EFOC17 Coax-Counterpoise Antenna Installation Guide
EFOC17 Coax-Counterpoise Antenna Installation Guide
Install the EFOC17 as one complete RF system: a radiator measured and adjusted toward the 16.8 m starting dimension, an integrated 4:1 UNUN, a deliberately controlled section of coax shield and a choke at the boundary of that return path.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
This guide applies specifically to the coax-counterpoise version of the EFOC17. The separately sold two-wire version is an off-center-fed dipole: it uses a short radiator leg and a hard 1:1 current choke directly at the feedpoint. Do not mix the two installation methods.
Current coax-counterpoise configuration: use 16.8 m as the radiator’s starting dimension, approximately 16 m of EXTRAFLEX-BURY-7 coax, and the supplied ferrite choke positioned approximately 3.7 m from the feedpoint. The wire-cutting machine deliberately supplies extra length: unroll and measure the actual wire before marking, folding or cutting it. That first 3.7 m coax section is an intentional part of the RF return path.
Check the Selected Version Before Installation
The EFOC17 platform is available in different power and return-path configurations. Confirm the version on the order and on the current EFOC17 product page before cutting wire or routing coax.
| Configuration | Radiating structure | Required choke boundary |
|---|---|---|
| Coax-counterpoise version covered here | Radiator measured toward a 16.8 m starting dimension, plus the intentional first 3.7 m of coax exterior | Supplied ferrite choke at approximately 3.7 m from the feedpoint |
| Two-wire off-center dipole version | Long and short wire legs form the antenna | Hard 1:1 current choke directly on the coax side of the feedpoint unit |
The transformer assembly may have a much higher published power rating than the supplied baseline choke. The complete installed system is limited by its weakest component and by VSWR, duty cycle, cooling, connector condition and common-mode current. For the coax-counterpoise version, the supplied choke is a practical baseline for approximately 200 W SSB or 100 W FT8 in a clean installation. Use the current product documentation when choosing a higher-power choke.
Choose a Practical Wire Shape
The EFOC17 can be arranged as a flat top, inverted L, inverted U or sloper. These diagrams show useful layout families; they are not scaled construction drawings and do not promise a fixed radiation pattern. Height, bends, soil, nearby metal, vegetation and feedline routing all change impedance and pattern.
Flat top
Run the radiator as straight and clear as the site permits. A straight installation is usually the easiest baseline for tuning and later comparison.
Inverted L
Keep the vertical section clear of conductive masts, gutters, fences and wiring. The bend and the surrounding environment affect both current distribution and azimuth pattern.
Inverted U
Use an inverted U where two supports provide the available span. Avoid sharp folds and keep the descending sections away from reachable conductive objects.
Sloper
A sloper is often convenient with one high support. Its directional behaviour depends on height, slope, ground and nearby structures, so verify the installed result rather than assigning a direction from the drawing alone.
Install the Intended Return Path
Measure before you cut: the supplied antenna wire is intentionally machine-cut longer than the stated radiator dimension. Do not assume that the length in the package is already 16.8 m. Unroll it without stretching it, lay it out straight, measure from the feedpoint connection end, mark the 16.8 m starting point and leave the surplus folded back for initial tuning.
- Mount the EFOC17 enclosure securely and provide strain relief so the wire load is not carried by the connector.
- Unroll and measure the delivered wire. Mark the 16.8 m starting dimension, connect and route that measured length in the chosen shape, and leave the surplus at the far end folded back.
- Route the first approximately 3.7 m of coax from the feedpoint as the intentional return-path section. Keep that geometry stable while tuning.
- Install the supplied ferrite choke at approximately 3.7 m from the feedpoint. The cable beyond that choke is the station feedline, not an uncontrolled extension of the radiator.
- Route the remaining feedline away from the radiator where practical. Add station-boundary or equipment-entry choking only when the measured current map shows that another boundary is needed.
The return section may be routed vertically, horizontally or on a slope to suit the installation, but changing its height, proximity to ground or proximity to other conductors can retune the system. Do not tune with one route and then permanently install it in another.
Commission and Tune the Antenna
Measure at low power with the antenna in its final position. Keep the radiator shape, the first 3.7 m of coax, the choke and the remaining feedline fixed during each comparison. A shack-end SWR reading includes the feedline, so use a calibrated antenna-side reference plane where possible or at least document the cable used.
- Sweep the complete 40 m band first and record both the minimum and the band-edge values.
- Check 20 m, 15 m and 10 m, then check 30 m, 17 m and 12 m if those bands are required.
- Use folded-back wire for the first adjustments. Shortening generally raises resonant frequencies, but the multiband response does not move as one perfectly rigid block.
- Change only a small amount at a time, return the antenna to the same height and geometry, and repeat the complete sweep.
- Use an external tuner where the current product guidance or the installed impedance requires it; a tuner does not correct excessive exterior feedline current.
Do not chase one attractive SWR dip at the expense of the other intended bands. The useful result is stable operation across the required band segments with the return path and choke still doing their intended jobs.
Verify Common-Mode Control
Stable SWR does not prove that the choke is effective. If possible, measure net current around the complete coax at several points before and after the 3.7 m choke. Repeat the measurements with the same transmitter power and cable geometry. Unexpected current below the choke, RF in the shack or strong retuning when the feedline is moved indicates that the installed boundary needs attention.
Mechanical, RF and Weather Safety
- Never install or lower the antenna where any wire, support or tool could contact an overhead power line.
- Keep the radiator and feedpoint out of reach during transmission; end-fed and off-center systems can develop high RF voltage.
- Use supports, insulators and strain relief suitable for the wind load and the installation period. Do not overtension the wire.
- Disconnect transmitting equipment before changing wire, coax or choke placement.
- Weather-seal the SO-239 connection correctly and provide a drip loop without trapping water against the enclosure.
- Lightning protection, bonding and disconnection must follow the station’s local electrical and structural requirements; an RF choke is not a lightning-protection device.
Mini-FAQ
- Which EFOC17 version does this guide cover? The coax-counterpoise version, using 16.8 m as the measured radiator starting dimension, approximately 16 m of coax and the supplied choke at approximately 3.7 m.
- Is the supplied wire already exactly 16.8 m? No. RF.Guru deliberately supplies extra wire. Unroll and measure it before marking, folding or cutting.
- Can I use this guide for the two-wire version? No. The two-wire version is an off-center-fed dipole and uses its hard 1:1 current choke directly at the feedpoint.
- Must the EFOC17 be perfectly straight? No. Flat-top, inverted-L, inverted-U and sloper layouts are possible, but geometry changes impedance and pattern and must remain fixed while tuning.
- Should I cut the wire immediately? No. Start with folded-back adjustment, make small changes and recheck every intended band before cutting.
- Does a low SWR prove that the choke works? No. Check exterior coax current and installation sensitivity as well as SWR.