1. Off-Center Fed Dipoles
Practical Applications
Presented by:
Mickey Westover, W1LB
&
Greg Algeri, WA1JXR
Additional material provided by
Mike Bernock, N1IW (SK)
N1IW

2. Multi-Band Antenna alternatives

3. Multi-Band Antenna alternatives

4. Off Center Fed Dipole Basics
Half wave resonant antenna at lowest frequency of operation
Even Harmonic resonant
160M: 160, 80, 40, 20,10
80M: 80, 40, 20, 10
Fed 1/3 of the way from the end vs. the middle
Feed point impedance is approximately 200Ω. 4:1 current balun does the trick
N1IW

5. Geometry 80M design: 45.3 ft, 90.7 ft 160M design: 88ft, 176 ft
Both use 4:1 Guanella balun design
Feed point impedance at resonance drops as effective height above ground decreases
Recommend 1/3 λ high for 4:1 balun to work well
N1IW

6. Why They Work BUT! It’s just a dipole!
½ wave resonant element, + harmonic wire
Voltage/Current relationship at 1/3 feed point provides constant ratio on even harmonics
Broadside null on 2nd and higher harmonics
At higher frequencies pattern gets “spikey”
N1IW

7. Current & Voltage at FundamentalI V
N1IW

8. I & V at 2nd Harmonic I V
N1IW

9. I & V at 4th Harmonic I V
(2X Expanded Scale)
N1IW

10. I & V at 8th Harmonic I V
(2X Expanded Scale)
N1IW

11. I & V at 6th Harmonic (Bad Dog!)
(2X Expanded Scale)
Feed point is at a current minimum. Very high impedance
N1IW

12. I & V at 3rd Harmonic (Bad Dog!)
Odd Harmonics Same problem: Current minimum
N1IW

13. 160M OCFD on 160M (x1) AZ EL
N1IW

14. 160M OCFD on 80M (x2) EL
N1IW

15. 160M OCFD on 40M (x4) EL
N1IW

16. 160M OCFD on 20M (x8) EL
N1IW

17. 160M OCFD on 10M (x16) EL
N1IW

18. Why use an OCF Dipole? 10% of QSO quality is radio and operator.
90% of QSO quality is ANTENNA!
A traditional equal length dipole is easily tuned to one specific band and marginal on others without aggressive use of a tuner.
The OCF Dipole has reasonable VSWR on other bands above its length.

19. The Original Windom Antenna of The 1930's
The original Windom was a Zepp-type antenna fed 14% off-center with a single wire.
The single-wire feeder radiated RF all the way into the operating position. (YIKES!)
A very undesirable side-effect that the Carolina" version not only eliminated but transformed into an extra asset by...

20. The Original Windom Antenna of The 1930's
…transforming the 200 ohm feed point impedance with the 4-1 current balun to match 50 ohms…
and choking off the RF on the coax before it enters the shack… (YEAH!)
thus forcing the choked off RF to travel instead toward DX stations.

21. Off Center Fed Calculation Chart
Short Side = 468 divided by the frequency,
then multiply by .36 (equals length in feet)
Long Side = 468 divided by the frequency, then multiply by .64 (equals length in feet)
Or you can go to the next slide…..

22. Off Center Fed Lengths Freq. Short Long VSWR 160 Meters 93.6' 166.4'
1.8
80 meters
45'
90'
1.6
40 meters
23.4'
41.5'
30 meters
1.4
20 meters
11.8'
21.1'
1.5
17 meters 6
12 meters
10 meters
1.3
6 meters

23. Typical Results w/50’ LMR-240

24. Parts for an 80 meter OCF Total cost around $65 Spilt Bolts
$2 each. If you
want to ‘play’ with
the tuning
14 or 12ga
THHN Wire
$50 for 500’
An 80 meter OCF will use approximately $14 of the wire
4-1 Current
Balun
Approx $45
Total cost
around
$65
5 Dog Bone
Insulators
$1 each
Burndy’s aka:
Bootlace Furrules
10 cents each

25. Tools Crimper Tape Measure 100’ if you have it Pliers Bandaids
as required
Adjustable Wrench

26. Step One Insulator and burndy at the balun end of wire.
Leave about a 6” tail 26

27. Step Two Crimp burndy to secure the insulator to the wire.
Leave about a 6” tail 27

28. Step Three Install second insulator on end of antenna in case you
have to let part of the antenna hang down. 28

29. Step Four If you have to let part of the antenna hang down you can
simply tie a knot in the wire to support the horizontal section. 29

30. Step Five Strip ½” of insulation from the ‘tail’ at the balun end and
Crimp a ring connector to the wire. For corrosion
resistance solder the connection. 30

31. Step Six Secure ring connector to one side of the balun and secure
the center insulator to the balun. 31

32. When preparing to build your OCF Antenna be sure to…

33. When preparing to build your OCF Antenna be sure to…
Plan Ahead

34. DISCLAIMERS Read at your own risk
Objects in mirror are close…real close
Batteries not included
Prosecutors will be violated
Prohibited where void
Keep out of reach of children and teenagers.
(think about that one)
Only you can prevent forests
Tax, title, and dealer prep extra
Going up the down staircase prohibited
Caution…just caution.
Shake. Well before using.