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4NEC Model of droopy 1/4 problems

Jun 16th 2015, 23:18

KK6QHZ

Joined: Nov 18th 2014, 12:23
Total Topics: 0
Total Posts: 0
I was decided I to put a 2 meter antenna in my attic for use with my hand held and after browsing the web, decided a 1/4 wave "droopy" with 4 radials might be a good choice.

Conventional wisdom is that the vertical should be slightly less than 1/4 wave. The radials should be a 1/4 wave and angled down at 45 degrees for impedance matching. It seemed like an educational venture to model the antenna with 4NEC2 to learn about antenna modeling. My optimized results do not agree with conventional wisdom and I am stumped as to why. Below are setup info, results and the card deck for NEC.

Suggestions appreciated and thanks in advance.

Jerry
KK6QHZ


4NEC2 version 5.8.15

Optimization settings
SWR=75
Gain=25
Rin=50
All others 0
Freq 146

0.6165 = Monopole length - Optimized
0.1993 = Radial length - Optimized
30.0 = Radial droop angle - Optimized
3.1 = Antenna height - estimated height in attic from true ground


From 144 to 148Mhz

SWR < 1.25
Gain essentially 4.7
Z 46 to 52 ohms


NEC card deck

CM 2 Meter 1/4 wave
CM 4 radial drooping
CM
CE
SY high=3.1 'Antenna height
SY wlen=0.6165 'Monopole length
SY rlen=0.1993 'Radial length
SY droop=30.0 'Radial droop angle
SY drpang=cos(droop) 'Adjustment for droop
SY r1x=rlen*1.0*drpang 'Radial 1 X
SY r1y=rlen*0.0*drpang 'Radial 1 Y
SY r2x=rlen*-1.0*drpang 'Radial 2 X
SY r2y=rlen*0.0*drpang 'Radial 2 Y
SY r3x=rlen*0.0*drpang 'Radial 3 X
SY r3y=rlen*1.0*drpang 'Radial 3 Y
SY r4x=rlen*0.0*drpang 'Radial 4 X
SY r4y=rlen*-1.0*drpang 'Radial 4 Y
SY rz=high-(sin(droop)*rlen)
GW 1 32 0 0 high 0 0 wlen+high 2.588mm 'Monopole
GW 2 16 0 0 high r1x r1y rz 2.588mm 'Radial 1
GW 3 16 0 0 high r2x r2y rz 2.588mm 'Radial 2
GW 4 16 0 0 high r3x r3y rz 2.588mm 'Radial 3
GW 5 16 0 0 high r4x r4y rz 2.588mm 'Radial 4
GE 1
GN 0 0 0 0 5 0.001
EK
EX 0 1 1 0 1 0 0 'Voltage source (1+j0) at wire 1 segment 1.
FR 0 0 0 0 146 0
EN
Jun 17th 2015, 00:44

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Is the length in wavelengths? If so, it looks like it optimized it for something around 5/8 wavelength, which is known to have more gain.

Zack W1VT
Jun 17th 2015, 04:32

KK6QHZ

Joined: Nov 18th 2014, 12:23
Total Topics: 0
Total Posts: 0
The lengths are in meters and 5/8 wavelength would be somewhere between 1.2 and 1.3 meters. (Your idea does raise the question of could a 5/8 wave work?)

By my calculations, 0.6165 meters is about 3/10 of a wavelength. With these numbers in mind, I'm not sure that the 5/8 theory is the solution.

Since I am new to 4NEC2, the problem could easily lie with how I modeled the antenna.

Thanks for the idea Zack.

Jerry
KK6QHS
Jun 17th 2015, 15:52

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
It looks like you found a combination in which the antenna is slightly long and the radials are short to compensate. I've found that short radials can be effective, but they don't decouple the feedline like 1/4 wave radials. In fact, modeling will show that grounding the feedpoint of a short radial system can add losses that aren't an issue with 1/4 wave radial systems.

Zack W1VT
Jun 17th 2015, 23:49

KK6QHZ

Joined: Nov 18th 2014, 12:23
Total Topics: 0
Total Posts: 0
For my on sanity I reset the starting values to the conventional values and ran the optimization again. This time it converged near the expected values. I compared the results (SWR, Gain, Z) of the two sets of values and they quite similar.

I can come up with several reasons why something like this could happen but have no clue as to which it might me. I will play around a bit more for my own education but conventional beats wierd results every time (-;

Thanks for the conversation Zack and 73's.

Jerry
KK6QHZ

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