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

Multi-Band Antenna alternatives

Multi-Band Antenna alternatives

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

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

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

Current & Voltage at Fundamental I V N1IW

I & V at 2nd Harmonic I V N1IW

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

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

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

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

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

160M OCFD on 80M (x2) EL N1IW

160M OCFD on 40M (x4) EL N1IW

160M OCFD on 20M (x8) EL N1IW

160M OCFD on 10M (x16) EL N1IW

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.

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...

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.

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…..

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

Typical Results w/50’ LMR-240 http://www.dj0ip.de/off-center-fed-dipole/80m-ocf/

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

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

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

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

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

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

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

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

When preparing to build your OCF Antenna be sure to…

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

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.