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.
The so-called "OCF windom" is also a misnomer, for the same reasons.
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 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.
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, 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.
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 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 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.
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.
After examining all the graphs above, we can see that:
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:
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.
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.
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.
In light of the above, I believe that:
73 de VE2DPE
7, Rue de la Rive, Notre-Dame-des-Prairies, Québec, Canada J6E 1M9
QTH Locator: FN36gb
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