A dummy load is a resistive device (typically a 50-ohm resistor for most ham transmitters) mounted in a shielded enclosure that absorbs the transmitter's RF energy and converts it to heat. Because it presents the proper impedance to the transmitter and is non-radiating, a dummy load lets you operate and test a transmitter (measure output power, adjust settings, or troubleshoot) without sending signals out over the air and potentially causing interference.

Memory aids (if helpful):

• "Dummy load = dead antenna" — it looks like an antenna to the transmitter but doesn’t radiate.
• Think "load" as a device that takes power (and turns it into heat), not radiated signal.

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An antenna analyzer is a handheld tool used to check whether an antenna is properly tuned — that is, resonant at the frequency you want to use. When an antenna is resonant it presents the appropriate impedance (often near 50 ohms for most transceivers) and shows a low standing wave ratio (SWR), so it can transmit and receive efficiently. An antenna analyzer measures the antenna's impedance and SWR so you can adjust the antenna length or feed system for the best performance on the desired frequency.

Be aware that measurements of impedance and SWR are indicators of resonance but can sometimes be misleading. For example, a dummy load is essentially a 50 ohm resistor and will show a perfect SWR or impedance on an analyzer, yet it does not radiate like a real antenna and therefore will not function as one.

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A typical RF dummy load is a 50-ohm non-inductive resistor mounted on a heat sink so it can absorb and safely dissipate the transmitter's RF power without radiating it. "Non-inductive" means the resistance is provided by a resistive material or construction that does not include coils of wire, because coiled wire has inductance that would change the impedance at RF and make the load frequency-dependent. The heat sink (or sometimes an oil bath or copper plates) is used to get rid of the heat produced when the transmitter's power is turned into heat. Dummy loads are made to match the 50-ohm impedance that most transmitters and coaxial feedlines expect, so the transmitter sees a proper load even though no antenna is connected.

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SWR stands for standing wave ratio, which indicates how much of the transmitter's energy is actually delivered to the load (the antenna) versus how much is reflected back. A standing wave is created when energy is reflected from the antenna system, and the SWR is the ratio of the maximum to the minimum amplitude of that standing wave along the feed line. The second number in the ratio is always 1, so a perfect impedance match is when the first number is also 1 — written as 1:1 — meaning virtually all the transmitted power is delivered to the antenna with no reflected power. For more detail see: http://en.wikipedia.org/wiki/Standing_wave_ratio

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The standing wave ratio (SWR) measures how well the transmitter output impedance is matched to the antenna system. When the impedances are matched, power transfer to the antenna is most efficient.

If the antenna system is not well matched (high SWR), some of the RF energy is reflected back toward the transmitter. Those reflected waves can produce large voltage and current swings in the transmitter's output stage. Solid-state output transistors are sensitive to those stresses and can be damaged by excessive reflected power, so modern solid-state transmitters automatically reduce output power as SWR rises to limit the reflected energy and protect the RF output transistors.

Reducing transmitter power does not change the SWR or improve the impedance match; its purpose is to prevent damage to the transmitter, not to alter the antenna system or to satisfy unrelated regulatory requirements.

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SWR (Standing Wave Ratio) is a measure of how well the antenna system is impedance-matched to the transmitter. A low ratio (1:1 is ideal) means most of the transmitted energy is delivered to the antenna and radiated. A high ratio means a significant portion of the energy is reflected back toward the transmitter rather than being radiated. An SWR of 4:1 therefore indicates a substantial impedance mismatch, with much reflected power, possible heating or damage in the transmitter, and reduced effective radiated power. SWR is a ratio, not a dB value or a gain figure.

Memory aids:

• Think of the antenna system as a window: a perfect window (1:1) passes all the light (energy) through; a mirror reflects most light back (high SWR).
• A practical “good” SWR is around 1.1:1 to 1.5:1; values like 4:1 indicate a problem that should be corrected.

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All feed lines (coax, twin‑lead, etc.) have some resistance and dielectric loss. When RF power is lost while traveling through a feed line, that electrical energy is converted into heat in the conductors and insulating materials. Longer or thinner cables, poor connectors, and lossy dielectric materials increase the amount of power dissipated as heat, which appears as attenuation (reduced delivered power) at the far end of the line.

Memory aids / mnemonics:

• No memory aids provided.

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SWR is short for Standing Wave Ratio.

A directional wattmeter is used to measure how much RF power is traveling in each direction on a feedline — forward (from the transmitter toward the antenna) and reflected (power that is returned toward the transmitter when the antenna/feedline is not perfectly matched). By comparing forward and reflected power the amount of reflected energy can be determined and the SWR calculated (a high reflected power means a high SWR; very little reflected power indicates a good match).

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Moisture in coaxial RF cables can cause feed line failure because water alters the cable's electrical properties and can damage connections. Water inside the cable or at connectors can create conductive paths that short the inner conductor to the shield, and it can corrode or oxidize connectors so they develop higher resistance or even open circuits. Moisture also changes the dielectric properties (capacitance) of the insulation between conductor and shield, which changes the characteristic impedance and leads to mismatches and increased losses. To avoid these problems, seal cable ends and connectors to keep moisture out.

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If you've ever seen something left in the sun and become hard and brittle, you can understand why that is harmful for a coax cable jacket. Coax connected to an outside antenna is exposed to sunlight, so UV resistance in the outer jacket will significantly delay (or prevent) the jacket becoming brittle and cracking.

Once the jacket becomes brittle and cracks, water can enter the cable. Water inside a coax can short the conductors, change the cable's characteristic impedance, increase attenuation of RF signals, and cause corrosion — all of which can lead to cable failure. To avoid this, outdoor coax should have a UV-resistant jacket or be run inside protective conduit to keep sunlight off the jacket.

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Foam-dielectric coax has less loss per foot than solid-dielectric coax of the same size, especially at higher frequencies. That’s the advantage the question is looking for.

In exchange, foam types can be more easily damaged or contaminated by moisture, so they need good connectors and sealing.

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