G8JNJ 432 MHz Mixed Mode Helical Antenna NEC Model

Illustration of NEC-2 GH Helix Card

by Dr. Carol F. Milazzo, KP4MD (posted on 04 Nov 2015)
E-mail: [email protected]

CONTENTS
• Introduction
• 4nec2 Model
• References
• INTRODUCTION

To illustrate the application of the NEC-2 GH helix command in the 4nec2 antenna modeling program1, here is an annotated 4nec2 model of an interesting omnidirectional circular polarization 70 cm helical antenna by Ehrenfried2 that demonstrates the use of the NEC-2 GH command3 to automate the helix segment construction and the GM command4 to elevate the helix above ground. This method is superior to manually calculating the coordinates of each segment.  I also demonstrate the use of the GH command to construct loops as zero pitch helices in my Universal Magnetic Loop Antenna models5 and 144 MHz Halo model analysis6.

SWR and Reflection Coefficient vs. Frequency

Impedance vs. Frequency

Model Calculation

Gain vs. Frequency

G8JNJ 432 MHz Mixed Mode Antenna Model

CM Helix over perfect ground
CM Illustrates use of GH (helix) input
CM Illustrates use of GM (move wires) input
CE
SY f=432 'Input Frequency MHz
SY k=308.756 'Input Velocity constant ~300 Mm/s
SY w=k/f 'Calculate Wavelength in space
SY hw=0.5*w 'Calculate Half wavelength in space
SY t=3 'Input Number of helix turns
SY ts=18 'Input Segments per helix turn
SY n=t*ts 'Calculate Segments total
SY wl=0.5 'Input Antenna height in wavelengths
SY l=wl*w 'Calculate Antenna height in meters
SY s=l/t 'Calculate Turn spacing in meters
SY r=sqr(hw*hw-s*s)/2/PI 'Calculate Helix radius in meters
SY d=.0025 'Input Wire radius in meters
SY h=2 'Input Height above ground in meters
GH 1 n s l r r r r d
GM 0 0 0 0 0 0 0 h 0
GE 1
LD 5 0 0 0 58000000 'copper
GN 1
EK
EX 0 1 n/2 0 1. 0 0
FR 0 0 0 0 f 0
EN

REFERENCES

1. 4nec2 Antenna Modeling Program, Voors, A
2. "70 cm Mixed Mode Helical Antenna", Ehrenfried, M, G8JNJ
3. Helix Spiral Specification (GH), NEC-2 Reference
4. Coordinate Transformation (GM), NEC-2 Reference
5. "A Universal HF Magnetic Loop Antenna NEC Model", Milazzo, C, KP4MD
6. "144 MHz Halo Antenna", Milazzo, C, KP4MD