w8io

The W8IO Antenna Site - Optimized LPDA

(updated 18 June 2017)


Optimized LPDA for narrow band
This project consists of an attempt to design a Narrow-Band Optimized Log-Periodic Dipole Array which has the same or better performance characteristics (VSWR, Gain, Front-to-Back, Front-to-Rear, high G/T, etc) as those of a similar boom-length monoband Yagi.  I call this new type of antenna - NBO-LPDA.

Normally, log-periodic dipole arrays are designed to cover a large frequency range, such as 14 - 30 MHz or  50 - 200 MHz. LPDA's produce moderate gain, moderate F/B and low VSWR over almost all frequencies within the frequency range that they are designed for.  They are similar in shape to Yagi beams, however, Yagis almost always have more gain and more F/B within the band that they are designed for, compared to a similar size LPDA which contains the band.

While evaluating some high-Tau LPDA designs in 2013, I found that I could obtain gain values within 1 dB of similar length high performance VHF/UHF Yagis. These high-Tau designs also used a Sigma value near optimum and produced extremely high gain and F/B and F/R and extremely clean patterns in NEC simulations. Since the gain was so close, I decided to inspect what parameters could be changed to improve the gain on a specific frequency (e.g. 144.1 MHz) without sacrificing too much F/B or F/R, or possibly improving the F/R along with the gain. The display of current on each element suggested that the longest 2 or 3 elements could be tweaked for slightly better F/R without affecting the overall LPDA performance. Also the front elements could also be tweaked to improve gain.  The results of my optimization were very encouraging.  I now have several optimized designs for both 144 and 432 MHz Amateur bands, and I now plan to build and test some of these to verify the real performance vs. the NEC simulation. I will post more information here after this verification process.

In addition, I have also been looking at the HF spectrum to see if I can do something similar there also.

20 Meter NBO-LPDA's
I first chose to look at creating an NBO-LPDA for the 20 meter Amateur band.  After a quick look at available large commercial 20 meter Yagis, I chose the M2 20M6 Yagi as my "standard" to compare to.  My first attempt produced an 8-element NBO-LPDA on a 65 foot boom which produced 11.2 dBi peak gain and over 25 dB F/R across the 14.0-14.35 MHz band.  My second attempt produced a 7-element NBO-LPDA on a 56 foot boom with 11.27 dBi peak gain, more than 11.0 dBi from 14.0-14.35 MHz and 26 dB F/R (peak) and more than 22 dB F/R across the band.  I first optimized the design in NEC2 with linear-taper elements, then converted these to a standard element-taper based on Hy-Gain 20 meter diameters.  The tapered-diameter design was simulated in NEC 4.1.  The plots are shown below. I will supply NEC files upon request.

LP14-7
LP14-7 layout diagram

VSWR
LP14-7 VSWR

gain and F-b
LP14-7 Gain (dBi) and F/B and F/R

az pattern
LP14-7 azimuth pattern at 14.150 MHz

elevation pattern
LP14-7 elevation pattern at 14.150 MHz







Comments are welcome!

contact Roger: email to 

rgcox2 (at) gmail.com

Roger Cox W8IO - Spring Lake, MI