UHF 7-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.
Raw Gain of 8 dBi (+/- 0.2 dB) was VERY FLAT, slightly more than irregular Raw Gain in VA3RR's 8-El LPDA. F/B & F/R Ratios were also improved compared to VA3RR's. Both had excellent SWR.
Note that this design ASSUMES the use of a "Shorting Stub" at the back of the Antenna (0.609-inch long). If this is omitted, there will be a severe mid-band Raw Gain HOLE and a mid-band SPIKE in the 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 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 3.9 in the Optimized LPDA...slightly towards the REAR.
I also conducted optimization runs with Angle fixed at 15, 30, 40 & 45 degrees...none were better.
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.
BTW: Initially I tried an optimization matched to 300-ohms, but the SWR was somewhat higher than desired (3.3:1), with a slightly better match to 75-ohms, with the resultant design best matched to 150-ohms. So 300-ohm LPDA is possible, but trying to match impedance degrades trying to maximize Gain, F/R & F/B Ratios. So LPDA's are best when matched to 75-ohms.
I also tried optimizations which didn't try to also search for best F/B & F/R Ratios. Raw Gain was nearly 1 dB higher, but F/B & F/R Ratios had dips to only about 6 dB, so it's not reported here (just like LPCAD34's 7-El LPDA analyzed is a separate album).
EDIT (13Jun2013): Filename was mislabeled...results are for 7-Element LPDA and NOT 8-Element, making the performance improvement even more impressive.....
Date(s): 13 Jun 2013. Album by holl_ands. 1 - 19 of 19 Total. 2912 Visits.
enlarge 77KB, 1024x731 1 UHF 7-El VEE LPDA w Stub - Optimized 3D View
enlarge 289KB, 1024x956 2 UHF 7-El VEE LPDA w Stub - Optimized Top View - Note 3.9-deg REARWARD Sweep [1 large square = 1.25 inches]
enlarge 77KB, 1152x265 3 UHF 7-El VEE LPDA w Stub - Optimized Front View Optimum Shorting Stub is only 0.609-in Long. [1 large square = 1.25 inches]
enlarge 183KB, 1152x678 4 UHF 7-El VEE LPDA w Stub - Optimized UHF Raw Gain = 8.0 dBi (+/= 0.1 dB) Very Flat Frequency Response F/B & F/R Min = 26 dB is EXCEPTIONAL
enlarge 193KB, 1152x678 5 UHF 7-El VEE LPDA w Stub - Optimized UHF SWR (75-ohms) Under 1.6 is Exceptional
enlarge 234KB, 1152x678 6 UHF 7-El VEE LPDA w Stub - Optimized UHF Impedance
enlarge 301KB, 1024x1024 7 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 470 MHz
enlarge 300KB, 1024x1024 8 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 530 MHz
enlarge 300KB, 1024x1024 9 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 590 MHz
enlarge 300KB, 1024x1024 10 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 650 MHz
enlarge 302KB, 1024x1024 11 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 698 MHz
enlarge 303KB, 1024x1024 12 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 758 MHz
enlarge 304KB, 1024x1024 13 UHF 7-El VEE LPDA w Stub - Optimized Azimuthal Pattern at 806 MHz
enlarge 32KB, 791x1024 14 UHF 7-El VEE LPDA w Stub - Optimized 4nec2 File
enlarge 22KB, 791x1024 15 UHF 7-El VEE LPDA with Shorting Stub - Optimized Contents of *.bat (Text) File
enlarge 185KB, 1152x678 16 UHF 7-El VEE LPDA - IF SHORTING STUB MISSING UHF Raw Gain mid-band HOLE in Response
enlarge 212KB, 1152x678 17 UHF 7-El VEE LPDA - IF SHORTING STUB MISSING UHF SWR (75-ohms) mid-band SPIKE in Response
enlarge 232KB, 1152x678 18 UHF 7-El VEE LPDA - IF SHORTING STUB MISSING UHF Impedance
enlarge 76KB, 1152x264 19 UHF 7-El VEE LPDA w Stub - Optimized Side View [1 large square = 1.25 inches]
