Off-Center Fed Dipoles

1. Off-Center Fed Dipoles

Practical Applications
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2. Why?

Looking for solutions for low band antennas
Was abused by a counterpoise as a child
Looking for multiband solutions
Traditional low band wire arrays use dipoles
or inverted vees
Applications to driven & parasitic arrays
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3. Off Center Fed Dipole Basics

Half wave resonant antenna at lowest
frequency of operation
Even Harmonic resonances
(V/I ratio is approximately constant)
–
–
160M: 160, 80, 40, 20, 17, 12, 10
80M: 80, 40, 20, 10
Fed 1/3 of the way from the end vs. in the
middle
Feed point impedance is approximately 200Ω
–
4:1 current balun does the trick
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4. OCFD Data

80M
–
–
–
–
Used for 10 years
136 feet in length (located at abt 30 feet)
Fed 44.5 feet from one end
Resonant on 80M, 40M, 20M, 10M (no tuner)
160M (measured data)
–
–
–
–
–
Recent addition
264 feet long (located at abt 80 feet)
Fed 88 feet from one end; 4:1 homebrew balun
200Ω on 160M and 166Ω on 80M at resonance
2:1 BW: >200 kHz on 160M, 260 kHz on 80M
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5. Why They Work

It’s just a dipole!
But
–
–
–
½ wave resonant element, then harmonic wire
Voltage/Current relationship at 1/3 feed point
provides essentially constant ratio on even
harmonics
Broadside null on harmonics
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6. Current Distribution on a Dipole

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7. Current & Voltage at Fundamental

Current & Voltage at Fundamental
1
0.9
I
0.8
0.7
0.6
V
0.5
0.4
0.3
0.2
0.1
0
0
60
120
180
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8. I & V at 2nd Harmonic

I & V at 2nd Harmonic
1
0.9
I
0.8
0.7
0.6
0.5
V
0.4
0.3
0.2
0.1
0
0
60
120
180
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9. I & V at 4th Harmonic

I & V at 4th Harmonic
1
0.9
I
0.8
0.7
0.6
0.5
V
0.4
0.3
0.2
0.1
0
0
(2X Expanded Scale)
60
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10. I & V at 8th Harmonic

I & V at 8th Harmonic
1
0.9
I
0.8
0.7
0.6
V
0.5
0.4
0.3
0.2
0.1
0
0
(2X Expanded Scale)
60
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11. But what about the 6th harmonic?

Feed point is at a current minimum
Very high impedance
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12. I & V at 6th Harmonic (Bad Dog!)

I & V at 6th Harmonic (Bad Dog!)
1
0.9
V
0.8
0.7
0.6
0.5
0.4
0.3
0.2
I
0.1
0
0
(2X Expanded Scale)
60
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13. And Odd Harmonics?

Same problem: Current minimum
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14. I & V at 3rd Harmonic (Bad Dog!)

I & V at 3rd Harmonic (Bad Dog!)
1
0.9
V
0.8
0.7
0.6
0.5
0.4
0.3
0.2
I
0.1
0
0
60
120
180
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15. Basic Gain Plots

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16. 160M center fed dipole at 80 feet

AZ
EL
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17. 160M OCFD at 80 feet

AZ
EL
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18. 160M OCFD on 80M (x2)

EL
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19. 160M OCFD on 40M (x4)

EL
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20. 160M OCFD on 20M (x8)

EL
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21. 160M OCFD on 10M (x16)

EL
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22. Geometry/Height

Old 80M design: 45.3 ft, 90.7 ft; 30 ft high
New 160M design: 88ft, 176 ft; 80 ft high
Both use 4:1 Guanella balun design
–
80M variation: 45/65 ft flat+26 ft dropper
Feed point impedance at resonance drops as
effective height above ground decreases
–
Recommend 1/3 λ up for 4:1 balun to work well
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23. 4:1 Guanella Current Balun

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24. 80M OCFD Construction Detail

~8 Wraps, covered with
heatshrink tubing
45’ Leg
90’ Leg
“Dogbone”
Insulator
#12 stranded/insulated copper
Stainless hardware
4:1 Current Balun
(PVC enclosure)
UHF
Connector
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25. 160M OCFD Construction Detail

~8 Wraps, covered with
heatshrink tubing
88’ Leg
176’ Leg
“Dogbone”
Insulator
#12 stranded/insulated copper
Stainless hardware
4:1 Current Balun
(PVC enclosure)
UHF
Connector
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26. 160M OCFD Normalized VSWR Plots

3.0
160M
2.8
80M
40M
2.6
2.4
VSWR
2.2
40M
2:1 BW = 370 kHz
fr = 7260 kHz
80M
2:1 BW = 260 kHz
fr = 3610 kHz
2.0
160M
2:1 BW > 200 kHz
fr = 1825 kHz
1.8
1.6
1.4
1.2
Good correlation of VSWR curve mid points
1.0
1.75
1.80
1.85
1.90
1.95
2.00
Frequency (MHz) @ 160M [x2 for 80M, x4 for 40M]
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27. OCFD Orientation Issues

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28. Single 160M OCFD at 125º, 160M/80M

How to deal with the
broadside null…
Rotate antenna to
achieve best gain
compromise
El of 30º
Europe (50º)
80M
160M
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29. Disclaimer…

Your mileage may vary
Batteries not included
Some assembly required
Professional driver, closed course
Void where prohibited
Do not dispose of in fire
Taxes, titles, license fees extra
10M band openings longer than 2 hours
require immediate medical attention
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