Connectors and cables

Coax is the most common feed line in amateur radio (Technician pool, T9B03), and the connectors at each end decide a lot about how much power reaches the antenna. Water in connectors, extra connectors and high SWR all add loss in a feed line (Technician pool, T9B08), and a loose connection shows up as erratic SWR (Technician pool, T9B09). This guide shows the connectors you will meet, how to fit and seal the common one, and how to choose the cable. The antennas guide explains SWR and feed lines, and putting up antennas covers routing coax and lightning protection.

Plug, jack, male and female

Connector names describe two things. Plug and jack describe the coupling: a plug has its thread inside a coupling nut, and a jack has its thread on the outside. On standard SMA, and on the UHF connectors below, the plug also has the center pin and is called male, and the jack has the center socket and is called female. Reverse polarity SMA (RP-SMA) swaps only the center contacts and keeps the body names: an RP-SMA female is a jack with an inner pin, and an RP-SMA male is a plug with a center receptacle (Taoglas). So name an RP-SMA connector by its body, not by its pin. The PL-259 is the cable plug and the SO-239 the equipment jack (ARRL, Choosing a Ham Radio).

The common RF connectors

Threads, bayonet studs and pin diameters are from the interface drawings in MIL-STD-348B (DoD).

UHF connectors: PL-259 and SO-239

At HF, the standard connectors are the UHF family: the PL-259 cable plug and the SO-239 equipment jack (ARRL). Despite the name, they are not the connector for the UHF bands. A 1965 military handbook describes them as general-purpose connectors that are “not constant-impedance connectors and will therefore introduce some voltage reflection.” It rates them “generally satisfactory at frequencies up to 200 megacycles” (MHz), usable “with caution up to 500 megacycles,” at peak voltages up to 500 volts, and says they “should not be exposed to the weather” (MIL-HDBK-216). That matches the pools: PL-259s are common at HF and VHF, and Type N is the connector for frequencies above 400 MHz (Technician pool, T9B06 and T9B07).

For mobile antennas, the ARRL names a threaded base, PL-259/SO-239 and NMO as the common mounting methods, and the antenna must match the mount (ARRL).

SMA and reverse polarity SMA

Reverse polarity SMA (RP-SMA) uses the same bodies as SMA but swaps the center contacts: an RP-SMA jack has an outside thread and a center pin, and an RP-SMA plug has an inside thread and a center socket. “Reverse polarity” refers only to the gender of the center contact, not to any signal polarity. RP-SMA was introduced to keep consumer wireless gear from being connected to high-gain antennas, and 2.4 GHz products use it to meet FCC Part 15 (Taoglas). That rule requires an unlicensed transmitter to be designed so that no antenna other than the one furnished can be used with it. A permanently attached antenna or one with a “unique coupling” is considered sufficient, and “a standard antenna jack or electrical connector is prohibited” (47 CFR 15.203). An SMA plug threads onto an RP-SMA jack, but the two pins do not mate. Before you buy an antenna or adapter for a handheld radio, look at the radio’s connector: thread inside or outside, and a pin or a socket in the center.

Other connectors on a radio

Low-frequency and DC signal connections to a transceiver commonly use RCA phono connectors (General pool, G6B12). DC power connectors are covered below.

Adapters

An adapter adds two more connections to the line. Multiple connectors in a line are a source of loss, every connection is one more place for water to get in, and a loose one causes erratic SWR (Technician pool, T9B08 and T9B09). The handbook’s description of UHF connectors as not constant impedance covers the series’ adapters too (MIL-HDBK-216). Where you can, put the right connector on the cable instead.

Fitting a PL-259

Cables come with connectors already fitted, but the ARRL suggests learning to install your own, both for the skill and to save money (ARRL). The military handbook’s procedure for a solder-type PL-259 on RG-8 size cable (MIL-HDBK-216):

1. Cut the end of the cable square and remove 1-1/8 inch of the jacket.
2. Bare 5/8 inch of the center conductor and trim the braid. Slide the coupling ring onto the cable, then tin the exposed center conductor and braid.
3. Screw the plug body onto the cable. Solder the braid to the body through the solder holes, using enough heat to bond braid to shell, and solder the center conductor to the pin.
4. Screw the coupling ring onto the body.

Thinner cables need a reducing adapter that screws into the plug: the handbook lists the UG-175/U for RG-58 and the UG-176/U for RG-59. With an adapter you remove 3/4 inch of jacket, slide on the coupling ring and adapter, fold the braid back over the adapter and trim it to 3/8 inch, then bare 5/8 inch of center conductor and solder as before (MIL-HDBK-216). Crimp connectors and other makers’ designs use their own dimensions and tools, so follow the instructions packed with the connector you buy.

Weatherproofing outdoor connections

Every connector used outdoors needs careful weatherproofing, whether it is a PL-259, BNC or Type N (Technician pool, T9B01), even though Type N is moisture-resistant (General pool, G6B07). Water that gets into a connector adds loss and eventually ruins the cable (Technician pool, T7C09 and T9B08).

One method uses rubber mastic tape covered by vinyl tape. 3M’s instructions for its Scotch 2228 mastic tape, which it makes for electrical insulation and moisture sealing, are to apply it in half-lapped layers, stretched to three quarters of its original width, then overwrap it for mechanical protection with two half-lapped layers of its Super 33+ vinyl electrical tape (3M).

Choosing coax

The common coax sizes, from smallest to largest, are RG-58, RG-8X and RG-8 or RG-213. The ARRL’s advice: use RG-58 only for short runs of 50 feet or less, and never at the output of an amplifier; RG-8X will carry full legal power but is not a good choice for runs over 100 feet; RG-213 suits all amateur HF uses except very long feed lines (ARRL). Check the datasheet for your band, though: Belden rates its RG-8X (9258) at 1,000 W at 10 MHz but 370 W at 50 MHz and 250 W at 100 MHz (Belden 9258), below the 1,500 W PEP that 47 CFR 97.313 allows on most bands. The most common impedance for amateur coax is 50 ohms (Technician pool, T9B02).

Loss grows with frequency and length. Coax loss increases with frequency (Technician pool, T9B05), and RG-213 has less loss than RG-58 at any given frequency (Technician pool, T9B10). The chart plots the published figures:

Loss is per 100 feet, nominal, from each cable’s datasheet (8240, 9258, 8267, 8214, 7810A). Other brands of the same RG type differ, so check the datasheet for the cable you buy.

What the loss means. A loss in decibels converts to a power ratio as 10 raised to the power of (dB / 10) (ARRL Radio Mathematics). Feed 100 W into 100 feet of each cable on 6 meters (50 MHz) and, by the published figures, about 56 W reaches the antenna through RG-58 (2.5 dB), 62 W through RG-8X (2.1 dB), 74 W through RG-213 (1.3 dB) and 81 W through RF-400 (0.9 dB). The difference grows with frequency. High SWR adds more loss on top of this (Technician pool, T9B08).

Foam versus solid dielectric. Foam dielectric coax has less loss per foot and a higher velocity factor than solid dielectric coax of the same size, but a lower safe maximum operating voltage (Technician pool, T7C11; Extra pool, E9F08). The two RG-8 size cables above show it: with the same 0.285 inch insulation, the foam cable has a thicker center conductor, a velocity factor of 78 percent against 66 percent, and lower loss at every frequency in the table. Velocity factor matters when you cut coax to an exact electrical length; the antenna calculator uses it.

Lowest loss. Air-insulated hardline has the lowest loss of the common feed lines (Technician pool, T9B11), and parallel-conductor line has lower loss than coax with a plastic dielectric (Extra pool, E9F07).

Outdoors. Pick a cable whose datasheet rates it for outdoor use, with a jacket that resists ultraviolet light: UV damage lets water into the cable, and moisture is a cause of coax failure (Technician pool, T7C09 and T7C10). Of the cables above, Belden rates RG-213 (8267) for indoor use only and RF-400 (7810A) as sunlight and UV resistant (8267; 7810A).

DC power connectors

For 12 V equipment, ARES and RACES groups have a standard way of assembling Anderson Powerpole connectors, described below. The PP15/45 series, the smallest Powerpole housings, accepts wire from 20 to 10 AWG and is rated up to 55 A per pole (the UL rating for 10 AWG wire-to-wire) (Anderson Power Products). It takes 15, 30 and 45 A contacts (West Mountain Radio, hosted by ARRL).

The red and black housings slide together on dovetails, and how you pair them matters. The ARES and RACES standard orientation is correctly polarized and genderless, so any two cables built to it plug together; pairs assembled another way may be mirror images rather than genderless mating pairs (West Mountain Radio, hosted by ARRL). Copy the orientation from a cable your group already uses, and check polarity before you plug in a radio (ARRL-hosted installation tips).