Antenna Advice

Reming870

Petty Officer 2nd Class
Joined
Jul 15, 2010
Messages
191
My antenna broke in storage for Radio #1. I do know it was a Shakespeare but no idea what # it was.

Radio #2 has a 8-ft Shakespeare # 5206-C.

I am looking for advice if I should take the opportunity to change Radio #1 to something different than Radio #2.

Boat usage involves 30-40 mile offshore day trips.

Specs on Radio #2 antenna 5206-C are:
6dB Gain, 8', fiberglass finish VHF marine band antenna.
Coax cable element.
Nylon ferrule with standard 1"-14 thread.
Bandwidth: 3 MHz.
SWR: 1.5:1 @156.8 MHz.
Impedance (Ohms): 50.
Max. Input (Watts): 50.
DC Ground: YES.
Two year warranty.
 

JoLin

Vice Admiral
Joined
Aug 18, 2007
Messages
5,146
Re: Antenna Advice

Antenna height will do more for you than anything else. I'd go taller than 8' if I could.
 

Reming870

Petty Officer 2nd Class
Joined
Jul 15, 2010
Messages
191
Re: Antenna Advice

Its on top of a pilot house. I calculated it to be 16 ft from flat water to antenna tip with an 8 ft antenna.

Would I be better with a 9db rating to get further range?
 

jhebert

Ensign
Joined
Jul 24, 2005
Messages
902
Re: Antenna Advice

It[']s on top of a pilot house. I calculated it to be 16 ft from flat water to antenna tip with an 8 ft antenna.

Would I be better with a 9db rating to get further range?

In your case the antenna base is about 8-feet above the water. This is a reasonably good height for a small boat antenna. You can figure the effective antenna height by using the mid-point of the radiating portion of the antenna. For example, if you used a four-foot extension mast on your base, and mounted a 3-foot antenna on the extension, the center of the radiating part of the antenna would be about 8 + 4 + 1.5 = 13.5-feet above the water. Or, let's assume you used a common "8-foot" antenna which has about 2-feet of non-radiating segment at its base and a 6-foot antenna at the top. This antenna is too heavy for an extension, so we mount it on your base. Its height is going to be 8 + 2 + 3 = 13-feet. In either case, your antenna is going to be about 13.5 to 13 feet above the water.

The range to the radio horizon is found from the relationship

Range-in-miles = ( 2 x height-in-feet)^0.5

If you could double the height of the antenna, the range to the radio horizon would increase by a factor of 1.4. In other words, to get a 40-percent increase in range, you would have to increase the antenna height to 27 or 26 feet. That is typically not a practical installation on a small boat. We can say that when the antenna height is more than 10-feet above the water, a point of diminishing returns has been reached for improving range by increasing antenna height. It is just not practical to run around on a small boat with a 27-foot high antenna.

All antennas create an effective gain by concentration of the radiation into a main lobe and reducing the radiation in all other directions. In a vertical antenna the concentration is to narrow the range of vertical angle radiation coming from the antenna, and to concentrate all radiation, as much as possible, into one main lobe aimed at the horizon. The more gain increases in the main lobe, the less radiation there is everywhere else. As long as the antenna remains stationary and its main lobe is directed toward the horizon, the result is a beneficial outcome: more signal is produced. If the boat is in motion and the orientation of the antenna changes from vertical, the main lobe of radiation moves away from the horizon. If the main lobe of radiation is very narrow, it takes only a few degrees of change in orientation to moved away from the peak signal and into the region where there is much less signal. For small boats there is always some motion unless at the dock or operating in extremely calm water, and for this reason the use of high-gain antennas is often seen as a disadvantage. Lower-gain antennas have a much broader main lobe, and even when the boat rocks 10-degrees, the change in gain at the horizon is minimal.

The improvement in range that can be obtained by increasing antenna gain is hard to calculate. The basis for a range increase due to gain increase depends on the path loss for a certain distance. There are several ways to model path loss. In the most favorable model, path loss is calculated as increasing at a rate of 20 x log(distance); this is called the free-space model. This gives us a relationship between path loss and path distance. If we use this model and calculate the outcome for a loss of 3-dB, we find the distance would be in the ratio of 1.4:1. That means an increase in gain of 3-dB ought to compensate for 3-dB of additional path loss, which, with this most favorable model, implies the path length could increase by 1.4-times. That sounds rather good. It may be too good.

The actual path loss is not always so easily modeled. Some models for actual path loss suggest the path loss is related to distance by a relationship more like 40 x log(distance). That implies that a 3-dB increase in gain would make the path length increase to 1.19-times.

It is hard to say exactly which model best describes any particular boat-to-boat communication path. As a result, we are left with making something of a guess about how increase in antenna gain will improve range. We can say that doubling the antenna gain will likely improve the range by about 20-percent to perhaps 40-percent at best.

By the way, your original antenna, the Shakespeare model 5206-C is probably one of the least expensive, least durable antennas available. It may also not be a particularly good antenna. I don't recommend that antenna.

The claims made by antennas manufacturers about the gain of their antennas are all really quite fanciful, and there is little rigorous testing or verification. It is my opinion that at the very least, all of these antennas have their gain overstated. You should subtract 3-dB from any claimed gain.

Let me turn to your initial question: is it better to use a 9-dB gain antenna to get further range?

Any increase in antenna gain helps to increase range, as I demonstrated above. These "9-dB" antenna are typically not really 9-dB gain; they're probably more like 6-dB. They are physically longer or taller. The taller antenna comes with an increase in antenna height, but the effective height is really the mid-point of the radiator, not the height of the tip of the radiator. The increase in gain comes with the narrowing of the main lobe. If you have a very stable boat, you may find that the narrow lobe does not create a problem.

My general recommendation is to use a smaller antenna, typically a 3-foot metal whip or a 4-foot fiberglass enclosed antenna, and mount it on a 4-foot extension, in preference to using an 8-foot or 16-foot antenna. I also like the durability of the metal whip antennas. The metal whip will just bend and not break if you hit something.

For more on estimating path loss, see

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

For more on how antenna gain affects main lobe width, see

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

For more on how to estimate the range of communication, see

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

For a derivation of how the radio horizon is calculated, see

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

On my own boat I use a 3-foot metal whip antenna on a 4-foot extension mast. This has proven to be a very durable antenna. It has been in use for seven seasons of boating. I do a lot of trailering and it has survived many thousands of miles on the highway. The performance of the antenna has been extremely good. I can communication boat-to-boat at a range of more than 15-miles. I can typically receive NOAA weather radio broadcast transmissions at ranges of 75-miles or longer on a consistent basis. I describe the particulars of my antenna installation at

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

and I have no reservation at all in recommending those components. They are not expensive, they are durable, and they work extremely well.
 
Last edited:

UncleWillie

Captain
Joined
Oct 18, 2011
Messages
3,995
Re: Antenna Advice

My antenna broke in storage for Radio #1. I do know it was a Shakespeare but no idea what # it was.
Radio #2 has a 8-ft Shakespeare # 5206-C.
I am looking for advice if I should take the opportunity to change Radio #1 to something different than Radio #2.
Boat usage involves 30-40 mile offshore day trips...

Are you looking to just replace the Antenna or the Radio Transceiver.
The differences in the Radios is the the user features. They all TX at 25 Watts Max. All RX about the same.

At 40 miles off shore you may be able to hear the CG and the WX channels but they will not hear you.
The only people that are going to hear you that far out are other boaters that are within 20 mile of you.
 

jhebert

Ensign
Joined
Jul 24, 2005
Messages
902
Re: Antenna Advice

At 40 miles off shore you may be able to hear the CG and the WX channels but they will not hear you.
The only people that are going to hear you that far out are other boaters that are within 20 mile of you.

Of course, the NOAA Weather Broadcasts are just that, broadcasts. NOAA Weather Stations are transmit-only; they are never listening.

The Coast Guard RESCUE 21 system is designed for a minimum range of 20-miles for detection of a 1-second transmission of a 1-watt signal with an antenna 6-feet high. Even with those restrictions, some of the stations have a predicted range of 40-miles. On a boat with a 25-watt radio, a gain antenna, and a good antenna height, it should be reasonably probable that 40-mile range can be obtained in many of the installations.
 

UncleWillie

Captain
Joined
Oct 18, 2011
Messages
3,995
Re: Antenna Advice

Of course, the NOAA Weather Broadcasts are just that, broadcasts. NOAA Weather Stations are transmit-only; they are never listening...

Totally Agree!
The Reference to hearing WX broadcasts is that some users may have the false sense of security that they are still within radio range of shore if the WX broadcasts are still receivable; Or, if broadcasts from the CG can be heard.

VHF is primarily line of site.
Once you are over the radio horizon; A stronger signal is not going to help.
A better antenna or a stronger Transmitter will not get the message through.
You can raise your antenna a few more feet and the Rescue 21 system surely has their antennas up hundreds of feet to improve the reception.

It also works the other way around.
Just because you do not hear a reply, doesn't mean the signal wasn't received.
If you have an urgent message, transmit it even if there is no response.

Yes, there are many nuances to to the effects of the atmosphere on the radio waves, but they can not be relied upon.
Scattering, Ducting, Skip, all produce amazing reception at times.
I have heard CG transmissions from nearly 100 miles away on occasions.
But I suspect I will be hearing from "Murphy" when it is really needed. :blue:
 

jhebert

Ensign
Joined
Jul 24, 2005
Messages
902
Re: Antenna Advice

Once you are over the radio horizon; A stronger signal is not going to help.
A better antenna or a stronger Transmitter will not get the message through.

This is not true. It is quite normal to hear NOAA Weather Broadcasts that originate from far beyond the radio horizon. The stronger transmitter and better antenna are what gets their message through. In the same way, a good installation on a boat, with a good antenna nicely installed as high as possible, will expand the range. It is quite normal for boat-to-boat communication to occur at distances greater than the calculated radio horizon.
 

UncleWillie

Captain
Joined
Oct 18, 2011
Messages
3,995
Re: Antenna Advice

This is not true. It is quite normal to hear NOAA Weather Broadcasts that originate from far beyond the radio horizon. The stronger transmitter and better antenna are what gets their message through. In the same way, a good installation on a boat, with a good antenna nicely installed as high as possible, will expand the range. It is quite normal for boat-to-boat communication to occur at distances greater than the calculated radio horizon.

I still Agree!

As I stated Murphy works both side of the street.
Some days it is great, others not so much.
But there are no guarantees.

The WX broadcasts are part of the Emergency Broadcast System.
They are directed at the general public in their homes.
The Transmitters are often in the 1000 watt category with antennas often co-located on TV broadcast towers at heights approaching 1000 feet.
The NOAA WX tower closest to me has a 50 mile footprint over land.

Which is why I cautioned that Receiving a WX broadcast is not an indication that you are within radio range of shore with your VHF Radio.

:joyous: I am not disagreeing with you, I am just stating for the masses, that there is a reason that VHF Marine Radios are only expected to cover 20-25 miles with any degree of certainty.

They will certainly go farther under ideal conditions.
Imaging how far you could talk Ground to Air, to a Rescue Chopper at 10,000 feet!:D
 

jhebert

Ensign
Joined
Jul 24, 2005
Messages
902
Re: Antenna Advice

The WX...Transmitters are often in the 1000 watt category with antennas often co-located on TV broadcast towers at heights approaching 1000 feet. The NOAA WX tower closest to me has a 50 mile footprint over land. Which is why I cautioned that Receiving a WX broadcast is not an indication that you are within radio range of shore with your VHF Radio.

My local NOAA Weather Radio station is KEC63, which serves a metropolitan area of several million listeners. It transmits with 330 watts, which is typical for a high-power NOAA station. You have to be careful when you look up the antenna heights as they are typically given in feet above sea level. KEC63's antenna is listed as 1370-feet, but its tower is at 685-feet above sea level. The antenna is 685-feet above ground. The coverage map predicts prime signal levels out to about 45-miles. Usable signal levels are predicted to over 60-miles.

Also, even with an antenna at 685-feet, the radio horizon distance for KEC63 is only 37-miles. This is a good example of how increasing height begins to have diminishing returns.

By the way, I recommend using NOAA Weather Radio broadcast signals as test signals for checking the performance of your receiver and antenna. If you can't hear NOAA broadcasts from several stations more than 50-miles away, there is probably something wrong with your antenna or your receiver. Here is an article describing testing a receiver with NOAA signals, and it includes some recordings of the signals to give an idea of how they are being received.

Receiver Testing - Moderated Discussion Areas

Radio signals routinely propagate over the horizon or beyond the "line of sight" distance due to refraction in the atmosphere. There are actually three different distances to the horizon based on the height of the observer:

--the geometric horizon distance

--the optical horizon distance, somewhat farther due to refraction of light in the atmosphere, and

--the radio horizon distance, longer still due to even greater refraction of radio waves in the atmosphere

You might find the separate calculation of these three distances to be of interest:

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

jhebert

Ensign
Joined
Jul 24, 2005
Messages
902
Re: Antenna Advice

With regard to being able to communicate with the Coast Guard radio station, if you can hear them, then they should be able to hear you, if your station is using 25-watts and your antenna is working properly. The path loss should be the same in either direction, in theory. In practice there may be some asymmetry in the path loss due to unusual effects--what is called one-way propagation. But if your transmitter is working properly, delivering the rated 25-watts to the antenna, and if the antenna is working properly, your transmitted signal should be strong enough to be heard by the Coast Guard radio station if you can hear them.

If your antenna is not working properly, your receiver can compensate with more gain, but your transmitter power cannot compensate. This will result in an asymmetry in your receiver--transmit range. For a more thorough explanation of this condition, see


VHF Marine Radio: Asymmetry in Receive and Transmit Range Due to Bad Antennas
 

ssobol

Chief Petty Officer
Joined
Sep 3, 2010
Messages
503
Re: Antenna Advice

Sea Tow has a number of automated radio check stations around the US. Basically you transmit a voice message and it sends it back to you. You can check the range of you radio system by checking with multiple stations or checking the same station at various ranges. These stations in combination with a GPS unit or chartplotter should give you a good idea how well your particular radio set up works.

Automated Radio Check Service | Sea Tow
 
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