I found an old VHF Marine Band antenna that I had obtained years ago and disassembled. It took me a bit of searching to find this, but, as Chris says, I never throw out anything.
This antenna was a mid-priced eight-foot antenna encased in a fiberglass housing from a major manufacturer, probably Shakespeare. The fiberglass housing (or radome) failed, probably due to some sort of impact or stress. The fiberglass portion was fitted and adhered to a metal ferrule that could be threaded onto a 1-inch x 14 threaded mount--in other words, a very typical antenna often described as a "6 dB gain" antenna.
From top to bottom, here is what I found:
- the top 7.25-inches were waxed string, with one end wrapped to the radiator below and the other end exiting the top of the fiberglass housing and secured there; this string suspended and insulated the radiating part of the antenna inside the fiberglass housing;
- a 25-1/2-inch long radiating element which had been broken off of the lower part when the antenna failed; the radiating element is made from brass tube with an outside diameter of 5/32-inch. The reason the tube broke at this point is likely due to a portion of the tube wall having been removed, creating a hole. This created a weakness, and the upper part of the brass tube broke off the lower part. The reason for the hole was to allow an insulated conductor travelling inside the tube to exit and be soldered to the brass tube at this point;
- a 10-1/2-inch long radiating element, the portion below the breaking point; this section has a second brass tube soldered in parallel at the bottom for 2-1/2-inches. The purpose of this second tube segment was probably to create a "trombone capacitor." (I will explain this more below.)
- a length of 50-Ohm coaxial transmission line similar to RG-58 with white outer insulation but having no markings, which extended for about 52-inches until it exited the antenna housing at the metal ferrule base, evidenced by a permanent bend and some discoloration in the insulation from passing through a rubber grommet.
Here are three close-up views of the antenna components I describe above:
- The top section with a seven-inch long waxed string attached to suspend the antenna from the top of the radome
- antennaTop.jpg (63.75 KiB) Viewed 13175 times
- The broken junction in the radiating element; the center conductor of the lower element exited via a hole and was soldered to the radiator here.
- midPointJointConnection.jpg (43.91 KiB) Viewed 13175 times
- The lower end of the radiator, with a trombone capacitor and transmission line. The details of how the transmission line and trombone capacitor were connected have been lost because the transmission became disconnected from the antenna.
- antennaBaseFeedlineConnection.jpg (47.24 KiB) Viewed 13175 times
Analysis of antenna structureThe overall length of the radiating portion of the antennas is 36-inches, the combined lengths of the 25-1/2 broken top section and 10-1/2-inch length of the associated bottom section. In terms of radio wavelength at 157-MHz, this represents a length of just under one-half wavelength. The antenna can be described as an end-fed half-wavelength vertical.
Of the overall eight feed of antenna housing, we see that only three-feet of the assembly is the actual antenna radiating element. The upper 7-inches are just a piece of string. The next 36-inches are the antenna radiator, and the remaining 52-inches are just transmission line. The overall housing can be described as a three-foot antenna mounted 4-foot 6-inches above the housing base by a string suspension.
Because there was damage to the connections at the base of the antenna, it is not possible to be certain exactly how the antenna was connected to the transmission line. The base seems to provide for a trombone capacitor, a mechanical method of constructing a small capacitor value by inserting a conductor with good insulation inside a tube, making a coaxial capacitor. The capacitor is formed by the capacitance between the inner and outer conductors, and the longer the tube the conductor can be inserted creating more capacitance. In this instance the value of capacitance would be small, a few picofarads. The capacitor may have been part of an impedance matching network to better match the radiation resistance of the antenna to the 50-Ohm transmission line. I suspect that the length of the tube was determined by experiment when the antenna was designed and prototyped.
The existence of a conductor running up inside the radiating element and connecting to the radiator at an elevated point is also likely a manifestation of a impedance matching technique. If the transmission line connected to the very end of a half-wavelength radiator, the impedance at that point would be extremely high, several thousand Ohms, resulting in a very poor match to 50-Ohms. By moving the actual connection of the transmission line to a higher point on the radiating element, the impedance would begin to drop. I suspect that this matching point was determined by extensive trial and error experimentation when this antenna design was developed and prototyped.
Conclusion
This common "8-foot" fiberglass-encased antenna reveals itself upon disassembly to be a three-foot antenna suspended seven-inches below the top of the housing.
The use of a string to suspend the top of the antenna inside the housing is probably only workable at low power levels like 25-Watts. The end of the radiating element is the point of highest voltage on an antenna. The string must be kept dry and have good insulating characteristics to not break down under the voltage stress created by the antenna currents.