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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The characteristic impedance is more or less frequency independent. While technically the dielectric constant of the dielectric is frequency dependent, the effect is so small that if you kept increasing the frequency of your signal, you'd surpass the maximum rated frequency of the cable before you noticed a change in the dielectric constant, and thus this effect is practically always ignored. The characteristic impedance depends on the geometry of the transmission line, the dielectric constant of the dielectric used, and the resistance of the conductors used, and since none of these change over frequency the characteristic impedance does not change over frequency.
As frequency increases, the resistance of a conductor increases due to something called the skin effect. At DC the entire cross section of a cable will carry current, but as frequency increases the current tends to stick closer to the outer areas of the conductor, not utilizing the middle of the conductor. The depth that current penetrates into the cross-section of the conductor is known as the skin depth and varies with the square-root of the frequency. As less conductor is used, the resistance of the conductor increases. For example, RG-58 loses 0.048dB/ft at 100MHz, but loses 0.354dB/ft at 2400MHz.
Note that this RF impedance is not the same as characteristic impedance.
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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-213 is thicker, less flexible, and has lower loss than the much thinner RG-58 coax.
RG-213 is similar to RG-8 in terms of thickness and other properties. The PVC jacket of RG-213 will hold up longer than that of RG-8.
Just remember that as a general rule of thumb, a larger cable means less loss.
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Flexible coaxial cables use a dielectric (in the form of teflon or foam usually) to separate the center conductor and outer shielding. This dielectric contributes loss due to small amounts of conductance and reactance.
The multi-conductor unbalanced cable does not trap the signals between wires as well as coaxial cable does, leading to some losses in the form of radiating signals.
In the air-insulated hard line, the air is very non-conductive which reduces any losses associated with the dielectric. The outer shielding conductor prevents signals from radiating away, reducing losses from radiation.
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