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Author Topic:   VHF Radio Antenna for Small Boats
jimh posted 07-10-2006 11:28 PM ET (US)   Profile for jimh   Send Email to jimh  
This discussion is for comments or questions related to the REFERENCE article

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.

[A entire sidebar of discussion about my old antenna has been deleted from this thread. The article is about the new antenna, not about the old antenna--jimh]

swist posted 07-11-2006 07:00 AM ET (US)     Profile for swist  Send Email to swist     
What you have proven is there is no substitute for height. It is perhaps counterintuitive that a 4' antenna should perform better than the standard 8', but in 99% of the installations the short antenna is mounted low, thus eliminating any performance advantage it might have.

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.

Putting a 4' antenna on a 4' base extension gives you an 8' structure, however you look at it.

jimh posted 07-11-2006 07:21 AM ET (US)     Profile for jimh  Send Email to jimh     
The dominant influence of height was shown in another article:

Marine VHF Radio Communications
by James W. Hebert

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

This new article shows the benefits of using the small antenna from GAM Electronics to realize added height and superior electrical performance in preference to the more commonly used antennas. The SS-2 antenna from GAM Electronics is a very well made, moderate cost, small, and lightweight antenna that is perfectly suited for use on small boats. The article tries to bring these qualities of the GAM Electronics antenna to a wider audience.

17 bodega posted 07-11-2006 05:52 PM ET (US)     Profile for 17 bodega  Send Email to 17 bodega     
Good article. I may switch to that antenna for my boat. Currently my long whip is mounted to the top of my console bars, which are already at about 6 feet or so above the floor of the deck. This would optimize the performance of my radio even further.

Steve

Bulldog posted 07-11-2006 07:11 PM ET (US)     Profile for Bulldog  Send Email to Bulldog     
Nice job on the article Jim, that seems like a great set up! I had two antenna mounts on my Revenge at about the same spot, I relocated my VHF antenna to the port side and used a 4' Galaxy on a one foot stainless extension. I put a matching galaxy 4' AM/FM antenna on a stainless extension on the starboard side. In hindsight, which is always 20/20 I should have gone with the two foot extensions at the same price, or the GAM setup. Everything works fine and there is other stuff to do!...Jack

Chuck Tribolet posted 07-11-2006 10:12 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The Shakespeare Galaxies have RG-8X, the cheaper antennas have RG-58. I've had both, they weighed about the same. The Galaxy seemed to work better. The Galaxy did about as well as the GAM in the test you [provided a direct hyperlink] to.


Chuck

jimh posted 07-11-2006 10:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--The GAM Electronics antenna outperforms most of the other antennas in the test (to which a direct link is given in my article), even though it is mechanically smaller. Its electrical performance is very good. It is on a par with the best of the eight-foot antennas in the test, and better than most of them. This test is about the only one I've seen where actual field strength readings from the antennas on transmit were taken.

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.

For small boats going offshore, I suggest trying an installation like this but using an eight-foot extension. This would raise the average height of the antenna even more. With such a long mast, a secondary support would be needed, but it should be possible to locate the mast in such a way to provide for this.

bigjohn1 posted 07-12-2006 06:36 AM ET (US)     Profile for bigjohn1    
Great article Jim...

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.

While there are several clean antenna installs this site has showcased over the years, many can still be problematic for offshore fishermen using an 8' antenna and a bimini top. I offer the above as one potential solution. I saw this install recently on an 18' Maritime Skiff and will seriously consider going that route on my next antenna.

Chuck Tribolet posted 07-12-2006 10:57 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I would summarize your article as saying "8-foot antennas are junk and this 4-foot antenna is better", when what you actually demonstrated was "there was a cheap old beat up antenna that wasn't working right, and this 4' antenna is better".

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


Chuck

alfa posted 07-12-2006 12:01 PM ET (US)     Profile for alfa  Send Email to alfa     
Good article Jim.
A good antenna system is a feeder AND antenna which impedance is 50 ohm (matched impedance), and generally a half wave (electrical). The little coil is used to shorten the radiator (4'). Elevation is a must for vhf.

Alain

jimh posted 07-12-2006 07:48 PM ET (US)     Profile for jimh  Send Email to jimh     
Chuck's summary of my article is flawed.

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 to 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

--not interfere with the tachometer on transmit.

vink posted 07-13-2006 10:55 PM ET (US)     Profile for vink  Send Email to vink     
Jim - I was delighted to see your article on the Gam Electronics antenna. These antennas are used quite exclusively for mobile applications by the local amateur radio community in this area. This antenna was originally designed by a member of our local area amateur radio club, Andy Nuttle. Before his passing Andy lived on Lake Winnipesaukee. We often visited each other. I loved to tap his memory banks and discuss antena designs, propogation and the building of home brew amateur radio repeaters. One day some twenty years ago he gave me one of his original design Gam antennas. This antenna must be at least thirty or so years old and I've had it on three of my boats. It is currently mounted on my Sport 150. With the bimini down I mount it on the rail and with the top up I mount it on one of the supports. This antenna although thirty years old still works perfectly.
Since I don't know how to add photos to my reply I'll send you a couple of photos of this original design Gam antenna by e-mail. You will be able to see how the design has changed over the years. It is also possible to see the embossed serial #2 on this antenna. Maybe if you choose to you can add these photos to your thread.
jimh posted 07-13-2006 11:30 PM ET (US)     Profile for jimh  Send Email to jimh     
vink--Please send the pictures. I look forward to seeing them. I have been an active radio amateur for 42-years, and I have probably spent more time reading about, building, and erecting antennas than another other facet of amateur radio. I have also been a licensed Broadcast Engineer for 30 years. I am glad to hear of the GAM antenna's amateur radio heritage.

de K8SS

vink posted 07-14-2006 06:19 AM ET (US)     Profile for vink  Send Email to vink     
CQ, CQ, CQ de N1FQP
BarryGreen posted 07-14-2006 08:49 AM ET (US)     Profile for BarryGreen  Send Email to BarryGreen     
de W1JFK
handn posted 07-14-2006 10:37 AM ET (US)     Profile for handn  Send Email to handn     
The whole point of a 4 foot antenna is to mount it high on a sailboat. Because the radio energy is less focused, a 4 footer works better when the boat heels over and the radio energy is focused back toward the water. If an antenna must be mounted on a poor location on a small power boat, a four footer may offer an advantage because it might perform better in a low or obstructed location.
If I had a small boat and depended on my vhf radio for any reason other than to talk to my friends, I would invest in a radar arch and a 9 foot Galaxie.
Even in extremely rough conditions, a power boat doesn't heel over enough to negate the power advantage of a longer antenna if the antenna can be mounted on top of the boat, at least six feet above the water.
Coming back from Baja recently in very rough conditions (5'-6' wind blown chop), I was still able to communicate with my friend on a less than perfect land base station 31 natical miles away. The signal wasn't perfect, but it didn't fade in and out as the boat rolled as one would expect if the signal was being effected by the movement of the boat.
I have a Galaxie 8 footer mounted on top of my t-top. The bridge clearance on my boat is about 9 feet. If I had a four foot Galaxie on a four foot extension in the same place, all that I would accomplish is cut my output in half and reduce the maxium distance of my comunications, not necessarily by half but by some significant amount.
Jim, come to San Carlos Sonora in the fall and bring your four footer. If it performs better than my 8 footer mounted in the same place on my boat, I will rename my boat, the "Four Footer" publicly on the forum.
If you come in late October or November, chances are good that we can raise a marlin or even a yellowfin tuna.
Hank Ellwood
alfa posted 07-14-2006 11:01 AM ET (US)     Profile for alfa  Send Email to alfa     
de F1BZF
sail16 posted 07-14-2006 11:31 AM ET (US)     Profile for sail16  Send Email to sail16     
Not to add fire to the flames, but I think you'll find the range difference between 3db and 6db really isn't that great (and certainly *far* less than 2x).

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.

My conclusion is that antenna height and antenna + cable quality are critical to determining range, while gain is mostly of help with signal quality at the very extreme of range. [Caveat: I'm not an RF engineer.]

jimh posted 07-14-2006 11:48 PM ET (US)     Profile for jimh  Send Email to jimh     
In any radio circuit, a stronger signal is always useful, and it does not matter at all if the added signal strength comes from the gain of the antenna, from its more advantageous installation at a greater height, or from increased transmitter power. To say that gain in the antenna is not important is not logical. A reduction in antenna gain will result in a reduction in signal strength, and a reduction in signal strength will result in reduced communication range. The nice part about antenna gain is that it is bi-directional. It provides a boost on both transmit and receive. Increased transmitter power only helps on the transmit side of the circuit.

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


jimh posted 07-15-2006 12:10 AM ET (US)     Profile for jimh  Send Email to jimh     
I reviewed the linked USENET article and I do not find that it contains any calculation of interest at all. I am confused by the reference to it.

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.

So the answer to the question, "How much (distance) will my range increase with a 3-dB increase in antenna gain," is hard to quantify for all situations, but any increase in signal always helps, and especially so in marginal conditions.

sail16 posted 07-15-2006 08:59 AM ET (US)     Profile for sail16  Send Email to sail16     
The calculator is in the 17th post of the usenet thread. It's here http://www.artscipub.com/simpleton/simp.range.html
jimh posted 07-15-2006 09:43 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the direct link to the nomogram. I did not follow the original link you gave to the second page of the thread and didn't see it. The granularity of the nomogram data is a bit coarse for the low antenna heights common in marine use. The nomogram is also based on 1-microvolt received level, which is not particularly strong in FM work. Using the 25-watt scale, an antenna height of 10-feet, and an antenna gain of 3-dB, the range computes to about five miles.

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

This is on a free space, line of sight path. If the antenna gain increases 3-dB, the range will increase by a factor of 1.4-times.

jimh posted 07-15-2006 10:35 AM ET (US)     Profile for jimh  Send Email to jimh     
I am in total agreement that mounting a high-quality antenna to a base on a radar arch is probably the optimum configuration if your boat allows for it. There is nothing better than height to improve range on ground-wave communication links.

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

Regarding this last point, I had observed that there were effects from the boat in the prior installation on the antenna's radiation pattern. The antenna always worked better in the direction toward starboard, where there was nothing in its path, than it did toward port, where the antenna was looking through the boat, the windshield, the frame of the flying top, and the occupants of the cockpit. In the new antenna installation, the antenna base is elevated above all of these structures, and there should be much less influence from them on the antenna's radiation pattern.

handn posted 07-15-2006 12:41 PM ET (US)     Profile for handn  Send Email to handn     
Jim, I don't want to nitpick your excellent article and thoughtful discussion which benefits all small boat owners, but I do question one thing that you state. I paraphrase, Gam produces gain that is on par with the best of the 8-foot antennas and better than a poor 8-footer. If the efficiency of the Gam antenna was 100%, its output would be limited to 3 dB. There is no way quality construction can increase the gain of the antenna which is limited by its length. I don't know if a Galaxie matches the efficiency of the Gam, but an 8-foot Galaxie operating at a less than 100% efficiency would still have a sizable gain advantage. Of course gain isn't everything. You may hear a transmission from a poor high-dB-gain antenna but the signal and reception from a poor quality rig probably contains static and distortion. I know from experience that a Galaxie is not a poor quality antenna.

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.

For most boaters in the U.S. going forth near a major metropolitan are, a basic rig is probably sufficient. For those intending to boat "off the grid" as I do, the several hundred dollars you spend upgrading your radio and antenna is the best investment you can make.

davej14 posted 07-15-2006 12:41 PM ET (US)     Profile for davej14  Send Email to davej14     
Getting the antenna base away from a compass on the console is mandatory. I found that the base of a Shakespeare 5240-R 3' VHF antenna had a significant influence on my compass if it was within one foot. This is the case without power. I had to relocate the antenna to a rail mount on the side opposite the compass. Now I think I will get an extension mast to increase the range and minimize any possible magnetic effects.
jimh posted 07-15-2006 01:48 PM ET (US)     Profile for jimh  Send Email to jimh     
handn--I was surprised at the actual measured performance of the antennas compared in the test. On paper and in theory, a longer antenna ought to have more gain, but that is not automatically realized in practice. In order to make most effective use of a longer antenna, the distribution of the antenna currents has to be managed carefully so that the phase and amplitude of the antenna currents throughout the array are optimized for most effective pattern and gain. This, too, does not happen automatically. There is also the consideration of the actual efficiency of the antenna. In an antenna with ohmic loss, the efficiency will decrease. As I mentioned above, in the economy 8-foot antennas the actual radiating elements often consist of nothing more than the center conductor of the feedline, which is a rather small diameter copper conductor. Finally, the antenna has to be properly matched to the transmission line at its feed point, so that there will be an effective transfer of power from the transmitter to the antenna. All of these variables affect the overall performance of the antenna, and it is too simplistic to say that any larger antenna will outperform any smaller antenna. The results of the measured testing show this. The GAM antenna produces stronger signals than some other larger antennas. No where do I state that the GAM antenna will produce stronger signals than every 8-foot antenna, and this, too, is shown in the test results. The better 8-foot antennas produce stronger signals.

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.

Please don't attribute these test results to me--I did not conduct the tests. The results are one of the few places where you can see actual measurement data about a VHF Marine Band antenna's performance.

jimh posted 07-15-2006 05:23 PM ET (US)     Profile for jimh  Send Email to jimh     
Added data from VSWR and FORWARD POWER measurements. These were made with a DIAWA directional wattmeter using a 20-watt scale. (I was planing on using a better directional wattmeter, I couldn't find the right slug for the Bird Model 43.) The battery voltage was under 14 volts. The power output of the transmitter is quite sensitive to battery voltage. It would probably produce a few more watts if the battery voltage were run up to around 14.2-Volts.

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.

I would rate the VSWR performance of the SS-2 as excellent.

jimh posted 07-17-2006 08:59 AM ET (US)     Profile for jimh  Send Email to jimh     
An additional advantage to be gained from an installation such as I describe with the GAM Electronics SS-2 as compared to a typical 8-foot fiberglass whip antenna results from the distribution of current in the antenna. I explain:

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.

My feeling is that both of these factors tend to favor the SS-2 type of antenna, and this may help explain why it works well.

davej14 posted 07-18-2006 07:33 AM ET (US)     Profile for davej14  Send Email to davej14     
jimh:

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.

I am somewhat confused by your last comments on power distribution. If an 8 ft antenna has two elements in series I would surmise that applying power to them would necessarily cause equal current to flow simultaneously in each element. I do not see how current could flow in one element first. Now if the elements are wired in parallel I see how this would be possible. Could you explain this phenomenon?

jimh posted 07-18-2006 12:29 PM ET (US)     Profile for jimh  Send Email to jimh     
The radio frequency current distribution in a monopole antenna is not uniform. Depending on the length of the radiator and how it is fed, the maximum current node can be located in different places.

An antenna does not act like a DC circuit. In a DC circuit the current in devices connected in series would be the same in all device. In a radio antenna, energy is leaving the antenna and being propagated outward. Some of the energy applied to the lower element leaves the array before it can flow to the upper element.

jimh posted 10-09-2006 06:05 PM ET (US)     Profile for jimh  Send Email to jimh     
I discovered another advantage to the stainless steel whip antenna: it is very resistant to breaking due to contact with overhead objects.

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.

The learning curve on this boatwell was rather high, too. I must have forgotten to lower the antenna about half the time I put the boat in the slip. That would have been an expensive week of broken fiberglass antennas!

jimh posted 10-27-2007 01:21 PM ET (US)     Profile for jimh  Send Email to jimh     
I added a new image showing the extension mast being used to fly a courtesy ensign. This is another benefit of the installation.
jimh posted 11-11-2007 10:35 AM ET (US)     Profile for jimh  Send Email to jimh     
Some readers requested a precise measurement of the length of the GAM Electronics antenna assembly. I will be glad to furnish this measurement as soon as I have an opportunity to take it.
sitotis posted 11-18-2007 12:11 AM ET (US)     Profile for sitotis  Send Email to sitotis     
On Jim's recommendation I purchased the GAM SS-2 antenna assembly described above. The boat is in the garage for the winter, so I cannot yet comment on the signal quality or performance. The overall length of the assembly is 89". I also added a 12" SS extension at the base to bring it to 101" total. I also installed a backing plate to reduce stress points on the fiberglass.

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.

2. The whip bounces around a lot when the antenna is in the horizontal position. Depending on the location of the antenna, this may not be an issue for some people. For me though, the lowered antenna is close to the gunwale. I am worried that the bouncing whip may either chip my gelcoat or after repeated bouncing on my Sunbrella cover (when up) may wear a hole through it. The solution is to trailer with it in a higher ratchet position, but then I begin to worry about wind resistance on the highway while trailering. For long trips, I will probably remove the whip from the horizontal mast.

sternorama posted 11-21-2007 01:36 PM ET (US)     Profile for sternorama  Send Email to sternorama     
sitotis-this may not be an option for you, but I had a similar towing issue with my 16' and the antenna. I used velcro wrapping straps to pad and secure the antenna to the railing while towing.
Chuck Tribolet posted 11-21-2007 03:39 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
West used to have some nice straps for tieing the antenna down,
But I can't find them in the current catalog. They had a
rubber piece that went between the antenna and the rail, and
a velcro strap for each side. I've been using them for years.
They come in two flavors, one for the antenna parallel to
the rail and one for perpendicular.


Chuck

Chuck Tribolet posted 11-21-2007 03:52 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
West still has them, just not in the catalog as far as I can
tell. I dug out an old (2001) West Catalog, and found them.
It showed the Manufacturer's p/ns as LOCKB PARALL and LOCKB
CROSS. I Googled and got two hits, one was West!

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.

Chuck

floater88 posted 09-18-2013 07:47 AM ET (US)     Profile for floater88  Send Email to floater88     
I'm wondering though how you secure the GAM when you fold it down? On my [Boston Whaler boat] there is a little clip the end of the 8-foot antenna slips into. As the GAM appears thinner than the fiberglass, how do you secure it? Did you just replace that clip with a thinner one? Thanks
jimh posted 09-18-2013 10:17 AM ET (US)     Profile for jimh  Send Email to jimh     
On my boat, I use a short length of cord to secure the 4-foot extension portion to a pad eye. The tip of the antenna whip is free to move about. Normally when the boat is not in use there is a cover over the whole topsides. The antenna is under the cover, and the whip is in contact with the cover.
jimh posted 09-18-2013 10:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Following up on the installation of the GAM SS-2 antenna, it has now been in service on my boat for seven years. The antenna works as well as it did on initial installation. The mechanical durability of the antenna and the extension mast are excellent. They both show no signs of wear. The electrical performance of the antenna continues to be excellent. On receive the antenna picks up NOAA weather broadcast stations from extremely long range. On a recent cruise, we were in a rather remote location on Lake Superior and cruising with some other boats. One of the other boaters commented that he could not receive any NOAA weather radio stations. I checked my radio and found that I could receive weather radio broadcasts on eight of the ten weather radio channels. On transmit the antenna continues to work well, as judged by my ability to easily communicate with any other VHF Marine Band station that I can hear.

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

I continue to be pleased with the mechanical durability of the GAM SS-2 and with its electrical performance, and I continue to recommend this antenna and 4-foot mast combination as an excellent choice for installation on a small boat.

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