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Author Topic:   VHF Antenna Installation: Feed lines
jimh posted 03-28-2012 07:00 AM ET (US)   Profile for jimh   Send Email to jimh  
Most VHF Marine Band antennas are supplied with a coaxial transmission line that is permanenet connected in an unrepairable manner to the antenna. Typically the feed line is about 20-feet in length and is RG-58-type or RG-8X type coaxial cable. In the case of RG-58-type coaxial cable, if the cable is of a good grade, say RG-58C/U (a Mil-Spec grade), the transmission line loss will be slight at 156-MHz, only about -1.05 dB. To appreciate this, if your radio produces 25-watts at the output, then 19-watts will reach the antenna. If you were to cut off ten feet, the attenuation would decrease to about -0.6 dB, and with 25-watts output the antenna would receive 22-watts. It is hard to imagine a situation in which a change in signal level of half a decibel would make any difference.

In the case of RG-8X there is even less loss, so cutting off a few feet makes even less difference. On recreational boat the coaxial transmission line is seldom of cable larger than RG-8X-type.

A problem caused by cutting the feed line to be shorter is usually reattachment of the connector at the transmitter end. The PL-259 connector can be a problem for many boaters to properly attach. There may be more harm caused by a poorly fitted connector than good done by cutting off a few feet of feed line.

Most VHF Marine Band antennas are intended to be used with 50-ohm or 52-ohm coaxial transmission line. Coaxial transmission lines of 75-ohm impedance should not be used. Curiously, a 75-ohm cable is optimized for lowest loss, but a 50-ohm cable is better for handling power; most transmitters are standardized for 50-ohm antenna systems.

Although one is often suggested or cited, there is no magic length for the transmission line when connected to a tuned and resonant antenna where the antenna impedance and the feed line impedance are closely matched (as in the case in VHF Marine Band antennas), and, in general, the transmission line should only be as long as needed. However, the antenna should not be located too near the transmitter. Most manufacturers specify at least a three foot separation and more is preferred. Having a few feet of extra cable can also be useful in the event you decide to move the radio or the antenna.

The concern for the transmitter to be too near the antenna is not for anything related to the transmission line, but that the transmitter will be in the radio-frequency field of the antenna. Putting the transmitter in a strong RF field may cause problems if the RF interferes with the transmitter's own circuitry. This is called RF Feedback. If can result in bad modulation or erratic operation of the transmitter.

The transmission line should be led away from the antenna in a manner that tends to reduce the exposure of the transmission line to the radiated signal from the antenna. For a vertically polarized end-fed antenna, like almost all small boat VHF Marine Band antennas, you just want to avoid running the transmission line parallel to the antenna and close by it. The more vertical separation, the better. On my boat the radio is located about six feet below the base of the antenna and about two feed horizontally away from it. There is no sign of any RF feedback.

A good cable loss calculator is at the Times Microwave website

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