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UHF Portables: Why Only 4 Watts Instead Of 5?

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#1
How come some UHF portables only give you 4 watts of transmit power instead of the full 5 watts? Sometimes you'll have a radio model that comes in VHF and UHF versions. The VHF version will do 5 watts, but then they'll often make the UHF one only 4 watts.

Why?
 
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#2
Not going to be much difference between 4 and 5 watts :roll:
How come some UHF portables only give you 4 watts of transmit power instead of the full 5 watts? Sometimes you'll have a radio model that comes in VHF and UHF versions. The VHF version will do 5 watts, but then they'll often make the UHF one only 4 watts.

Why?
 

SteveC0625

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#3
How come some UHF portables only give you 4 watts of transmit power instead of the full 5 watts? Sometimes you'll have a radio model that comes in VHF and UHF versions. The VHF version will do 5 watts, but then they'll often make the UHF one only 4 watts.

Why?
Not really kosher to start a second thread on the same topic in another sub-forum, as in it's against the rules.

http://forums.radioreference.com/bu...why-only-4-watts-instead-5-a.html#post2646003

To answer your question, due to the higher frequency of the radio signals, and the limitations of battery capacity, UHF portables have been 4 watts for a long, long time. Remember that they use the same batteries and physical case size as their lower frequency VHF sisters. The only way to get the same battery life at both frequency ranges is to limit UHF to 4 watts.
 
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#4
That was an accidental duplicate post, but I thought it was in the same forum?

So higher frequencies use more battery current for the same amount of transmit power as lower frequencies? There are some UHF portables that do give you the full 5 watts, just not all of 'em.
 

GTR8000

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#5
There are some UHF portables that do give you the full 5 watts, just not all of 'em.
You say "the full 5 watts" as if it's some sort of gold standard. It's not, it's just an arbitrary number that some of the more popular portable radios were capped at because of battery limitations, as Steve pointed out.

Generally speaking, the lower the frequency the higher the power output. However,that is not always the case.

For example, an APX 8000 puts out a maximum of 6.3 watts on VHF, 5.3 watts on UHF, 2.65 watts on 700, and 3.3 watts on 800. In low power mode, it's capable of going as low as 1 watt on all bands.

Don't get too wrapped up in numbers. There is a lot more to the effective power of radios than raw wattage, not the least of which is the antenna system.
 
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#6
One really good reason is to meet FCC certification for a handheld radio the transmit power must not exceed:
VHF 6 watts, UHF 4 watts

The limits are part of RF Exposure Limits.
 
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#7
One really good reason is to meet FCC certification for a handheld radio the transmit power must not exceed:
VHF 6 watts, UHF 4 watts

The limits are part of RF Exposure Limits.
I've seen this mentioned several places, but I've never seen it in Part 90, Part 2 or any of the other places I've looked. Can you provide a source? No doubting you, just looking for a source I can use for work related stuff. "Some guy on the internet said it" doesn't work as justification for work related decision making.
 
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#8
I've seen this mentioned several places, but I've never seen it in Part 90, Part 2 or any of the other places I've looked. Can you provide a source? No doubting you, just looking for a source I can use for work related stuff. "Some guy on the internet said it" doesn't work as justification for work related decision making.


It might not be in the regulations. The 4 & 5 watt limitations have to do with SAR, but I can not remember what it is, or where the SAR limits are defined, possibly the FDA(USA)... Not sure.
 
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#12
OK, that's the stuff I've read before and I'm failing to find the part where it says what you guys are saying.

There are a lot of variables involved, and I'm just not seeing the VHF = 6 watts, UHF = 4 watts and 800 = 3 watts thing that people talk about.

I'm open to someone educating me on this.
 

gewecke

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#13
How come some UHF portables only give you 4 watts of transmit power instead of the full 5 watts? Sometimes you'll have a radio model that comes in VHF and UHF versions. The VHF version will do 5 watts, but then they'll often make the UHF one only 4 watts.

Why?
I have a few ham radios which transmit 5 watts on Uhf, and some rated for only 4 watts out and even 2 watts. There's really no gain between 4-5 watts, but there is quicker battery drain. Rf exposure limits are supposed to dictate the output, but make little difference at low outputs. Take your pick. :) 73, n9zas
 
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#14
I've been working on commercial 2-way radios since the 1970s and back then there were no exposure limits to consider. Maximum power in a hand held radio was limited partially by transistors and Ni-Cad batteries at the time and 5w was max for VHF and 4w was it for UHF.

Time marches on, batteries are much better as are transistors but now we have FCC and OSHA RF exposure limits in effect. The old levels around 5w on VHF and 4w on UHF into the typical rubber antenna and held against your face happens to be right about at the 5mw/cm2 6min FCC exposure limit for occupational/controlled exposure.
prcguy
 
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#15
The old levels around 5w on VHF and 4w on UHF into the typical rubber antenna and held against your face happens to be right about at the 5mw/cm2 6min FCC exposure limit for occupational/controlled exposure.
prcguy
Thanks, that makes sense. Still, variables and such. CCR's with 8 or more watts, etc.
 
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#17
One thing to remember is that in a perfect world into a dummy load the radio may do 4,5,6 watts depending on the band, directly coupled with no cable loss. But in reality with the rubber duck threaded on a majority of that is dumped as heat back into the radio because of the inefficiencies of the antenna design, especially lower frequencies. Some radios radiate better than others.

A lot of newer HT's are advertised as 6 Watts VHF and 5 watts UHF.
 
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