Transmit power

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kyparamedic

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Ok, I'm sure this is a dumb question but I'm still a radio novice and can't seem to find an answer to this. Why does the transmit power seem to decrease the higher up the spectrum you go? For example, most VHF portables have a max output of 5W, UHF 4W, and 800 3W. My understanding is that the higher the frequency, the less penetrating power it has and the more subject it is to interference and other nasty effects. So wouldn't higher frequencies need more power? It just seems contrary to the little i've read and understand about radio propagation.

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nd5y

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It is mainly due to RF exposure guidelines. In the past it was because RF circuits are less efficient and have more loss at higher frequencies.
 

chrismol1

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I would think its sorta like when the higher the frequencies the more that it fries ur brain on near the microwave bands. so the exposure is wanted to be minimal, unfortunatly the power doesnt seem to go with the certian types of freq, I guess this would only apply to handhelds as they are closer, but its also the same with VHF and UHF. why should a VHF mobile at 110watts over a 450Mhz 45 watt as the UHF are line of sight, but its the applications that matter, in a residential city with building penetration
 

fineshot1

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nd5y said:
It is mainly due to RF exposure guidelines. In the past it was because RF circuits are less efficient and have more loss at higher frequencies.
Not exactly folks. The higher in frequency you go the more milliamp hour energy it takes to make a watt of power. The batteries in portables can only be a certain small size so in a lot of cases the reduction of power output the higher in frequency you go is a compramise in order to keep things to a reasonable level, otherwise if you designed a portable on 800 or 900 to be able to have power out at 5 watts the battery would last you a much shorter time and you would have some very unhappy customers.
 

kb2vxa

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Cummon guys, let's get serious. It is all in the efficiency of RF amplifiers, the higher in frequency you go the lower the efficiency so RF output power gets lower as DC input power remains the same. Due to losses the components are "derated" meaning lowering input power to remain within heat dissipation parameters. Put them together and overall transmitter power drops drastically so to maintain output more and heftier amplifying stages are needed, not to mention a beefier power supply. In a compact device this is impractical so output is severely limited while power on a "base" transmitter is theoretically unlimited.
 
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kyparamedic

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Ok, so it's not that the radios operating at higher frequencies wouldn't benefit from transmitting at a higher power, it's more that it's not as technically feasible, correct? Would an 800 MHz portable operating at 5W as opposed to 3W make much of a difference? Does this have anything to do with the reason you need more sites on an 800 MHz system to maintain the coverage you have on a VHF system? I know there are other reasons VHF tends to have better range, just wondering if the lower operating power has much, if anything to do with it.

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chrismol1

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Mostly the wavelength or the two different bands makes the difference of range, sorta like, which one is the better? The jump that goes 2 feet or the jump that goes 4 feet?
 
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