UHF 8-Element "VEE" LPDA (Log Periodic Dipole Array) with Shorting Stub and Variable Sweep Angle, Optimized using nikiml's Python Scripts, then analyzed using 4nec2. Note that Shorting Stub orientation can be above, behind, below or even ALONG the boom. See 4nec2 File for Dimensions: about 16-in Wide x 12-in Wide.
Raw Gain of 8.6 dBi (+/- 0.2 dB) was VERY FLAT, about 1 dB more than irregular Raw Gain in VA3RR's 8-El LPDA (used in oldsparks build and GHZ24 Parabolic designs). F/B & F/R Ratio Minimum = 27.2 dB and SWR (75-ohms) under 1.4 were also improved compared to VA3RR's, although both had excellent SWR.
Note that this design ASSUMES the use of a "Shorting Stub" at the back of the Antenna (0.68-inch long). If this is omitted, there will be a severe problems in Raw Gain and SWR. A design without the Shorting Stub would require a different optimization.
"VEE" LPDA myth would lead us to believe that a forward sweep in the range of 30-45 degrees would provide the best performance. HOWEVER, in the several runs I've conducted for 7-El & 8-El LPDAs with and without Shorting Stubs and various optimization parameters, the "best" angle was found to be in the range of +/- 4 degrees....very close to a non-swept LPDA. I was surprised to see a Sweep Angle of MINUS 2.7 in the Optimized LPDA...slightly towards the REAR.
L.B. Cebik discussed this myth in the fol. white paper: http://w4rnl.net46.net/download/v1.pdf Equations, Charts and other info re LPDA found here: http://www.salsburg.com/Log-Periodic.pdf
In search of the optimum Forward Sweep Angle in a "VEE" LPDA, I constructed a 4nec2 file with SIX VARIABLES that could be Randomly searched using nikiml's Python Optimization Scripts to find the "best" combination of Forward Sweep Angle (from Y-Axis), Tau =Li/Li+1 = Dij/Djk, Sigma = Dij/2*Lj, Feedline Impedance, Length of Longest Element (determined by Lowest Design Freq) and Length of the Shorting Stub (which I found improved SWR and esp. Gain smoothness).
It was NOT necessary to stipulate either the Boom Length or the Highest Design Freq, the above parameters providing a complete description of the LPDA, relying on Python Optimization to do a "best fit" against desired Frequency Response stipulated in *.bat file.
Date(s): 12 Jun 2013. Album by holl_ands. 1 - 15 of 15 Total. 2573 Visits.
enlarge 81KB, 1024x775 1 UHF 8-El VEE LPDA with Shorting Stub - Optimized 3D View
enlarge 255KB, 1024x809 2 UHF 8-El VEE LPDA w Shorting Stub - Optimized Top View - Note 2.7-deg REARWARD Sweep [1 large square = 1.25 inches]
enlarge 83KB, 1152x282 3 UHF 8-El VEE LPDA w Shorting Stub - Optimized Front View Optimum Shorting Stub is only 0.68-in Long. [1 large square = 1.25 inches]
enlarge 189KB, 1152x678 4 UHF 8-El VEE LPDA with Shorting Stub - Optimized UHF Raw Gain = 8.4 to 8.8 dBi UHF F/B & F/R Ratio Minimum = 27.2 dB
enlarge 210KB, 1152x678 5 UHF 8-El VEE LPDA with Shorting Stub - Optimized UHF SWR (75-ohms) is Under 1.4
enlarge 232KB, 1152x678 6 UHF 8-El VEE LPDA with Shorting Stub - Optimized USB Impedance
enlarge 300KB, 1024x1024 7 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 470 MHz
enlarge 299KB, 1024x1024 8 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 530 MHz
enlarge 300KB, 1024x1024 9 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 590 MHz
enlarge 299KB, 1024x1024 10 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 650 MHz
enlarge 300KB, 1024x1024 11 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 698 MHz
enlarge 303KB, 1024x1024 12 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 758 MHz
enlarge 304KB, 1024x1024 13 UHF 8-El VEE LPDA with Shorting Stub - Optimized Azimuthal Pattern at 806 MHz
enlarge 36KB, 791x1024 14 UHF 8-El VEE LPDA with Shorting Stub - Optimized 4nec2 File
enlarge 22KB, 791x1024 15 UHF 8-El VEE LPDA with Shorting Stub - Optimized Contents of *.bat (Text) File
