antennas are only marginally inferior to a resonant structure, and it is a shame to spoil the case by offering invalid experimental data. >>>>>. |
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>>> reciprocity. S-points. Why can't you transmit into a ferrite rod. Definitions: Near (induction) field. Far (radiation) field. Resonance. Choosing the feedpoint. Impedance and EMC considerations. The role of the balun transformer. Bad folklore from bad materials science (from people who should know better). Input impedance of a dipole (doublet). (on Log and linear freq. scales) Getting started with NEC. ("EZ" pronounced "easy". Exporting data to spreadsheet) >> Resonance dictated by total conductor electrical length on the antenna side of any chosen pair of terminals. LCR models for antennas. Parameter extraction. Can consider the feedline as part of the antenna: Wire length (electrical) dictates the resonances. Forming part of that length into a transmission line reduces the radiation resistance, but does not greatly alter resonances (esp. if velocity factor of line is taken into account.). Can also consider line to be separate from antenna. Line becomes an impedance transformer. Overall effect is the same as considering the line to be part of the antenna. Transmission Lines: Surge resistance R0 = √L/C Ideal TL equation. Properties of λ/2 and λ/4 lines. (Theory of reflection coeff, SWR, return loss. - moved to CH 6.) (Basics of reflectometry - moved to CH 6.) Loss mechanisms. Resistivity, skin effect, dielectric loss. Complex surge impedance (by analogy with complex permittivity to represent dielectric loss and complex permeability to represent magnetic loss). General TL equation. (intro to Cosh and Sinh functions). Properties of the TL equation. (Extracting the loss resistance component, good SWR bandwidth is a sign of excessive loss) Transmission line models for antennas. The vacuum is a transmission line. Parameter extraction. Radiation resistance vs Loss resistance, efficiency. Multiband and broadband antenna systems. Why magic lengths of twin-line are unsatisfactory. Traps are useful for controlling radiation pattern, but are lossy and enhance harmonic radiation. Parallel connected antennas. Double-tuning systems for increasing SWR bandwidth (might make a nice worked example for loaded verticals in Ch 1). General coverage: Antennas don't need to be resonant. Feedlines do not have to have 1:1 SWR. (linear distortion effects) nothing wrong in using electrically-short antennas. Short antennas for 160m and 80m. Why you don't need an 80m long garden to work successfully on 160m. Awkward impedances General qualitative ATU efficiency issues. >> High efficiency is not mandatory for low power installations. ERP and receiver sensitivity are the real issues. . |
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Model: g5rv.ez . Model: zs6bkw.ez . Model: 80m_dipole.ez . 5.7dBi. Model: 80m_prox.ez . Dipole in free space: Model: freespace.ez . Z data: freespace.ods . . |
© D W Knight 2009.
David Knight asserts the right to be recognised as the author of this work.
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