Connectors, Adapters and Pigtails: QMA, SMA, N-type and TNC in Ground Station Feed Lines

A ground station's feed line usually fails at its ends rather than along the cable. A connector occupies a few centimetres of length, but within those centimetres the characteristic impedance changes three times, and the mechanics absorb every mating cycle, every vibration and all the moisture. In the "Cables and Adapters" category the connector side is represented by 20–30 cm QMA/SMA/N-type adapters, 50 cm flexible cable transitions, and connectors matched to specific cable sizes. What follows covers how these interfaces differ, how to assemble them correctly, and why a chain of three adapters is worse than one crimped connector.

What a connector does to the signal

A coaxial line works as long as the characteristic impedance along it stays constant. A connector is the place where the cable's construction (centre conductor, dielectric, shield) passes into an entirely different geometry: a contact pin, a PTFE washer, a threaded or snap-on coupling, and back into cable. Every one of those transitions produces a small reflection, and all the reflections add up.

Three parameters describe the quality of that transition. VSWR shows how much power is reflected back: a good N-type is specified at no worse than 1.1 up to 3 GHz and no worse than 1.3 across 3–11 GHz. Insertion loss describes how much power simply turned into heat in the contacts. Passive intermodulation (PIM) arises when an imperfect junction between two metals behaves as a weak non-linear element: two strong signals generate a third one on it, which lands in the receive band. For N-type the typical third-order intermodulation figure is around −125 dBm. At a position with a repeater, where transmitter and receiver operate simultaneously, an oxidised or under-torqued connector becomes a source of self-interference, and no antenna swap will fix it.

Contact inside a connector is made on the centre conductor and the outer conductor at the same time. The centre contact in the jack is formed from spring fingers that grip the pin of the plug. The outer contact in N and TNC is either solid or slotted. A slotted contact acts as a spring and touches both the base and the side wall of the mating part; a solid one touches only the base. The second option is cheaper and electrically no worse while the joint stays tight, but when vibration loosens it the signal degrades sharply. For vehicles and masts the slotted outer contact is the recommended choice.

Connector families

N-type. A threaded 50-ohm medium-size interface, working from DC to 11 GHz in standard brass construction and to 18 GHz in extended-range versions. Nickel- or silver-plated brass body, PTFE insulator, operating temperature −65 to +165 °C, durability around 500 mating cycles. Current series are rated IP67 once mated. This is the primary field connector: the large body is easy to handle in gloves, the thread holds under vibration, and weatherproofing boots are available for it. The Avenger remote antennas and the trunk assemblies in the range are built predominantly on N-type.

TNC. The same size and the same mechanics as N, but with a finer thread and a more compact body; standard brass construction is rated for use below 11 GHz. Precision stainless steel versions per MIL-T-81490 and MIL-C-87104 work to 18 GHz and tolerate a higher installation torque, which is why they go onto airborne and heavily vibration-loaded equipment.

SMA. A miniature threaded connector developed in the 1960s, originally for use up to 12 GHz, with current variants reaching 18 and 26.5 GHz. Durability up to 500 mating cycles, provided the correct torque is applied. It is the most common interface on controllers themselves, on amplifiers and on small antennas.

QMA. The quick-disconnect version of SMA, created by the Quick Lock Formula alliance in 2003. The internal geometry copies SMA, but a latch replaces the thread: the joint is made by pushing, without a wrench, and after mating the connector rotates freely through 360°, which takes torsion off the cable. Amphenol's documentation gives up to 6 GHz for QMA series connectors and up to 18 GHz for adapters in the same series; either way there is margin at 2.4/5.2/5.8 GHz. The QLF standard provides for intermateability between manufacturers. The compact latch allows connectors to be spaced 12.4 mm apart, which is why QMA took hold on Alientech amplifiers and similar equipment where several ports must fit on a small housing.

Clamp-style connectors such as UHF (PL-259) are not used in these feed lines: the construction does not maintain constant characteristic impedance and suits frequencies below roughly 300 MHz.

Gender, polarity and common mistakes

English-language documentation uses plug and jack, while Ukrainian and Russian shop practice uses "male" and "female" in the literal sense. The two pairs do not always line up: in some series the plug is the part with the external thread regardless of whether it carries a pin or a socket. When ordering it is safer to state both attributes, the thread type and the centre contact type.

Reverse polarity is a trap of its own. RP-SMA and RP-N have an ordinary coupling but a swapped centre contact: the plug carries a socket and the jack carries a pin. The interface was introduced to make it harder to attach third-party antennas to consumer equipment, and it is widespread in the Wi-Fi and FPV world. Mechanically an RP-SMA threads onto an ordinary SMA, after which the line stays open, because two sockets or two pins cannot meet. Torque for RP parts is the same as for standard SMA, since the thread is identical.

The third mistake is 75-ohm connectors from television equipment, which look much like 50-ohm ones. A 50/75-ohm mismatch produces a permanent reflection at every junction.

Pigtails and adapters

A pigtail is a short length of flexible cable with different connectors at each end. In the range these are the 50 cm flexible QMA male to N-Type female transition and the universal 20–30 cm QMA/SMA/N-type adapters in every pin/socket combination. They are built on RG-223: roughly 5.3 mm in diameter, silver-plated copper conductor, double silver-plated shield, working range up to 6 GHz. RG-223 attenuation is about three times that of RG-8 size trunk cable, but over 20–50 cm that amounts to tenths of a decibel, and the gain in flexibility outweighs it.

A pigtail solves two problems. The first is joining dissimilar interfaces, where the amplifier has QMA and the remote antenna has N-type. The second is mechanical relief: stiff trunk cable screwed directly onto a controller's connector breaks it at the first sharp movement, whereas a flexible insert absorbs that load. This is why the transitions are made for specific amplifiers: Alientech Duo 2 and Duo 3, Alientech Deimox, Avenge Angel Avenger, Acasom ROC-4, 2E Mavka, 4Hawks Raptor.

Trouble starts once there is more than one pigtail. Every mated pair adds its own reflection and its own transition region, and in the field it adds a point that can work loose or take in moisture. A QMA→SMA→N chain built from two separate adapters is always worse than a single QMA-to-N transition of the right length, even when the combined loss looks negligible on paper.

Assembly: crimp, solder, clamp

A connector is selected for a specific cable size, not for a "roughly similar diameter". Cable of the KSR400 class (also known as RG-8 or LMR-400) takes connectors from a series designed for a 10.3 mm outer diameter with foamed PE; KSR500 and 1/2-inch super-flexible feeder each have series of their own. A mismatch leaves an air gap under the foil and raises VSWR.

The three attachment methods differ in how well they suit field work. Crimping calls for a crimp tool with the correct die, but gives a repeatable result and does not risk overheating the dielectric. Soldering calls for skill: foamed polyethylene flows under excess heat and the conductor drifts off axis. A clamp connector assembles without tools but holds up less well under vibration.

Cable preparation matters more than the attachment method itself. The layers should be removed with a calibrated strip tool to a set length, the conductor end deburred, and the foil left undisturbed. Manufacturers supply dedicated tooling for this: a combination strip tool for crimp and clamp connectors, a crimp tool with a die for the specific series, crimp rings, a cable cutter and a deburring tool for the centre conductor. Assembly with a utility knife and pliers produces a connector that works on the bench and falls apart on the mast.

Installation torque

An under-torqued connector gives an unstable contact and rising insertion loss; an over-torqued one deforms the thread and cracks the dielectric.

SMA is tightened with a 5/16 inch torque wrench. A second wrench holds the cable-side body so the torque is not transferred to the crimp. Before mating, the internal surfaces are blown out with compressed air: a grain of sand inside a connector ruins both the contact geometry and the thread.

Weatherproofing and vibration

Mated N-type connectors from current series are rated IP67, but that covers the interface itself rather than the point where the cable enters. Manufacturers supply rubber boots for specific series (separately for plug and jack) and boots with bend relief. The classic field alternative is self-amalgamating tape over the joint followed by a wrap of electrical tape, laid so that water runs past the junction rather than into it.

Vibration backs the coupling nut off regardless of the initial torque. Aviation historically dealt with this by lock-wiring the connector to its mate; self-locking connectors, in which a locking mechanism engages once the nut is torqued, are now more common. In a ground station it is enough to clamp the cable immediately behind the connector so that the weight and movement of the run are not transferred to the thread.

Practical conclusions

N-type goes at the antenna end of the run: it weatherproofs well, holds under vibration and has good durability. QMA is more convenient on the controller side, mating one-handed and rotating after the joint is made. The transition between them is made with a single pigtail rather than two adapters.

A pigtail is a consumable. SMA and N are rated at around 500 mating cycles, and with a station struck daily that allowance runs out in a couple of years, in practice sooner, because real cycles come with misalignment and contamination. A spare transition in the kit costs less than a lost deployment.

A connector with a slotted outer contact is the choice for anything that moves or stands on a mast. Solid contacts are left for fixed indoor connections.

RP variants are checked before purchase. Visually RP-SMA and SMA differ only in whether a pin sits inside the coupling, and in the field that difference surfaces at the worst possible moment.

Summary table