The Off-Center-Fed Dipole
(OCFD)
Understanding How It Works
Can Pay Off

I like the off-center-fed dipole (OCFD) because it performs quite well on most amateur radio frequency bands... if you are not afraid to stray away from the "traditional" way of doing things.

Before going any further, I should mention here that the OCFD is sometimes falsely referred to as a "windom" antenna.

  • A windom antenna is a single-wire-fed antenna which is worked against ground.
         It's a grounded vertical wire connected to a horizontal portion acting as a top hat capacitance.

  • On the other hand, an OCF dipole is an ungrounded dipole antenna fed with twin-conductor line, either coaxial or open wire parallel conductors.

The so-called "OCF windom" is also a misnomer, for the same reasons.

Recall...

A half-wave dipole is usually fed at its center because this is where maximum current occurs. Feeding the dipole at that point yields a minimum (free space) impedance of 70-72 ohms. This makes it possible to use the popular 50 ohms coaxial with relatively low mismatch SWR levels... on the fundamental frequency and odd harmonics thereof (ex. a 7MHz half-wave dipole will also work on 21 MHz).

But keep in mind that the impedance of a half-wave dipole varies with height above ground. For example, when it's installed at a height of 0.13 wavelengths above average ground, its impedance will be closer to 50 ohms which is perfect for 50 ohms coaxial line.

The Harsh Reality

The impedance of a half-wave dipole rises as we move the feedpoint off-centre - away from maximum RF current (minimum impedance of 70-72 ohms)... and into increasing RF voltage territory.

Graph of RF current distribution on an 80 M half-wave dipole, showing where off-center feedpoints at 20%, 25% and 33% from one extremity intersect with the RF current curve.

Manufacturers of off-center-fed dipole antennas choose an offset feedpoint where the theoretical impedance is approximately 300 ohms on the antenna's *fundamental* frequency of resonance. They will then be able to use a 6:1 current balun (RF choke) at the feedpoint which will conveniently allow the use of 50 ohm coax... which most ham radio operators like to use. This convenient feedpoint impedance is located about 1/3 (33%) of the dipole length away from one end.

  • But, remember that a dipole's feedpoint impedance will vary with its height above ground, and with the dielectric constant of the ground below the antenna. Therefore, the 6:1 balun will likely be transforming impedances lower than 300 ohms... which makes the impedance-transforming ratio unpredictable. In such a case, the current balun will only serve as a RF choke… which is better than nothing, I guess.

  • Also, please note here that this "neat" 6:1 arrangement falls to pieces as you try to use the same OCF antenna on other frequency bands than the fundamental frequency for which it was intended. The impedance will *not* be 300 ohms on the other frequency bands - the main reason behind using an OCFD.

Off-center-fed dipole (OCFD) Guiding Principle

If there is RF current present at the chosen offset feedpoint of an 80 M half-wave off-centre-fed dipole (OCFD), then this feedpoint is "usable".

But, is it the best feedpoint position for multiband use?

Let's find out by examining different offset feedpoints on an 80 M half-wave dipole for each amateur radio HF band. Offsets of 33%, 25% and 20% will be illustrated.

Off-Center-Fed Dipole
(OCF) Graphs

Even Harmonics of (f0) 3.5 MHZ (80 M Band)

Second harmonic (2xf0) = 40 M Band (7 MHz)

Graph of the 40 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 40 M RF current curve.

Fourth harmonic (4xf0) = 20 M Band (14 MHz)

Graph of the 20 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 20 M RF current curve.

Sixth harmonic (6xf0) = 15 M Band (21 MHz)

Graph of the 15 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 15 M RF current curve.

Eighth harmonic (8xf0) = 10 M Band (28 MHz)

Graph of the 10 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 10 M RF current curve.

Odd Harmonics of (f0) 3.5 MHZ (80 M Band)

The Third Harmonic (3xf0) of 3.5 MHz

Graph of the 30 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 30 M RF current curve.

The third harmonic of 3.5 MHz (3xf0) is 10.5 MHz but our amateur radio 30 M band allocation is a shade lower in frequency at 10.1 - 10.15 MHz. The 10.1 MHz current curve is shown in lighter grey on the graph, a shade to the right of the 10.5 MHz (3xf0) curve. As you can see on the graph, the antenna is *physically* a little too short (3.96%) to fully accommodate three complete half waves at 10.1 MHz.

The Fifth Harmonic (5xf0) of 3.5 MHz

Graph of the 17 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 17 M RF current curve.

The fifth harmonic of 3.5 MHz (5xf0) is 17.5 MHz but our amateur radio 17 M band allocation is a shade higher in frequency at 18.068 MHz. The 18.068 MHz current curve is shown in lighter grey on the graph, a shade to the left of the 17.5 MHz (5xf0) curve. As you can see on the graph, the antenna is *physically* a little too long (3.14%) at 18.068 MHz.

The Seventh Harmonic (7xf0) of 3.5 MHz

Graph of the 12 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 12 M RF current curve.

The seventh harmonic of 3.5 MHz (7xf0) is 24.5 MHz but our amateur radio 12 M band allocation is a shade higher in frequency at 24.89 MHz. The 24.89 MHz current curve is shown in lighter grey on the graph, a shade to the left of the 24.5 MHz (7xf0) curve. As you can see on the graph, the antenna is *physically* a little too long (1.57%) at 24.89 MHz.

Use Of An 80 M Off-Center-Fed Dipole On A Non-Harmonic Frequency

Graph of the 60 M RF current distribution on an 80 M half-wave dipole - also showing where 20%, 25% and 33% off-center feedpoints intersect with the 60 M RF current curve.

The 60 M band (5.3-5.4 MHz) is higher in frequency than the 80 M band but not quite twice (even harmonic) of 3.5 MHz. Therefore a full wave at 5.3 MHz does not quite fit within an 80 M dipole. In other words, the 80 M dipole is *physically* too short to fully accommodate two complete half-waves at 5.3 MHz. Nonetheless, the 80 M dipole - fed off-centre - will definitely be suitable for use on 60 M.

Preliminary Conclusions

After examining all the graphs above, we can see that:

  • A 33% offset feedpoint will not allow use of the 80 M OCFD on 30 M and 15 M where the RF current is zero, resulting in unworkable impedances in the thousands of ohms.
  • A 25% offset feedpoint will not allow use of the 80 M OCFD on 20 M and 10M for the same reason.
  • A 20% offset feedpoint will not allow use of the 80 M OCFD on the 17 M band due to lack of enough RF current.

But a closer look at each graph indicates - at least in theory - that feeding an 80 M half-wave off-center-fed dipole somewhere between the 20% and 25% offsets would make it usable on all HF ham bands (except 160 M), because there would be enough RF current in all cases.

Of course, any feedpoint where the RF current will be less than maximum - while still greater than zero - will exhibit higher impedance than the theoretical 70-72 ohms of a center-fed half-wave dipole in free space. An open wire high impedance feed line is highly recommended.

That's why I chose to feed my OCFD with 450 ohm ladder line:

  • its characteristic impedance will likely be closer to the higher impedances of an off-centre fed dipole than coaxial feed line;
  • the ladder line's extremely low loss characteristic means that even high SWR values will result in negligible losses. In plain language, I couldn't care less about the OCFD's impedance on the frequency bands I wish to use the antenna on.

Of course, in practice, height above ground, ground conductivity and the presence of structures within a half wavelength of the antenna will have an influence on the impedance of any feedpoint one might choose.

BALUN OR UNUN?

Must I point out here that an off-center-fed dipole antenna is inherently an unbalanced antenna!

Therefore, using a balun - a BALanced to UNbalanced transformer - with such an antenna caters more to popular misconceptions than to common sense. A UNUN - at the OCFD antenna feedpoint - would be much more appropriate for those who absolutely insist on using lossy coax transmission line with an antenna. A UNUN would, at least, serve to (somewhat) prevent RF currents from travelling down the outer shield of a coaxial feed line... and interfere with equipment in the shack and/or in the house... or a neighbour's house!

On the other hand, a 9:1 UNUN would make more sense at the transceiver end of the 450 ohm transmission line... as a necessary attempt to match with the 50 ohm RF terminal of a transceiver while also serving as a RF choke.

Admittedly, the complex impedances which will appear at the end of the 450 ohm transmission line cannot be transformed into a purely resistive 50 ohm impedance with a UNUN.

Therefore, it will still be necessary to use an antenna tuner to transform the complex impedances to something within the range of impedances that the transceiver can safely work with.

A further argument in favor of 450 ohm ladder line is that a low-loss transmission line is an absolute *must* in receive mode. When I'm working QRP, I need the few microvolts of RF "juice", that my off-center-fed dipole antenna has managed to catch, to reach my receiver instead of being dissipated as heat along a coaxial transmission line.

Generic 450 ohm ladder line has a matched loss of 0.08 dB/100 feet at 10 MHz... while 100 feet of RG-8X coax will squander 0.7-1.0 dB at the same frequency... an order of magnitude more than ladder line! I rest my case here on transmission line efficiency.

To avoid the inevitable (small) RF losses caused by a UNUN between the ladder line and the final relatively short length of coaxial line entering the radio room, I chose not to use one.

Instead, I chose to connect the ladder line directly to the coax using a homemade weatherproofed junction box outside the house. Then, to prevent RF (travelling along the coax shield of the lead-in) from entering the shack and crating havoc, I use snap-on ferrite RF chokes on the coax before it enters the house.

I leave the task of matching the complex impedances - appearing at the XCVR end of the coaxial transmission line - to my LDG AT-600PRO (L-network) automatic tuner.

Summary

In light of the above, I believe that:

  • An 80 M half-wave off-center-fed dipole, with a feedpoint at about a 22.5% offset from one end, will give access to all HF bands from 80 meters up to 10 meters.
  • The best transmission line to use in such a case is 450 ohm ladder line for its extremely low loss characteristic.
  • The ladder line should be connected directly to the coax lead-in, using a weatherproofed box outside the shack.
  • Snap-on ferrite chokes be used on the coax lead-in to prevent parasitic RF currents from entering the house and interfering with equipment in the radio room, and other electronic devices elsewhere in the house and a neighbour's house.
  • A good quality wide range antenna system tuner should be able to handle the complex impedances which will inevitably appear at the XCVR end of the transmission line coming down from an 80 M half-wave off-center-fed dipole being used as a multiband HF antenna.


73 de VE2DPE
Claude Jollet
7, Rue de la Rive, Notre-Dame-des-Prairies, Québec, Canada J6E 1M9

QTH Locator: FN36gb





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