Component 1-A — Wall transition choke (required)

1:1 current choke, balanced studs in, SO-239 out

Spec: 1:1 current balun (choke) · rated for your power · 1.5–54 MHz · weatherproof housing · Greyline ships RF choking with every antenna; higher ratings on the site

What it does: Manages the balanced-to-unbalanced transition at the shack wall. Presents a high common-mode impedance to current trying to flow on the outside of the coax braid. Without this device, the coax braid becomes a third conductor — an unintended antenna radiating back into the shack.

Why 1:1 and not a transformer: The impedance on the ladder line varies widely across bands on a non-resonant antenna, and the wide-range ATU exists to handle exactly that. A 1:1 current choke does the one job the transition needs — stopping common-mode current — without adding a transformation the tuner did not ask for. Some operators run a 4:1 or 9:1 at this point for their own reasons, and it can work; it is not something we prescribe.

Placement: Outside the shack wall, at the exact point where the ladder line arrives. Connect the two ladder line conductors to the balanced stud terminals. Run the shortest possible coax from the SO-239 output into the shack. Weatherproof. Permanent outdoor installation.

Component 1-B — Antenna feedpoint choke (required)

Greyline feedpoint choke — included with every antenna; upgrade by power

1:1 current choke · 1–61 MHz · ships in your box · higher ratings: the 3500W Maxi and the 5kW beads

What it does: A 1:1 current choke at the antenna feedpoint chokes common-mode current at the source, before it can travel down the ladder line toward the shack. This is the most important choke in the system for an OCF antenna.

Why feedpoint placement is primary: Because the VDA is off-center fed, the feedpoint is where current imbalance originates on some bands. The feedline between the antenna and any downstream choke can itself behave as part of the antenna on certain frequencies — effectively making the choke at the wall less effective for those bands. Placing the choke at the feedpoint stops the problem at its origin.

Placement: At the antenna feedpoint connections, between the antenna's feed studs and the ladder line conductors. No coax involved at this position.

Any equivalent 1:1 current choke with balanced input and output, rated for your power level and 1.5–54 MHz, does the same job. Call us if you are unsure which rating you need.

Component 1-C — Station entry choke (optional, recommended at amplifier power)

Greyline Maxi Line Isolator, or any 5kW 1:1 coax current choke

1:1 current choke · coax in/out · 3500W Maxi · equivalents approx. $110–$125; choose a low-band or high-band core to match where you operate most

What it does: A second choke on the coax run between the wall choke (1-A) and the ATU. Provides a second line of defense against any residual common-mode current on the short coax run inside the shack. At legal-limit power with an amplifier, this is a meaningful addition.

Placement: Between the short coax coming from Component 1-A and your ATU input. Can be installed inside the shack at the ATU input or just outside the shack entry point.

Wiring diagram — signal path, Kit 1

Setup sequence — Kit 1

1. Install Component 1-B at the antenna feedpoint before any RF is applied.
2. Route 450-ohm ladder line from feedpoint to shack wall. Keep it clear of metal.
3. Install Component 1-A at the shack wall exterior.
4. Run shortest possible coax from 1-A to ATU input. Install 1-C here if using it.
5. First session: 20 meters, 100 watts maximum. Tune. If the ATU tunes on 20M, it will tune on every band.
6. Check for RF feedback: touch the mic, the key, the computer — no tingle means clean.
7. If specific bands behave differently (won't tune, noisy, feedback), note which ones. That pattern tells you which choke position needs attention or upgrading.
8. Once confirmed clean at 100W, work up through the bands.
9. Add the amplifier last. Start at reduced power. Confirm each band. Then full power.

Total investment — Kit 1: the feedpoint choke ships with the antenna; add a 1:1 wall transition choke rated for your power. With the optional station-entry choke, two chokes. Choose the 3500W or 5kW-rated chokes for amplifier operation.

Component 2-A — Antenna feedpoint choke (required)

Greyline RF Choke / Line Isolator — choose by power

1:1 current choke · SO-239 in/out · 1–61 MHz · 500W Mini · 3500W Maxi · 5kW slip-on beads · or any 5kW 1:1 coax current choke of the same class, approx. $110–$125

What it does: Stops common-mode current at the antenna feedpoint, preventing the coax outer braid from becoming a third conductor and an unintended radiator. Without this device, RF travels down the outside of the coax shield into the shack. The result is noise on receive, RF feedback on transmit, and equipment problems at high power.

Which rating: 500W for a barefoot station. 3500W Maxi for an amplifier. The 5kW beads for legal-limit operation with margin. Rate the choke above your amplifier's peak, not at it.

Placement: At the antenna feedpoint, coax side. This is the first device the coax connects to when it leaves the antenna. Mount it securely at the base of the antenna mast or at the feedpoint connection point. Weatherproof.

Component 2-B — Station entry choke (optional, recommended at amplifier power)

A second Greyline choke, matching the rating of 2-A

What it does: A second choke at the shack entry — between the coax coming in from outside and the ATU input. Catches any residual common-mode current picked up along the coax run (from nearby wiring, grounding differences, or induced RF from the antenna near-field). At legal-limit power, this is a worthwhile addition. If you run 100W it is optional. If you run a kilowatt, add it.

Coax selection guide

PVC-jacketed coax is not recommended for permanent outdoor or buried HF runs — UV degradation and moisture ingress over time compromise the cable's electrical performance. Use polyethylene-jacketed or foam-dielectric direct-burial coax rated for burial.

Wiring diagram — signal path, Kit 2

Setup sequence — Kit 2

1. Install Component 2-A at the antenna feedpoint before any RF is applied.
2. Run coax from 2-A to shack. Bury if possible — direct-burial-rated cable.
3. Install Component 2-B at the shack entry or ATU input (optional, add now or later).
4. First session: 20 meters, 100 watts maximum. Tune. Your ATU will find a match. If it tunes on 20M, it tunes everywhere.
5. Check for RF feedback. No tingle means clean installation.
6. Work through the bands at 100W. Then add power. Add the amp last.

Total investment — Kit 2: a Greyline choke from $65 (500W) to the 3500W Maxi. With optional 2-B, two chokes. Plus coax: LMR-600 class runs approximately $2.50–$3.50/foot depending on source. A 150-foot run = $375–$525. Budget accordingly for your run length.

Why not PVC wire insulation?

PVC is the most common wire insulation and is disqualified for two reasons. First, most PVC-insulated wire is not rated for the high voltages present on a feedline operating at elevated SWR with full power. Second, PVC has a poor loss tangent at RF frequencies — it absorbs a measurable fraction of the RF energy passing through it as heat. At 60 Hz the loss tangent is negligible, which is why PVC works fine in household wiring. At HF and above, it is significant and measurable. Silicone rubber avoids both problems.

Component 3-A — Wire (required)

Silicone rubber-insulated 2-conductor wire — #16 AWG minimum, #14 AWG preferred — high-voltage rated

Search: "silicone mini split wire 2-conductor" or "16 AWG silicone high-temperature 2-conductor" — available from electrical suppliers. High-voltage rating of several thousand volts required.

Why silicone: Silicone rubber provides excellent voltage breakdown and adequate loss tangent up into the VHF range. It is also available in #16 AWG copper, the optimal wire size for this balanced-line application. Reasonable flexibility for pulling through conduit; reasonable cost. The dielectric comparison is standard material in Zavrel W7SX (ARRL) and the ARRL Handbook.

Wire gauge: #16 AWG is the minimum and preferred gauge. #14 AWG is also acceptable and reduces conductor resistance on longer runs.

Component 3-B — Conduit (required)

Metallic flex conduit with PVC jacket — ½" or ¾" ID

Flexible metallic conduit with attached PVC jacket — any electrical supplier or home center — approx. $0.75–$1.25/ft. Both ½" and ¾" interior sizes available.

Why metallic conduit: Burying a balanced line directly in soil creates problems — soil conductivity and moisture variation alter the line's impedance, and proximity to soil causes loss. The metallic conduit forms a shield around the balanced pair, decoupling the line from soil effects entirely. The outer PVC jacket weatherproofs the assembly for direct burial.

Size: ½" ID for two #16 AWG conductors. ¾" ID for two #14 AWG conductors or runs with multiple bends.

Alternative: Separate metal conduit inside non-metallic conduit achieves the same shielding at lower material cost. The jacketed combined product is easier to install.

Component 3-C — Shack-end transition (required)

Balanced-line ATU — or a 1:1 current choke at the wall transition point

Two paths work. If your ATU has balanced output terminals, the wire runs directly to them. If your ATU has coax output only, a 1:1 current choke at the wall (same spec as Kit 1 Component 1-A) handles the balanced-to-unbalanced transition; some operators choose a 4:1 or 9:1 here for their own reasons, which we do not prescribe. The Johnson Matchbox remains the classic example of an excellent balanced-line tuner — it tunes balanced line directly and uses an air-core balun rather than ferrite. The specific hardware choice is the operator's; the principle is to maintain the balanced-line characteristics until the ATU.

About half of Greyline buyers use a shack desktop ATU successfully. See the ATU selection guide for the full options discussion.

Why lower loss than coax under high SWR — the physics

A coaxial cable operating at elevated SWR loses more power than its matched-load specification indicates. The additional loss scales with SWR and run length. A balanced open-wire line with low-loss dielectric insulation has only conductor losses — no significant dielectric loss — and those conductor losses are substantially lower under high-SWR conditions than equivalent coax. This is well-established transmission line physics, consistent with Kraus W8JK ( Antennas ) and the ARRL Handbook transmission line loss tables.

The metallic conduit solves the one problem that makes direct-buried open wire impractical: soil coupling. Without the metallic shield, soil conductivity and moisture variation alter the line's impedance. The conduit prevents that, giving a balanced line the burial characteristics of coax while retaining the loss advantages of open wire.

Wiring diagram — signal path, Kit 3

Installation notes

1. Pull both wire conductors through the conduit before burial. Use wire-pulling lubricant on longer runs.
2. Leave 18–24 inches of wire extending from each end for connections.
3. Seal conduit ends with weatherproof silicone sealant to prevent moisture ingress.
4. Mark conductor polarity (which wire is which) at both ends before burial and keep it consistent throughout the installation.
5. Connect conductors directly to the VDA feedpoint studs. Weatherproof with self-amalgamating tape.
6. At the shack end, connect to balanced ATU terminals or to the balanced studs of a 1:1 current choke.
7. Bury at least 6 inches deep, 12 inches preferred.

Kit 3 vs. Kit 2 (coax): Under the high-SWR multiband conditions of a VDA installation, Kit 3 has lower feedline loss and lower material cost per foot than coax, and can be buried permanently. The trade-off is installation discipline — wire must be pulled through conduit, connections must be carefully weatherproofed, and conductor polarity must be tracked from feedpoint to shack. Kit 2 (coax) is plug-and-play. For short runs at modest power the practical difference is small and coax is the simpler choice. For long permanent runs at full power, the physics favors Kit 3.