UHF 9-Element Log Periodic Dipole Array (LPDA) with (and without) Shorting Stub analyzed using 4nec2. Without the Shorting Stub, performance was significantly degraded.
UHF Raw Gain = 9.0 to 9.2 to 8.6 to 8.8 to 8.0 dBi, F/B & F/R Ratios Under 20 dB and SWR (75-ohms) Under 1.8.
Dimensions calculated by LPCAD34, entering Boom Length (18-in), Number of Elements (9), Rear Element Diameter (AWG10) and High/Low Frequency Rolloffs (470/698) (see 4nec2 File). [Other dimensions will generate different results.] Characteristic Impedance = 75-ohms. LPCAD34 could not calculate a solution for 300-ohms.
LPCAD34 generated a 4nec2 file, but the Shorting Stub was missing the Shorting Wire (GW10), resulting in a strange attachment off to the side of the front element. I added a definition of GW10 to the 4nec2 file, with the stub pointed directly upwards from the rear element to facilitate varying the length and provide better visibility.
Since LPCAD34 did NOT calculate the requisite Shorting Stub Length, I defined "SLL" as a SYmbol variable and performed an optimization using nikiml's python scripts. Over range of 0.1-5-in, 1.04-in was optimum.
Looking at the charts, it was determined that a more optimized performance was possible if the size were SMALLER by F=0.97, which reduced SWR and improved F/B & F/R Rations on the highest freqs. Apply this correction to all dimensions in the 4nec2 file. It is already applied to below diagrams.
Below LPDA_uhf.doc contains *.bat file for running the scripts, where it was necessary to DISABLE Autosegmentation (OFF=0), since nikiml's scripts did not properly keep the Transmission Lines (TL Cards) on the middle segments when autosegmenting.
Note that the target-function is a joint optimization of Mismatch-Loss, Gain and Front-To-Rear Ratio.
To increase Gain on higher frequencies, a new design with a 10 to 30-percent larger high frequency rolloff parameter (FH) should be investigated.
Date(s): 20 Dec 2011. Album by holl_ands. 1 - 14 of 14 Total. 3944 Visits.
enlarge 93KB, 1024x912 1 UHF 9-El LPDA w Shorting Stub - LPCAD34 3D View
enlarge 245KB, 1024x724 2 UHF 9-El LPDA w Shorting Stub - LPCAD34 Top View [1 big box = 1.25-inches]
enlarge 69KB, 1152x197 3 UHF 9-El LPDA w Shorting Stub - LPCAD34 Side View [1 big box = 1.25-inches]
enlarge 140KB, 1152x584 4 UHF 9-El LPDA w Shorting Stub - LPCAD34 UHF Raw Gain = 9.0 to 9.2 to 8.6 to 8.8 to 8.0 dBi UHF F/B & F/R Ratios Under 20 dB
enlarge 164KB, 1152x584 5 UHF 9-El LPDA w Shorting Stub - LPCAD34 UHF SWR (75-ohms) under 1.8
