Component 1-A — Wall transition balun (required)

Balun Designs Model 4114

balundesigns.com · approx. $90 · 4:1 current balun · 5kW PEP · 1.5–54 MHz

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. Also provides a 4:1 impedance ratio that translates the wide-ranging impedance on the ladder line to a range your ATU can handle efficiently. Without this device, the coax braid becomes a third conductor — an unintended antenna radiating back into the shack.

Why 4:1 and not 1:1: A pure 1:1 choke at the wall transition does nothing to address the impedance mismatch between the 450-ohm line characteristic impedance and the 50-ohm coax. The 4:1 ratio handles the transition more gracefully and reduces the burden on the ATU. On a multiband non-resonant antenna the impedance presented by the ladder line varies widely across bands — the 4:1 keeps it in a range the ATU can manage cleanly.

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)

Palomar MC-1-1500LL MAXI-CHOKER (balanced stud output version)

palomar-engineers.com · approx. $90 (1.5kW) / $110 (3kW) · 1:1 current choke · 1–61 MHz · ladder-to-ladder

What it does: A 1:1 current balun with balanced ladder line terminals on both sides — ladder line in, ladder line out. Installed at the antenna feedpoint, it 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. The choke has balanced stud terminals on both input and output sides. No coax involved at this position.

If Palomar is unavailable: Balun Designs makes equivalent 1:1 current chokes with balanced input/output. Call them directly. Specify: 1:1 current balun, balanced in, balanced out, rated for your power level, 1.5–54 MHz.

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

Balun Designs Model 1171 (low bands) or Model 1161 (mid/upper HF)

balundesigns.com · approx. $110–$125 · 1:1 ATU current balun · 5kW · coax in/out

What it does: A second choke on the coax run between the wall balun (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.

Which model: Model 1171 cores are optimized for 160M through 40M (1–35 MHz). Model 1161 cores are optimized for 30M through 10M (3–54 MHz). If your primary operating is on the low bands, use 1171. If you are primarily a high-band DX operator, use 1161. If you operate all bands at high power, one of each in series is the complete solution.

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: Components 1-A + 1-B required = approx. $180–$200. With optional 1-C = approx. $290–$325 depending on power ratings selected. Choose 3kW or 5kW-rated components for amplifier operation.

Component 2-A — Antenna feedpoint choke (required)

Balun Designs Model 1171 (low bands primary) or Model 1161 (mid/high bands primary)

balundesigns.com · approx. $110–$125 · 1:1 ATU current balun · 5kW · SO-239 in/out

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 model: 1171 for 160M through 40M primary operation. 1161 for 20M through 10M primary operation. Both cover the full range — the difference is which end of the spectrum has the highest choking impedance. If you operate all bands equally, either works. If you operate both extremes regularly at high power, consider one of each in series.

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)

Balun Designs Model 1171 or Model 1161 (matching selection from 2-A)

balundesigns.com · approx. $110–$125

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. LMR-400-DB and LMR-600-DB are both rated for direct 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 — LMR-400DB or LMR-600DB for direct burial.
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: Component 2-A required = approx. $110–$125. With optional 2-B = approx. $220–$250. Plus coax: LMR-600 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. Reference: Zavrel W7SX, Greyline feedline paper (Plum Valley Systems LLC, January 2025).

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 on Amazon, Grainger, and 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 Zavrel paper documents the comparative analysis.

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

Southwire flexible metallic conduit with attached PVC jacket — Home Depot — 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. Reference: Zavrel W7SX, Greyline feedline paper.

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 Southwire combined product is easier to install.

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

Balanced-line ATU — or a balun 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 4:1 current balun at the wall (same as Kit 1 Component 1-A — Balun Designs Model 4114) handles the balanced-to-unbalanced transition. The Zavrel paper notes the Johnson Matchbox as an example of an excellent balanced-line tuner — it tunes balanced line directly and uses an air-core 50-ohm balun rather than a ferrite balun. 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 4:1 current balun.
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.