Mounts, Feedline, and Getting the Coax Inside

Introduction

The hardware between the antenna and the radio is where installations quietly fail. An excellent antenna in an excellent position, fed by a poor cable through a badly made entry, performs worse than a mediocre antenna done properly — and unlike position, none of this is adjustable later without redoing the work. This volume covers the mounts, the cable, the connectors, and the hole in the truck.

4.1 Mounts

NMO is the right standard for the whips. It is the professional mobile mount, it is mechanically robust, the antenna unscrews from the base for garage clearance, and every relevant whip is available with an NMO base. The mount requires a 3/4-inch hole in whatever it is mounted to.

That hole should go in a fabricated plate, not the rack and not the truck. Volume one made this argument on reversibility grounds; it holds equally on electrical grounds, because the plate is also the ground plane. A single part solves both problems:

Aluminium, thick enough not to oil-can — 1/8 inch is ample, 3/16 is better if it also carries a whip’s leverage.
Sized for the band: the guideline is roughly 6 inches of surface around the base at UHF, 18 inches at VHF, more for 2 m. A plate that satisfies 2 m properly is a large plate, and this is where the no-ground-plane antennas earn their price.
Anodising removed under the NMO mount and under every bolt, because anodised aluminium is an insulator and an NMO mount relies on the electrical continuity of that joint. A scuff with abrasive paper and a smear of anti-seize is the whole technique.
Bolted into the T-slot with T-nuts so it can be repositioned.

For the no-ground-plane antennas, a bracket is enough. The CompACtenna tri-bander does not need the plate and is on the list precisely for that reason. The cellular MIMO panel will almost certainly bolt flat to a rack member or a small sub-plate with its own hardware.

Avoid mag-mounts as permanent fixtures. They need steel — the rack is aluminium — and they walk, scratch, and depart at highway speed. A mag-mount on the cab roof is a perfectly good temporary answer for a GMRS antenna that gets used occasionally, and that may be the right call given GMRS is undecided.

4.2 Feedline

The Cellular Gateway dive established the cable choice for the cellular runs. It applies to the whole rack.

Use LMR-240 or an equivalent, not RG-58. RG-58 loses roughly 9.8 dB per 100 ft at 440 MHz where LMR-240 loses about 4.4 dB, and the gap widens with frequency, so the penalty at 700 MHz and again at 1900 MHz is worse than those figures imply. On a fifteen-foot run the absolute loss is a fraction of a decibel either way at VHF and rather more at cellular frequencies — but it is subtracted from a link budget that is already failing, and on the cellular side it is subtracted from all four chains simultaneously.

LMR-400 is better still and is stiff enough to be a genuine routing nuisance in a vehicle. At these lengths it is rarely worth the trouble except on the HF run if one is built.

Rules that matter more than cable grade:

Keep runs short. Every foot not run is loss not incurred, and it is the strongest practical argument for mounting the gateway in the cab rather than at the tailgate.
Buy antennas with integrated pigtails where the option exists. Most vehicle MIMO antennas ship with a specified length of attached cable, which removes four connector pairs and four chances of a bad crimp.
Respect the bend radius. LMR-type cable has a foam dielectric that deforms permanently when kinked, and a kinked cable is a mismatch that no amount of tuning fixes. Sweeping curves, not corners.
Minimise adapters. Each is loss and a failure point on a vehicle that vibrates continuously.
Leave a service loop at both ends. Vehicles flex, racks are removed, and a cable cut exactly to length is a cable that will be cut too short the first time anything moves.

4.3 Connectors, and a gender trap

The gateway’s cellular ports are SMA-female, so cable ends must be SMA-male. Its Wi-Fi ports are RP-SMA-female, which look nearly identical and are not interchangeable. Ham and scanner gear will generally be UHF (PL-259/SO-239) or BNC, and NMO bases are supplied with a choice of pigtail terminations.

The failure mode is banal and common: an antenna arrives with RP-SMA leads, or a run is built with the wrong end, and it is discovered at a campsite four hours from a supplier. Write the required connector at each end of every run on the layout drawing before ordering anything.

Weatherproofing matters on a truck that is driven through weather by design. Outdoor connections get self-amalgamating tape over the joint and electrical tape over that — in that order, because the amalgamating tape is the seal and the electrical tape is the UV jacket.

4.4 The hole in the truck, which is the one irreversible decision

Coax has to get from the rack to the radios inside, and this is the step that cannot be undone.

Options, best first:

An existing grommet or wiring pass-through. Every truck has factory penetrations into the cab and the bed. Finding one and enlarging or reusing it is vastly preferable to making a new hole, and it is almost always possible. This is worth an hour with a flashlight before any drill is picked up.
A proper bulkhead feed-through plate in a location chosen once, carrying all runs together, sealed with a purpose-made gland. If a hole must be made, one well-made hole carrying every cable is far better than several.
Under a door seal. Workable for one or two thin cables, marginal for LMR-240, and a source of slow leaks and pinched jackets. Acceptable as a temporary arrangement, not as the final one.
Through the tailgate gap or an open bed. Fine for a temporary setup, bad permanently — the cable chafes and the bed is not sealed.

Whichever is chosen: seal it properly, from both sides, with a drip loop. Water runs along cable jackets, and a cable that enters the cab on an upward slope delivers water into the cab. A loop below the entry point makes water drip off rather than travel in.

Corrosion protection at every bare-metal cut is not optional on a vehicle that sees Michigan road salt. Any hole drilled through painted steel gets deburred, primed, and sealed.

4.5 Bonding and grounding

Distinct from the ground plane discussed in volume one, and worth doing regardless:

Bond the rack to the truck body with a proper strap — braid or heavy wire, short, to clean metal at both ends. The rack sits on rubber-isolated mounts and anodised hardware and cannot be assumed to be electrically connected to anything.
Bond the ground plates to the rack at the plate, not through the antenna mount.
Common-mode current on the outside of the coax braid is the usual cause of “the radio makes noise on the truck’s electronics” and “the truck’s electronics make noise on the radio”. A ferrite choke or a few turns of coax through a large ferrite core near the feedpoint is a five-dollar fix for a class of problem that is otherwise very hard to diagnose. The baluns-ununs dive in the Antennas project covers why.
Modern vehicles are electrically sensitive. A 2026 truck is full of CAN bus and driver-assistance electronics. Never take a radio’s ground from a random body bolt near a control module; run the negative back to the battery or to a properly established distribution point. This ties into the Vehicle Power dive.

4.6 A note on the awning, which will be in the way

The 83-inch Topoak occupies the driver’s side and its brackets need 4 inches of clearance from the corner. It also sweeps through a large volume when deployed, and it is a fabric structure that will chafe against anything sharp.

Two consequences for cabling: no coax should be routed where the awning arms travel, and any run along the driver’s side should be inside the T-slot channel or under a cover rather than exposed. It is worth deploying the awning fully once, before any cable is secured, and marking with tape where the arms actually go. That five-minute exercise prevents a category of damage that is otherwise discovered by hearing it.