Routing Coax for HF Antennas (1–30 MHz)

Listen to enough antenna installations and you will hear sensible operators give opposite instructions. “Bury the coax to stop RF.” “Never bury coax because the SWR changes.” “A dipole feed line must leave at exactly a right angle.” “An EFHW needs one magic cable length.” Most of the conflict comes from answering two questions at once.

The first question is mechanical: will the cable survive water, sunlight, pulling, crushing, bending, animals, vehicles, temperature, fire rules and future maintenance? The second is electromagnetic: which current modes exist, what couples to nearby conductors and where does the antenna end? A good route must answer both, but a trench cannot replace a current boundary and a choke cannot make an unsuitable jacket safe underground.

Start With the Two Coaxial Current Paths

In the wanted coaxial transmission-line mode, current flows on the centre conductor and the inner surface of the outer conductor with an equal-and-opposite terminal relationship. The electric and magnetic fields are largely confined between those conductors in a well-constructed line. Nearby objects do not become part of that ideal differential path merely because the jacket passes near them.

The outer surface of the shield can support an additional current relative to the mast, earth, station and other conductors. Call it common-mode, sheath or feedline-exterior current—but state what is being measured. ITU-T K.136 explicitly treats unwanted common-mode current on the external conductor of a shielded transmission line as an EMC test variable.

The modes can coexist. A 50 Ω-looking differential load does not prove that exterior current is zero, and exterior current does not mean that normal transmission inside the coax has stopped. Routing changes matter electromagnetically when they change the exterior-current path, induce current on the shield or nearby conductors, or alter the antenna environment.

Mechanical Protection and RF Control Need Separate Evidence

Do not use an RF symptom as the only mechanical acceptance test. A cable can show a good SWR while its jacket admits water or its bend has begun to deform the dielectric. Conversely, a mechanically excellent trench can change an antenna whose coax exterior was deliberately or accidentally carrying current.

Parallel Runs Increase Opportunity for Coupling

Two nearby conductors couple through electric and magnetic fields. The result depends on separation, common parallel length, orientation, conductor geometry, terminations, frequency and the current/voltage distribution at that location. “Parallel is bad” is directionally useful, but it is not a distance specification.

Near a high-voltage region of an antenna, capacitive coupling may dominate. Near a high-current region, inductive coupling may be more important. A shield can reduce transfer into the wanted internal coax mode, yet the shield exterior is itself a conductor that can carry induced current. A mast, fence, gutter or conduit can also become an induced-current path.

Reduce unnecessary close parallel length when the layout permits. When it does not, model the conductors or compare exterior current and station behaviour before and after the route change. State distance in metres and wavelengths, because the same physical gap means something different at 1 MHz and 30 MHz.

A Right-Angle Crossing Is a Preference, Not Isolation

Crossing two locally straight conductors near 90° can reduce accumulated mutual coupling compared with a long close parallel run. It does not make electric-field coupling, fringing fields, common impedance or a safety conflict disappear. The benefit also shrinks when the conductors curve back together or remain close elsewhere.

Never use the RF preference to override the required separation from mains, lightning conductors, buried utilities or other services. IEC TR 61000-5-2 and ITU-T K.37 treat routing, separation, bonding and cable class as a system EMC problem. The applicable electrical and construction rules decide which crossings and shared pathways are permitted.

Feedpoint Departure Has No Magic Angle or Distance

For a straight dipole, leaving approximately perpendicular to the wire is a useful first geometry because it avoids an immediate long parallel run beside either arm. It is not necessary to achieve exactly 90°, and “a few metres” is not the same electrical distance on every HF band.

The best departure depends on the current distribution, antenna height, feed-line-exterior current, mast, support and nearby structures. Route so the line does not become a strongly coupled third conductor, while preserving strain relief, bend radius and safe access. Verify the result rather than trimming the garden to fit a diagram.

Physical symmetry does not prove current balance. A centre-fed dipole can still drive exterior current through unequal arm environments or an asymmetric feed transition. An off-centre-fed dipole does not demand one universal transformer ratio or choke location. The feed impedance, transformer topology and installed common-mode loop must be declared.

Place a Choke at the Boundary You Intend

A common-mode choke inserts complex impedance into one exterior-current path. It does not “force the coax to be coax” in every possible loop, and its label does not establish its impedance, loss, voltage, current or temperature on 1–30 MHz.

If the coax exterior must be excluded from the antenna, the choke normally belongs at the intended antenna/feed-line boundary. If a deliberate length of coax exterior is part of an end-fed return structure, the choke may define the far end of that section. A second choke at the building entry is justified only when it establishes another measured EMC boundary; it is not an automatic cure and it is not a lightning protector.

Measure exterior current at several cable positions before and after the change. Standing-wave maxima and minima move with frequency and route, so one quiet point is not enough. Check complex feed-point impedance at the same reference plane, receive noise, pattern or station RFI as appropriate. For transmitting systems, qualify choke heating, insulation and common-mode voltage/current at the actual power, duty cycle and mismatch.

End-Fed Antennas Need a Declared Second-Terminal Structure

An EFHW matching network has two terminals on each side even when one radiator wire dominates the drawing. Current at the transformer’s reference side couples into some combination of a deliberate counterpoise, enclosure, transformer capacitance, coax exterior, station wiring and earth. The route becomes part of the antenna when material current flows on it.

There are two valid high-level strategies. Define a separate return structure and place a choke where the long feed line begins; or deliberately include a declared section of coax exterior and place the common-mode boundary after it. Both require installed measurement. “No counterpoise” merely leaves the second-terminal coupling unspecified.

Coax length needs careful interpretation. A transmission line transforms complex impedance between reference planes even when exterior current is negligible. Loss also changes the measured SWR magnitude. Therefore, a different impedance reading after adding cable does not by itself prove common mode. Sensitivity to cable route or shape, confirmed exterior current, pattern change or a controlled choke test is stronger evidence that the outside of the line is participating.

Verticals and OCFs Do Not Get Exemptions

A ground-mounted vertical with radials may provide a strong deliberate return structure, but radial count, soil, base bonds, nearby conductors and feed-line route still decide exterior current. An elevated vertical can be well controlled with a designed radial/counterpoise system; a “no-radial” label does not explain its return path.

Off-centre-fed dipoles often present a more challenging balance and transformation problem because the arm impedances and environmental coupling differ. That does not prove that every OCF needs a 4:1 transformer, a feed-point choke and another entry choke. Use the design impedance and topology, then measure exterior current across every operating band.

For every antenna family, the practical rule is the same: name the intended radiator and return conductors, place a current boundary where the design needs one and route the remaining feed line as a protected transmission line.

Burial Starts With the Exact Cable Part Number

“Outdoor coax,” “direct burial,” “watertight,” “riser” and “general purpose” are not interchangeable jacket claims. The Times Microwave LMR catalogue, for example, lists distinct LMR-400 variants for outdoor, outdoor/watertight and indoor/outdoor riser use. That is evidence for reading the exact datasheet—not for assuming that every black 10 mm coax can enter soil.

Use a cable explicitly rated for the wet, chemical, temperature, UV, crush, pulling and fire environment it will encounter. Underground conduit is commonly wet from condensation or water entry, so conduit does not automatically turn a dry-location cable into a wet-location cable. Metallic conduit also introduces a bonded conductor that can change exterior-current coupling; non-metallic conduit still changes spacing and mechanical conditions.

Burial depth, separation, warning tape, conduit, vehicle loading and permissible co-location depend on jurisdiction, soil and land use. Locate every underground service and obtain the required permission before digging. Do not publish or copy one universal depth.

Buried splices and connectors are not universally forbidden, but they must be part of a manufacturer-rated sealed assembly and installation method for that environment. An accessible above-ground enclosure is usually easier to inspect and replace. Use compatible sealing systems; some tapes, mastics and solvents attack jackets or trap water when applied incorrectly.

Bend Radius Is a Datasheet Quantity

Coax can be damaged without an obvious kink. Excess bending can move the centre conductor, ovalise the shield, crush foam dielectric, change impedance or weaken the jacket. Connector transitions and repeated-flex locations may require more space than a one-time installation bend.

Use the exact manufacturer’s minimum installation radius, repeated-bend radius, pulling tension, crush and torsion limits. They vary substantially among cables that look similar. The current Times Microwave LMR guide, for example, lists different installation and repeated-bend radii for several members of one product family. No generic “ten cable diameters” rule can replace the selected cable record.

Support the cable so connectors do not carry its weight. Allow a drip path where appropriate, but use a proper entry gland or rated feedthrough rather than relying on a loop alone. Keep outdoor connectors inspectable and follow the connector manufacturer’s preparation, torque and sealing instructions.

Building Entry Is a Safety and EMC Boundary

The entry point must manage water, strain, fire spread, bonding, surge protection and cable routing together. The required sleeve, gland, fire stop, cable jacket transition, bonding conductor and surge protective device depend on the building and local rules.

Lightning protection is not achieved by adding a common-mode choke or by burying the feed line. IEC 62305-3 and IEC 62305-4 treat external lightning protection, bonding, separation, lightning protection zones and surge protection as coordinated parts of a system. ITU-T K.21 defines equipment resistibility tests for external coaxial ports; passing or citing an equipment test is not proof that an amateur installation is protected.

Diagnose the Path Before Moving Hardware

A change when you touch or move coax is a clue, not proof of common mode. It can also reveal a loose connector, damaged shield, changing capacitance, a shifting reference plane or a faulty instrument setup. RF in the shack, receive-noise changes and cable-length sensitivity likewise have several possible causes.

Use a controlled sequence:

1. Document the route. Record cable part number, length, connector/splice locations, burial/conduit, antenna geometry, bonds and nearby conductors.
2. Inspect mechanically. Check jacket, water evidence, support, connector preparation, bend radius and strain before interpreting RF.
3. Fix the reference plane. Calibrate at the feed point or de-embed the measured cable, and record complex impedance—not only the radio’s SWR display.
4. Scan exterior current. Use the same calibrated or repeatable clamp-on probe at several marked positions on every band of interest.
5. Change one variable. Compare route, temporary support, choke position or deliberate return geometry with rapid A/B/A measurements.
6. Measure the objective. Record exterior current, accepted power, field/pattern, receive SNR or station RFI under unchanged settings.
7. Finish mechanically. Only after the RF boundary is stable, complete permanent supports, sealing, burial and labelled entry work.

A route is successful when the exact cable survives its environment, the antenna currents follow the declared design, the station meets its RF objective and the entry satisfies the governing safety system. That answer will remain useful long after one-line routing rules have contradicted each other again.

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