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Author Topic:   Antenna For VHF Marine Radio Service
DWS posted 12-22-2005 08:59 PM ET (US)   Profile for DWS   Send Email to DWS  
What parameters do I need to consider when choosing an antenna for [the VHF Marine Radio Service]? The antenna will be on a classic 17-foot Boston Whaler boat; it will be mounted on a stainless arch over the console. I suppose should be 8 feet in length. The reply back from tech at [a radio manufacturer] said [the selection of the antenna] depends on the boat and to ask the sales guy at my local marine store. But we all know who works there. I was looking to see what gain, length, and type the manufacturer would suggest, but the answer was very curious.
jimh posted 12-22-2005 09:51 PM ET (US)     Profile for jimh  Send Email to jimh     
I would prefer a 4-foot antenna from one of the better makers. DIGITAL or SHAKESPEARE come to mind. For some reason the 4-foot antennas are made only in the premium quality brands, and tend to cost more than the more common 8-foot antennas. I like the shorter antenna mounted higher, particularly on a small boat. The motion on a small boat is generally greater than larger boats, so the broader radiation pattern of the smaller antennas will be a plus on a small boat.
sail16 posted 12-22-2005 09:51 PM ET (US)     Profile for sail16  Send Email to sail16     
I think your choice is relatively straight forward.

In terms of length, you really have only two practical choices-- 3'(metal whip) to 4' (fiberglass), or 8'. Length and gain are directly related--t he shorter antennas will be 3db-- perfectly fine in range unless you are going far offshore; the 8'antenna will be 6db-- better range, but quite a load on your grab rail if that's what you were getting at with your mounting description. Also, the 8' antenna will give you spotty transmit performance in moderate to rough seas.

Personally, I would recommend a 3' metal whip unless range is really important to you. The metal whip will take more abuse, esp. if it is mounted where people might grab onto it. In terms of brands, I've always had great luck with Metz, but I am sure other brands are close to or equal in terms of quality.

For more info, you might check out //www.westmarine.com/webapp/wcs/stores/servlet/westadvisor/10001/-1/ 10001/selecvhf.htm

Chuck Tribolet posted 12-22-2005 10:36 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I have an 8' 6dB gain Shake Galaxy on my 16'7" Montauk. I
don't have problems with spotty transmission or reception,
and it rocks a good deal in Monterey. The wider beam of the
4' 3 dB gain antennas is more for sail boats that may be
heeled over 45 degrees.


Chuck

rtk posted 12-22-2005 10:43 PM ET (US)     Profile for rtk  Send Email to rtk     
I chose a 4 foot Digital VHF antenna exactly for the reasons Jim gives. The 4 foot is typically a lower gain antenna compared to the 8 foot, but my research showed on a boat that is subject to alot of rocking, the lower gain antenna may be a better choice over a higher gain antenna.

I have a t-top that the antenna is mounted on, so I am starting out with the antenna mount being around 7' above sea level.

If you look at VHF antennaes designed to be mounted on top of a sailboat mast, I believe the gain is very low, and it is usually a whip type antenna. The reason for the low gain I believe is the effect of the mast swaying side to side. A 40 foot mast will sway a good distance side to side when the boat rocks.

There is a good amount of information available on the topic, and I think few people take the movement of the antenna into consideration when choosing one. As I undertand, height is not the only factor to consider.

www.westmarine.com/webapp/wcs/stores/servlet/westadvisor/10001/-1/10001/ selecvhf.htm

"Selection criteria:

dB Rating

An antenna's dB rating (or antenna gain) indicates the apparent increase in transmitting power due to its ability to focus energy. Antennas with a high dB rating concentrate energy perpendicular to the antenna shaft in a field that is shaped like a disk. This makes your radio signal appear stronger to receiving stations around you. It also reduces the amount of energy transmitted above or below the antenna, which can be a problem if your boat is pitching or rolling in seas. A concentrated signal can actually fade in and fade out as the boat rolls and pitches.

As the dB rating of the antenna increases, so does the height of the antenna, but the horizontal angle diminishes.

The broad radiation pattern from a low dB antenna allows a sailboat to heel over and still send signals toward the horizon. Sailboats should use a 3dB antenna mounted at the top of the mast whenever possible."

Rich

DWS posted 12-23-2005 03:06 PM ET (US)     Profile for DWS  Send Email to DWS     
OK. it looks like it will be a shakesphere in the 4 to 6 foot range, as the 7 foot mounting hight over the console should allow for good line of sight qualities, and a relatively short coax run up from the radio. Thanks, DWS.
David Livingstone posted 12-23-2005 08:39 PM ET (US)     Profile for David Livingstone  Send Email to David Livingstone     
DWS, I mounted a 8'on the rail of my Montauk http://ca.pg.photos.yahoo.com/ph/montauk1998/detail?.dir=/a8b5&. dnm=1288.jpg . It's a 5226 Shakespear Galaxy. Same as Chuck's but in black and has the 20' cable that reachs the consol through the rigging tunnel . It stows along the rail and doesn't interfer with the bimini. I fish 5 - 10 miles off and went with the 8'.

All the best

David

DWS posted 12-24-2005 12:48 AM ET (US)     Profile for DWS  Send Email to DWS     
Yup Dave, that was my first choice for mounting and stowing of the antenna but since I have already bent the stainless for the arch to hold rod holders, GPS mushroom, I thought I would go ahead and put a small loop of stainless to accept the fold down antenna bracket.and keep the rigging tunnel a bit less full and at the same time keep the coax run short as I can. And an 8' unit is not out of the question, thanks for the numbers. DWS.
handn posted 12-28-2005 08:10 PM ET (US)     Profile for handn  Send Email to handn     
I agree with Chuck and David. I have an 8 foot Galaxie antenna and boat on the Sea of Cortez where your radio is your lifeline since there is no Vessel Assist, Coast Guard and scant boat traffic away from the major ports.
With an Icom 602, my radio performance is outstanding. I am constantly relaying messages from and two other boats and am able to regularly communicate with a high ground land based station up to 300 nautical away in rough seas or smooth water.
For a few dollars more, the Galaxie is worth every penny. You never know when you abosolutely positively need every bit of performance your radio can deliver.
jimh posted 12-28-2005 11:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Direct VHF radio propagation over 300 miles paths is very unusual. It is more likely you have communicated with distant base stations using some intervening repeater or relays stations. During some enhanced atmospheric conditions, over-the-horizon propagation at VHF frequencies is possible. This may also have been a factor.

However, it is totally wrong to infer that use of a certain type or brand of antenna will permit marine VHF radios to have ranges of 300 miles to shore stations.

handn posted 12-29-2005 09:19 AM ET (US)     Profile for handn  Send Email to handn     
Jim, vhf reception is line of sight as you know. The potential range is well in excess of 300 n.m. if there are no obstructions.
The land based station is on a cliff above the ocean, about 500 feet above sea level, here in San Carlos. A unknown brand 25 watt vhf is hooked up to a 12 foot Galaxie antenna on top of a house owned by a friend of mine.
Last May we went to Bahia Muertos about 50 miles south of La Paz, almost 300 miles SSE of San Carlos. I called my friend every morning and was able to reach him always. I had to get out in the ocean away from coastal obstructions for a successful transmission. Rough water or smooth water, it didn't seem to make a difference. Absent a skip, I am never able to talk to surface boats at these distances but am sometimes able to talk to friends on boats at distances up to 90 n.m.
A companion boat was also able to reach the station most mornings. My friend has a 12 foot Galaxie mounted on an extension and an Icom 602 in a 43 sportfisher. His antenna is 25-30' off the water. I believe his installation is less than perfect because his radio should outperform mine because his antenna is longer and higher and his radio and antenna are the same brand.
Icom advertises superior receiver performance for their 602 radios. My reception of the distant base station was always clear. I don't know if Icom claims a transmission advantage as well, but the distant station reported my transmissions were clear.
I am aware of no performance testing reports for vhf antennas. Every one down here who has a quality radio installation uses a Galaxie antenna. Hank Ellwood
sail16 posted 12-29-2005 12:26 PM ET (US)     Profile for sail16  Send Email to sail16     
Handn,

You are correct that VHF is essentially line of sight.

Assuming your antenna is 10 feet above the water, a 300 statute mile transmission would require the other station to be 45,000' above sea level. In other words, 300 miles on VHF is only reasonable if one of the station is an airplane.

More realistically, you should expect to get up to about 30 miles of range if the other station is 300' above sea level.

Of course, these are only estimates based on the distance to the horizon, but in the absence of other unusual factors, you should assume that these are the maximum transmission distances given the antenna heights.

You can experiment with other antenna heigh combinations at
http://www.qsl.net/kd4sai/distance.html

Buckda posted 12-29-2005 12:47 PM ET (US)     Profile for Buckda  Send Email to Buckda     
I don't know if it's from a repeater or not, and I don't know if they could hear me if I tried to contact them, but I can pick up Coast Guard transmissions (pan-pan's, etc.) out of Grand Haven, Michigan, Milwaukee, Wisconsin and Chicago, Illinois on Lake Michigan when boating out of St. Joseph. Milwaukee must be more than 80 miles away. They're either broadcasting from a 2,500 foot tower (my 8' fiberglass Shakespeare 5225 XT antenna tops out at about 16 feet above water on the arch), or I'm getting a bounce from a repeater or off the atmosphere...but it happens with regularity.

Two years ago, I also responded to a late-in-the-year radio check from Chicago Harbor for a vessel in Calumet Harbor (Indiana), which is more than 10 miles away...and that was before I had the arch, just an 8' Shakespeare 5101 mounted to the gunwale.

As for the rec on the Montauk - I'd go with the antenna that will give you the greatest range. That may be an 8' antenna. The VHF, first and foremost, is a safety device.

By the calculator provided, that gives you about 8 miles of room to reach another boat with a similar setup, and a bit more to talk to a shore station.

If you're out there alone often, or out there pretty far from home port, you need maximum reach to reach help. Remember, plan for poor conditions, not ideal ones. On that front, JimH's advice for a broader radiation pattern is sound - in 6-8 footers with your boat pitching and rolling and a dead engine, what antenna is going to reach help?

Sorry for the doom and gloom, but that's the mindset I try to put myself in when selecting safety gear.

Dave

jimh posted 12-29-2005 01:00 PM ET (US)     Profile for jimh  Send Email to jimh     
Antenna height is the greatest factor in determining the range of VHF groundwave propagation. The higher an antenna is mounted the farther its range.

A 4-foot antenna mounted on a 4-foot extension will actually have better range than an 8-foot antenna mounted to the same base. The average height of the 8-foot antenna will be 4-feet. The average height of the 4-foot antenna mounted to a 4-foot extension will be 6-feet.

handn posted 12-30-2005 08:59 AM ET (US)     Profile for handn  Send Email to handn     
I checked the distance from San Carlos to Bahia Muertos on my gps and it is in fact 247 n.m. so I fudged a little but now want to set the record straight. Verdad, that's my story and I am sticking to it.
The radio transmissions are without a repeater. There are no repeaters in the Sea of Cortez.
The year before another friend communicated with the same land based station from Cabo Pulmo which is almost to Cabo San Lucas and 330 n.m. from San Carlos. He has a 48 foot boat with a 12 foot Galaxie antenna on an extension and an older Icom radio. He took particular care to ground his radio with a grid of copper plates in the bilge.
A four foot antenna on a four foot extension would give better transmit performance than an 8 footer in calm seas because it concentrates the signal but it would provide inferior reception capacity. A longer antenna has better reception. In rough seas, the pitch of the boat beams that concentrated signal from that short antenna down into the water or up into the sky. A wider beamed signal transmits better in rough conditions. If space allows, the longer the antenna the better.
For all around real world performance on a small boat an 8 foot Galaxie is hard to beat.

Chuck Tribolet posted 12-30-2005 10:09 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Buckda: The USCG has an extensive repeater network. You can
puzzle out the locations of the repeaters using the charts at
http://www.navcen.uscg.gov/marcomms/cgcomms/nds.htm .

VHF is NOT strictly line of site. I regularly talk from
Monterey Bay to Carmel Bay. Linear distance is about 6nm,
but there's a 400' granite ridge in between. Both boats do
need to be a bit offshore, if either one is close to shore,
it doesn't work.

We should note that Handn says he has a 12' antenna, which
would be 9 dB gain.

Handn: it's the longer antenna that has the more concentrated
beam, not the shorter, and both transmit and receive is
a narrower beam.


Chuck

sail16 posted 12-30-2005 10:29 AM ET (US)     Profile for sail16  Send Email to sail16     
A quick sanity check on range.

How far can you pick up a commercial FM radio station? 75 miles? 100 miles? Definately not 300 miles.

Now, consider that commerical FM radio has many advantages over marine VHF, including:

* ~60mHz lower frequency
* Significantly higher transmit towers
* Significantly Higher wattage (1kw+)
* Horizontally polarized antennas

Marine VHF is inferior along these critical dimensions, and as a result, its range will fall below that of commercial FM.

handn posted 12-31-2005 08:36 AM ET (US)     Profile for handn  Send Email to handn     
I know everyone up north whose Whalers are frozen in drydock thinks I have fried my brain with tequila and there may be some truth in that. However, the transmissions by myself and others did occur and I'll wager anyone the price of a new radio that they will occur again when we go to Baja in the spring.
There must be some refraction of vhf signals particularly in the morning.
jimh posted 12-31-2005 10:37 AM ET (US)     Profile for jimh  Send Email to jimh     
I do not believe there is any advantage to be gained in a VHF Marine radio installation by use of a copper grid in the hull of the boat. Such systems are commonly used to create a counterpoise system for low and medium frequency vertically polarized antennas, but generally this is unheard of in VHF work.

The electrical behavior of an antenna is reciprocal. If it has gain on transmit it has the same gain on receive. If it has a particular pattern of radiation on transmit, it has the same pattern on receive. To perform otherwise would require some new laws of physics.

Extended propagation at VHF frequencies is not unknown, and I think that given the marine setting there could be some enhancement due to weather effects. A temperature boundary in the air can enhance propagation. This is particularly true over water. These phenomena occur regularly, and particularly could be enhanced by diurnal variations. A layer of cool air over warm sea water could set up this effect, known as trophospheric ducting.

Conditions that allow for extended propagation are known as "band openings", and when they occur it is quite amazing how far one can reach.

One reference on trophospheric ducting explicitly mentions the Gulf of Mexico as being a prime region for this enhanced propagation to occur.

Cf.: http://home.cogeco.ca/~vem3ont22/propagation/tr-modes.htm

"Dry Mexican air flowing across the Gulf of Mexico or Dry Saharan air flowing across the Meditteranean are two examples of prime tropo-producing conditions."

Amateur radio operators on the west coast have reached stations in Hawaii on frequencies adjacent to the VHF Marine band. This is a path of over 1,000 miles. It is not routine, yet it is not unheard of.

Here in SE Michigan during the summer it is fairly common to be able to receive television signals from stations in Cleveland (over 100 miles away) on late nights or very early mornings when the weather conditions across Lake Erie are favorable.

But all that aside, it is still inappropriate to infer that use of a particular type and brand of VHF Marine Band antenna will result in the ability to reach shore stations at extended ranges like 250 to 300 miles. Anecdotal reports of such communication are interesting, but they don't cause me to change my recommendation for an antenna for use on a small Boston Whaler boat.

Moe posted 01-06-2006 09:03 AM ET (US)     Profile for Moe  Send Email to Moe     
We put a 4' Digital brand antenna on a ratchet rail mount on the bimini frame of our 150 Sport. It can be vertical with the bimini open or closed in the "radar arch" position, and even when the whole bimini is folded forward. In the latter position, the antenna is next to the bow rail. It can also be folded down along side one of the rails whether the bimimi frame is up or fully down, for trailering.

Here's a picture or two. Note that we were one tooth off on the ratchet mount when we raised it and didn't get the antenna purely vertical in the first picture.

http://www.engr.udayton.edu/staff/lriggins/Whaler/36.jpg

http://www.engr.udayton.edu/staff/lriggins/Whaler/4.jpg

If you're putting together a radar arch, you could mount a 4' antenna so it folded down across the boat on top of the arch, as well as fore-aft, the way ours does.

--
Moe

Buckda posted 01-06-2006 11:53 AM ET (US)     Profile for Buckda  Send Email to Buckda     
Chuck -
Thanks for that link to the USCG repeater network.
jimh posted 01-08-2006 11:56 AM ET (US)     Profile for jimh  Send Email to jimh     
As I mentioned before (and explained in detail in the Reference article), antenna height is the most important factor in determining the range of communications from a small boat. The reasons for preferring a 4-foot antenna mounted on a 4-foot extension over an 8-foot antenna are:

• The 4-foot elevation will move the base of the antenna higher. Most of the radiation occurs in first two feet of the antenna. If you were to carefully measure the current flowing in the antenna, you will find that the current decreases as you approach the end of the antenna. In a simplistic way, you can consider that the radio frequency energy applied to the antenna is escaping into the ether, and as it travels along the antenna, more of it escapes. Thus, the power in the upper half of the antenna will tend to be less than the power in the lower half. In many small boat installations, particularly ones where the antenna is mounted at the gunwale level, this puts the "sweet spot" of the antenna very low. Elevating this portion of the antenna to a higher level level will improve the range.

• In terms of gain, a 4-foot antenna mounted on a 4-foot extension will probably be nearly the equivalent of an 8-foot antenna mounted to the same base. In the 8-foot antenna, consider that in the best case about half of the power goes to the upper 4-foot section and half to the lower 4-foot section. If the lower portion of the antenna is obstructed by other components of the boat, like the console, the bimini top frame, railings, etc., it will not work as well as the upper 4-foot of the antenna. It is probably better to put all the power into a 4-foot antenna mounted on a 4-foot extension, rather than to put half of your power into the upper half of an 8-foot antenna.

• There is really little evidence that an 8-foot antenna will have 3-dB gain over a 4-foot antenna. Creation of gain in the antenna is a sure fire thing in theory, but in actual practice it is much harder to obtain. Manufacturers of marine antennas are held to absolutely no standard in their claims of gain for their antennas, and, in fact, they seem to operate on their own astral plane in terms of claims for gain from simple vertical antennas. Measurement of actual gain from an antenna is difficult to document. At least one respected radio publication refuses to print any claims of antenna gain in advertising due to the impossibility of verifying it by any reasonable means.

• Any gain from an antenna, if it exists at all, refers to gain in the main lobe of the antenna. In order to achieve gain in the main lobe, the antenna has to reduce signal in the other lobes. This is not a problem if you can orient the antenna so that the main lobe always points in the direction of the desired communication path. Most VHF Marine Radio antennas are vertically polarized and should have a uniform pattern in the azimuthal or horizontal plane, but they will not have a uniform pattern in the take-off angle or vertical plane of radiation. In fact, it is in the vertical plane where all of the adjustment of the radiation pattern is made to create gain. In theory it would be best if the antenna could be made to concentrate as much energy as possible at the horizon, that is, at a very low vertical angle. This would provide the best enhancement possible. However, there is nothing in the claims of VHF Marine antenna manufacturer's that ensures that the main lobe of their antenna (where all of the gain is) will be oriented at the horizon. It might very well be that in a typical installation the main lobe may not be aimed at the horizon, but in fact elevated at a few degrees. It would be quite interesting to compare antennas in a controlled fashion to see if they really did develop maximum gain at a 0-degree angle, i.e., toward the horizon.

In order to create gain in the main lobe, the antenna must remove power in other lobes. Also, the longer the antenna becomes, the more lobes it has. Often there are deep nulls between lobes. In a high gain antenna it is really quite difficult to avoid creating a radiation pattern with many minor lobes and with deep nulls. It takes careful design an construction to avoid this.

Gain is also very dependent on frequency, and it is very difficult to construct an antenna which has uniform gain over a wide band of frequencies. It is not unusual for the gain of an antenna to vary by several decibels across its intended band of operating frequencies. Thus an antenna rated to have a particular gain may indeed have that gain, but only at a particular target frequency. At frequencies plus or minus a few percent, the gain may deteriorate.

On a small boat in motion in a seaway, the antenna will be in motion. Even if the gain of the 8-foot antenna turned out to be in a main lobe which was exactly where it was most effective, at the horizon, as the boat pitches and rolls, the antenna will be aiming its main lobe somewhere else. It could just as easily occur that in the process the antenna instead orients a deep null at the horizon. At that moment the antenna will not only have no gain, but it could very well have significant attenuation compared to a smaller antenna whose radiation pattern was broader.

• On transmit, elevating the antenna on a 4-foot extension will raise the main current lobes of the antenna above other electronic devices on the boat. For example, on a Boston Whaler REVENGE the standard antenna mounting location is adjacent to the instrument panel. When a 25-watt marine radio is in transmit, all of the electronics on the instrument panel are about a foot away from the base of the antenna. It is very common to see indications of radio interference from the antenna current getting into other electronics, including the transmitter itself, a special case known as RF Feedback. This often results in very distorted modulation, which, unfortunately, is frequently heard on transmission from small boats with poor antenna installations.

• Antennas are affected by their surroundings. If an 8-foot antenna is mounted to a low base, other objects around the antenna may affect and distort the antenna's pattern. Any antenna will work better if mounted in a clear setting. Thus using a 4-foot extension to elevate the antenna can help to remove the influence of surrounding objects.

Now all of this said and considered, if an 8-foot antenna were mounted on a 4-foot extension it would probably perform better than a 4-foot antenna mounted there. However, a 12-foot antenna and extension is a rather unwieldy mechanical combination on a small boat.

deepwater posted 01-12-2006 07:30 PM ET (US)     Profile for deepwater  Send Email to deepwater     
[Expressed thanks for comments in this article.]
jmorgan40 posted 01-19-2006 06:31 PM ET (US)     Profile for jmorgan40  Send Email to jmorgan40     
DWS,
Below is a link to how I mounted my 4 ft Shakespeare although I would go with Digital if I did it over.
http://morganfamily.home.mindspring.com/id1.html

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