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The delta loop antenna, like the Greek uppercase letter delta ( ) from which it draws its name, is triangle-shaped (Fig 14-8) The delta loop is a full wavelength, with elements approximately 2 percent longer than the natural wavelength (like the quad) The actual length will be a function of the proximity and nature of the underlying ground, so some experimentation is necessary The approximate preadjustment lengths of the sides are found from: L1 L2 437 ft FMHz 296 L3 FMHz [148] [147]
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The delta loop antenna is fed from 52- coaxial cable through a 4:1 balun transformer The delta loop can be built in a fixed location, and will offer a bidirectional pattern
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L1 = 437 Feet FMHz L2 = L3 = 296 Feet FMHz
4:1 BALUN
14-8 Delta loop antenna
Half-delta sloper (HDS)
The half-delta sloper (HDS) antenna (Fig 14-9) is similar to the full delta loop, except that (like the quarter-wavelength vertical) half of the antenna is in the form of an image in the ground Gains of 15 to 2 dB are achievable The HDS antenna consists of two elements: a /3-wavelength sloping wire and a /6 vertical wire (on an insulated mast), or a /6 metal mast Because the ground currents are very important, much like the vertical antenna, either an extensive radial system at both ends is needed, or a base ground return wire (buried) must be provided The HDS will work on its design frequency, plus harmonics of the design frequency For a fundamental frequency of 5 MHz, a vertical segment of 33 ft and a sloping section of 66 ft is needed The lengths for any frequency are found from 328 3 FMHz [149]
296 Large wire loop antennas and 164 FMHz [1410]
The HDS is fed at one corner, close to the ground If only the fundamental frequency is desired, then you can feed it with 52- coaxial cable But at harmonics, the feedpoint impedance changes to as high as 1000 If harmonic operation is intended, then an antenna tuning unit (ATU) is needed at point A to match these impedances
Bisquare loop antenna
The bisquare antenna, shown in Fig 14-10, is similar to the other large loops, except that it is /2 on each side, making a total wire length of two wavelengths This antenna is built like the diamond loop shown earlier (ie, it is a large square loop fed at an apex that is set at the bottom of the assembly) In this case, the loop is fed either with an antenna tuning unit (to match a 1000- impedance) or a quarter-wavelength matching section made of 300- or 450- twin-lead transmission line A 1:1 balun transformer connects the 75- coaxial cable to the matching section
Good electrical and mechanical connection
3 6 Metal mast or wire
Earth
52 Coax
Buried ground return wire (bare)
14-9 Half-delta loop antenna
Bisquare loop antenna 297
Each side is /2 2 L side = 480 FMHz
/ Matching section 44
14-10 Bisquare 2 antenna
square loop
1:1 BALUN transformer
52 Coaxial cable to receiver
The bisquare antenna offers as much as 4-dB gain broadside to the plane of the antenna (ie, in and out of the book page), in a figure-8 pattern, on the design frequency It is horizontally polarized When the frequency drops to one-half of the design frequency, the gain drops to about 2 dB, and the antenna works like the diamond loop covered previously
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Small loop receiving antennas
Radio direction finders and people who listen to the AM broadcasting bands, VLF, medium-wave, or the so-called low-frequency tropical bands are all candidates for a small loop antenna These antennas are fundamentally different from large loops and other sorts of antennas used in these bands Large loop antennas have a length of at least 05 , and most are quite a bit larger than 05 Small loop antennas, on the other hand, have an overall length that is less than 022 , with most being less than 010 The small loop antenna responds to the magnetic field component of the electromagnetic wave instead of the electrical field component One principal difference between the large loop and the small loop is found when examining the RF currents induced in a loop when a signal intercepts it In a large loop, the current will vary from one point in the conductor to another, with voltage varying out of phase with the current In the small loop antenna, the current is the same throughout the entire loop The differences between small loops and large loops show up in some interesting ways, but perhaps the most striking is the directions of maximum response the main lobes and the directions of the nulls Both types of loops produce figure-8 patterns but in directions at right angles with respect to each other The large loop antenna produces main lobes orthogonal, at right angles or broadside, to the plane of the loop Nulls are off the sides of the loop The small loop, however, is exactly the opposite: The main lobes are off the sides of the loop (in the direction of the loop plane), and the nulls are broadside to the loop plane (Fig 15-1A) Do not confuse small loop behavior with the behavior of the loopstick antenna Loopstick antennas are made of coils of wire wound on a ferrite or powdered-iron rod The direction of maximum response for the loopstick antenna is broadside to the rod, with deep nulls off the ends (Fig 15-1B) Both loopsticks and small wire loops are used for radio direction-finding and for shortwave, low-frequency medium-wave, AM broadcast band, and VLF listening The nulls of a loop antenna are very sharp and very deep Small changes of pointing direction can make a profound difference in the response of the antenna If you point a loop antenna so that its null is aimed at a strong station, the signal strength of
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