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Author Topic:   VHF Radio Propagation
chopbuster posted 02-02-2008 07:39 PM ET (US)   Profile for chopbuster   Send Email to chopbuster  
For all of you radio heads in here: All marine VHF radio signals are line of sight only, [true or false]?

How further over the horizon, at sea level, will a 4' vs 8' antenna reach with 25W?

Is there a "most efficient" height for a antenna?

What portion of the antenna radiates more efficiently than the remainder?

Bella con23 posted 02-02-2008 08:45 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
There is a rather technical and very informative article in the reference section of Continuous Wave related to the installation and setup of VHF radios in small boats.

The following is an excerpt from that article, and should answer your questions.

"If we want a line of sight path between two boats, their antennas must both be 12.4 feet above the water. If using the typical 8-foot whip antenna, and considering the radiation to come from the center of the whip, this means the base of the antenna must be 8.4-feet above the water. This is a typical mounting height on a small boat with a radar arch or hard top. Such a configuration is about the tallest that can be easily achieved on a small boat. On boats with the antenna mounted directly to the boat's hull, the antenna height will probably be much lower, and the corresponding line of sight range will be shorter. Note that range changes as a function of the squareroot of the antenna height, so to double the range you have to raise the antenna four times higher. If an antenna is located 6-feet above the water, its LOS range will be about 3.5 miles. Thus two boats with antennas of this height will have a range of about 7 miles.

"As shown above, the path loss over 10 miles was not great enough to affect the received signal level, so it follows that over a shorter path the path loss in line of sight range will be even more favorable."

jimh posted 02-02-2008 11:16 PM ET (US)     Profile for jimh  Send Email to jimh     
The radio horizon is slightly longer than the optical horizon, and it varies with frequency. Propagation of signals over the horizon is both possible and likely.

The length of the antenna has no effect on propagation of a radio wave. Once the wave leaves the antenna it propagates the same as any other wave; it does not know anything about the antenna that produced it except for the characteristics of its polarization. Radio waves are considered to be either linearly or circularly polarized. If linear they are generally either horizontal or vertical polarization. If circularly polarized they are either right hand or left hand (or clockwise or counter-clockwise) polarity. VHF Marine radios generally all use linear polarization with a vertical orientation.

The distance to the radio horizon is determined by the height above terrain or the sea.

Height does not affect the efficiency of a VHF Marine Band antenna, except in the sense that the radiation resistance tends to vary slightly with height. Roughly speaking, the portion of the antenna in which the greatest antenna current flows is most significant in determining the strength of the signal produced in a remote receiver.

To improve the range of your VHF Marine radio you should increase the height of the antenna.

In the usual vertical Marine antenna, the actual antenna portion is only in the upper part of the antenna. In many so-called "8-foot" antennas the actual radiator of the antenna is in the upper 6-feet. In many "4-foot" or "5-foot" antennas the actual radiator is in the upper 3-feet.

When you ask about the "efficiency" of the antenna you are not really asking about the actual efficiency of the antenna, most likely, but instead you are using the term "efficiency" in a strange sense to characterize the effective coverage area produced by the antenna. The actual efficiency of the antenna is related to the strength of the electromagnetic wave it creates for a given amount of radio frequency power applied. In your case, you are probably really inquiring about how much signal an antenna will produce at a remote receiver location. This is mainly related to its height.

An antenna should be mounted as high as possible and as much in the clear as possible. An antenna which is mounted in such a way that it is surrounded by other conductive objects will not work as well as an antenna mounted in the clear.

AZdave posted 02-03-2008 01:06 AM ET (US)     Profile for AZdave  Send Email to AZdave     
The good news is that you are likely to be communicating with a very tall antenna in an emergency situation. I think the Coast Guard station on the west end of Bogue Bank, NC has a tower between fifty and one hundred feet high.
jimh posted 02-03-2008 06:49 AM ET (US)     Profile for jimh  Send Email to jimh     
Antennas for U. S. Coast Guard shore stations are generally favorably sited to have excellent coverage toward the sea or coastal area they are intended to cover. In addition, the USCG radio system often has multiple remote receive and transmit sites which are linked into a single radio system. This extends the coverage area of a particular station and the radio watch it keeps.
jimh posted 02-03-2008 07:00 AM ET (US)     Profile for jimh  Send Email to jimh     
If one assumes that a so-called 8-foot antenna has slightly more gain than a so-called 4-foot antenna (but this is not always the case in actual practice), then if the two antennas are located at the same height, the 8-foot antenna will have a slightly longer range. This is due to the gain of the antenna.

All antenna gain occurs by concentration of the radiation in a particular direction and elimination or reduction of radiation in other directions. To achieve this gain in practice, the favorable lobe of the radiation from an antenna with gain must be pointed in the direction of the receive location.

Anytime the favorable lobe of a high-gain antenna is not pointed directly at the receive location, the high-gain antenna will produce lower signals at the receive location. The higher the gain of a high-gain antenna becomes, the narrower the lobe of the antenna pattern becomes, and the more precisely it has to be oriented in the favored direction.

In a small boat there is generally a great deal of motion, so antennas with a high-gain, and thus a small, concentrated main lobe, may not be as effective as an antenna with lower gain but a broader pattern of radiation.

chopbuster posted 02-04-2008 12:22 PM ET (US)     Profile for chopbuster  Send Email to chopbuster     
Thanks for all the valuable on topic feedback guys, I'm
in the planning stages of a VHF install on a 2007 150 Sport and due to the low console/gunnel design, I need to
have the optimal configuration for the antenna.
swist posted 02-05-2008 09:23 AM ET (US)     Profile for swist  Send Email to swist     
I think the definition of "line of sight" is a bit complicated for VHF radio propagation situations. As jim suggested, there is some bending going on, but there must be other factors at play also....

I boat in midcoast Maine, an area with a lot of islands and fingers of land which tend to be long, narrow, and elevated - by hundreds of feet in some cases. There is such an island between my home dock and Boothbay Harbor - a real boating mecca in the Summer. I can hear (and be heard) easily on my VHF over a distance of only 3 miles, with the above-mentioned 150' high island in the way. (My antenna is a one of those 4 ft shorty jobs mounted to the side of the Montauk console). Is the signal going around the ends of the island? That's a big detour. And the direct "line of sight" path would have to bend up and then down by a considerable amount for that path to work.

I know little about radio wave propagation but am guessing that if you are close enough to the target and have enough power (25W over only 4 miles), then the "line of sight" rules don't come into play.

jimh posted 02-05-2008 10:46 AM ET (US)     Profile for jimh  Send Email to jimh     
When radio waves propagate they can be bent sightly by some atmospheric conditions, but they mainly travel in a straight line. Radio waves can be reflected by other objects.

On short paths there is an extreme amount of reserve gain in the circuit if 25-watts of transmitter power is used, so it is possible to communicate over short paths which have very high path loss. Such a path might exist due to reflections or other unanticipated effects.

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