A Captivating Saga of VHF Antennas
Posted: Sun Aug 09, 2020 10:57 am
As I mentioned recently, I had a failure of the antenna extension mast on my boat's VHF Marine Band radio antenna installation. Repairing this failure has led me to an entirely new approach to antenna installation. I will explain in a multi-part narrative.
The 13-year-old antenna installation consisted of a GAM ELECTRONICS SS-2 antenna and ADAP-II base adaptor, mounting atop a four-foot extension mast Shakespeare model 468. In trying to disassemble these three components, there was quite a problem. The transmission line between antenna and radio was captivated in place in two positions: the feed line ran down the inside of the extension mast and exited at a hole in the base; and the feed line passed through a bulkhead fitting on the cabin topside. The diameter of both these passages was only large enough for the coaxial cable itself to pass through. In radio work, we say the transmission line was captivated in place by these two small passages. In order to extract the coaxial cable from the extension mast, I had to cut off the (very recently installed) connector at the radio end of the cable, pull the cable out of the cabin feed-through, and then remove the ADAP-II adaptor from the extension mast and extract the cable from the extension mast.
The connector at the radio end of the cable was a FME (For Mobile Equiment) female cable-end straightconnector that I had just installed a few months ago. (More information about FME connectors is available in a prior article in this forum.) Although the cost of this connector is very low (only about $3) and I had more of them on-hand, it was irksome to have to cut-off a perfectly good connector in order to perform a routine repair to the extension mast. Also, the connector used a solder-on center conductor and a crimp-on shield conductor. I had the proper crimp tool on hand, but the soldering iron I had available had a tip that was much too large to work on the very small center pin contact. Also, I decided I was not going to install another connector just to re-mount the GAM SS-2 antenna without the mast, as later I would have to cut-off that connector again to put an extension mast back into the system.
A further problem encountered was the need to unthread the ADAP-II adaptor from the upper end of the extension mast. The two parts fit together using the standard 1-inch x 14 pipe threads. The lower end of the transmission line passed through a hole at the bottom of the extension mast. Unthreading the ADAP-II adaptor from the extension mast would require about 14-turns, with each turn imparting a twist to the coaxial transmission line. I did not want to impart 14-revolutions of twist into the transmission line, so it became necessary to first remove the extension mast from the steel antenna base, a typical ratchet mount.
The point of failure of the extension mast was actually at the base in the section that threads onto the steel mounting base. The base of the extension mast is made of a soft material, some sort of plastic that might have originally been PVC. But 13-years of exposure to sunlight and weather seemed to have softened the plastic material somewhat. The friction between the threads of the plastic base and the steel mount was very high. Removal by hand was impossible. A large slip-joint pliers was needed to gain enough purchase and leverage to unthread the mast. Of course, pressing the pliers onto the plastic base with enough force to be able to rotate the base to unthread it created further friction. The room available for the pliers was also limited by the location of the mount. I had to unthread the mount about 1/8th-turn at a time. The 14-revolutions took about 100 or more 1/8th-turns on the base--a very tedious process. All the time the extension mast was being rotated, the coaxial cable feed line had to be carefully allowed to wind up in a spool around the extension mast, as it was still captivated at the base of the mast.
Aggravating the unthreading of the base from the steel mount was a partial fracture in the plastic base. This caused the upper part and lower part of the base to be threaded at different points on the steel threads of the base, further increasing the friction between the plastic and steel parts. Getting the extension mast unthreaded while not damaging the coaxial cable transmission line captivated in the base required careful use of tools.
Of course, eventually I got the extension mast off the mount. The next step was to unthread the ADAP-II mount from the top of the extension mast. Again, about 14-revolutions were needed. And again the transmission line would be rotated or twisted with each revolution. I pushed the transmission line back through the exit hole at the base of the extension mast to free it from being captivated there. I took all the twists out of the cable, and laid it out in a straight line. Then I began to unthread the ADAP-II adaptor from the top of the extension mast. Each rotation of the adaptor during unthreading imparted a twist to the coaxial cable. After about two rotations I stopped and untwisted the transmission line two turns. Then back to unthreading the adpator from the extension mast two more turns. This was repeated until the ADAP-II adaptor was completely unthreaded from the mast.
With the broken extension mast removed, I began to re-assess the antenna system installation. Repair of the broken extension mast had required too much disassembly of the antenna system to effect a repair. I determined that I would re-design the antenna system so that in the future the transmission line could be run from antenna to radio via passages that would be large enough to allow the FME female cable-end connector to pass. The FME connector will fit through a hole of 3/8-inch diameter. In this way the antenna and its transmission line could be removed without have to cut off the connector.
[More installments to come.]
The 13-year-old antenna installation consisted of a GAM ELECTRONICS SS-2 antenna and ADAP-II base adaptor, mounting atop a four-foot extension mast Shakespeare model 468. In trying to disassemble these three components, there was quite a problem. The transmission line between antenna and radio was captivated in place in two positions: the feed line ran down the inside of the extension mast and exited at a hole in the base; and the feed line passed through a bulkhead fitting on the cabin topside. The diameter of both these passages was only large enough for the coaxial cable itself to pass through. In radio work, we say the transmission line was captivated in place by these two small passages. In order to extract the coaxial cable from the extension mast, I had to cut off the (very recently installed) connector at the radio end of the cable, pull the cable out of the cabin feed-through, and then remove the ADAP-II adaptor from the extension mast and extract the cable from the extension mast.
The connector at the radio end of the cable was a FME (For Mobile Equiment) female cable-end straightconnector that I had just installed a few months ago. (More information about FME connectors is available in a prior article in this forum.) Although the cost of this connector is very low (only about $3) and I had more of them on-hand, it was irksome to have to cut-off a perfectly good connector in order to perform a routine repair to the extension mast. Also, the connector used a solder-on center conductor and a crimp-on shield conductor. I had the proper crimp tool on hand, but the soldering iron I had available had a tip that was much too large to work on the very small center pin contact. Also, I decided I was not going to install another connector just to re-mount the GAM SS-2 antenna without the mast, as later I would have to cut-off that connector again to put an extension mast back into the system.
A further problem encountered was the need to unthread the ADAP-II adaptor from the upper end of the extension mast. The two parts fit together using the standard 1-inch x 14 pipe threads. The lower end of the transmission line passed through a hole at the bottom of the extension mast. Unthreading the ADAP-II adaptor from the extension mast would require about 14-turns, with each turn imparting a twist to the coaxial transmission line. I did not want to impart 14-revolutions of twist into the transmission line, so it became necessary to first remove the extension mast from the steel antenna base, a typical ratchet mount.
The point of failure of the extension mast was actually at the base in the section that threads onto the steel mounting base. The base of the extension mast is made of a soft material, some sort of plastic that might have originally been PVC. But 13-years of exposure to sunlight and weather seemed to have softened the plastic material somewhat. The friction between the threads of the plastic base and the steel mount was very high. Removal by hand was impossible. A large slip-joint pliers was needed to gain enough purchase and leverage to unthread the mast. Of course, pressing the pliers onto the plastic base with enough force to be able to rotate the base to unthread it created further friction. The room available for the pliers was also limited by the location of the mount. I had to unthread the mount about 1/8th-turn at a time. The 14-revolutions took about 100 or more 1/8th-turns on the base--a very tedious process. All the time the extension mast was being rotated, the coaxial cable feed line had to be carefully allowed to wind up in a spool around the extension mast, as it was still captivated at the base of the mast.
Aggravating the unthreading of the base from the steel mount was a partial fracture in the plastic base. This caused the upper part and lower part of the base to be threaded at different points on the steel threads of the base, further increasing the friction between the plastic and steel parts. Getting the extension mast unthreaded while not damaging the coaxial cable transmission line captivated in the base required careful use of tools.
Of course, eventually I got the extension mast off the mount. The next step was to unthread the ADAP-II mount from the top of the extension mast. Again, about 14-revolutions were needed. And again the transmission line would be rotated or twisted with each revolution. I pushed the transmission line back through the exit hole at the base of the extension mast to free it from being captivated there. I took all the twists out of the cable, and laid it out in a straight line. Then I began to unthread the ADAP-II adaptor from the top of the extension mast. Each rotation of the adaptor during unthreading imparted a twist to the coaxial cable. After about two rotations I stopped and untwisted the transmission line two turns. Then back to unthreading the adpator from the extension mast two more turns. This was repeated until the ADAP-II adaptor was completely unthreaded from the mast.
With the broken extension mast removed, I began to re-assess the antenna system installation. Repair of the broken extension mast had required too much disassembly of the antenna system to effect a repair. I determined that I would re-design the antenna system so that in the future the transmission line could be run from antenna to radio via passages that would be large enough to allow the FME female cable-end connector to pass. The FME connector will fit through a hole of 3/8-inch diameter. In this way the antenna and its transmission line could be removed without have to cut off the connector.
[More installments to come.]