Three-Element, Collapsible YagiFor Two Meters

(And an Introduction to NEC2)

Mac A. Cody / AE5PHMarch, 2016

Taking the Challenge(s)

● "Sherman, set the Wayback Machine to October, 2013 ..."

● Reach more repeaters with a 5W handheld

● Roll-up Jpole on a mast could reach a number of repeaters

● How much better could I do with a Yagi?

● Construct my first Yagi antenna

● Must switch easily between vertical and horizontal polarization for future 2M SSB communications

● Must be collapsible for easy transport and storage

● Make it myself, because I am cheap thrifty!

● Learn something about antenna design and analysis software

● Many free antenna design software tools are available

● NEC2 is the antenna analysis software tool of choice

Yagi Antenna Design Tools

● Yagi-Uda Antennas by VE3SQB - What I used● MS Windows software, but runs on Linux using Windows

emulation (Wine)

● Free software at http://www.ve3sqb.com/yagi.exe

● A lot of other antenna design tools on his website

● VHF/UHF Yagi Antenna Design by Martin E. Meserve (K7MEM)

● Part of his Javascript Electronic Notebook website● http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf.html

● Yagi Uda Antenna Calculator by Ajarn Changpuak

● Part of his ELECTRONICS-LAB website

● http://www.changpuak.ch/electronics/yagi_uda_antenna.php

● This is just a small sample of the many design tools available

Three-Element Yagi Design

CENTER OF 2-METER BAND

APPROXIMATE TAPER OF THE COLLAPSIBLE WHIP

6 mm 2 mm

NEC2 Antenna Analysis Tools

● NEC2++ 1.3.1 (Currently at version 1.7.0) – Used for my original analysis

● C++ implementation of NEC2

● Runs on Linux or on MS Windows

● Free software at (via link to a Github Project Page) http://elec.otago.ac.nz/w/index.php/Necpp

● Qantenna 0.2.1 (Currently at version 0.3.0) – Used for my original analysis

● Process and visualize NEC2 files

● Runs on Linux only and is also a bit unstable

● Free software at http://qantenna.sourceforge.net/

● 4nec2 by Arie Voors (Currently at version 5.8.16) – Used for this presentation

● FORTRAN implementation of NEC2 with a lot of supporting tools

● Runs on MS Windows and Linux (via Wine)

● Free software at http://www.qsl.net/4nec2/

What is NEC2?

● Numerical Electromagnetics Code (version) 2● Comprehensive package for the analysis of the electromagnetic

properties of structures

● Can analyze radiating properties i.e. antenna gain, as well as scattering properties of structures

● Based on the method of moments solution of the electric field integral equation (EFIE) for thin wires and the magnetic field integral equation (MFIE) for closed, conducting surfaces

● Uses an iterative method to calculate the currents in a set of wires and the fields that result

● NEC2 was originally written in FORTRAN (also C, C++)● Does not model tapered elements such as those made of

telescoping aluminum (not directly, anyway)

● Does not model buried radials or ground stakes

Using NEC2

● Due to its FORTRAN heritage, NEC2 utilizes 'cards' – a.k.a. formatted lines of ASCII text

● Each 'card' specifies either an antenna element, a stimulating source characteristic, or an analysis command

● If the antenna being analyzed is simply a collection of 'wires', generating a NEC2 'card deck' is a fairly simple process

CM NEC Input File for three-element, 2-meter collapsible YagiCM PT control card suppresses printing of element currentsCM TL control card specs transmission line in terms of Z,length,and shunt YCM Whip antenna specifications (sections fully extended):CM Segment 1 is 0.122 meters long and 0.00300 meters in radiusCM Segment 2 is 0.101 meters long and 0.00250 meters in radiusCM Segment 3 is 0.099 meters long and 0.00200 meters in radiusCM Segment 4 is 0.097 meters long and 0.00150 meters in radiusCM Segment 5 is 0.102 meters long and 0.00100 meters in radiusCM Reflector center hex stand-off specifications:CM 0.02540 meters long and approximately 0.00367 meters in radiusCM Driven element hex stand-off specifications:CM 0.01111 meters long and approximately 0.00367 meters in radiusCM Simplifying assumption is non-tapered elements 0.00200 meters in radiusCM The reflector should be 1.01080 meters longCM The driven element should be 1.00030 meters longCM The director should be 0.91113 meters longCM The reflector should be 0.56281 behind the driven elementCM The director should be 0.30746 meters in front of the driven elementCM The yagi is suspended in free spaceCEGW 1 11 -0.56281 0.50540 2.00000 -0.56281 -0.50540 2.00000 0.00200GW 21 25 0.00000 0.50015 2.00000 0.00000 -0.50015 2.00000 0.00200GW 41 11 0.30746 0.45557 2.00000 0.30746 -0.45557 2.00000 0.00200GE 0 FR 0 1 0 0 146EK 1EX 0 21 13 0 10RP 0 90 1 0 90 1 0EN

Simplified Three-ElementYagi NEC2 Input

End Comments

Reflector Element

Driven Element

Comments

End GeometryFrequency of 146 MHz

Voltage on 13th Segment

End Card

Request Radiation Pattern

Director Element

Extended Thin-Wire Kernel

X1 X2Y1 Y2Z1 Z2 RADIUS#SEGTAG#

FREQUENCY

VOLTSTAG# SEG#

4nec2 Geometry (F3) Display

● The upper-left element is the reflector

● The driven element is in the center

● The lower-right element is the director

● The tag numbers and wire segments are displayed

● The coordinate axes indicate that the Yagi is in a horizontal orientation

Calculating a Field Pattern (F7)

● The Generate Window (F7) enables the user to determine what function will be calculated

● What is present in the original file can be used

● The Far Field pattern is for a specific frequency

● Models the antenna's radiation pattern

4nec2 Pattern (F4) Display

● The top window shows the horizontal radiation pattern

● The bottom window shows the vertical radiation pattern

● About 7.9 dBi (decibels relative to isotropic) gain obtained along the forward direction, below -10 dBi in the back end

● An isotropic antenna has equal gain in three dimensions and is an idealized abstraction!

4nec2 3D Viewer (F9) Display

● The 3D view provides another means of visualizing the antenna radiation pattern

● The vertical, colored scale indicates the gain in dBi

● Currents present at different points on the antenna are shown on top of the antenna

Calculating SWR Over Frequency

● The Generate Window (F7) also allows estimation of antenna performance over a range of frequencies

● In this example, a sweep from 144 MHz to 148 MHz in 1 MHz steps is performed

SWR/Gain/Impedance (F5) Display

● The SWR at 144 MHz is about 1.1, which is good

● The SWR at 146 MHz is about 1.5, which is acceptable

● The SWR at 148 Mhz is about 2.2, which marginal

● The reflection coefficent gets progressively worse (higher) with increasing frequency (from below -25 dB to about -8 dB)

Gain and Impedance

Smith Chart (F11) Display

144 MHz

148 MHz

Review of Simplified Yagi Model

● Reasonably good radiation pattern over frequency range● 7.6 dBi gain, 18.6 dB front to back @ 144 MHz

● 7.9 dBi gain, 22.1 dB front to back @ 146 MHz

● 8.2 dBi gain, 18.3 dB front to back @ 148 MHz

● Variant SWR over frequency range● 1.1 @ 144 MHz

● 1.5 @ 146 MHz

● 2.2 @ 148 MHz

● Variant Impedance over frequency range● 46+j3 @ 144 MHz

● 50+j21 @ 146 MHz

● 32+j36 @ 148 MHz

● Remember, the antenna geometry has been approximated● Let's get more accurate...

CM NEC Input File for three-element, 2-meter collapsible YagiCM PT control card suppresses printing of element currentsCM TL control card specs transmission line in terms of Z,length,and shunt Y...CEGW 2 4 -0.56281 0.50140 2.00000 -0.56281 0.43170 2.00000 0.00100GW 3 4 -0.56281 0.43170 2.00000 -0.56281 0.33470 2.00000 0.00150GW 4 4 -0.56281 0.33470 2.00000 -0.56281 0.23570 2.00000 0.00200GW 5 4 -0.56281 0.23570 2.00000 -0.56281 0.13470 2.00000 0.00250GW 6 4 -0.56281 0.13470 2.00000 -0.56281 0.01270 2.00000 0.00300GW 7 1 -0.56281 0.01270 2.00000 -0.56281 -0.01270 2.00000 0.00367GW 8 4 -0.56281 -0.01270 2.00000 -0.56281 -0.13470 2.00000 0.00300GW 9 4 -0.56281 -0.13470 2.00000 -0.56281 -0.23570 2.00000 0.00250GW 10 4 -0.56281 -0.23570 2.00000 -0.56281 -0.33470 2.00000 0.00200GW 11 4 -0.56281 -0.33470 2.00000 -0.56281 -0.43170 2.00000 0.00150GW 12 4 -0.56281 -0.43170 2.00000 -0.56281 -0.50140 2.00000 0.00100GW 22 4 0.00000 0.49615 2.00000 0.00000 0.43170 2.00000 0.00100GW 23 4 0.00000 0.43170 2.00000 0.00000 0.33470 2.00000 0.00150GW 24 4 0.00000 0.33470 2.00000 0.00000 0.23570 2.00000 0.00200GW 25 4 0.00000 0.23570 2.00000 0.00000 0.13470 2.00000 0.00250GW 26 4 0.00000 0.13470 2.00000 0.00000 0.01270 2.00000 0.00300GW 27 1 0.00000 0.01270 2.00000 0.00000 -0.01270 2.00000 0.00367GW 28 4 0.00000 -0.01270 2.00000 0.00000 -0.13470 2.00000 0.00300GW 29 4 0.00000 -0.13470 2.00000 0.00000 -0.23570 2.00000 0.00250GW 30 4 0.00000 -0.23570 2.00000 0.00000 -0.33470 2.00000 0.00200GW 31 4 0.00000 -0.33470 2.00000 0.00000 -0.43170 2.00000 0.00150GW 32 4 0.00000 -0.43170 2.00000 0.00000 -0.49615 2.00000 0.00100

Detailed Three-ElementYagi NEC2 Input (Part 1)

End Comments

Reflector Element

Driven Element

Comments

X1 X2Y1 Y2Z1 Z2 RADIUS#SEGTAG#

GW 42 4 0.30746 0.45157 2.00000 0.30746 0.43170 2.00000 0.00100GW 43 4 0.30746 0.43170 2.00000 0.30746 0.33470 2.00000 0.00150GW 44 4 0.30746 0.33470 2.00000 0.30746 0.23570 2.00000 0.00200GW 45 4 0.30746 0.23570 2.00000 0.30746 0.13470 2.00000 0.00250GW 46 4 0.30746 0.13470 2.00000 0.30746 0.01270 2.00000 0.00300GW 47 1 0.30746 0.01270 2.00000 0.30746 -0.01270 2.00000 0.00367GW 48 4 0.30746 -0.01270 2.00000 0.30746 -0.13470 2.00000 0.00300GW 49 4 0.30746 -0.13470 2.00000 0.30746 -0.23570 2.00000 0.00250GW 50 4 0.30746 -0.23570 2.00000 0.30746 -0.33470 2.00000 0.00200GW 51 4 0.30746 -0.33470 2.00000 0.30746 -0.43170 2.00000 0.00150GW 52 4 0.30746 -0.43170 2.00000 0.30746 -0.45157 2.00000 0.00100GE 0 FR 0 1 0 0 146EK 1EX 0 27 1 0 10RP 0 90 1 0 90 1 0EN

Detailed Three-ElementYagi NEC2 Input (Part 2)

End GeometryFrequency of 146 MHz

Voltage on 13th Segment

End Card

Request Radiation Pattern

Director Element

Extended Thin-Wire Kernel

X1 X2Y1 Y2Z1 Z2 RADIUS#SEGTAG#

TAG# SEG#

4nec2 Geometry (F3) Display

● The positions of the reflector, driven element, and director are as before

● Note the individual wires used to form each element (solid dots)

● Note the greater number of modeling segments

● A larger number of segments yields a more accurate model, but takes longer to calculate

4nec2 Pattern (F4) Display

● Again, the top window is the horizontal radiation pattern and the bottom window is the vertical radiation pattern

● Now about 8.4 dBi gain in the forward direction, but only -7.9 dBi gain in the back end

● This antenna provides better forward gain, but doesn't suppress the rear lobe as well

4nec2 3D Viewer (F9) Display

● The 3D view again provides the antenna radiation pattern

● The vertical, colored scale indicates the improved gain

● Note that the sections of the whip antennas have been accentuated

SWR/Gain/Impedance (F5) Display

● The SWR at 144 MHz is about 1.5, which is acceptable

● The SWR at 146 MHz is about 1.3, which is acceptable

● The SWR at 148 MHz is about 1.4, which is acceptable

● The reflectance coefficient is comparatively consistent over the frequency range and not too high (less than 3 dB variation)

Gain and Impedance

Smith Chart (F11) Display

144 MHz

148 MHz

Review of Collapsible Yagi Model

● Reasonably good radiation pattern over frequency range● 8.2 dBi gain, 12.6 dB front to back @ 144 MHz

● 8.4 dBi gain, 16.3 dB front to back @ 146 MHz

● 8.6 dBi gain, 20.2 dB front to back @ 148 MHz

● Relatively constant SWR over frequency range● 1.5 @ 144 MHz

● 1.3 @ 146 MHz

● 1.4 @ 148 MHz

● Variant Impedance over frequency range● 40-j14 @ 144 MHz

● 38-j5 @ 146 MHz

● 35+j5 @ 148 MHz

● How well did the actual antenna work?

Yagi Construction Details

Yagi Testing – Horizontal Polarization

Yagi Testing – Vertical Polarization

29.078 Miles from QTH to W5WB/W5AUY Repeater (SW Dallas ARC)

Now you can get started creatingthe next great antenna design....

Thanks for your time!

Questions?