Everything That Wants a Place on the Rack

Introduction

Before any layout can be drawn, the candidates have to be enumerated honestly — including the ones that are still open questions, because a position left unplanned is a position that will be occupied badly later. This volume inventories every antenna this build might carry, what each one needs, and which of them are transmitters, because that last distinction organizes everything in the volume that follows.

2.1 The candidates

Table 1 — The candidates

Antenna	Band	Role	Ground plane	Status
Cellular MIMO array, 4 lead	600 MHz – 6 GHz	Receive-critical, transmits at low power	Independent designs available	Considering
CompACtenna 2M/220/440 tri-band NMO	144 / 222 / 440 MHz	Transmit, up to 50 W	Not required — chosen for this	Considering
Scanner antenna, NMO 150/450/758	Wideband	Receive only	Required	Considering
Discone	25 MHz – 1.3 GHz	Receive only	Varies by design	Open — “can’t find it”
HF adjustable-coil whip	3–30 MHz	Transmit, up to 100 W	Vehicle body needed	Considering
GMRS antenna	462 / 467 MHz	Transmit, up to 50 W	Depends on model	Open — no radio chosen
GPS puck	1575 MHz	Receive only	Usually integrated	Not recommended — see the Cellular Gateway dive

Seven candidates, four of which transmit, on a rack roughly 60 inches long whose driver’s side is spoken for by an 83-inch awning.

That is the problem in one sentence. There is not room for all of this done properly, and the volume that follows is about which compromises are acceptable.

2.2 The receivers, and what they are worth

The cellular MIMO array is the most demanding item on the list and the least tolerant of a bad neighbourhood. It wants four spatially separated, cross-polarized elements; it wants to hear signals down around −110 dBm; and its front end is wideband by construction — specified 600 MHz to 6 GHz — which means it cannot filter out a strong nearby transmitter simply by being tuned elsewhere. It is simultaneously the antenna most likely to be ruined by its neighbours and the one whose whole purpose is recovering signals too weak to hear otherwise.

The scanner is the second victim. A Uniden SDS100 covers roughly 25 MHz to 1.3 GHz continuously. Wideband receivers have no out-of-band rejection worth the name; that is the price of being wideband. A scanner sharing a rack with a 50 W transmitter will be deaf while that transmitter keys, and there is no configuration that prevents it — only mitigation.

The discone is the same story, more so. A discone is deliberately wideband and deliberately unselective. It is a superb receive antenna in a quiet location and a liability next to transmitters. If one is eventually sourced, it belongs as far from the transmitting antennas as the rack permits, and realistically it may belong on a mast at camp rather than on the truck at all — which is a legitimate answer and worth considering before buying a mount for it.

The Antennas project’s discone-wideband dive covers the antenna type itself; the point here is placement, and the placement answer for a discone on a small transmitting rack is “reluctantly, and far away”.

2.3 The transmitters, and what they cost their neighbours

The tri-band ham antenna is the main aggressor. A mobile rig on 2 m, 1.25 m, or 70 cm at up to 50 W, radiating from a whip a few feet from a wideband cellular receiver, is a serious interference source — not because anything is defective, but because the power ratio is enormous. The gateway is trying to resolve signals around a ten-billionth of a watt while a transmitter a metre away emits fifty watts. Even excellent isolation leaves a large signal.

The HF setup is the most severe and the least compatible with a rack. A Xiegu X6100 driving a 100 W linear, into an adjustable-coil whip, is a different class of problem. HF mobile antennas are electrically short and heavily loaded, which means very high voltages at the coil and a large reactive near field. On HF the vehicle body is genuinely part of the antenna system, so a good installation wants the feedpoint bonded to the truck rather than isolated on an anodised aluminium rack.

The honest recommendation is that HF should not share the rack with the cellular array. The two most natural answers are a hitch-mounted or bumper-mounted HF whip at the rear — as far from everything as the vehicle allows and bonded properly to the frame — or, better for a rig that also camps, treating HF as a stationary-camp activity with a wire antenna thrown into a tree and the X6100 operated from a table. The second option is free, performs far better than any mobile HF whip, and removes the problem from this dive entirely. The random-wire-end-fed and multi-band-dipoles dives in the Antennas project cover that case.

GMRS is undecided and that is fine, but reserve the space. No radio has been chosen. GMRS at 462 and 467 MHz sits close enough to the 70 cm ham allocation that it has essentially the same placement requirements, and a rig carrying both a 70 cm capable ham antenna and a separate GMRS antenna is carrying two antennas that will fight each other for very little gain. Worth deciding before the layout is fixed: whether a single well-placed dual-service antenna, or accepting the ham antenna for ham and a small mag-mount for GMRS when it is actually needed, is better than permanently allocating a rack position to a radio that has not been bought.

2.4 The GPS puck, dismissed on the record

Included in this table for completeness because it appeared on the equipment list, and dismissed for the reasons given at length in the Cellular Gateway dive: on the GL-X3000 the GNSS input shares the diversity port, so feeding it costs receive diversity — the improvement that matters most in exactly the weak-signal conditions this build targets. The truck already carries GNSS in the eTrex, the Gaia tablet, the phone, and the inReach.

No GPS antenna on this rack. If a combination antenna is bought because it is the best cellular antenna available, its GPS lead gets terminated and left unconnected.

2.5 What this leaves

Stripping out what should not be on the rack, the realistic occupancy is:

1. Cellular MIMO array — the receive-critical item, needing the best position available.
2. Tri-band ham antenna — the main transmitter, needing maximum separation from item 1.
3. Scanner antenna — receive-only, tolerant of a mediocre position, intolerant of being close to item 2.
4. Possibly GMRS, if a radio is bought and a dual-service compromise is rejected.

Three, maybe four antennas, on a 60-inch rack with one long side unavailable. That is a solvable problem, and the next volume covers the physics that determines how.