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Old 04 January 2012, 16:55   #21
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SPR, sorry I meant 71's not 8's
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Old 04 January 2012, 17:01   #22
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I'm being serious. Is it serious enough to move it. VHF doesn't get used often.
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Old 04 January 2012, 17:06   #23
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Quote:
Originally Posted by maxhar
I'm being serious. Is it serious enough to move it. VHF doesn't get used often.
I was too! but only device I know needs mounts at least 6 foot away is radar!

The amount of time you use your radio, I think not an issue...

s.
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Old 04 January 2012, 17:10   #24
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I'm being serious. Is it serious enough to move it. VHF doesn't get used often.
Google VHF health issue or harmful and see what comes up, nothing of any relevance to VHF radios.
I think how close the VHF aerial is to your head is would be the least of my worries these days....
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Old 04 January 2012, 17:13   #25
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Well Garmin seem to think the aerial should be mounted at least 1.5m from any persons in their instructions.

I've always been under the impression that it should be away from people when transmitting. 25watts at whatever radio frequency is quite a lot of power.

Quote:
…..”All the effects appear to be worse as the frequency is raised. A serious side effect of exposure at frequencies above 150 Mhz, (that is VHF and above), is that body parts can become resonant absorbers, which raises the amount of energy taken in, and reduces the threshold exposure level at which effects are seen.”
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Old 04 January 2012, 17:22   #26
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I'm being serious. Is it serious enough to move it. VHF doesn't get used often.
Then I wouldn't worry about it. If the aerial were continuous duty it would be of greater concern.

There are RF limits and a 25W aerial 1m from your head definitely exceeds what the HAM radio community would do for instance. Here is a table for the US occupational health standards.
Section

Marine VHF is in the 156-174Mhz range so the middle line on the table.

The problem is knowing the radiation pattern of your aerial. For simplicity sake I am going to call it 1/2 of a sphere. It does not transmit up or down for instance. A 1m diameter sphere has a surface area of about 12.5m or 125000 cm2. 1/2 of that is 62500 cm2.

25w maximum power = 25000 mW
25000/62500 = 0.4 mW/cm2

About 2x what you'd be allowed to exposure your neighbor to if you mounted the aerial on an apartment building. But a little less than half of a "controlled" occupational exposure. This depends significantly on the radiation pattern of your aerial.

Since your duty cycle is small I wouldn't worry about it. With any kind of RF energy, use the low power setting if you can communicate with that and only use 25W if required. There's no RF issue with receiving full time.
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Old 04 January 2012, 18:16   #27
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There are RF limits and a 25W aerial 1m from your head definitely exceeds what the HAM radio community would do for instance.
Have you met "ham radio guys"? I'm not sure if they start out like that or its the exposure...
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Old 04 January 2012, 18:47   #28
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Who told you this as it sounds like bollocks to me.
A radar is transmitting all the time it is turned on, even IF a VHF did cause a health hazard it would only do this when it is transmitting.
OK, that's because you don't know the effect of radiation on the body.

It is because the antenna converts currents which comes from the radio device into radio waves. The increased signal strength of the handheld radio antennas is achieved by concentrating the signal into various narrow discs and therefore, the signals are much more
focused. So it is received better by the receiver. At the same time the signals are moving to all directions (horizontally => line-of-sight system).

In medicine, Electromagnetic radiation and health are discussed quite widely and something called "Specific absorption rate" (Watt/kg) is used to measure it. So it measure the rate of enegry absorbed by the body or even tissue. That was a hot topic in 1990s when mobile phones first came!

Therefore, most radio manufacturers follow the guidance of US military/IEEE which has set out some limits on how much especially the handheld radios can be powered. Potentially, one reason behind not having a 25W handheld radio other than the limited bettery life could be this.

Whether it harms your brain or not, most researchers have found little evidence, but the limits have not yet been changed!
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Old 04 January 2012, 18:48   #29
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You need a license to transmit at any power level up to 25W. Anything over 25W is illegal, whether you have a license or not.
We have 50W or 100W+ station radios. I meant the station radio license not the VHF user license.
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Old 04 January 2012, 19:29   #30
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Originally Posted by SPR

yes - and that mobile phone you use, x-rays and microwaves are not helping much either....

not forgetting the Wi-Fi and Cordless phone waves....

hence I wear a tin foil hat!

S.
I can vouch for the power of microwaves at least, buried a good friend of mine last week after he decided to open up his oven and have tinker with it
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