Transmission Line Loss by Cable Type

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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Location: Michigan, Lower Peninsula
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Transmission Line Loss by Cable Type

As a general rule in designing a radio transmission system, the loss in the transmission line should be kept to a minimum. There will always be some loss in the transmission line, but a line loss figure of -1 dB is a reasonable goal.

A loss of 1 dB means the power will be reduced by a factor of 0.74. For example, if a transmitter produces 25-Watts at its output and is connected to a load by a transmission line with a loss of 1 dB, the load will receive only 25-Watts x 0.794 or 19.8-Watts of power. The 5.2-Watts of power lost goes mostly into warming up the transmission line. If the loss in the transmission reaches 3 dB, then half of the transmitter power is lost in the transmission line, and for a 25-Watt transmitter only 12.5-Watts would reach the antenna.

Because transmitters in the VHF Marine Band are rated for their output power, not for power at the antenna, any loss in the transmission line reduces the effective power of the transmitter. Also, transmitters in the VHF Marine Band must not make more than 25-Watts, so they typically are adjusted to be conservative in power out. Further, power output tends to vary with the supply voltage. For a supply voltage of 12.0-Volts, which might be typical for the actual voltage available at the radio after allowance for the state of charge of a 12-Volt battery and voltage drop in the power distribution wiring, the power output of a VHF Marine Band rated for 25-Watts might be closer to 20-Watts. A transmitter making 20-Watts and connected to the antenna by a transmission line with a loss of 1 dB would only deliver 15.9-Watts to the antenna.

Transmission line loss also causes a loss of received signals. To some extent, a receiver can overcome transmission line loss with increased gain. Unlike a transmitter which operates at a fixed gain and fixed output, a receiver has variable gain and also has enormous reserve gain. Loss of the received signal in the transmission line between the antenna and the receiver can be compensated by increase gain in the receiver, but that process reaches a limit when the input signal is very weak. Those weak signals at the threshold of reception will be most affected by loss in the transmission line, and as a result, transmission line loss effectively limits the ultimate sensitivity of a receiver. When receiving very strong signals, transmission line loss on receive really has no effect.

This article examines typical 50-Ohm coaxial cable types and finds the maximum length for use at 150-MHz (the VHF Marine Band) where loss will not exceed -1 dB.

RG-58C/U is rated at -6.1 dB/100-feet; maximum length for -1 dB is 16.4-feet. Outside diameter is 0.195-inch.

RG-8X is rated at -4.6 dB/100-feet; maximum length for -1 dB is 22-feet. Outside diameter is 0.242-inch.

LMR-240 UltraFlex is rated at -3 dB/100-feet; maximum length for -1 dB is 33-feet. Outside diameter is 0.240-inch.

RG-213/U is rated at-2.8 dB/100-feet; maximum length for -1 dB is 36-feet. Outside diameter is 0.405-inch.

LMR400 UltraFlex is rated -1.5 db/100-feet; maximum length of -1 dB is 67-feet. Outside diameter is 0.405-inch.

Data for cable loss taken from a vendor's website and represents good quality cable. Inferior quality cable sold with similar type designators will likely have higher loss.

jimh
Posts: 11862
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Transmission Line Loss by Cable Type

Power ratios expressed in decibels can be converted to a numerical power ratio (gain or loss) . The formula is

Power ratio = 10(dB/10)

For example, a power ratio of 5 dB is

Power ratio = 10(5/10) = 3.16

Another example: a power ratio of -2 dB is

Power ratio = 10(-2/10) = 0.63

Power ratios expressed as numerical ratios can be converted to decibels. The formula is

dB = 10 log (PowerRatio)

For example, if power increases to 15-Watts from 9-Watts, the power ratio is 15/9 or 1.66. Expressed in decibels this is

db = 10 log(1.66) = 2.2 dB

If power decreases to 16-Watts from 25-watts, the power ratio is 16/25 or 0.64. Expressed in decibels this is

dB = 10 log(0.64) = -1.94 dB