.net barcode reader library Stacked dipoles in Software

Encoder Data Matrix in Software Stacked dipoles

Stacked dipoles
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Figure 6-16 shows a double dipole (ie, two half-wavelength dipole antennas spaced a half-wavelength apart) The transmission lines are connected in parallel at the receiver This antenna provides about 3-dB gain over a single dipole, and it adds a bit of fade protection because two side-by-side antennas provide a bit of space diversity (see the end of Chap 2 for information on diversity reception)
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Other dipoles 161
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10 K 9 8 7 6 5 4 10% Inductive reactance (XL) of coil (ohms) 3
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Coil location as a percentage of element length
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6-14 Inductive reactance versus coil location for shortened inductance-loaded dipole
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162 High-frequency dipole and other doublet antennas
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R = rope I = insulator Coax to RIG or RCVR
DC motor
L' = 505/FMHZ 6-15 Electrically reducing the length of a dipole antenna by placing a capacitor at the feedpoint 1:1 BALUN 1:1 BALUN
coax
Box (see text)
Rope Insulator
XMTR 6-16 Phased dipoles provide a 3-dB gain over single dipole
The two dipoles are supported by a common structure consisting of ropes (R) and end insulators (I) to support masts on the ends In the center, a half-wavelength space is taken up by a rope so that the structure is maintained The space is determined by 492 Spacefeet = [623] FMHz and, for the dipoles, Lengthfeet = 468 FMHz [624]
The stacked dipole gets a bit lengthy on low-frequency bands, but is easily achievable by most people on the upper HF bands
Off-center-fed full-wave doublet (OCFD) antennas 163 The feedlines for the two dipoles can be connected directly in parallel and fed from the transmitter, provided that the antenna tuning unit will support one-half the normal expected impedance Alternatively, a box can be provided that includes a matching transformer for 1:2 ratio These can be built like a balun transformer on a toroidal core A trifilar winding is used Alternatively, a phasing box can be built that will allow altering the directionality of the antenna by 90 This is done by using the switching circuit to reverse the sense of L3
Off-center-fed full-wave doublet (OCFD) antennas
An antenna that superficially resembles the Windom is the off-center-fed doublet (OCFD) antenna of Fig 6-17 It is a single-band antenna, although at harmonics it will begin to act as a resonant, standing wave, longwire antenna The overall length is one wavelength long: 936 Lfeet = [625] FMHz This antenna works best at heights of at least /2 above ground, so practical considerations limit it to frequencies above about 10 MHz (ie, 30-m band) The feedpoint of the antenna is placed at a distance of /4 from one end, and is a good match for 75- coaxial cable A 1:1 balun transformer at the feedpoint is highly recommended The pattern of a 1 antenna is a four-lobe cloverleaf, with the major lobes being about 53 from the wire The gain is about 1 dB
2 Rope EI 1:1 BALUN
2 EI Rope
coax
6-17 Off-center-fed 1-wire antenna
164 High-frequency dipole and other doublet antennas
Off-center-fed nonresonant sloper (OCFS)
Perhaps more viable for many people is the nonresonant off-center-fed sloper (OCFS) antenna of Fig 6-18 This antenna consists of a wire radiator that must be longer than 3 /2 at the lowest frequency of operation The feedpoint is elevated at least /4 above ground at the lowest operating frequency The antenna is fed with 75- coaxial cable The shield of the coax is connected to a /4 resonant radial (counterpoise ground) There should be at least one radial (more is better) per band of operation The far end of the radiator element is sloped to ground, where it is terminated in a 270- noninductive resistor The resistor should be able to dissipate up to onethird of the power level applied by the transmitter
Double extended Zepp antenna
The double extended Zepp antenna (Fig 6-19) provides a gain of about 2 dB over a dipole at right angles to the antenna wire plane It consists of two sections of wire, each one of a length 600 L1feet = [626] FMHz
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