VHF Radio Antenna Location

lcookie

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Mar 14, 2012
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Okay folks,

I know this has been beat to death, but I cannot find anything on the web to answer my question so i am bringing it to the experts. The manual says the MPE radius is 10 ft horizontally away from and 16 ft vertically above a persons location when transmitting. Well my boat is only 16 ft, I do not have a tee top, and my only mounting location is along the gunnel. Now in the book is also says that this range is needed for a 25 watt radio with a 9 db antenna. (200 watts transmission). I have a 25 watt radio with a 3 db antenna. (50 watts transmission). Since my actual broadcast power is 25% of the maximum value can I reduce my MPE radius to 25% (2.5 ft) or am I over simplifying things? Likewise, if the radio is broadcasting at 1 watt the total transmission would be 2 watts. Would the radius decrease to 0.1 ft (1.2 in)? I have searched for an equation that I could use to determine the radius but I have had no success. If my estimation is correct I could get the horizontal separation but the vertical will still be a problem. So could I keep moving the antenna away horizontally to compensate for the vertical deficiency? Lastly, am I correct is assuming the MPE only applies to when I am actively transmitting and not when I am in standby or receiving a transmission?

Thanks in advance
 

Bondo

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Re: VHF Radio Antenna Location

Well my boat is only 16 ft, I do not have a tee top, and my only mounting location is along the gunnel.

Ayuh,... Welcome Aboard lcookie,.... Maybe ya oughta just buy a Hand-held radio,..??

My antenna is mounted forward the windshield, on the side of the cabin,...

What the heck is a MPE, Anyways,..??
 

lcookie

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Re: VHF Radio Antenna Location

Maximum Permissible Exposure Radius. I had a handheld but it was not powerful enough.
 

Pez Vela

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Re: VHF Radio Antenna Location

... am I over simplifying things?

I'm afraid that you are. You're going to have to provide considerably more detail before the MIT graduates can weigh in on your problem. Posting a reference to the manual you are consulting would be a start. At only 5' 9", I too have a vertical deficiency, which I have solved by wearing elevator shoes. The same thing applies to a VHF antenna.
 

Bondo

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Re: VHF Radio Antenna Location

Maximum Permissible Exposure Radius. I had a handheld but it was not powerful enough.

Ayuh,.. Donno,... I been sittin' inside those numbers for years..... am I killin' myself, 'n passengers,..??
 

R Socey

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Re: VHF Radio Antenna Location

I always heard the higher the better - mine will go on top of t-top. If I didn't have one I'd put it on top of the console, but I might be way off?
 

lcookie

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Mar 14, 2012
Messages
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Re: VHF Radio Antenna Location

Found this in a Raymarine Manual

This system has a Maximum Permissible Exposure (MPE) Radius of 1.5
meters (per OET Bulletin 65), assuming the maximum power of the radio
and antennas with a maximum gain of 3dBi. Accounting for the height of an
average adult (2 meters) the minimum height of the antenna above the deck
to meet RF exposure compliance requirements is 3.5 meters.

5 ft it is.
 

bruceb58

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Re: VHF Radio Antenna Location

It only matters when you are transmitting. I can count on one hand how often I have transmitted.
 

Fireman431

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Re: VHF Radio Antenna Location

It only matters when you are transmitting. I can count on one hand how often I have transmitted.

And if you are transmittting that much, you have bigger problems then where your antenna is. Mount it on the gunwale or the side of the console.
 

Outsider

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Re: VHF Radio Antenna Location

What legal 25 watt radio do you have that transmits at even 25 watts, much less 200? :confused:
 

lcookie

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Re: VHF Radio Antenna Location

I have a entry level Cobra radio. The 200 watt is the magnification of the signal through a 9 db antenna as I understand it.
 

bruceb58

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Re: VHF Radio Antenna Location

I have a entry level Cobra radio. The 200 watt is the magnification of the signal through a 9 db antenna as I understand it.
There is no magnification of the signal. The antenna is still transmitting 25W assuming no loss(which there will be some) its just that the antenna pattern is basically shaped so that instead of equal power in all directions it has 9db of gain in the directions you want.

Here is a picture that describes what I am taliking about.
vhf23.jpg

Obviously radiation is wanted in the direction you are wanting to transmit/receive with. Of course, with a higher db antenna, it is more critical to have the antenna exactly vertical.

So basically, you are transmitting 25 Watts...thats it! No ampification is occuring with the antenna.
 

jhebert

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Re: VHF Radio Antenna Location

...basically, you are transmitting 25 Watts...thats it! No ampification is occuring with the antenna.

This is pure nonsense. The author must have no idea how radio communication works. Do not believe him.

A transmitter power of 25-watts into an antenna of 9-dB gain produces a field equivalent to a 200-watt transmitter. If there were no gain (which is another word for amplification) in an antenna, there would be no reason to use antennas high directivity.

Concepts like this are so basic that it is staggering to read people's comments who want to refute the Laws of Physics.
 

NYBo

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Re: VHF Radio Antenna Location

This is pure nonsense. The author must have no idea how radio communication works. Do not believe him.

A transmitter power of 25-watts into an antenna of 9-dB gain produces a field equivalent to a 200-watt transmitter. If there were no gain (which is another word for amplification) in an antenna, there would be no reason to use antennas high directivity.

Concepts like this are so basic that it is staggering to read people's comments who want to refute the Laws of Physics.
And you are misreading what the issue at hand is. The OP is concerned about the potential damage from RF absorption by the human body a few feet from the antenna, not effective radiated power as it relates to range.
 

jhebert

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Re: VHF Radio Antenna Location

And you are misreading what the issue at hand is. The OP is concerned about the potential damage from RF absorption by the human body a few feet from the antenna, not effective radiated power as it relates to range.

More nonsense. The human body does not care how the RF field was created. Do you think the human body knows that the RF field was created by a 25-watt transmitter and an antenna with 9-dB gain instead of a 200-watt transmitter with an antenna of unity gain?

Let me ask you this: when you see the light from a lighthouse, do your eyes say, "Oh, that's just a little 100-watt lamp making that light, and even though it has been focuses into a tight beam by all those lenses in the lamp, it still is no brighter than a 100-watt lamp would be at the same distance."

What an antenna with gain does is just light what a lens does with light: the antenna focuses the radiation into a tigher, more intense beam of energy.
 

foodfisher

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Re: VHF Radio Antenna Location

Are these things safe. IE cancer risk? You guys are starting to cause worry.
 

Outsider

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Re: VHF Radio Antenna Location

This is pure nonsense. The author must have no idea how radio communication works. Do not believe him.

The 'author' is correct in that there is no actual increase in RF. There would, potentially, be an increase in effective radiated power (ERP), which is a standardized theoretical measurement of radio RF energy. Look it up, way too complicated and way outside the measurement ability of any average boater. The 'author' also showed there is an effective radiation 'pattern' of different antennas. You might disagree, but Shakespeare doesn't, and they, too, have some idea how radio communication works.

For the OP, what you're reading is likely a warning required by the FCC, in my opinion along the lines that California requires a cancer warning on just about anything sold there. If you have the room, an 8' antenna (typically 6db) mounted vertically will yield your best performance. If space is cramped, a 3db (usually 3') might be your best option although your effective range will be less due to actual realized gain, radiation pattern, and antenna height. The important thing is to have a quality installation of a quality antenna as high above the water as you can manage, the rest will be up to your radio ... ;)
 

jhebert

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Re: VHF Radio Antenna Location

there is no actual increase in RF.

Wrong again. How can an antenna with gain produce an increase in the strength of a signal at a remote receiver if there is no actual increase in the radio frequency energy? The answer: there is an increase in the radio frequency energy. That is how gain in an antenna produces a benefit. If there were no gain in an antenna in the radio frequency energy being radiated from the antenna, there would be no reason to use an antenna that has gain.
 

bruceb58

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Re: VHF Radio Antenna Location

This is pure nonsense. The author must have no idea how radio communication works. Do not believe him.
Apparently you didn't read what I was saying. I am VERY familiar how communications works. The gain of the antenna is as described in the picture I posted. It is exactly the same as your light example. The radio is transmitting 25W but the beam has 9 dB gain in the direction of the main lobe. The power outside the main lobe is smaller than it would be if you didn't have. The cumulative energy of the antenna transmitted is still 25W. That was my point.

If there were no gain in an antenna in the radio frequency energy being radiated from the antenna, there would be no reason to use an antenna that has gain.
He was talking about the cumulative(average) power radiated from the antenna, in all directions, up, down sideways. The gain of the 9db antenna is the gain in one particular direction referenced to a hypotheical antenna that radiates in all directions equally, which obviously can't physically exist.

I am an EE like you are and have a communications and radar background. Wasn't trying to get this discussion up to this technical a level. Not really sure why it was needed for the OP's understanding.
 
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