fm rf transmission lines
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Transmission lines are one of the maincomponents in the RF transmission plant ofa broadcast station.
Acting as the connecting link between thetransmitter and the antenna, thetransmission line plays a critical role in boththe quality and reliability of the broadcast
signal; therefore, the proper choice of atransmission line type to be used can havea significant impact on the success of thestation.
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With the development of digital formatsfor transmitting broadcast signals, it isimportant to review the existingtransmission line performance
specifications and any measured dataon the installed line at the broadcast siteprior to completing plans for
implementing digital transmission.
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The choice of transmission line is typicallydecided based on the following criteria:
Frequency of operation Power handling
Attenuation (or efficiency)
Characteristic impedance Tower loading (size and weight)
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Semiflexible coaxial cables are designedwith soft-tempered copper inner and
outer conductors. This type of transmission line is used
extensively for radio and low powertelevision broadcasting, as well as inter-element feeders for some antennatypes.
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Rigid coaxial lines are designed withhard-tempered inner and outer
conductors. They have high power handling
capabilities and low attenuation values.
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Coaxial transmission lines used for shortinterconnections between equipment,
typically inside buildings, havetraditionally been referred to as RFcables.
They are small in diameter, making themvery flexible and very useful in areas withminimum space. The
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Globally, semiflexible coaxial lines arethe most popular choice for
broadcasting applications. It is ideal fora wide variety of low and mediumpower transmission systems.
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Foam dielectric lines are designed forsystems that normally do not require a
gas pressure path to the antenna orother connected components. Typicalapplications include FM radio and lowpower television where the antennas do
not require pressurization.
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Air dielectric cables utilize a spiraldielectric material to separate the
conductors. Because the majority of thevolume between conductors is now air,these cables have lower attenuationand higher average power handling
than foam dielectric cables.
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The phase stability of the cable over longperiods of time is extremely important.
Ordinary coaxial cable should not beused in these instances, as it is subject tosignificant phase variation withtemperature change during its initial
usage.
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These cables have been subjected to aprocess that cycles the cable through a
wide temperature range to remove thephase instability caused by hysteresis(movement between the inner andouter conductors).
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It is important to make all connector andsplice attachments with the cable ends
in a horizontal position. This minimizes the chance of metal chips
falling into the cable during the trimmingprocess.
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Proper attachment of the finishedconnector to the antenna or transmitter
prevents loosening of the connectionfrom vibration or other stresses.
These connections should be securelytaped to prevent loosening.
Tighten bolts that are opposite (notadjacent) to the proper torquespecification.
Overtightening can result in a warpedflange.
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Two principal methods for FM DigitalTransmission are employed:
One is combining the analog and digitalsignals using a hybrid combiner at theoutputs of the analog and digitaltransmitters and then using a single
transmission line to a common antenna. The second involves installing a second
antenna and transmission line to transmitthe digital signal.
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The frequencies of operation fallbetween 88 and 108 MHz, and the
characteristic impedance of the cablewill almost always be 50 ohms.
Increased size of the cable used is apositive for power handling (higherratings) and attenuation (higherefficiency) but is a negative regardingline costs and higher loading on the
tower.
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A proper installation can be the difference
between a smooth running transmitter site
and a continuous effort to correct failures in
the transmission line and antenna system.
Environmental changes results in substantial
stress on the cable from corrosion, vibration,
atmospheric contaminants, and thermalchanges that are not present with indoor
installations.
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GROUNDING: Because the cablerepresents a direct connection betweenthe outside and the transmitterequipment, proper grounding isnecessary to protect the equipmentfrom the large currents caused by a
lightning strike. ELECTRICAL TESTING: When the line has
been installed and mechanically
inspected, an electrical check isnecessary for maintaining a reliablesystem.
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If the installation of the coaxial cable isdone properly, then basic maintenance
procedures will keep it operating troublefree for many years.
Repair procedures usually involvetightening and replacement of
hardware and damaged hangers.
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It is important that installers follow themanufacturers recommended
procedures to avoid damaging the line
by denting or kinking and to correctlyattach it to the tower using theappropriate hangers and spacing forlong-term reliability.