SWR or Standing Wave Ratio is a measure of how well an antenna system matched to the transmitter and is an indication of efficiency. A low ratio indicates that the transmitted energy is effectively being delivered to the antenna and beyond. A higher number is an indication that something is not matched and that some of the transmitted energy is returning to the transmitter or reflected back. You can understand that if we short out the end of the coax that no energy will be put into the air, also if we leave the connector open and not connected to the antenna that energy is not transmitted to the air; instead it goes back into the transmitter and is dissipated as heat. A very bad SWR can also damage your transmitter.
A very good SWR is 1.1:1, a good SWR is anything less than 1.5:1 and you should be concerned if the value is ever more than 2:1.
Comments about the Distractor answers: A high SWR will not necessarily increase television interference since less energy is actually radiated into the air. The same is true for prolonging the life of the antenna, less energy is delivered to the antenna so it won't shorten the life of the antenna (weather is the greatest concern for antenna life).
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The most commonly used coaxial cable impedance for amateur radio installations is 50 Ohms. We have pretty much standardized on this value. Just memorize this fact.
The other answers are just there to confuse you. 8 Ohms is common for your stereo speakers, 600 Ohms is common for the old wired telephone lines and 12 Ohms is there to confuse you with the 12 Volts used in your automobile's battery system.
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Though there are some impedance considerations, when it comes to radios and their 50 Ohm antenna outputs I am going to focus on the installation of these feedlines to help you remember.
When it comes to ham radio there are two types of feed-lines typically used, balanced feedlines, such as twinlead and ladder-line or coax-cable such as RG-58, RG8 and LMR cables.
For longer feedline runs a balanced feedline like ladder-line will tend to be less lossy. However, there is one major exception to that rule. Balanced feedlines may exhibit significant signal loss called "attenuation" if it runs near or along metal objects.
To help you remember, think back to the older houses that used the twinlead or "ribbon" to connect their TVs to the roof mounted antennas. (the same stuff we use today for portable J-Poles)
A good antenna installation on those older houses always included "standouts" or "standoffs" which consisted of an eyelet on about a 4 inch screw. This would hold the twinlead conductors out away from the house a few inches and provide clearance around metal objects such as rain-gutters, conduits and metal siding. This was done to avoid attenuation caused by running the feedline near or along metal surfaces. Additionally special care was taken when the feedline was routed through windows to get it into the house.
Now coax-cable on the other hand shows very little effect running directly on top of or around metal objects. That's why the answer states it "requires few special installation considerations".
In fact most of us run coax right through the firewall of our cars or under the floor mats and even pinched between the metal car door and door jam up onto the metal car roof to the magnet mounted antenna sitting on top of the metal car. Other than avoiding situations that will physically damage the coax cable there are very few "special considerations" to worry about when using coax cable.
Another note: The other answers all use the phrase "any other." When it comes to equipment especially, be wary of an answer that implies that something is better, worse, cheaper, more capable, etc. than "any other" option out there.
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The best transfer of power occurs when the entire system has the same impedance. Impedance is similar to resistance, except that it varies with the frequency of the signal. Impedance is created by a combination of capacitance and inductance, and Amateur Radio systems all run at 50 ohms, though some types of feedline may differ, such as twin-lead ladder-line, which is 300 ohms. In these cases, something is needed to match the impedance to the rest of the system so that the power can be efficiently converted into a radio frequency (RF) signal.
Because impedance is a function of 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 used to adjust the antenna system's impedance to match the transmitter's impedance. This can allow a radio operator to use an antenna on a frequency that it is not tuned for (has the wrong impedance at that frequency). Some operators even use long random lengths of wire as an antenna, using an antenna tuner to match the impedance.
Some antenna tuners are automatic, while others require you to adjust knobs and watch an SWR meter to correctly tune your antenna.
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There are losses in wires whether it be for electrical power distribution or RF energy, but RF energy has another loss based on frequency. The coax has inductance in the center wire and in the shield around it. It also has capacitance between the center wire and the shield. As frequency increases, the reactance of an inductance increased and is in series with the transmitter and the load (antenna) which reduces the current that can flow. Also as frequency increases the capacitive reactance decreases which takes energy from the center conductor to the shield. This working against resistance in the wires and the effect on the inductance makes the cable lose energy as the frequency increases.
That's more than you wanted to know so here is a simple example to help illustrate this. When you go shopping, the doorway lets people exit the building easily. If more people are going through and it gets crowded then things can slow down and some of your precious time is lost.
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Despite its name, the UHF connector is not well suited for frequencies above 300 MHz, as explained on wikipedia. Neither RS-213 nor DB-25 refer to connectors that can reasonably be used with an antenna.
The Type N connector was designed to handle signals at microwave frequency ranges, and is an excellent choice for RF above 400 MHz.
If you need a last minute aid, it's the only answer without a number.
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PL-259 (which is the male side of the UHF connecter that mates to the SO239 - female side) is the most commonly used connector type for mobile and tabletop amateur radio rigs. They are certainly not watertight and should be protected against water intrusion when installed where they may be exposed to the weather. They are decent for UHF frequencies, but they are almost universally used at HF frequencies. Though they are often called "UHF connectors", that name came from a time when UHF referred to frequencies over 30 MHz (according to wikipedia).
In practice, the PL-259 connector can have a lot of loss at frequencies above 300 Mhz, so usually when you find a radio with a PL-259 connector used for the antenna connector of a UHF (usually 70 cm) rig it's in order to make it easy to use a multi-band antenna, since PL-259 is used so commonly at VHF and HF frequencies.
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If you get water inside your coax connector it can cause a partial or complete short between the center conductor and the shield of the coax connector, which would definitely result in feed line loss.
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When you have a loose (or intermittent) connection in your antenna or in a feedline, connector, or adapter, the SWR (standing wave ratio) readings can change every time your cable gets bumped, vibrated, or jiggled. In this case as in many cases, the simplest answer is often the correct one. A Loose Connnection in an antenna or a feed line
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These are probably the two most commonly used 50 ohm coax types; the RG-8 is thicker, less flexible, and as such has lower loss than the RG-58 coax. Just remember that as a general rule of thumb, a larger cable means less loss.
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75-ohm coax should never be used for ham radio because it's the wrong impedance. The correct impedance for an antenna is 50 ohms, so the closer you are to 50 ohm impedance the better signal you will receive. Multi-conductor unbalanced cable should be eliminated from the possibilities for the same reason; in order to use it you would need to have an impedance match circuit (such as an antenna tuner) and since it's unbalanced (lacking a ground for each line) there is nothing to shield it from interference which would cause even more loss.
This question is a little bit of a trick question, however, because most beginners who have studied will know the above and will thus assume that the 50-ohm flexible coax is the way to go. In fact, though 50 ohm flexible coax is probably the most commonly used feedline, an air-insulated hard line has much lower loss. Air is actually a very good insulator, and the rigidity of the feedline helps keep the loss very low. The primary disadvantages to an air- insulated hard line are that they are rigid (you can't adjust the installation easily, or move it) and the possibility of getting water inside, which can short out the feed line.
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