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The W8IO Antenna Site - LOGGI - a Log-Periodic / Yagi Hybrid Antenna

(updated 4 Mar 2016)


What is a Loggi?

Normally, log-periodic dipole arrays (LPDA) are designed to cover a large frequency range, such as 14 - 30 MHz or  50 - 200 MHz. LPDA's produce moderate forward gain, moderate F/B and low VSWR over almost all frequencies within the frequency range that they are designed for.  LPDA's have all elements connected to a feeder (transmission line). This feeder has a feedpoint at the front of the array. LPDA's are designed based on mathmatical ratios such as TAU (scale factor) and SIGMA (spacing factor).

Yagi beam antennas are designed to cover a very narrow frequency range, typically 2%.  Yagi beams generally cover only one Ham band at a time, although traps, sleeves and interlaced parasitic elements can make multiband Yagis possible. Yagi beams can be designed to produce extremely high forward gain and high F/B, however this will also reduce the useable bandwidth by doing so. Yagi's are generally fed toward the rear of the array on one particular dipole element. However, some hybrid Yagi's such as the Hy-Gain TH11DX use a log-cell as the driven element. These antennas are sometimes called Log-Yag antennas.

While evaluating some high-Tau LPDA designs, 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 many front elements could also be tweaked to improve gain and F/R over a narrow band.  This new design is called a Loggi. This stands for a LOG - yaGI hybrid. The Loggi is different from other designs called Log-Yag antennas in that all of a Loggi's elements are driven.  There are no directors or reflectors. It is actually more of a narrowband optimized LPDA than a real hybrid. The front driven elements in the Loggi driven array seem to act like Yagi directors. Also, the rear driven elements in the Loggi driven array seem to act like Yagi reflectors.

log -yagi comparisonLPDA - Log Periodic Dipole Array:
Yagi beam antenna:
Loggi beam antenna:
Log-Yag beam antenna:

currents currents
                               Example of element currents in normal LPDA                                                                                     Example of element currents in Loggi beam
                                                                   

HF Examples:

VHF and UHF Examples:

Comments are welcome!

contact Roger: email to 

rgcox2 (at) gmail.com

Roger Cox W8IO - Spring Lake, MI