The Coaxial Feed Line in a Ground Control Station: Cable Classes, Losses and Connectors

A signal booster and a remote antenna are joined by cable, and whatever that cable absorbs is not recovered by a better antenna or a stronger transmitter. The "Cables and Adapters" category holds at least five structurally different classes, from a half-metre pigtail to a 7/8-inch corrugated feeder, and they are not interchangeable. What follows covers where the losses come from, what every metre of the run costs in decibels at 2.4 and 5.8 GHz, and the logic behind assembling a line for a ground control station or a repeater.

How a coaxial cable is built

A coaxial line has four layers. The centre conductor carries the signal; in most cables of this class it is solid copper-clad aluminium (CCA), less often stranded copper. The dielectric holds the conductor on axis and sets the characteristic impedance. The outer conductor (the shield) carries the return current and blocks external fields. The jacket protects all of it from ultraviolet, moisture and mechanical damage.

The key difference between cheap and expensive cable sits in the dielectric and the shield. Solid polyethylene gives a velocity of propagation around 66 %, foamed polyethylene 84–87 %. Foamed PE is half gas, so dielectric losses in it are markedly lower. Shields come in three forms: a single braid (typically around 40 dB of shielding), foil plus tinned braid (over 90 dB), and a corrugated copper tube (hermetic, with the best figures, but with a large bend radius).

Where the decibels come from

Cable loss has two components. Conductor loss rises with the square root of frequency, because the skin effect confines the current to an ever thinner layer of metal. Dielectric loss rises linearly with frequency. Times Microwave publishes an exact attenuation formula for LMR-400 (the size class that RG-8 and Kingsignal KSR400 belong to), in dB per 100 feet:

A = 0.12229 · √F(MHz) + 0.00026 · F(MHz)

At 900 MHz this gives 3.67 + 0.23 dB, so the dielectric accounts for 6 % of total loss. At 2400 MHz it is 5.99 + 0.62 dB (9 %), at 5800 MHz 9.31 + 1.51 dB (14 %). This is why cheap cable with a solid dielectric behaves reasonably at VHF/UHF and falls apart at 5.8 GHz.

Converting decibels is simple in practice: 3 dB is half the power, 6 dB a quarter, 10 dB a tenth. Feed-line loss works in both directions. On transmit it reduces radiated power, on receive it reduces the level arriving from the aircraft, so 6 dB in the feeder eats into the link margin twice per communication cycle.

Shielding and VSWR

At a position where a repeater, an EW system and several controllers all operate side by side, shielding stops being a secondary parameter. Single-braid cable lets a neighbouring transmitter's own radiation into the receive path, and the operator gets a raised noise floor instead of range. The second quantity is characteristic impedance. All equipment in this class is built for 50 ohms; KSR400 is specified as 50 ± 2 ohms up to 3 GHz with VSWR no worse than 1.28. Every mated connector pair adds its own mismatch, which is exactly why a chain of three adapters is worse than one properly crimped connector.

Cable classes in the category

Pigtails and super-flexible jumpers. Items such as the 50 cm flexible QMA male to N-Type female transition, or the universal 20–30 cm QMA/SMA/N-type adapter, are built on RG-223: roughly 5.3 mm in diameter, double silver-plated shield, silver-plated copper centre conductor. RG-223 attenuation is 8.8 dB per 100 feet at 400 MHz against 2.5 dB for LMR-400, three times as much. Over 30–50 cm that comes to tenths of a decibel, and the gain in flexibility is worth it. Over five metres the same cable is unacceptable.

The trunk class: RG-8 / LMR-400 / KSR400. This is the backbone of the range: QMA–QMA and N-Type–N-Type extension cables in 10, 15, 20, 30 and 40 metre lengths, sold in pairs of two runs. Construction: solid 2.74 mm CCA conductor, 7.24 mm foamed PE, aluminium foil with a tinned copper braid, 10.29 mm overall diameter. Characteristic impedance 50 ohms, capacitance 78.4 pF/m, velocity of propagation 84 %, shielding above 90 dB, operating temperature −40 to +85 °C. Bend radius is 25.4 mm during installation and 101.6 mm for repeated bending. Attenuation per the manufacturer's formula: 21.7 dB/100 m at 2400 MHz and 35.5 dB/100 m at 5800 MHz.

RG-213 in copper. QMA drone kits built on RG-213 from the Ukrainian manufacturer Odeskabel use a stranded copper conductor, solid PE and a single 96 % braid. Velocity of propagation is 66 % and attenuation 5.5 dB per 100 feet at 400 MHz, more than twice as lossy as LMR-400 of the same diameter. The penalty is offset by cold tolerance, resistance to repeated bending and field repairability, which is why these kits are stocked at 5 metres rather than 30.

KSR500 (LMR-500 size class). Diameter 12.7 mm, flexible PVC jacket, the same "foamed PE plus foil and braid" principle but with a larger cross-section and correspondingly lower ohmic loss. The dealer specification for a 20-metre N-male to N-male assembly gives no more than 3.4 dB at 2400 MHz and no more than 6 dB at 5800 MHz.

Corrugated 1/2" and 7/8" feeders. Kingsignal HCAAY-50-9 and the super-flexible HCAAYZ-50-9 in an LSZH jacket (Ø13.4 mm), HCAAY-50-12 (Ø15.7 mm) and HCTAY-50-22 (7/8", Ø27.3 mm). Their outer conductor is a corrugated copper tube, so the shield is hermetic and has no braid gaps. For HCAAY-50-12 the manufacturer specifies a 4.80 mm CCA conductor, 12.30 mm foamed PE, a 13.80 mm corrugated copper tube, capacitance 76 pF/m, velocity of propagation 87 % and a weight of 215 kg/km. Bend radius is 50 mm for a single bend and 125 mm for repeated bending, which is the main operational constraint of corrugated cable. A 20-metre assembly on 1/2" super-flexible feeder gives no more than 3.2 dB at 2400 MHz and 5.45 dB at 5800 MHz. HCTAY-50-22 is specified by its manufacturer over 100–5800 MHz.

Connectors. QMA was developed by the Quick Lock Formula alliance in 2003 as a quick-disconnect replacement for SMA: the same internal geometry, but a snap-on latch instead of a thread, with 360° rotation after mating. Amphenol's documentation gives up to 6 GHz for the QMA connectors themselves and up to 18 GHz for adapters in the same series; either reading leaves margin at 2.4/5.2/5.8 GHz. SMA works from DC to 18 GHz, N-type to 11 GHz. On a mast and in the field N-type is preferred: threaded coupling, a larger body, easier weatherproofing. On the controller side QMA is more convenient, mating without a wrench and not stressing the cable when rotated.

Calculating the run in decibels

The RG-8 figures come from the Times Microwave formula for bare cable; the other two rows are dealer specifications for finished assemblies that already include a pair of connectors. Direct comparison is only valid with that correction in mind.

For a forty-metre RG-8 run this works out at roughly 8.7 dB at 2.4 GHz and roughly 14.2 dB at 5.8 GHz. The first means losing about seven eighths of the power, the second over ninety-six percent. The familiar situation where the 2.4 GHz link holds while 5.8 GHz collapses at the same position is explained by this difference, not by a "bad amplifier".

Selection in practice

The amplifier belongs as close to the antenna as possible. A long cable is appropriate between the station and the mast, not between the amplifier and the radiator; in the latter case you amplify the signal only to throw it away immediately.

Length is chosen by actual distance with a modest allowance. Forty metres with twenty of them coiled under the table costs the full loss of forty metres. A coiled surplus also behaves as an inductance and degrades matching.

Cables are sold in pairs because the amplifier duplicates both antenna channels of the controller. Both runs must be the same type and the same length; a difference in attenuation between the branches unbalances diversity reception and defeats its purpose.

The jacket is chosen by installation site. PVC is cheaper and more flexible, PE resists ultraviolet and suits permanent outdoor routing, LSZH is mandatory wherever the cable runs inside a shelter, shelter-body or vehicle, since it releases no halogens when burning.

Corrugated feeder makes sense on a fixed mast where the run is laid once. For a mobile station struck every day, a 125 mm bend radius and 215 kg/km become a problem, and flexible KSR400 or KSR500 turns out more practical despite slightly worse figures.

Power handling is not a limiting factor in these applications: amplifiers of the Alientech DUO or Avenger Booster class operate at single-digit watts, while RG-8 size cable is rated for kilowatts of peak power. The limits are attenuation, shielding and mechanics.

Pigtails remain the weakest link in the chain. A half-metre transition with a poor crimp introduces more mismatch than twenty metres of trunk cable, and it is the part that most often fails from repeated mating.

Summary table

Which types need articles of their own

The category splits into three families, and mixing them into one text is a mistake.

The first is RF cable assemblies: QMA–QMA and N-Type–N-Type extensions in 10/15/20/30/40 m, RG-213 controller kits, finished assemblies on KSR400, KSR500 and corrugated feeder. A separate article should cover how to calculate the link budget and match length to a specific position.

The second is connectors, adapters and pigtails: QMA, SMA, N-type and TNC in all male/female combinations, plus 20–50 cm cable transitions. This needs material on compatibility, crimping, weatherproofing, and why a chain of adapters is worse than one correct connector.

The third is the non-RF cabling that sits in the same category but has nothing physically to do with coax: the Ops-Core AMP U174 headset adapter cable, ALIENTECH brackets and cables for the DJI Mavic 3 RCN1 controller, the housing with cables for the DJI Smart Controller, and power and data cables for charging stations and Starlink. Entirely different parameters apply there (conductor cross-section, pinout, current, connector vibration resistance), and each subgroup deserves its own text.