Hi-VHF 7-Element Log Periodic Dipole Array (LPDA) with (and without) Shorting Stub analyzed using 4nec2. Without the Shorting Stub, performance was significantly degraded.
Hi-VHF Raw Gain = 9.1 to 9.5 to 8.4 dBi, F/B & F/R Ratio MIN = 20 dB and SWR (75-ohms) under 2.3.
Dimensions calculated by LPCAD34, entering Boom Length (48-in), Number of Elements (7), Rear [= ALL] Element Diameters (3/8-in) and High/Low Frequency Rolloffs (174/216) (see 4nec2 File). [Other dimensions will generate different results.] Note that Zig-Zag Transmission Line (TL Card) model does NOT stipulate any Boom Crossection size or shape.
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 (GW8), resulting in a strange attachment off to the side of the front element. I added a definition of GW8 to the 4nec2 file, with the stub pointed directly upwards from the rear element to facilitate varying the length and provide easy 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 0.5-15-in, 0.53-in was optimum.
Looking at the charts, it was determined that a more optimized performance was possible if the size were SMALLER by F=0.98, which reduced SWR and improved F/B & F/R Rations on the highest freq. Apply this correction to all dimensions in the 4nec2 file. It is already included in below diagrams.
Below LPDA_hiv.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 - 15 of 15 Total. 3898 Visits.
enlarge 79KB, 1024x966 1 Hi-VHF 7-El LPDA w Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements 3D View
enlarge 200KB, 1024x720 2 Hi-VHF 7-El LPDA w Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Top View [1 big box = 5-inches]
enlarge 60KB, 1152x205 3 Hi-VHF 7-El LPDA w Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Side View [1 big box = 2.5-inches]
enlarge 146KB, 1152x584 4 Hi-VHF 7-El LPDA w Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Hi-VHF Raw Gain = 9.1 to 9.5 to 8.4 dBi Hi-VHF F/B & F/R Ratio MIN = 20 dB
enlarge 50KB, 1152x144 12 Hi-VHF 7-El LPDA w Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Front View [1 big box = 2.5-inches]
enlarge 181KB, 1152x584 13 Hi-VHF 7-El LPDA w NO Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Hi-VHF Raw Gain Loss, esp High Freqs F/B & F/R Ratio are Degraded, esp. High Freqs
enlarge 186KB, 1152x584 14 Hi-VHF 7-El LPDA w NO Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Hi-VHF SWR (75-ohms)
enlarge 153KB, 1152x584 15 Hi-VHF 7-El LPDA w NO Shorting Stub LPCAD34 Calc: 48-in Boom, 7-Elements Hi-VHF Impedance
