Achieving a low-SWR with an LPDA (or even analyzing) can be a bit of a challenge.
Running 4nec2 with TL Cards simulating a Zig-Zag feedine (like W-G antennas's) means you
ASS-U-ME that the real antenna will actuallly have the impedance in the TL Cards....
But probably not, esp if using a nearby metal boom....4nec2 can investigate sensitivity...
One could TRY to model the metal boom and zig-zag feelines using many-many wire
statements, but based on my experience, I think the quad-precision NEC4 engine is
needed to handle the mix of big & tiny wires, which apparently cause problems using
the double-precision NEC2 engine.....
Impedance of a Twin-Boom LPDA can be analyzed in LOGPERIO3.XLS spread sheet:
Here are some Hi-VHF & UHF examples I pulled from my archive:
Here's an alternative, very similar spread sheet calculator:
[On my system, GoogleDocs allows you to "Choose another application"....]
Note that at the highest frequencies, only the forward most elements provide significant
contribution to the overall Gain. Hence it is important to pick a high-frequency design
frequency that is 10-20 percent HIGHER than the max operating frequency.
The fol. LPDA Analysis addresses not only Impedance, but also optimization
and the use of the (optional) "matching stub" on the back of the LPDA:
Some alternative LPDA Analyses:
FYI: The "matching stub" was discussed earlier in this thread:
Mayes & Carrel's original Patent for a Swept Forward LPDA summarized how
the optimum swept forward angle depends on other LPDA design parameters:
Analysis of a Swept Forward LPDA with variable sweep can be found here:
I avoided the Impedance calculation in vicinity of a metal boom problem by simply
eliminating the boom entirely....the NO-BOOM ZIG-ZAG LPA, both a "flat, stacked"
version and a higher performance WEDGE:
It was also simple to construct....for indoor/attic use....
PS: Unfortunately, no doubt because it must TRANSMIT on Hi-VHF and UHF TV Bands, we
can't buy an inexpensive VSWR meter, such as is readily available for Ham Radio bands.....