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Assessing VHF Marine Band Antenna Performance Using Distant NOAA Weather Radio Broadcasts

by James Hebert

A common concern among recreational boaters is the performance of their VHF Marine Band radio. This concern is properly focused. It is my experience that a great many recreational boats have VHF Marine Band radios that don't function very well. The source of the problem is often the antenna. A simple method for assessing the performance of a VHF Marine Band antenna is presented.

Using NOAA Weather Radio

Antenna performance is reciprocal: it works the same on receive as on transmit. This allows the antenna to be evaluated by its receive performance. All that is needed is a distant station to cooperatively transmit for testing. Fortunately, there is a network of 1,000 NOAA weather radio broadcast transmitters that can provide a source of distant signals to use for testing reception. It is almost universal that VHF Marine Band radios include receive-only channels for the frequencies used by these NOAA stations.

To test your antenna, simply tune in a remote NOAA station and assess the signal strength obtained. To make this process easy, NOAA has provided coverage maps for all 1,000 of their stations. These coverage maps show the estimate signal strength of the station versus location. The maps are color-coded for easy interpretation. Here is a sample map:

Plot of estimated signal strength verses range for WXN69 in Sister Bay, Wisconsin. Original source http://www.nws.noaa.gov/nwr/wi/sisterbay.gif

The color code indicates anticipated signal level:

COLORSIGNAL LEVELRECEPTION
WhiteGreater than +18 dBuVReliable
Green0 dBuV to +18 dBuVPossible but unreliable
RedLess than 0 dBuVUnlikely

To test your system, look for a transmitter whose coverage should be in the green sector for your location. This gives you a signal level that requires your antenna and receiver to be working at a reasonable sensitivity in order to hear the station.

The coverage map gives the frequency of the transmission. You can convert to the channel number by consulting your particular radio's instruction manual. A typical channelization is according to this scheme:

WX-1 = 162.550 MHz
WX-2 = 162.400
WX-3 = 162.475
WX-4 = 162.425
WX-5 = 162.450
WX-6 = 162.500
WX-7 = 162.525
WX-8 = 161.650
WX-9 = 161.775
WX-10 = 163.275

In most areas there will be at least one station providing "white" level coverage. You should be able to receive this station with a very strong signal. In FM reception there is a quality of a receiver known as "capture ratio" which describes the ability of the receiver to demodulate only the stronger of two signals it is receiving simultaneously. When one signal is stronger than another on-channel signal by the level specified in the receiver's capture ratio, the weaker signal is ignored and only the stronger signal is demodulated. When two signals are near the same strength, you often can hear artifacts of the weaker signal on the stronger signal.

On some channels there may be more than one signal available. You may hear effects from the weaker signal modulating the stronger signal. Look for a channel on which there is only one signal likely to be available in your area. To verify the exact station you are receiving, it may be necessary to listen to the broadcast for several minutes. Most stations incorporate a station identification announcement into their transmissions every five minutes or so. To find a station in your area, use the coverage map index.

Practical Example

Using the above technique, I evaluated the performance of my boat's antenna. While sitting at the dock in Leland harbor, I tuned my radio to WX-4 and listened for NOAA Weather station WXN69 in Sister Bay, Wisconsin. According to the coverage map, my location was right at the fringe of the "green" coverage zone.

The green-shaded coverage indicates a signal level of at least 0-dBuV, or a sensitivity of 1-microvolt. Most VHF Marine Band radio receivers are rated for a sensitivity of at least 0.5-microvolts. This means that a typical VHF Marine Band radio should be able to receive signals in the green-shaded coverage zones with at least the minimum signal to noise ratio specified, typically 12-dB. The radio I used in my test has a sensitivity of 0.25-microvolt for 12-dB SINAD. The test produced a good copy on the transmission from WXN69 in Sister Bay. There was some fading in and out, as to be expected when at the extreme limit of the predicted range, but the signal was useable. This test verified that the receiver and antenna were performing as expected.

Using NOAA Weather Radio broadcasts can be an effective way to analyze the performance of your antenna. If you cannot receive distant NOAA Weather Radio transmissions which are in the green-shaded coverage zone, your radio or antenna are not performing as they should.

More on WXN69

I have investigated further into my reception at Leland harbor of NOAA Weather Radio WXN69 from Sister Bay, Wisconsin in order to gain a better understanding of the performance of my antenna on the path involved. The first step was discovering the precise location of WXN69. On-line resources were somewhat vague. Various listings provided latitude, longitude, and elevation, but when I checked those locations using GOOGLE EARTH, I could not see any radio tower. I finally resorted to an older technique: I made a few telephone calls. First I called the NOAA Weather office in Green Bay. There I spoke to a couple of guys who were familiar with the transmitter up in Sister Bay, but they didn't have the exact location of it. They gave me the telephone number of "someone who would know." That turned out to be an engineer who maintained the WXN69 transmitter facility. He zeroed me in to the exact location. He gave me the transmitter site with reference to some roads in the area, and, yes, I could see a tower on GOOGLE EARTH at that location! That also gave me the precise latitude and longitude.

The on-line listings mentioned the "elevation" of WXN69 as being 780-feet. This turned out to be the ground elevation above sea level of the base of the tower, not the antenna height above ground. The engineer told me the actual antenna was "about 300-feet up on the tower." He also told me that for a long time they used to run the transmitter at reduced power, but beginning in 2007 they got orders to turn it up to the full authorized power of 1,000-watts. WXN69 is now running a kilowatt of RF at the transmitter. I assume the have a few dB of feed line loss to the antenna, and the antenna probably has some gain. It would be reasonable to assume the effective radiated power is probably at least 2,500-watts. The engineer asked me why I wanted to know all this information, so I explained I was using WXN69 like a signal generator to check my radio and antenna performance. He told me his experience was that WXN69 really "gets out," and that he could "DX it at 90-miles." ("DX" is an old telegrapher's abbreviation for "distance," and it has always been in popular use in radio to mean reception of a distance station.)

Based on my conversation with the engineer, I computed WXN69's location as:

 45° 14' 10.50"N
087°  5' 16.26"W

For Leland Harbor, I used my position as:

 45°  1' 27.09"N
085° 45' 41.81"W

Then I used GOOGLE EARTH to check the path between those two. I discovered that the path crosses over the south shore of North Manitou Island. This screen grab shows how the path just nicks the sand dunes:

Plot of path between WXN69 at Sister Bay, Wisconsin and Leland, Michigan on GOOGLE EARTH photography.
Closer View

The distance is a little over 66-miles. The dunes are several hundred feet high, and very likely have some influence. Radio waves behave somewhat like waves at sea, and when they encounter an obstruction they will tend to be bent by it. You can imagine that radio waves from WXN69 will be bent or refracted slightly, and Leland harbor is not completely shadowed by the island.

Path Vertical Profile

Next I used a specialized application, RadioWORKS, to examine the vertical profile of the path in more detail. Here is a plot of the path produced by entering the location of the two points into RadioWORKS. The program then retrieves geographic data about all the intervening terrain from a geo-database. This first plot shows the path profile if the curvature of the earth is ignored:

Plot of vertical terrain in path between WXN69 and Leland, Michigan.

The sand dunes of North Manitou Island show up as that green blip on the right side at the 54-mile distance. WXN69 is shown at 1,075-feet above sea level, which is roughly 300-feet above the tower base elevation of 775-feet (obtained from the geo-database). My receiver is shown at being 8-feet above lake level, which is about 574-feet. The red oval encloses what is known as the Fresnel zone. This area is significant in determining influences from ground reflections. As you can see, the Wisconsin side of the path from WXN69 nicely clears all the local terrain.

This analysis shows that we have a line-of-sight path--except for that 200-foot high pile of sand--between my boat and WXN69's transmitting antenna, but, again, this ignores the curvature of the earth. RadioWORKS offers to include the curvature of the earth in the plot, but it does it in a way which also includes an allowance for what is known as the "four-thirds Earth" effect. This refers to a tendency for radio waves to propagate differently than visible light waves. Radio waves tend to be refracted somewhat by the atmosphere, and using a radius of the earth which is exaggerated by a factor of 4/3 is often done to approximate this effect. Here is the plot of the path with an allowance for curvature of the earth at the usual 4/3-Earth factor:

Plot of vertical terrain in path between WXN69 and Leland, Michigan with allowance for curvature of the earth with 4/3-radius effect.

This analysis shows that the path is no longer line-of-sight but involves significant over-the-horizon propagation. However, I was still able to receive the weather broadcast. That 1,000-watt transmitter and gain antenna were influential.

Conclusions

Using remote NOAA Weather Radio broadcast stations to test your VHF Marine Band radio is a simple way to determine if your antenna is working properly.

For further discussion on this topic please use the SMALL BOAT ELECTRICAL discussion which has already been started.

Addendum

In June 2011 I was back in Leland again, and made an additional check on my antenna and radio receiver capabilities. WXN69 from Sister Bay was still coming in with a good signal, easily copied, although not at full-quieting strength. I tuned to several other radio channels, and found a new station. WNG684 was broadcasting the Marquette NOAA weather radio program from a location at Smith Lake, near Manistique in Michigan's upper peninsula. WNG684 was coming in stronger than Sister Bay, with a full-quieting signal and rock solid, no fading. Of course, I had to investigate this transmitter, too.

A telephone call to Marquette's NOAA office gave me the antenna height, 370-feet, and some on-line research with GOOGLE EARTH located the tower adjacent to Smith Lake. The transmitter is listed at 300-watts. The path distance is over 78-miles. So WNG684 transmits with an antenna of similar height but with less power and is farther away than WXN69 in Sister Bay, but it was coming in better. Also, predicted coverage for Smith Lake is not as good as for Sister Bay. The estimate of coverage area for Smith Lake generally falls well short of Leland, while Sister Bay's coverage extends all the way across the lake to Leland and beyond. Smith Lake was really getting out on this particular day!

Plot of vertical terrain in path between WNG684 and Leland, Michigan without allowance for curvature of the earth with 4/3-radius effect.

Above is the path vertical profile without allowing for the curvature of the earth. It would be a clear shot across Lake Michigan to Leland from Smith Lake.

Plot of vertical terrain in path between WNG684 and Leland, Michigan with allowance for curvature of the earth with 4/3-radius effect.

Above is the path but with allowance for the curvature of the earth. The path is no longer a line-of-sight path, and Leland is over the horizon from Smith Lake.

Below are two predicted coverage maps. Note that there is some predicted coverage around Leland, but only inland and on certain hilltops far above lake level.

Plot of estimated signal strength verses range for WNG684 in Smith Lake, Michigan. Original source http://www.nws.noaa.gov/ Plot of estimated signal strength verses range for WNG684 in Smith Lake, Michigan. Original source http://www.nws.noaa.gov/

There are several explanations for the excellent signal I received. The reception occurred around 9 a.m. when there could have been a stable layer of cold air above the lake. Thermal stratification of the atmosphere tends to help radio signals refract over the horizon. There is also a chance the signal was being bounced off the tall sand dunes directly South of me. The shoreline about three miles South of Leland is a dune cliff about 300-feet high. It is hard to say if the reception was unusual and due to enhanced conditions, or if WNG684 roars into Leland every morning like it was that June. However, the reception is clearly a case of over-the-horizon coverage and far exceeds the usual line-of-sight predictions based on the radio horizon of each station.

Finding NOAA Weather Broadcast Transmitter Sites

For use with GOOGLE EARTH there is a keyhole mark-up language (KML) file of data about NOAA weather broadcast transmitter locations that can be helpful in finding the various NOAA stations. (From the linked page, use the hyper-link marked "KML" to save the file on your local computer. From the saved file you can import the data to your saved places for Google Earth.)

I found that in some cases the location of the transmitter is not quite exactly shown correctly, but it is generally close enough to be useful in estimating the distance to the station from you receiver for the purpose of assessing the path length.

A Further Example

I give a detailed description of a test of receiver sensitivity using this method in a separate article. The test found five weather stations at distances of up to 80-miles that could be received.


DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Copyright © 2009 by James W. Hebert. Unauthorized reproduction prohibited!

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Last modified: Tuesday, 02-Apr-2013 11:32:17 EST
Author: James W. Hebert
This article first appeared January 19, 2009.