490 MHz Crossed Dipoles (Interconnected with One Feedline), analyzed using 4nec2 after Dimensions determined using nikiml's Python Optimization Scripts.
Azimuthal Pattern for Horizontal Polarization is a TRUE OMNI with about -1 dBi Gain (+/- 1 dB) and very low SWR (Under 1.03). Horizontal Gain was only slightly reduced for moderate to large negative Elevation Angles, hence Look Down Angle coverage is Very Good.
Azimuthal Pattern for Right Hand Circular Polarization (RHCP) was not quite a True Omni with about -3 dbc Gain (+/- 1.5 dB) and very low SWR (Under 1.03). Note that as shown, RHCP covers LOWER Hemisphere. To change so RHCP covers UPPER Hemisphere, reverse Feedlines to Upper Dipole.
For about 3 dB more Gain, two Vertically Stacked Crossed Dipoles could be used. And for about ANOTHER 3 dB more Gain a Turnstile with FOUR Crossed Dipoles could be used.
SUMMARY OF DIMENSIONS: [From 4nec2 File] Relem=5 ' Element Radius (millimeters) VF1=1.00 ' Feedline Velocity Factor [OPEN LADDER] F=0.996 ' RESCALE FACTOR [487 TO 490 MHZ] HL=F*125=124.5 ' Half-Length of Dipoles (millimeters) (Max Y-axis Coordinates) Zdd=20 ' Z-Axis Dipole-to-Dipoles Separation L1=F*28.5=28.4 ' Feedine PHYSICAL Length (IN MM) Between Dipoles FL1=VF1*L1/1000=0.0284 ' Feedline Electrical Length (IN METERS) between Dipoles Zc1=342 ' Characteristic Impedance (ohms) of Feedline Between Dipoles
Resultant Optimization favored the lowest SWR, rather than Uniform Gain in all directions. Hence I applied a small RESCALE FACTOR to recenter "Best" performance from 487 to 490 MHz.
Due to short length (28.4-mm), construction of the Interconnecting Feedline is NOT critical, with 0.75-in separation between AWG10 or AWG12 or AWG14 working equally well.
3 May 201639 items2351 visitsAlbum by holl_ands
enlarge 54KB, 1024x840 1 490MHz Crossed-Dipoles + One TL 3D View
enlarge 102KB, 1152x332 2 490MHz Crossed-Dipoles + One TL Front View [1 large square = 25-mm]
enlarge 285KB, 1019x1024 3 490MHz Crossed-Dipoles + One TL Top View [1 large square = 25-mm]
enlarge 182KB, 1152x703 4 490MHz Crossed-Dipoles + One TL UHF Raw Gain from 486 to 494 MHz
enlarge 196KB, 1152x703 5 490MHz Crossed-Dipoles + One TL UHF SWR (50-ohms) from 486 to 494 MHz Under 1.03
enlarge 234KB, 1152x703 6 490MHz Crossed-Dipoles + One TL UHF Impedance from 486 to 494 MHz
enlarge 215KB, 1152x703 7 490MHz Crossed-Dipoles + One TL UHF Raw Gain is Wideband
enlarge 235KB, 1152x703 8 490MHz Crossed-Dipoles + One TL UHF SWR (50-ohms) is Wideband
