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Old 03-19-2014, 11:42 PM
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Question Why UHF @ 4 Watts and VHF @ 5 Watts?

Why are UHF transceivers generally, limited to 4 Watts while VHF transceivers are able to use 5 Watts? If this is due to a mandate by the FCC does anyone now the reasoning behind it?
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Old 03-20-2014, 12:14 AM
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No FCC mandate to it, just a matter of the abilities of the devices used for power output.
- 'Doc
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Old 03-20-2014, 12:51 AM
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The short answer - SAR. It has to do with the Specific Absorption Rate of the RF in the human body. RF causes heating of the tissues in the human body. The higher the frequency, the faster the tissue absorbs the RF, the faster the tissue heats. That's how microwave ovens work, and also why it it NOT A GOOD IDEA to stand in front of a microwave antenna, unless you want to get cooked!! So, manufacturers limit the power output at UHF to keep from frying your brain over the long term (aka liability).

Just go online and search for RF SAR for more information.
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Old 03-20-2014, 2:04 AM
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Actually some public safety VHF Icoms are 6 watts. Splitting hairs of course, but more power equals less transmit time on handheld radios. My old Icom IC2GAT would put out over 7 watts with a 13.2 volt battery pack. Too bad the "new" standard is 7.2 volts or two Lithium ions.

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Old 03-20-2014, 3:43 AM
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Quote:
Originally Posted by KJ6ZNS View Post
The short answer - SAR. It has to do with the Specific Absorption Rate of the RF in the human body. RF causes heating of the tissues in the human body. The higher the frequency, the faster the tissue absorbs the RF, the faster the tissue heats. That's how microwave ovens work, and also why it it NOT A GOOD IDEA to stand in front of a microwave antenna, unless you want to get cooked!! So, manufacturers limit the power output at UHF to keep from frying your brain over the long term (aka liability).

Just go online and search for RF SAR for more information.
Sources?

That can't be the reason UHF wattage is always less than VHF wattage on ham radios. I imagine for some reason it costs more money to make a radio produce more wattage in the UHF range.
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Old 03-20-2014, 8:46 AM
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The reason used to be the availability of of RF devices and current consumption and nothing to do with SAR.

When I started aligning and repairing radios almost 40yrs ago there was no SAR and nobody had a clue on RF safety. Going back in time it was a little less efficient to make power at UHF than VHF and Ni-Cad batteries were the best thing available to power hand helds.

Now we have more efficient PAs and higher capacity batteries so its possible to produce more power on UHF but SAR may have entered the mix late in the game.
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Old 03-20-2014, 8:52 AM
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The output of the PAs in the radios will be less efficient on the higher frequencies requiring more current drain. So the same output that gives you a 5w rating on144 is only going to give you 4w on 440 and 2w on 902. And it used to be that the transistors in the PA cost more for UHF, hence the reason UHF radios used to be more expensive than VHF radios. I don't think that's much of an issue any more for the reason PRCGUY explains below.

Quote:
Originally Posted by prcguy View Post
The reason used to be the availability of of RF devices and current consumption and nothing to do with SAR.

When I started aligning and repairing radios almost 40yrs ago there was no SAR and nobody had a clue on RF safety. Going back in time it was a little less efficient to make power at UHF than VHF and Ni-Cad batteries were the best thing available to power hand helds.

Now we have more efficient PAs and higher capacity batteries so its possible to produce more power on UHF but SAR may have entered the mix late in the game.
prcguy
THIS. ^^^


And btw, I have a several 5 watt UHF portables. I have not turned into radioactive mush yet.
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Old 03-20-2014, 9:11 AM
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Don't overlook the fact that the vast majority of (serious) UHF systems utilize repeaters, where far many fewer on VHF systems do. Under this scheme UHF needs to hit the closest tower/repeater to boost the TX power to rebroadcast it, where (non-repeatered) VHF systems fully depend on straight/direct radio-to-radio transmissions.
The added benefit is of course TXing at the lower power extends battery life.
This is a broad/general statement; not ALL UHF systems are repeatered, nor ALL VHF systems non-repeatered. It's the nature of the beast with RF.
I believe that this is the quick, easy answer.
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Old 03-20-2014, 9:53 AM
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The higher you go in freq's, the less efficient things get. So like was mentioned above, 800/900 is 2.5, or 3 watts, depending on the radio.
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Old 03-20-2014, 5:10 PM
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It takes a specific amount of energy to generate an electromagnetic wave. The more cycles (Hertz) per second, the more energy consumed per second.
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