VHF Antenna

[Targa17]

I plan to purchase a VHF marine radio in a couple of months. I will be using my boat on resevoirs as well as Lake Erie. Which type of antenna would you suggest, the 8ft fiberglass or the 36" stainless steel? Thanks for your input.

 

[jhebert]

Re: VHF Antenna

Here is a description of my antenna installation on a small boat.

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

The results have been outstanding. This information may be useful to you.

 

[arks]

Re: VHF Antenna

The end of the antenna must be as high as possible for maximum range, so the 8 footer would be your best choice IMO.
Unless you have a sailboat

 

[Boatist]

Re: VHF Antenna

I Like a 8 foot 6DB gain antenna but get a good quality like the Shakespears 5225.
The antenna is strong and flexes very little for long life.
I started with a small felexable 8 foot 6 db gain and it worked pretty well for a number of years. As time went on fishing 10 miles north of port I could here other boat getting station I could not pick up. Made sure the radio was not in local mode but still did not receive very well.

I finally got a 5225-XT and installed it and could not believe the difference. I could now here every thing and in camp had people asking me why I could here stations they could not pick up. I just explained what a big difference the 5225 antenna made to my boat.

That said a 8 foot antenna mounted 3 to 4 feet above the water has a range of
4.7 to 4.9 miles plus the range of the other Station.
So boat to boat range just under 10 miles.
Boat to Coast Guard out here is about 80 miles because their antenna are mounted on top of mountains along the coast.
Just North of San Francisco one of Coast Guards antennas is mounted on top of a 3000 foot high mountain.

The 3 foot high base loaded Stainless Steel antennas are use mostly on top of Sail boat mast.
They are a 3 db gain antenna.
On top of a 25 foot mast range is about 7.1 miles plus the range of the other station.
On Top of a 50 foot mast range is about 10 miles plus the range of the other station.
Now the 8 foot 6 DB gain antenna has effective radiated power out of 100 watts.
The 3 Foot Stainless 3 DB gain antenna has a effective radiated power of 50 watts.

From long distance the higher power out can make a difference in weather you can raise the Coast Guard or not. While power out has very little effect on range it can make a big difference in you getting thru to the Coast Guard.
I wil try and explain. Let say we are 70 miles North of San Francisco and with the height of their antenna the range is 80 miles.
Also there are two boats in this fishing area 70 miles to the north, one has a 3 foot 3db gain antenna and the other has a 8 foot 6 db gain antenna.
Out of no where a large rogue wave crashes on both boats filling them half full of water. Both boats turn on their bilge pumps and turn their boat into the swells.
Now they both get on the radio and call Mayday at the same time.
Even if the guy with the 3 db gain antenna has his antenna higher the guy with the 6 db gain will be the boat that get thru to the Coast Guard because he has a double the Effective Radiated Power. His radio will over power the other boat and be the one that gets heard.

This would be a very rare thing, but what does happen often in the San Francisco area there are weekends where there may be 2000 boats out on the water.

An example would be the Navy Fleet week in October. The Navy has several Air Shows over the Bay like the Blue Angels and a WWII simulated air fight. Coast Guard Block off an area where several Navy ships will pass by. The bay has so many boats outside the restricted area there is only 10 to 20 feet between boats.
Inside San Francisco Bay there could be 1500 boats.

With all these boats making Calls on Channel 16 there is always traffic on channel 16.
Now if your 70 miles north of the bay you can not here any of these boats in the bay and your not going to be able to override there signal but you still have a chance of overriding boats a good distance from the bay with a 3 db antenna.
If the Coast Guard ever hears any part of a MAYDAY call they will tell everyone to stop their traffic and now you can get thru to them.
You can also try and call on Channel 22 as they sometimes have a watch on 22 also and it has a lot less traffic. Last and not legal call on Channel 21 because durning these events they will have every Coast Guard boat out on patrol and will be useing channel 21.

Anyway the point is this is a case where a 6DB gain antenna will give you a better chance than a 3 db antenna even if the 3 db antenna is higher.

They also make 9 DB gain antenna with a ERP of 200 watts but these antenna are not recomended on small boats. This is because they transmit in a very narrow Horizontal band and as the boat rocks and rolls a distance antenna will not be in the radiation pattern of the 9 DB antenna.

Hope this makes some sense.

 

[jhebert]

Re: VHF Antenna

...the point is this is a case where a 6DB gain antenna will give you a better chance than a 3 db antenna even if the 3 db antenna is higher.

I disagree. For a more precise analysis of how antenna gain can affect range, please refer to my article:

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

and see the material under the subheading

Calculating Influence of Antenna Gain on Range

As shown in the above article, doubling the antenna gain improves range by about 20-percent. Now compare this to the influence of doubling the antenna height. Doubling the antenna height improves range (by increasing the line-of-sight path) by factor of 2^0.5 = 1.41, or 41-percent.

I much prefer greater antenna height over greater antenna gain.

In addition, all antenna gain results from a narrowing of the radiation pattern, and the gain occurs only in the focus of the main lobe. Anytime the main lobe is not oriented directly at the remote station, gain suffers. In comparing an antenna with 6-dB gain to one with 3-dB gain, once the remote station is ten degrees off-axis the difference between two antennas in minimal, only about 0.7-dB. If 20-degrees off-axis, the 6-dB gain antenna actually produces less signal toward the remote station. Given the instability of a small boat, the broader pattern of a lower gain antenna is desirable.

For a detailed comparison of antenna pattern as a function of gain, see

VHF Marine Band Antennas
http://continuouswave.com/whaler/reference/antennaVertical.html

Even without undertaking any attempt to understand the underlying theory of radio propagation regarding range, height, and antenna gain, it should be completely obvious that height is more important than antenna gain. If height were not important, the Coast Guard station would simply use a high gain antenna mounted at a modest height, instead of going to all the trouble of building 300-foot tall towers. This same analysis applies to television broadcast stations. If gain were more important than height, why would broadcast stations build 1000-foot tall towers if they could simply improve their range with a higher gain antenna? The obvious answer is height is more important in determining range, so any station interested in maximizing its range chooses to maximize the antenna height.

 

[ENSIGN]

Re: VHF Antenna

(Sounds like we got a antenna theory class)Just get the eight footer and mount it as high as possible.Good luck,Ted KB3QVL

 

[Boatist]

Re: VHF Antenna

Jhebert
We will just have to agree to disagree.
While the higher antenna can transmit farther with no other radio traffic the radio with double the power out will be the signal that the Coast Guard will here.
The 6 DB antenna on small boats will usually not rock and rolls enough to not hit and antenna on the horizion.
That is a problem with the 9 DB gain antenna and is the reason they are not recomemded for small boat.
I agree with most of the stuff in your post but will have to disagree on this point.

 

[jhebert]

Re: VHF Antenna

While the higher antenna can transmit farther with no other radio traffic the radio [with] double the power out will be the signal that the Coast Guard will [hear]....I agree with most of the stuff in your post but will have to disagree on this point.

Your interpretation is fanciful. The reason range increases is because the transmitter signal is strong enough to be received. The receiving station cannot tell if the signal is strong because the antenna gain was greater or because the antenna was in a higher location. The receiving station just knows the signal is strong enough to be received. A transmitter with a higher antenna that produces longer range cannot produce weaker signals than a transmitter with a lower antenna. Those two positions are contradictory.

Again, antenna gain is nice, but antenna height is nicer.

Also, claims of antenna gain are really just that: claims. Many antennas sold as having 6-dB gain do not actually have that much gain. Tests which measure antenna gain have shown that some antennas advertised as "3-dB" gain antennas produce stronger signals than other antennas sold as "6-dB" gain antennas. You can find a pointer to some actual antenna test results in the article I linked above.

 

[Boatist]

Re: VHF Antenna

Jhebert
I really did not want to get into this but antenna range and power are two totally diffenent things.
VHF signal is line of site so a higher antenna will have more range because it can see farther over the horizon than the lower antenna.
This does not make the signal stronger at all.

As long and both station can see the receiving stations antenna then the one with more power will be the vessel heard.

In my example the the Coast Guards antenna was 3000 feet high so has a range of 77 miles plus the range of the other vessel.
Both vessel were 70 miles away so both could hit the Coast Guards antenna with their signal.
The 3 DB Gain antenna will have a Effective Radiated power of 50 watts.
The 6DB Gain antenna will have a Effective Radiated power of 100 Watts.
If both Boats are talking at the same time the Coast Guard will hear the boat with twice as much power and not here the second boat at all.

I have a very good article from West Marine that address these issue that I will copy over.
It is too long to copy the whole article but here the parts that talk about height verse Gain.
The last paragraph explains what I am saying.
You can disagree with this but this is the way VHF Marine Radios work.
So if your in an Area like California like I am you want a 6 DB antenna of good quality.
If Your in a area with very few stations the 3 DB will work good.



"A gain increase of 3dB means a doubling of signal strength: 6dB is a fourfold increase, and 9dB is an eightfold increase. The actually watts transmitted doesn't increase; it's just that the power is concentrated, much like a reflector on a flashlight, in a more concentrated beam. Therefore, a radio with a 6dB antenna will sound like it has a larger transmitter than the same radio with a 3dB antenna."

"Elements Inside the Antenna

There are three main styles of electrical elements: those using cut lengths of coax cable, those using a simple brass radiator, and those using a more complicated copper and brass radiator. While all provide acceptable performance, better antennas use brass or copper inside the fiberglass for maximum strength and durability."

"Even if you could increase your power, it wouldn't help much. VHF radios operate on a line of sight principle between stations, meaning that the signals do not bend around obstructions or over the horizon (Okay, they do a little, but think of them as traveling about as straight as a beam of light.)

? This means that antenna height, more than any other factor, is responsible for determining how far you can transmit. An antenna mounted up high can "see" farther over the horizon.

? Under the right conditions, a very small transmitter can communicate over vast distances. For example, the signal from an EPIRB (emergency beacon) is less than one tenth of a watt, yet it is picked up by satellites orbiting 528 miles above the earth.

? So, the wattage of the radio is much less important than antenna height in determining range. Antenna gain is important, however. This is a measure of how much the antenna focuses the radio's energy in the horizontal direction. If several otherwise equal radios try to contact a single receiving radio, the higher gain antenna is the one the receiving radio will hear."

 

[Seon]

Re: VHF Antenna

The problem I had with the 8' whip was when towing the boat, I'd position it down but it'd get either stepped on or guest would get hung up on it with their clothing when boarding :redface:.
I ended up changing over to a 32" that I mounted on top of the cabin. It's not as high but since swapping I haven't noticed any change in transmitting or receiving.

 

[jhebert]

Re: VHF Antenna

SEON--You have brought up another problem with the typical eight-foot fiberglass whip antenna: they are very prone to damage. If I had been using an eight-foot fiberglass whip antenna in place of the installation I have now, I probably would have broken the eight-foot fiberglass whip at least four times. I would have shattered it on some low branches while the boat was on the trailer, or on some low bridges while passing under them, or on the low overhangs of some cover boat wells I dock in. Getting away from the eight-foot fiberglass antenna was the best money I ever spent.

BOATIST--Your theory that ground wave VHF radio communication range is increased more by antenna gain than by antenna height is not a widely held opinion, at least among people who don't base their radio propagation theory on what they read on a WEST Marine website. Again, just look around and notice all the very tall towers on which radio antennas are located. Why would anyone go to the trouble and expense of building and maintaining a tall tower for their radio antenna if the smart money was to use an antenna with double the gain at half the height?

 

[steelhead2000]

Re: VHF Antenna

If we all agree that the statement below is acccurate then I think Boatist has a point

VHF radios operate on a line of sight principle between stations, meaning that the signals do not bend around obstructions or over the horizon.

This would explain why radio towers are built on mountains or great elevation to make sure that the line of sight is not obstructed so whoever is trying to receivve or transmit signals are not blocked by structure. So assuming from Boatists example if both boaters were equal distance and had a clear line of sight to the tower 70 miles away the greater gain would drown out the 3Db.

 

[Boatist]

Re: VHF Antenna

BOATIST--Your theory that ground wave VHF radio communication range is increased more by antenna gain than by antenna height is not a widely held opinion, at least among people who don't base their radio propagation theory on what they read on a WEST Marine website. Again, just look around and notice all the very tall towers on which radio antennas are located. Why would anyone go to the trouble and expense of building and maintaining a tall tower for their radio antenna if the smart money was to use an antenna with double the gain at half the height?

Jhebert
Please do not misquote me.
I never said communication range is increased by antenna Gain ever.
What I said was increasing antenna height will increase range to the horzion.
Increasing antenna gain can help you get thru on busy radio channels.
Increasing Antenna gain does not increase range but does help you get thru on a busy channel.

The example was a Coast Guard antenna on top of a 3000 foot high mountain has a range of 77 miles.
On the VHF Marine raido
Two boats 70 miles away trying to contact that Coast Guard Station at the same time.
One boat with a 3 DB gain antenna mounted higher up.
Second boat with a 6DB gain antenna mounted mounted lower.
Then the station the Coast Guard will here is the boat with the 6DB Antenna.

Out here on a weekend or if we ever have another salmon there are many boats out.
Where we fish out of is about 25 miles North of San Francisco and has a Coast guard station but their antenna is not very high.
So if you need to call the Coast Guard will often get Group San Francisco instead of Bodega Bay.
San Francisco will often have many boats out. They may have Navy fleet week and have thounds of boats out.
They could have a Sail boat race and have many spectators.
In swells less than 5 feet will always have lots of fishing boats out.
To Raise them on the VHF Radio from 70 to 80 miles away can be hard but your chances are much better with a 6 db gain antenna.
You could raise a 3 db antenna 50 feet and it would not help at all.
The problem is not range as they are in range with even a low antenna.
The problem is being heard thru the traffic.
On your boat You can not here traffic more than about 12 miles away so do not know if someone is on channel 16 or not.
Coast Guard can here boats 77 miles to the north and 77 miles to the south.
Aslo 77 miles West out to sea and more than 80 miles inland. They have at least 6 antennas.
So the chances that someone is talking on channel 16 on a nice summer weekend are pretty high.
My chances of being heard by them is much better with a 6DB Gain antenna.

This is why most boat to boat transmisions should be made on 1 watt.
Boat to Boat is normally 10 to 12 miles range at best and 1 watt works just fine at that range.
This allows a boat sending a MayDay call with 25 watts and 6 DB gain for a effective radiated power of 100 Watts to be heard by the Coast Guard over the call to your buddy to switch and answer on channel 68.
MayDay of other saftey Transmissions should be made on 25 watts

Many Places inland you can listen all day and never hear another boat and only hear the Coast Guards Marine information brodcast.

In these locations a 3 DB Gain antenna would work very well.

I also never said I based My Radio Propagation theroy on what West Marine Said.
The West Marine Information was just to let you hear the same thing I said from a difference source wrote a little different to help you understand.
I could have wrote the article but everything they say is correct.

The 3 foot 3DB antenna can be the a good antenna for many. Most sail boat use it on the mast head.
Many who do not like the looks of a 8 foot antenna also use it.

The 8 foot 6DB is the best antenna for many also. I lay my antenna down on the deck before I hit the road so it is not going to hit anything.
Most times also cover the antenna with a 1 inch nylon tube that has two snaps.
A snap at the top that snaps to my deck where the camper cover would snap on and a snap at the base.
The cover keeps the sun off, stops antenna from scratching the deck and hold antenna in place.

My information is based on 50 years of Radio classes and experience.
40 Years of Ham radio.
a Master Course in Communication Electronics.
An Antenna Course and building and tuning many antennas.
I started with Building 2 CB radios in the 1960 and then moved to Ham Radio.
To get Your Novice Licence you have to understand Radio Propagation, it is a big part of the Novice Ham Radio test.
VHF and UHF are LINE of SITE and that is why they are mounted on high buildings or mountain tops.
Repeater also on top of high building or mountains but also have many rules on Antenna gain and radation pattern and power out.
HF will bend over the horzion Some and also SKIP.
Long wave and Low bans will bend over the horzion.

 

[Mark_VTfisherman]

Re: VHF Antenna

Well I learned a lot here, once again. But I also observe that just because something is logical does not mean it makes sense

 

[jhebert]

Re: VHF Antenna

I apologize if somehow my summary of Boatist's general argument was not consistent with his actual position on the question of which factor was more effective in increasing the range of VHF Marine Band communications, antenna gain or antenna height. I have consistently stated that I much prefer to increase antenna height as a way to increase range, as opposed to trying to increase range by increasing antenna gain. In this regard I am quite confident I have both the theory and practice (of thousands of stations) to support my position. I will no longer attempt to describe Boatist's position, other than to say from his repeated replies he apparently takes a contrary stance to mine.

As for credentials, I will stipulate that that Boatist's are very impressive. For my presentation, however, I will just rely on the Laws of Physics and the past and current practice of thousands and thousands of radio stations who rely on antenna height to increase their range more so than antenna gain to affirm my position.

As for the original inquiry in regard to Lake Erie, you can find the U.S. Coast Guard radio coverage maps here:

http://www.navcen.uscg.gov/marcomms/cgcomms/nds.htm

The chart for western Lake Erie is here:

http://www.navcen.uscg.gov/marcomms/cgcomms/charts/09detros.jpg

The plotted coverage is for a "mobile transmitter power of 1 watt at sea level over water." As you can see, the coverage is quite good. Stations with a 1-watt radio held at the surface of the lake will be in communication range over most of the lake. This back and forth about the desperate need to buy a 6-dB gain antenna is really of little importance. The Coast Guard will copy you just fine.