EFOC17 Off-Center Dipole Installation Guide

The RF.Guru EFOC17 is a 4:1 fed off-center dipole intended for practical multiband operation from 40 through 6 meters when installed correctly. This guide explains what the antenna is, how the supplied wire should be cut, where the choke belongs, and how to choose between the known working cut families.

Important: the hard 1:1 current choke belongs at the feedpoint, directly on the coax side of the 4:1 feedpoint unit. It is not just a general feedline accessory. It is part of the intended antenna system.

What the EFOC17 actually is

The EFOC17 is not an EFHW. It is a two-leg off-center-fed dipole using a 4:1 feedpoint transformer. The long leg and the short leg are both active parts of the radiating structure. Some operators loosely call the short leg a counterpoise, but electrically it is still the short radiator leg of the dipole.

Because the feedpoint is intentionally moved away from the center, the impedance is higher than with a center-fed dipole. That is why a 4:1 unit is used. Because the antenna is also asymmetrical in practice, common-mode control matters. That is why the hard 1:1 current choke belongs right at the feedpoint.

What wire is supplied

The EFOC17 is supplied with at least:

• 17 meters of wire for the long element
• 5 meters of wire for the short element

These are supplied as usable stock lengths, not as one single mandatory final cut. Depending on the installation and the band family you want to favor, the user cuts the wire to the desired final dimensions.

In other words: the supplied wire is intentionally there to be trimmed to suit the chosen setup. The antenna is not locked to one single pre-cut factory split.

Choose your cut family before trimming

Before you start cutting wire, decide what kind of behavior you want from the antenna.

• If you want to favor the more classic harmonic set, the 16.14 m / 4.04 m family is the traditional choice for 40, 20, 15, and 10 meters.
• If you want to pull the behavior more toward bands such as 17 and 12 meters, the 16.8 m / 3.4 m family can be a better fit.
• If you are starting from scratch and want to experiment, the supplied stock lengths give you room to trim toward the family that best suits your installation.

This matters because an off-center-fed dipole is not just about total length. The long-to-short ratio also affects which bands tend to land more naturally.

Feedpoint unit and choke placement

The intended system is a 4:1 feedpoint unit with a hard 1:1 current choke placed immediately at the feedpoint, on the coax side of that unit. Keep any coax between the 4:1 unit and the choke as short as practical.

Recommended choke for this antenna family: 5 kW ICAS Quad Core High Bands 40-10 m QRO 1:1 current balun/choke.

The reason is simple: without a serious choke at the feedpoint, the outside of the coax can start acting like part of the antenna. That usually makes the result more dependent on the house, mast, roofline, station ground, and cable routing. A second choke near the shack can still help in difficult installations, but it does not replace the feedpoint choke.

Installation shapes that work well

The EFOC17 can be installed in several practical ways:

• As an inverted V
• As a sloper
• As an offset horizontal wire
• As a shallow V
• As a gently bent wire where space is limited

A perfectly straight textbook installation is not required, but cleaner geometry usually gives more predictable multiband behavior. If bends are unavoidable, try to keep the first part of each leg leaving the feedpoint as straight and as clear as possible.

Placement guidance

Put the feedpoint as high and as clear as practical. Keep both wire legs away from gutters, downspouts, metal fences, masts, solar hardware, mains wiring, metal roofs, and large conductive surfaces. Try not to run the short leg tightly along the coax or hard against a wall.

If one side of the antenna must be more compromised than the other, it usually makes sense to give the long leg the clearest route. The short leg still matters, but the long leg tends to dominate more of the overall physical layout.

The coax is the feedline, not the intended missing radiator. If the coax has to radiate to make the antenna work, the choke strategy is not doing what it should.

How to measure the cut lengths

Measure each leg from the electrical connection point at the feedpoint unit to the far end of the wire. If you use loops or folded ends, count the electrically active length, not just the straight mechanical span.

It is good practice to leave a little extra folded back during first installation. That allows you to tune without cutting too early.

How to tune the EFOC17

Always tune the antenna in its final installed position. Do not trim it on the ground and expect it to land in the same place once it is raised.

Start by checking 40 meters first. The lowest intended band sets the overall scale of the antenna. Once 40 meters is reasonably where you want it, re-check the higher bands and see whether the chosen family is landing where expected.

• If resonance on 40 m is too low in frequency, shorten the wire carefully.
• If resonance on 40 m is too high in frequency, lengthen it or unfold previously folded wire.
• Use folded-back tuning first. Cut only when you are sure.

For small final corrections, keep the general character of the chosen family intact. For larger changes in band emphasis, it is usually better to move toward a different known family rather than randomly trimming one side too far out of proportion.

What to expect from 40 through 6 meters

The EFOC17 is a practical multiband compromise antenna. The intended operating range is 40 through 6 meters, but the exact result depends on height, nearby structures, wire routing, and how well common-mode current is controlled at the feedpoint.

On some installations the antenna will cover a wide spread very naturally. On others, one or more bands may want the tuner more often than expected. Higher HF bands and six meters are especially sensitive to nearby coupling and feedline participation, which is one more reason the feedpoint choke matters so much.

Practical installation rule

Decide which band family you want to favor, cut the supplied wire accordingly, install the antenna in its real operating shape, place the hard 1:1 choke at the feedpoint, and only then do the final trimming. Notes taken during trimming are worth keeping, especially if you later experiment with a different long-to-short split.

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