posted 07-10-2006 11:28 PM ET (US)        

VHF Marine Band Antennas For Small Boats
by James W. Hebert
http://continuouswave.com/whaler/reference/VHFAntenna.html

As I collect more data about the electrical performance of the GAM Electronics SS-2 antenna, I will add it to the article.

posted 07-11-2006 07:00 AM ET (US)         

My last boat had an 8' antenna whose base was at about mid-console height (about even with the engine control). My Whaler has 4' loading-coil antenna mounted at about the same place. There is no comparision. I need to be really close to my target for the short one to work at all.

The problem most of us face is that putting a tall antenna on most small Whalers presents several problems, including inability to get it over a bimini, and necessity to fold it down under a console cover. A gunwale mount which would solve some of these problems is difficult on Whalers since there is no convenient way to run the cable.

posted 07-11-2006 07:21 AM ET (US)            

Marine VHF Radio Communications
by James W. Hebert
http://continuouswave.com/whaler/reference/VHF.html

posted 07-11-2006 10:52 PM ET (US)            

By the way, I have disassembled one eight-foot fiberglass tapered whip antenna. I had this opportunity after I hauled the boat home one afternoon from the ramp and forgot to lower the antenna. I made it about 45-miles on the highway, almost to my house, until the antenna met a low tree branch and fractured the fiberglass whip. Inside the antenna I found the radiating element to consist of just the center conductor material of the coaxial feed line, that is, a relatively small diameter radiator.

The VSWR of an antenna is not a surefire bench mark of how well it works as an antenna. The VSWR is a measurement of how well the feed point impedance of the antenna matches the characteristic impedance of the feed line.

This weekend I plan to take some VSWR measurements with the new antenna using a professional grade directional wattmeter. I often see reports of antenna VSWR measurements made with $49 "marine" VSWR indicators, but I have very little faith in those measurements. Without going deeply into the theory of those measurements, it is very difficult to make measurements of standing wave ratios below 2:1 when the power levels involved are low (25-watt) as they are in this case. Here is a good technical paper which explains some of the problems in making accurate VSWR measurements:

http://www.bird-electronic.com/app_notes/StraightTalkAboutDirectivity. pdf

Someone with an electrical engineering background will find this interesting reading, but for most boaters it will have little meaning or application to their antenna installation.

An antenna which has a poor match to 50-ohm transmission line can be a problem, particularly with transmitters such as used in the VHF Marine Radio service that have a fixed output stage and no adjustment or tuning. High VSWR can reduce the power output from the transmitter. High VSWR can cause an increase in the feed line loss.

In my installation the feed line length could be reduced from the 20-feet of supplied cable to less than half of that. The difference in the rated loss of RG-58C/U and the non-Mil-Spec RG-8X is really insignificant; there might be 0.1-dB of difference. The Mil-Spec RG-58C/U cable is generally higher quality and is made with a non-contaminating vinyl jacket which will have superior longevity.

A great attraction of the GAM Electronics configuration is the separation of the antenna and the feed line. In many marine antennas the feed line is captivated in the antenna, and when the antenna is replaced, the feed line has to be replaced. While in my installation this is not particularly difficult to accomplish, in an antenna installation where the feed line is routed through tubing, such as in a radar arch, replacing the feed line could require some work.

posted 07-12-2006 06:36 AM ET (US)         

One thing I would mention is those looking for an inventive solution on where to install an 8' antenna (or a 4' antenna on a 4' entension) might want to consider installing a mini polling platform on the transom. The antenna can then be installed right on top of the platform giving you really good height. Of course there is the added expense of getting the platform fabricated but this option seems to solve many problems that plague fishermen using a bimini top BUT needing a tall antenna when running offshore. A couple of other positives with the aforementioned setup:

1. Since the antenna is way back aft, it is not subjected to the intense pounding like it would be if it were mounted up forward. This heavy offshore pounding has caused my Shakespear 5225XT to loosen up where the fibergalss section fits into the antenna base.
2. The coax can be run from the platform down to the rigging tube and then up to the console for a very clean look.

posted 07-12-2006 10:57 AM ET (US)            

I suspect a GOOD 8' antenna would have worked about as well.

posted 07-12-2006 07:48 PM ET (US)            

I don't make any comments about 8-foot antennas in general in the article. I simply refer to some actual test measurements made by someone else who tested several 8-foot antennas. There is no basis for the imputed statement "8-foot antennas are junk."

The old antenna I removed was in very good condition, both mechanically and electrically, and it had given good performance. There is no basis for the imputed statement "there was a cheap old beat up antenna that wasn't working right."

The reasons for changing were as stated in the article. The goal of the new antenna installation was to improve the range of communication. Let me quote myself:

"The primary goal of my search was to find a way to increase the overall antenna height above the water."

"The [existing] 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."

Both of these goals were achieved:

--the new installation has a higher average height, and

--the new installation does not cause interference to the tachometer.

I will offer Chuck this challenge: obtain any 8-foot antenna for the VHF Marine Band and ship it to my house. I will conduct a test to see if this antenna can accomplish the following:

--perform as well on transmit and receive as my new installation (when mounted on the same base), and

posted 07-14-2006 11:31 AM ET (US)            

There is a USENET thread on this topic that may be of interest. It includes a calculator you can use to test the argument yourself.

posted 07-14-2006 11:48 PM ET (US)            

In my previous article I showed how at most distances where the antennas of two stations were within their respective radio horizons there was sufficient signal level at the 25-watt power level so that the circuit had substantial fade margin. However, when the path is marginal, that is, the antennas are beyond the radio horizon, every decibel of signal level is important, and the more gain the better.

Because the VHF Marine Radio Service uses the frequency modulation technique of radio-telephony, there will often be a substantial improvement in the recovered audio signal-to-noise ratio when there is only a very minor improvement in signal. Frequency modulation (FM) can produce a very good recovered signal-to-noise (SNR) ratio when there is just a modest carrier-to-noise (CNR). However, once the CNR drops below the detector threshold, the SNR will degrade rapidly. As a result, FM is unsuited for weak signal use. This effect can be heard in some audio recordings provided by Shakespeare. In these recordings, a modest improvement in CNR of 3-dB produces a very dramatic improvement in SNR, probably on the order of about 10-dB. Listen for yourself:

Comparison of 3, 6, and 9-dB antennas
http://www.shakespeare-marine.com/electronics/sounds/3-6-9db.wav

Comparison of 3, 6, and 9-dB antennas with vessel rolling
http://www.shakespeare-marine.com/electronics/sounds/6-9dbrol.wav

The drawback, if there is one, to really high gain antennas is their directivity, which increases with the gain. A high-gain antenna may become too directive, and the motion of the boat may move the antenna in such a way that its major lobe no longer is oriented at the horizon, thus reducing the transmission range.

The point I make in my article about the GAM ELECTRONICS antenna is that the small antenna produces actual gain that is on par with the very best of the 8-foot antennas and GREATER than several common 8-foot antennas, yet its small size allows it to be mounted at a greater height. So you get the best of both worlds:

--you have actual (measured) gain on par with the physically larger antenna;
--you can mount the antenna higher due to its small size and weight.

Re the linked article mentioned by sail16

I did not find any calculator in the linked discussion. If there is some unique calculator that is provided in the thread you mention, could you explain what it does and how it complements the calculations I have already provided.

In my articles (see links above) you will find calculations for:

--radio horizon based on antenna height above the sea
--transmission line loss vs. frequency vs. cable type
--path loss vs. distance
--received signal level for a particular path, power, and antenna gain
--conversion from micro-volts to dBm levels in general and in 50-ohm systems

posted 07-15-2006 12:10 AM ET (US)            

The question addressed in that discussion is, "How much (distance) will my range increase with a 3-dB increase in antenna gain?" This is difficult to assess. There is no doubt that the range will increase, and, when the path is marginal, the difference of 3-dB can provide substantial improvement in the performance. You can hear this for yourself in the linked audio recordings I mentioned above which demonstrate the FM detector effect.

When the received signal levels are above the FM-detector threshold, a 3-dB improvement will not yield as spectacular an improvement, and, if the path is a really strong one, and the FM-detector has reached its limiting, there will be no improvement at all in the recovered SNR with a 3-dB increase, however the fade margin will increase.

When the signal path is less than the radio horizon, the increase in path loss with distance is easy to predict, and the relationship between distance and path loss is rather well defined

When the signal path is greater than the radio horizon, the increase in path loss with distance is more difficult to predict, and the relationship between distance and path loss become less well defined.

posted 07-15-2006 09:43 AM ET (US)            

To get back to the original question which the USENET thread asks,

"How much (distance) will my range increase with a 3-dB increase in antenna gain?"

you can reasonably estimate the answer to this from the equation for path loss which I show in

http://continuouswave.com/whaler/reference/VHF.html#pathLoss

From this equation you can infer that an increase in signal of 3-dB will compensate for additional path loss resulting from an increase in distance by a factor

3 = 20 log (d)
3/20 = log (d)
d = 10^(3/20)
d = 1.4 

posted 07-15-2006 10:35 AM ET (US)            

The reason for choosing the GAM Electronics antenna is its excellent measured performance in comparison to many other larger antennas. This leads me to believe that this antenna will perform very favorably in comparison to larger antennas, and, in the case of a radar arch mounting, it could again be mounted on a 4-foot extension mast. This would be a very interesting comparison. An 8-foot antenna on a radar arch versus the GAM 4-foot on a 4-foot extension on a radar arch. The GAM would again have an advantage in effective height.

But not all small recreational boats have a radar arch, and most have to mount their antenna on the deck or console. This was the case in my original installation. In this situation the base of the antenna is very close to other electronic devices, as well as being surrounded by other conductors such as the hand railings, the frame of the boat's canvas, or the windshield frame. By elevating the mounting on a 4-foot extension there are three very important benefits:

--antenna height is increased, improving range
--antenna base is removed from proximity to other devices, reducing radio-frequency interference to them
--antenna is separated from other conductors, improving the radiation pattern

posted 07-15-2006 12:41 PM ET (US)            

I don't have any scientific background but have lots of real world experience boating in a place where there is no Coast Guard or Vessel Assist and your VHF is your lifeline. On that basis, I offer my two cents worth.

On my first boat I had a budget radio rig, a $200 radio with an auxillary speaker hooked up to a $40 8-foot Shakespeare antenna mounted on the T-Top. It worked OK as long as ranges were short and there were no obstructions. Since I planed to cruise far from port on my new boat, I upgraded the radio to an Icom 602 and the antenna to a Galaxie 8-footer. My new boat has perhaps two more feet of bridge clearance. I expected a modest increase in radio performance but was amazed with what I got. Range and clarity have increased substantially both for receive and transmit.

West Marine publishes vhf receiver performance specificatons. About 95% of the radios have the same specifications. The Icom 602 and a couple of other premium radios offer greater sensitivy.

posted 07-15-2006 01:48 PM ET (US)            

The advertised gain for most VHF marine antennas is a confusing and often arbitrary number. I'd say the GAM antenna has 0-dB of gain, and use it as a reference because it is a half-wave antenna. In most real-world antenna measurements, the point of reference for gain measurements is considered to be a half-wave dipole antenna. The GAM antenna is a half-wave monopole antenna, slightly different, but close enough. A good 8-foot antenna ought to have about 3-dB gain in comparison.

If you look at the test data, you see these results:

Glassmaster      8-foot = -24.0 dBm (or 0.8 dB more signal than GAM)
Shakespeare 5225 8-foot = -24.3 dBm (or 0.5 more signal than GAM)
GAM SS-2         4-foot = -24.8 dBm
Digital 529      8-foot = -25.5 dBm (or 0.7-dB less signal than GAM)

All the other antennas tested had even lower signals, including several 8-foot antennas from various brands.

These results are the basis for my statement regarding the performance of the GAM antenna. I'll say it again: In an actual test of field strength the little GAM antenna produced signals that compared favorable with the best of the 8-foot antennas and better than the garden variety 8-foot antenna.

posted 07-15-2006 05:23 PM ET (US)            

The VSWR results are excellent. In reality, anything under about 1.5 is acceptable. The antenna showed the lowest VSWR around 157.4 MHz, or Channel 28.

posted 07-17-2006 08:59 AM ET (US)            

In a typical 8-foot fiberglass whip antenna there are actually two distinct radiating elements in the antenna, forming what antenna designers call an array. These elements are collinear, and are fed in series. The transmitter power is applied to the bottom element first, flows through this element, and is delivered to the top element. In an ideal configuration, the distribution of current in this collinear array would be uniform, that is, precisely equal currents would flow in each portion of the array.

The current flowing in the lower portion of the array will not be as effective in radiating because of the close proximity of the vessel's structure and the helmsman. These antenna currents radiate into surrounding objects which tend to reduce the propagation of radio frequency energy. Thus about half of the transmitter's power is used in the lower half of the antenna, where it does not work as effectively.

In contrast, the SS-2 antenna has only a single radiating element, the stainless steel whip, and all of the transmitter's power is applied to it. All of the power radiates from the highest portion of the antenna, where there are no surrounding vessel structures or people to block the propagation of the radio frequency energy.

An additional problem with most marine collinear array antennas is their series or end-fed configuration. The power is applied to the end of one of the elements in the array and must flow through the first element to reach the second. This tends to cause an uneven distribution in antenna current, and the usual situation is that more power flows in the first or lower element than does in the second or higher element. If this is the case, then most of the transmitter's power is radiated from the lower element whose height is significantly less than the upper element.

posted 07-18-2006 07:33 AM ET (US)            

This is an excellent discussion and I am almost ready to purchase a GAM SS-2. Do you think there would be a problem if I planned to remove the antenna from the base each time I lowered it for storage? If I could do this I would get a 4 foot extension mast, otherwise I would be limited to two feet.

posted 10-09-2006 06:05 PM ET (US)            

We rented a place on Watts Bar Lake in Tennessee for a week. The home came with a covered boat well. The antenna height was just a few inches too high for the boat well roof. The first time I put the boat into the covered well I discovered the antenna was about three inches too tall. The stainless steel whip on THE GAM antenna just bent over a bit. If I had a fiberglass whip I imagine it would not have been as pliant.

posted 11-18-2007 12:11 AM ET (US)            

A couple negatives I found though...

1. The wire inside the Shakespeare 498 4' extension rattles inside the tube when the tube moves side to side. I could imaging this to be annoying while rocking in the waves. I did find a solution for this though. At first I thought about spraying expanding foam into the tube. I decided this would potentially be too messy so I looked for another solution. I decided to attach large black zip-ties every 5" or so to the antenna wire along the 4' length where it passes through the Shakespeare extension (and in my case, also in the 12" SS extension tube). After attaching the zip-ties to the wire, I then cut the excess from each zip-tie, leaving them about 1.25" long. Then I fed the wire into the extension from the top, pusing in each zip-tie. The zip-ties now keep the wire stationary inside the tube and eliminate the rattling. With this modification, this rattling is no longer an issue for me.

posted 11-21-2007 03:52 PM ET (US)            

Current (and 2001) West p/ns are 247843 (parallel), $12.99, in
stock and 247850 (cross), $9.99, out of stock. West shows
the parallel as maded by Lockblock, the cross as Seafit.
I suspect if you found the Seafit p/ns, you could track them
down someplace other than West.

posted 09-18-2013 10:34 AM ET (US)            

In the course of seven years of use the metal whip has hit a few overhead obstructions and survived without damage.

posted 04-18-2015 10:12 AM ET (US)            

Regarding installation of VHF Marine Band radios on small boats, I have written a new article that gives a general overview of the installation process. See