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  VHF 8-Foot Antenna: What Is Inside

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Author Topic:   VHF 8-Foot Antenna: What Is Inside
jimh posted 10-19-2008 03:00 PM ET (US)   Profile for jimh   Send Email to jimh  
This weekend I disassembled a SHAKESPEARE brand 8-foot VHF Marine Band antenna. The antenna was the CENTENIAL 5101 model. This antenna is widely used and is well-known to boaters. Its is a tapering fiberglass mast with an attached chrome-plated mounting collar which is threaded internally to screw onto a 1-inch threaded antenna mount. The antenna had been broken when it hit an overhead obstruction, and it was given to me by a friend to investigate its construction.

The lower 4-feet 3-inches of the fiberglass mast contained nothing but the transmission line. This segment of the mast is not involved in the radiation of radio waves from the antenna. The lower 4-feet 3-inches are simply a way to raise the overall antenna height by that amount, the equivalent of using an extension mast of the same length.

The upper 7-inches of the antenna consists of nothing but twine which is used to hold the antenna assembly to the very tip of the tapering fiberglass mast that encases it. Again, this portion is not involved in any way with radiation of radio waves from the antenna.

The actual antenna portion of the mast consists of a brass tube about 5/32-inch OD and approximately 36-inches long. At the base another similar tube is soldered to the main tube and runs parallel to it for 2-1/2-inches. In terms of electrical length, we can analyze the antenna using the following relationship:

Wavelength in feet = 984 / Frequency in MHz

We can consider the antenna is designed for a target frequency of 156.8-MHz, roughly mid-band in the VHF Marine Band. In terms of that frequency, the dimension of a wavelength is thus

Wavelength in feet = 984/156.8
Wavelength in feet = 6.27-feet
Wavelength in inches = 75.3-inches

The 36-inch-long radiating portion of the antenna is thus

36/75.3 = 0.48-wavelength, or "one-half wavelength" or a "half-wave" antenna

When a half-wave antenna is fed at its end, the impedance of the antenna is very high because this is a high voltage point. In order to effectively feed a half-wave antenna at its end with 50-ohm transmission line, an impedance matching network is required.

The feed system for this antenna is complex. The center conductor of the transmission line is not connected directly to any portion of the antenna. (This gives it a DC open circuit when measured with a multi-meter.) Instead the center conductor is fed approximately 2-1/4-inch up the parallel brass tube. This forms a coaxial capacitor, coupling the transmission line to the radiator. The outer conductor of the transmission line is also not directly connected to the end of the radiator. Instead, the outer conductor or shield is soldered to a piece of RG-58/U center conductor which is fed inside the brass tube of the radiating element. This new conductor runs about 10-3/8-inch (0.138-wavelength) up the inside of the tube, then it is shorted to the brass tube radiator. This technique creates a lumped reactance which is used in the matching network.

With regard to gain, the manufacturer describes this antenna as:

Centennial Style 5101
8 ft. (2.43 M) VHF Marine Band 6dB
End-fed with matching stub

A new standard of excellence in an economical antenna! Shakespeare brings you the best of the high-end features in an 8ft antenna with a brass element and a smooth, high gloss, polyurethane finish that won’t turn yellow in the sun. This antenna delivers not only performance, but superb value.

http://www.shakespeare-marine.com/antennas.asp?antenna=5101

Many boaters seem to equate an 8-foot antenna as automatically being better than a 3-foot antenna (such as the GAM ELECTRONICS SS-2), but as this disassembly shows, this "8-foot" antenna really only contains a 3-foot radiating element.

Also see:

http://continuouswave.com/whaler/reference/VHFAntenna.html

for more discussion about VHF Marine Band antennas for use on small boats.

Chuck Tribolet posted 10-19-2008 08:34 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
It should be noted that this is a relatively inexpensive
antenna (About $45 at discount prices). It would be
interesting to see what's inside a Shake Galaxy. That's
described as a colinear phased 5/8 wave.


Chuck

swist posted 10-21-2008 02:45 PM ET (US)     Profile for swist  Send Email to swist     
I have been disassembling defunct electronic devices since I was five years old, always wondering what was inside. I am now 60 and the major conclusion I have come to is that the number of active components I find inside these things is inversely proportional to my age.

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