VHF Radio Antenna Location

[jhebert]

Re: VHF Radio Antenna Location

In the context of the radiation of a VHF Marine Band vertical boat antenna being harmful to humans, the human body does not absorb all the radiation from the antenna. The body only interacts with the radiation from the antenna that falls onto the body, and this is determined by the position of the antenna and the body. If I stand a few feet away from an antenna and I am standing in the path of the main lobe or beam of the antenna--and this is the typical situation on a small boat where people mount antennas to the gunwale--I will get the same amount of radiation from the antenna if it is a 9-dB gain antenna fed with 25-watts as I would if the antenna were a unity gain antenna fed with 200-watts. That is my point.

The best way to avoid being in the beam of the antenna is to raise the antenna above your head. Most vertical antennas have very little radiation in an axial direction. This is also a good idea to prevent the RF field from causing interference to other devices on the boat.

As the original poster asked, of course there is no concern about proximity to the antenna when receiving. The concern is only when transmitting.

The statements about RF exposure are now probably routinely included in the instruction manuals to satisfy some regulatory requirement or to give some wiggle room for lawyers.

 

[Outsider]

Re: VHF Radio Antenna Location

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?

RF energy is measured in watts, how does any antenna 'produce' more wattage? It's a wire ...

 

[Bondo]

Re: VHF Radio Antenna Location

Apparently you didn't read what I was saying.
Wasn't trying to get this discussion up to this technical a level.

Ayuh,.... Is this an Oil thread,..??




or just oily,..??

 

[NYBo]

Re: VHF Radio Antenna Location

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?

RF energy is measured in watts, how does any antenna 'produce' more wattage? It's a wire ...

Perhaps Mr. Herbert has stumbled onto the answer to all our energy woes: Feed 25 watts into a specially-constructed wire and get 200 watts out.

 

[Pez Vela]

Re: VHF Radio Antenna Location

The statements about RF exposure are now probably routinely included in the instruction manuals to satisfy some regulatory requirement or to give some wiggle room for lawyers.

That is probably the most inarguable point of the entire discussion. Hardly anyone reads that stuff, and fewer still are able to comprehend it. Here's what Raymarine says in one of its VHF radio manuals:

"Antenna mounting and EME exposure

Ensure that the antenna is connected to the radio before transmitting.
Raymarine declares a Maximum Permissible Exposure (MPE) radius of 1.5 meters (per OET Bulletin 65) for this system, assuming 25 watts output to an omnidirectional antenna of 3dBi gain or less.
For watercraft with suitable structures, the antenna base must be at least 3.5 meters above the main deck to meet the MPE for persons up to 2 meters tall. For watercraft without such structures, the antenna must be mounted so that its base is a minimum of 1.5 meters vertically from the heads of all persons.

Do not transmit when anyone is within the MPE radius of the antenna, unless they are shielded from the antenna field by a grounded metallic barrier."

Just to be extra safe, I'm going out to the kitchen and fashion myself a tinfoil hat.

 

[jhebert]

Re: VHF Radio Antenna Location

Perhaps Mr. Herbert has stumbled onto the answer to all our energy woes: Feed 25 watts into a specially-constructed wire and get 200 watts out.

Hi--You seem to be qualified to hold a cabinet position in the Obama Administration in the Department of Energy. You ought to apply.

Please spell my last name correctly. Thank you.

The concern in RF exposure is the intensity of the radiation, the frequency, and the time of exposure.

If you need help understanding how directional antennas increase the radiated power present in their main lobe, I suggest to read Frederick Terman, RADIO ENGINEERING. Mr. Terman's fine work was a standard text in electrical engineering for decades. I think it came out in the 1930's. Not much has changed in terms of the Laws of Physics, so it is still a good explanation of how antennas work.

For your assistance:
http://en.wikipedia.org/wiki/Frederick_Terman

http://www.amazon.com/Electronic-en...=sr_1_1?s=books&ie=UTF8&qid=1332027734&sr=1-1

 

[jhebert]

Re: VHF Radio Antenna Location

...RF energy is measured in watts, how does any antenna 'produce' more wattage? It's a wire...

The antenna concentrates the power applied to it into a more intense beam. It is exactly as I explained in the analogy to the light house, where light from a small lamp is focused by optical lenses into a very intense and narrow beam. Antenna structures perform the same sort of focusing of radio energy. It is actually not a very difficult concept to grasp. Have you ever seen the light from a lighthouse at a long distance when your eye is in the main beam of the light?

 

[NYBo]

Re: VHF Radio Antenna Location

The antenna concentrates the power applied to it into a more intense beam.

At a range of a few feet, the difference in beam size between a 3dB and a 9 dB antenna is one that makes little difference, Mr Hebert, which is what the OP's question was about.

Hi--You seem to be qualified to hold a cabinet position in the Obama Administration in the Department of Energy. You ought to apply.

You're too kind. And unable to detect a little ribbing, apparently.

 

[jhebert]

Re: VHF Radio Antenna Location

At a range of a few feet, the difference in beam size between a 3dB and a 9 dB antenna is one that makes little difference...which is what the OP's question was about.

The beam size is proportional at any distance. In the main lobe the field is always stronger in the higher-gain antenna than in the lower gain antenna--that is the definition of antenna gain.

The fellow originating the discussion seems to be more informed about RF fields than most of the people who responded. I don't think he got very good advice, which is why I joined the discussion. I don't really care about the usual radio fallacies that are passed around among boaters, but when boaters start sounding off on questions of RF exposure and are talking nonsense, I feel like this ought to be pointed out.

Yes, I sensed that you were trying to ridicule my answer. Please accept a little ribbing in return.

 

[jhebert]

Re: VHF Radio Antenna Location

As long as we are going on with the topic, I found the advice quoted from the manual of some radio regarding vertical separation distance compared to horizontal separation distance to make no sense. If you move a vertically polarized antenna that concentrates is radiation at the horizon to an elevated mounting over your head, you should be putting yourself into a portion of the pattern with a null in the radiation. If you put the antenna a few feet away from you horizontally at your same height, you will be in the main lobe of the radiation. That the manufacturer would call for more vertical separation than horizontal separation makes no sense to me.

At my workplace we have a 5-Megawatt UHF transmitter operating about 900-feet above us. If we were located 900-feet away from it horizontally and in its main beam, we'd all be brain dead in a few years from the exposure to RF.

My VHF Marine Band fixed-mount 25-watt radio just cautions that the distance to the antenna must be at least two feet. It does not specify vertical or horizontal.

I have investigated this topic further and calculated a few RF radiation densities for typical VHF Marine Band installations on small boats, and I compare them to the recommended exposure guidelines from the FCC. See my follow-on article at

http://continuouswave.com/ubb/Forum6/HTML/003120.html