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VHF Marine Band Antennas For Small Boats

by James W. Hebert

The range of communications using a VHF Marine Band radio is primarily determined by the antenna installation. This article explores an optimum configuration for a small recreational boat.

Choosing a VHF Marine Radio Antennas

Practically all recreational small boat installations use commercially manufactured antennas. There are several brands in very wide distribution, and choice of an antenna is often made from the selections offered by these manufacturers. For installation on my 22-foot Boston Whaler boat, I did not find any of the more commonly used antennas to be appropriate, and I searched for a better solution. The primary goal of my search was to find a way to increase the overall antenna height above the water. In a previous article, I demonstrated that the antenna height has the greatest impact on the communication range of the radio. Getting the antenna as high as possible was the most important factor.

The size, weight, and cost of the antenna were also considered. The boat's current antenna mounting hardware should be re-used, if possible, and the cost of the antenna must not be excessive. Because the boat can be operated at speeds of over 40-MPH on the water, and, if the antenna is accidently left in the upright position when on the trailer, at speeds of over 60-MPH on the highway, the wind loading of the antenna should also be minimized. The appearance is also moderately important. Finally, the electrical performance was paramount. The antenna must be an efficient radiator.

Higher Mounting

Many small boats are equipped with a radar arch, even if they lack any RADAR facilities. The arch makes a great place to install the mounting base for an antenna. It elevates the antenna base to about 7-feet above the water. Unfortunately, my boat does not have a radar arch, and for many reasons I probably will not install one. My antenna will have to be mounted elsewhere.

The current mounting location is at the base of the cabin superstructure on the starboard side. This location is just forward of the helm. It allows the helmsman to easily reach over the windshield to operate the tilting ratchet to lower the antenna. If at all possible, it was very desirable to re-use this location. Doing so would avoid having to make repairs to the hull (by filling the four 1/4-inch mounting holes), and the location had proven itself to be a very practical spot for an antenna.

What Was Wrong With the Current Antenna

As originally equipped, my boat had a very common eight-foot fiberglass tapered whip antenna. This is a so-called 6-dB gain antenna. The actual active part of the antenna is the upper 7-feet or so; this is where the radio frequency energy radiates from the antenna. This was precisely part of the problem to be cured. The antenna is just a foot removed from the helm dash panel which contains a number of instruments, including the engine tachometer. Whenever the radio transmitter was keyed, so much radio frequency energy was emitted from the base of the antenna that the tachometer gauge was affected. The indicator pointer on the tachometer would swing upward whenever the Push-to-Talk (PTT) button was pressed. This was a clear sign of interference from the antenna. Also, it demonstrated how some of the transmitter's power was being radiated from the base of the antenna, only one-foot up from the mounting.

If we allow that the base of the antenna was mounted about 4-feet above the waterline, and that the active portion of the antenna began about 1-foot up from the base, this means the antenna itself was located from 5-feet to 12-feet above the water. This gives an average height of (5+12)/2 = 8.5-feet above the waterline.

The search for a new antenna was focused on finding a four-foot antenna which could be mounted to a four-foot base extension installed on the original mount. This would put the average height of the antenna at 6-feet above the base, or at 10-feet above the water. This would increase the average antenna height by 1.5 feet above the water. While this does not sound like a huge increase, it will translate into greater range. With an average height of 8.5-feet, the radio horizon would be 4.14 miles. With an average height of 10-feet, the radio horizon will increase to 4.49 miles, an improvement of about 0.35 miles. Yes, it is not a fantastic difference, but it is an improvement!

The Ideal four-foot Antenna

The search for the ideal four-foot antenna proved to be difficult. There were many common four-foot antennas made with construction similar to the original antenna using a tapered fiberglass whip. However, most of these were too heavy to be supported by the original mount when placed atop a four-foot extension mast. They also had significantly more wind area, as well as significantly higher cost. For these reasons, the fiberglass whip with molded-in base style of antenna was eliminated from consideration.

The ideal antenna construction appeared to be one using a small diameter steel whip in conjunction with a base loading and matching coil. There were several choices of manufacturer for this style. Most of these antennas were intended for mounting on the top of sailboat masts, and they did not have a threaded coupling which would screw onto a standard mast extension. Some had adapters for this purpose, but they were judged to be awkward or a kludge. One antenna from a common manufacturer had all of the desired attributes, but its base coil was extremely large. This was both a size and an aesthetic problem.

The GAM ELECTRONICS Antenna

Finally, after exploring many websites and GOOGLE search results, the ideal antenna was found. A small manufacturer in New Hampshire, GAM Electronics, had the perfect solution. Their model SS-2 was exactly what I was looking for. It was a little over three-feet long, and in conjunction with a special mounting accessory model ADAP-II, it can be threaded onto the end of a standard extension mast. Mechanically, the SS-2 was exactly what I wanted. But what about its electrical properties?

I called GAM Electronics and spoke directly with Ed, the company owner. He told me about his product and proudly boasted that every antenna was hand tuned and tested during assembly! He also pointed me to an independent test of his antenna among a group of many popular antennas. The results were very impressive. The little SS-2 produced better signals than many much larger and more expensive antennas in actual field strength tests. And the VSWR was a very low 1.24:1. Electrically the antenna looked to be a better performer than most garden-variety 8-foot fiberglass whips. The antenna uses military grade RG-58C/U coaxial transmission line, too, which I strongly prefer over so-called "marine grade" cables.

The mechanical design is also quite optimum for a small boat. By separating the whip, the coil, and the mounting adapter into three components, the replacement of the antenna is modular. If the whip breaks--which is unlikely given its stainless steel construction--you need only replace the whip portion. If the coil breaks, just the coil can be replaced. You won't have to run all new feed line because that is part of the ADAP-II mounting.

The GAM Electronics SS-2 is a popular choice for an antenna by Boston Whaler Commercial and Government Products. Ed informed me that Whaler was a long-time customer of his. And, of course, with a name like GAM Electronics, what could be more appropriate for a Boston Whaler boat!

Installation

In early April I ordered a GAM SS-2 and ADAP-II mount. The antenna arrived a few days later. As soon as I took it out of the box I liked the look and feel of the little whip and coil. Its weight and wind loading would be about as light and small as one could hope for. Actual installation on the boat took longer than expected. It was mid-June before I had the new antenna installed. I mounted it atop a Shakespeare 498 four-foot white polycarbonate extension mast, using the original tilting base mount. I snaked the feed line down the extension mast, through the mount, and into the cabin. I soldered on a PL-259 connector (not supplied) and hooked it to my VHF Marine Band radio.

 

PHOTO: GAM Electronics SS-2 Antenna
GAM Electronics SS-2 Antenna; ADAP-II Base
The GAM Electronics SS-2 antenna screws into the ADAP-II base. The base in turn screws into a standard Shakespeare 498 four-foot extension mast. Relatively small size of the base loading/matching coil reduces the weight and wind loading. The stainless steel whip can be easily replaced or pruned for optimum resonant frequency. The coaxial cable feed line is integral with the ADAP-II base, and it is routed down the extension mast to the base.
Photo Credit: James W. Hebert

Sitting in my front yard about 22-miles from the Detroit River, I began to hear all of the commercial vessel traffic coming in loud and clear. I also could get good reception on Vessel Traffic Control up in Sarnia, Ontario, about 65 miles to the northeast. The little antenna was working well on receive.

Mechanically, however, I was worried. The total weight of the antenna and mast was probably greater than the original 8-foot whip, and the plastic extension mast was definitely stiffer and had more wind area. Moving the extension mast slightly produced some flexing in the cabin side, and this did not look like a good long-term situation. It could lead to problems in the laminate strength. I explored the back side of the mount. I found that the 1/4-inch mounting bolts were only retained by small washers and elastic stop nuts; there was no reinforcement. A backing plate was deemed necessary.

 

PHOTO: Shakespeare 498 four-foot extension mast
Shakespeare 498 Extension Mast and Base
The Shakespeare 498 plastic four-foot extension mast is an inexpensive ($30) way to raise the antenna. The mast threads onto the existing stainless steel ratchet mount. To reinforce the mount, a 4-inch by 6-inch backing plate was fabricated from 1/4-inch thick aluminum plate.
Photo Credit: James W. Hebert

A backing plate of about 4 x 6 inches was fabricated from a piece of scrap aluminum. Taking careful measurement of the hole layout of the existing mount, the four holes were laid out and drilled on the backing plate. The mounting screws were then slowly removed, until the antenna mount base was left in position, retained only by the adhesive sealant used to bed it to the hull. The sealant supported the entire weight of the mount and antenna! The backing plate was placed in position, and the four bolts re-installed and tightened. The whole operation had to be done by feel, as there is very limited visual access to the cabin interior where the antenna mounting bolts fasten. My hole layout was very good, and all four bolts fit perfectly into the backing plate holes, which were drilled only 1/16-inch oversize.

Performance

We just finished our first long weekend aboard the boat with the new antenna. So far the antenna appears to be working as expected. Our reception seemed on par with the two other boats in our company whose 8-foot antennas are mounted to bases on their radar arches elevated about seven feet above the cockpit deck. While siting at the main dock in Leland, Michigan, we were able to receive the NOAA weather radio station from Wisconsin (most likely WXN-69, Door County, 162.425 MHz), coming in from across the lake at a range of approximately 60 miles. (A chart of the expected coverage area of WXN-69 shows that Leland is at the fringe of this normal receive range.) In prior comparisons like this, these boats had always shown greater radio range. We have yet to make any meaningful measurements of transmission range. These will come in the future. As expected, operating the transmitter no longer produced any interference with the tachometer. The antenna is now several feet above the instrument cluster. I also anticipate the antenna will perform better on transmit because now all of the power is being directed into the upper portion of the antenna; none is wasted by radiating into the helm console.

The mechanical performance of the antenna was good. It did not have as much movement as I expected, and instead, when running at speed, the wind pressure seemed to steady the antenna. It does not look like there will be long-term mechanical effects from the slightly heavier antenna.

 

PHOTO: Profile view of antenna and extension mast mounted on Boston Whaler REVENGE 22 W-T WD
Boston Whaler and GAM Antenna
The actual antenna is now elevated well above the helm and its instruments and electronics. The extension mast creates a handy signal staff for flying of courtesy flags when in foreign water or for small pennants.
Photo Credit: James W. Hebert

VSWR Measurements

I made the following VSWR and POWER Measurements for the SS-2 Antenna:

GAM Electronics SS-2 VHF Marine Band Antenna
Transmitter: ICOM M-402
Directional Wattmeter: DAIWA CN-620
July 16, 2006
Vessel CONTINUOUSWAVE on its trailer

FORWARD POWER and VSWR versus FREQUENCY

CHANNEL MHZ     WATTS   VSWR
01A     156.05  18.5    1.35
06      156.3   18      1.3
09      156.45  18      1.27
12      156.6   18      1.25
16      156.8   18      1.2
20      157.0   18      1.2
28      157.4   18      1.16
88      157.425 18      1.18

The VSWR looks very good. This in itself is not a measure of the antenna's effectiveness, but an effective antenna will have a low VSWR and proper match to its feedline. The GAM Electronics SS-2 shows these qualities across the VHF Marine Band.

Plot: VSWR vs Frequency for GAM SS-2

Unanticipated Benefit

One unforeseen benefit of the new installation is the extension mast itself. It makes a perfect location for flying a courtesy flag when visiting Canada, which we often do. Because the mast is definitely not part of the actual antenna, there is no electrical or esthetic problem in attaching a small flag to it.

Installation Details

As shown above in the close up photograph, the stock Shakespeare 498 extension mast does not quite completely thread into the GAM ELECTRONICS ADAP-II base. To get a cleaner installation it is recommended (first suggested by Seth Campbell) that approximately four threads be cut off of the top of the Shakespeare 498 extension mast threaded fitting. This will allow the ADAP-II base to thread completely onto the extension mast. Also, there is a stainless steel mounting nut supplied with the SS-2 antenna. This should be removed before threading the antenna onto ADAP-II mount. This nut is used when the antenna is mounted on a different type of mount and is not needed with the ADAP-II base.

 

Photo: GAM SS-2 antenna, ADAP-II base, and Shakespeare 498 extenstion mast.
Shakespeare 498 Mount
From top to bottom: GAM SS-2 antenna, GAM ADAP-II mount, Shakespeare 498 extension mast. To get a cleaner installation the plastic threaded coupling on the 498 extension mast has been trimmed by about four threads.
Photo Credit: Seth Campbell

 

Photo: GAM SS-2 antenna, ADAP-II base, and Shakespeare 498 extenstion mast.
Courtesy Flag Staff
The four-foot extension mast used to raise and support the GAM SS-2 antenna also makes a perfect staff for flying a courtesy flag when cruising in foreign waters. The United States Power Squadron recommendations concur. Here is CONTINUOUSWAVE at a transient berth in Lions Head, Georgian Bay, Ontario, Canada.
Photo Credit: Christine Wilson

Follow-up

The 2014 boating season will be my eighth season using the GAM Electronics antenna and mounting system described above. All the original components of the system are still in use and working perfectly. The performance of this antenna system has been excellent. I can routinely communicate with any other boat or shore station that I can receive. On open water it is typical to communicate boat-to-boat at 15-miles or more. On receiving the antenna has been fabulous. I have good signals from NOAA weather broadcast transmissions at ranges of over 75-miles. I continue to strongly endorse this antenna installation for use on small boats.

Questions or Comments

Questions for the author or comments about this article may be added to the discussion reserved for that purpose.


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Copyright © 2006 by James W. Hebert. Unauthorized reproduction prohibited!

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Last modified: Wednesday, 06-Aug-2014 02:58:39 EDT
Author: James W. Hebert
This article first appeared July 10, 2006.