Any coax connector used outdoors should be carefully weatherproofed, no matter what type it is.

PL-259, BNC, and even weather-resistant type N connectors can all let water wick into the coax if the joint isn’t sealed. That leads to corrosion, higher loss, and weird SWR problems.

So the right idea (and the right answer) is:

All of them need weather protection when used outdoors.

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The standard characteristic impedance for most coaxial cables used in amateur radio is 50 ohms. This value has been widely adopted for radio-frequency transmission lines because it represents a practical compromise between power-handling capability and low signal loss, and equipment (transceivers, amplifiers, and antennas) is commonly designed for 50-ohm systems.

Memory aids:

• 8 ohms — common for stereo speakers
• 600 ohms — common for older wired telephone lines
• 12 (volts) — think of a car battery to avoid confusion

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There are two common types of feed line used in ham radio: balanced feedlines (like twinlead or ladder-line) and coaxial cable (such as RG-58, RG-8, LMR, etc.). Balanced feedlines can be less lossy than coax for long runs, but they are sensitive to nearby metal. If a balanced line runs along or near metal objects it can suffer significant signal loss (attenuation) unless it is kept away from those objects.

Because of that sensitivity, installations using twinlead or ladder-line often require special mounting hardware (standouts or standoffs) and extra care where the line passes near gutters, metal siding, conduits, or through windows. Coaxial cable, by contrast, is much less affected by nearby metal and is mechanically easier to route through buildings, through a vehicle firewall, under floor mats, or along metal surfaces without special clearances. For that reason coaxial cable is the most common feed line: it is easy to use and requires few special installation considerations.

Memory aids / mnemonics:

• Balanced feedline (twinlead/ladder-line) can be low-loss but must be kept away from metal to avoid attenuation.
• Old TV installations used twinlead with standouts/standoffs to keep the line a few inches from the house and gutters.
• Coax is tolerant of running along or over metal, so it needs fewer special installation precautions.
• Be cautious of statements that claim something is better than "any other" option — such absolutes are often misleading.

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The major function of an antenna tuner (antenna coupler) is to match the antenna system impedance to the transceiver's output impedance so maximum power is transferred. The best transfer of power occurs when the entire system has the same impedance. Impedance is similar to resistance, except that it varies with frequency. Impedance is created by a combination of capacitance and inductance.

Amateur radio equipment is designed to work into about 50 ohms, though some feedlines (for example, twin-lead ladder-line) have different characteristic impedances such as 300 ohms. In those cases, something is needed to match the impedance to the rest of the system so power from the transmitter is efficiently delivered to the antenna.

Because impedance depends on capacitance and inductance, a capacitor and/or inductor can be used to change the impedance. Antenna tuners contain variable capacitors and/or inductors and can thus be adjusted to make the antenna system present an impedance close to the transmitter's output impedance. That lets an operator use an antenna on frequencies where the antenna itself is not naturally resonant. Some operators use long random-length wires as antennas and rely on an antenna tuner to match the impedance.

Some antenna tuners are manual — requiring adjustment of knobs while observing an SWR meter — and some are automatic, making the matching process easier.

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The characteristic impedance of a coaxial cable is determined mainly by its geometry (diameters of the conductors and spacing) and the dielectric constant of the insulation. Those factors do not change appreciably with frequency in the range the cable is designed for, so characteristic impedance is essentially frequency independent.

Loss (attenuation), however, does increase with frequency. One major reason is the skin effect: at higher frequencies the RF current is confined to a thinner layer near the surface of the conductors instead of using the full cross section. The skin depth (how far current penetrates) decreases roughly with the square root of frequency, so the effective resistance of the conductors increases as frequency goes up, causing greater loss. Dielectric loss also tends to increase with frequency, adding further attenuation. For example, RG-58 has about 0.048 dB/ft loss at 100 MHz but about 0.354 dB/ft at 2400 MHz.

Note that the increased RF loss is not the same thing as a change in characteristic impedance.

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Despite its name, the UHF connector (PL-259/SO-239) is not well suited for frequencies above about 300 MHz, so it is not a good choice for RF above 400 MHz. Neither RS-213 nor DB-25 are connector types that are appropriate for antenna/coax RF use.

The Type N connector was designed to handle microwave frequency ranges and is an excellent choice for RF use above 400 MHz. It provides a reliable, weather-resistant threaded coupling and maintains consistent impedance at VHF/UHF and microwave frequencies, which is why it's commonly used for higher-frequency RF connections.

Memory aids:

• It's the only answer without a number.

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PL-259 is the male (plug) half of the UHF connector family and mates with the SO-239 (socket) receptacle. It is the most commonly used connector type for mobile and tabletop amateur radio equipment and is widely used at HF and VHF frequencies. PL-259 connectors are not watertight and should be protected from moisture when used outdoors. They are a threaded connector, not a bayonet-type, and their performance degrades at higher frequencies (they can have significant loss above a few hundred MHz), so they are not preferred for microwave operation.

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All feed lines have some loss due to the resistance of the conductors.

Water in or around coaxial connectors reduces the insulation resistance between the center conductor and the shield, creating leakage paths and increasing dielectric and conductive loss.

A high standing wave ratio causes higher-than-normal voltages and currents at points along the feed line; those increased voltages and currents result in greater heating and energy dissipation in the line, increasing loss.

Each connector or joint is an imperfect impedance transition, producing reflections and small resistive losses; having multiple connectors increases the total loss.

Because of these effects, all of the listed conditions are sources of loss in a coaxial feed line.

Memory aids:

• Water on insulators = leakage and more loss
• High SWR = higher voltages/currents → more heating/loss
• More connectors = more reflections and resistive loss

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A loose or intermittent connection in the antenna system or feed line causes the impedance seen by the transmitter to change when the cable or connector is moved or vibrated. Those sudden changes in impedance change the amount of power reflected back toward the transmitter, so SWR readings can jump around erratically. Tight, clean connections and undamaged feedline components prevent those intermittent impedance changes and give stable SWR readings.

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RG-213 has lower loss at a given frequency because it is a thicker, heavier-duty 50-ohm cable with larger conductors and a larger-diameter dielectric. Larger conductors reduce conductor (ohmic) loss and the larger spacing reduces dielectric loss, so the overall attenuation per unit length is less than for the thinner RG-58. RG-213 is similar in size and performance to RG-8, is less flexible, and has a more rugged jacket; RG-58 is smaller in diameter with smaller conductors and therefore exhibits more loss.

Memory aids:

• Thicker cable → less loss
• RG-213 ≈ RG-8 (thicker, lower loss)
• RG-58 is thinner and more flexible, but has higher loss

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Flexible coaxial cables use a dielectric (often Teflon or foam) between the center conductor and the outer shield. That dielectric material produces some loss because it is not a perfect insulator and has small amounts of conductance and dielectric loss.

Multi-conductor unbalanced cable does not confine the RF energy as well as coaxial cable, so some energy can be lost by radiation or by coupling between conductors, increasing loss compared with a well‑constructed coax or hardline.

Air‑insulated hardline has the lowest loss because air is an almost lossless dielectric compared with solid or foam dielectrics. In addition, the solid outer conductor of hardline prevents radiation of the signal, and the construction typically uses larger conductors which reduce resistive losses. These factors make air‑insulated hardline the lowest‑loss feed line of the types listed.

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Standing Wave Ratio (SWR) is a measure of how well the load (antenna and feedline) is impedance-matched to the transmission line and transmitter. The most efficient transfer of power occurs when the load and the transmission line have the same impedance; radios and most amateur equipment are designed for a 50-ohm system. If the antenna system is not matched to the feedline and transmitter, some of the transmitted energy is reflected back toward the transmitter instead of being radiated by the antenna.

SWR is related to the amount of forward (outgoing) energy compared to the reflected energy on the line: a low SWR means almost all the power goes to the antenna, a high SWR means a significant portion is reflected. Because impedance includes reactive components that vary with frequency, a system can have a good match (low SWR) at one frequency and a poor match (high SWR) at another.

Note on resistance vs impedance: resistance is a purely real quantity measured in ohms and does not vary with frequency, while impedance includes both resistance and reactance and generally does change with frequency. That frequency dependence is why SWR can change across a band.

Memory aids:

• Radios and most amateur gear expect a 50-ohm system — think “50 ohm match” when assessing SWR.
• Low SWR = most power delivered to the antenna; high SWR = more power reflected back.

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