# How to understand HF transmitter power consumption

#### PickyBiker

Joined Aug 18, 2015
109
My son bought a new mobile HF ham radio that has a maximum transmit power rating of 100 watts. We are trying to understand why the TX input power can be as high as 23 amps at 13.8VDC.

100w / 13.8v is only 7.25 amps. If you add an amp that it draws in RCV mode, you are still looking at about 8.25 amps.
My theory is that the radio's power rating is based on something like 100w audio input power to the final RF amplifier, not the total power output. That would mean the power consumed by the rf final stage would be added to the audio power.

Can someone tell us how exactly the TX power is calculated and why the max current draw is about 3x what would seem to be required?

#### sagor

Joined Mar 10, 2019
927
RF amplifiers are not 100% efficient. In fact, some are only around 70-75%, others as low as 50%. Worst case, for 100W out, you may be consuming 150W to close to 200W just for the final RF stages. That gives a current consumption in the 12A to 15A range alone, let alone powering other circuits in the radio.
The recommended 23A power supply is to ensure there is enough capacity for "peak" power usage to prevent voltage sag. Also, some radios have an accessory jack on the back that can supply up to 1A of power for accessories. Thus, recommended power supply to the radio is on the higher side for current recommendations.0
If you set the transmitter power down to a minimum level (say 5W), and measure the current draw, you will likely see 5A to 8A of current just to power the transmitter circuits without any major power out...

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#### PickyBiker

Joined Aug 18, 2015
109
The 23 amps is not the recommended power supply, it is the actual current draw at 100 watts TX. Even if it is 200w, it is still only 15 amps. There are still some amps to account for.

#### sagor

Joined Mar 10, 2019
927
The 23 amps is not the recommended power supply, it is the actual current draw at 100 watts TX. Even if it is 200w, it is still only 15 amps. There are still some amps to account for.
Re-read my last sentence in post #2. What is the idle current when transmitting wiith no power out? Add that to your numbers. Also, at 23A, is the power supply still 13.8V? I doubt it, as at that current you get voltage drops in the power cable, and the actual RF transistors see less voltage, perhaps even just 12V. Thus they have to draw more current to produce 100W output. The key is, what is the voltage at the power plug at the radio, not what the power supply meter says. You can easily drop a volt or two across the power cable between power supply and radio. Add to that, does the power supply itself remain at 13.8V? If not, you compound the voltage drops, and thus the radio needs more current.

Example:
Assuming a 14G stranded wire power cable, 10ft long between power supply and radio. Resistance counts both ways, red (positive) wires and black (ground/return) wire, you get a total resistance of about 0.054 ohms. At 23A, you get a voltage drop of almost 1.25V by the time it reaches the radio.

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#### PickyBiker

Joined Aug 18, 2015
109
I have asked my son to make the suggested measurements under minimum TX power. He will do that when he completes the new installation in his pickup truck. At home he had a 40A PS. In the truck he is planning on 10 gauge wire for power. I'll let you know what he finds.

#### drjohsmith

Joined Dec 13, 2021
852
how is the 100w stated ?
is it power into the antenna ?

can you post link to the device for us to look at ?

#### PickyBiker

Joined Aug 18, 2015
109
how is the 100w stated ?
is it power into the antenna ?

can you post link to the device for us to look at ?
Here you go.
Yaesu FT-891

#### drjohsmith

Joined Dec 13, 2021
852
Good bit of loose specs there..

13.8V +- 15 percent, is saying, its connected to a Lead acid battery.
TYPICAL 28 Amps Tx, 2 Amps RX.
Assuming thats at the -15 % , thats 12V,

Typical, thats saying , it needs a big cable to your supply,
it could be 5 Amps, it could be 40 amps, What does typical mean ?

Look at the other way,
at 28 Amps, at 13.8 + 15% is over 400 watts
if its putting 100 watts into the antenna, then its disipating 300 plus watts of heat,
A nice room warmer,
does it get hot ?

#### PickyBiker

Joined Aug 18, 2015
109
Clearly, it is generating some heat, but it reads 23 amps, not 28. These things are not as efficient as I thought.

#### vu2nan

Joined Sep 11, 2014
348
The 'rule of thumb' for current drawn by a transceiver, working off a 12 V DC supply, is 1A per 5W RF output.

There is no specific source for this information.

It has been arrived at from data furnished by reputed brands in their user manuals.

Some brands specify current drawn at 100W RF output and others at 200W DC input, though the rated RF output is 100W.

Thus, it may be inferred that the efficiency of the RF power amplifiers in these transceivers is only 50%.

The rule of thumb was found to be applicable even to transceivers with power outputs of 5W, 10W, 15W and 50W.

Nandu.

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#### sagor

Joined Mar 10, 2019
927
Again, we don't really know what voltage is being supplied to the radio "under load", at the radio itself. Measure the voltage at the radio power connector input under full 100W output. 12V at 23A is the same power as 13.8V at 20A. So, if your power supply sags under load, or you get voltage drop through the power leads, that may explain the extra current needed.
Again, a reminder that RF stages are about 60%-70% efficient, some are as bad as 50%-55%. Also, the rest of the radio circuits, including displays, processors, relays, filters, etc all use some power. Notice the spec sheet shows the RX using 2A just with receive. In TX the extra circuits may use 2A to 5A in the background, even before you get to the final RF stages and their power usage. Also, there are RF driver transistors that may be rated 5W to 20W driving the finals, and those may suffer the same poor efficiency. Those would use up a few Amps of current as well. If you put the radio in TX mode with no modulation (CW but no keying) and measure the current, that will show the overall current load of the radio before it even tries to produce 100W of RF power.
The spec sheet of 13,8V +/- 15% just means the radio will operate in that voltage range independent of what current is needed. The 23A TX rating is the current rating when the voltage is actually 13.8V at the radio. So, it sounds like the radio is operating as it should, using the power input as designed.

#### vu3jpd

Joined Apr 27, 2010
1
My question is similar type and case is similer but in different way.i have radio transmitting 300 w full and 150 watt mid range.it operate on 24 volt.i have 24 volt SMPS.i and not sure how much current it can supply.its 5 to 10 amp.my problem/question is i have to option 150 watt or 300 watt.i operate on 150 w.but i doubt weather it give full 150 watt or less? Actually I want to operate on 25 or 50 watt but no option .so is it safer to operate on 24 volt 5-10 amp power supply for 150 watt rf output? If yes then Second question , i understand that with this power supply it produce 50 to 100 watt .is it right.can anyone please throw some light?

#### drjohsmith

Joined Dec 13, 2021
852
My question is similar type and case is similer but in different way.i have radio transmitting 300 w full and 150 watt mid range.it operate on 24 volt.i have 24 volt SMPS.i and not sure how much current it can supply.its 5 to 10 amp.my problem/question is i have to option 150 watt or 300 watt.i operate on 150 w.but i doubt weather it give full 150 watt or less? Actually I want to operate on 25 or 50 watt but no option .so is it safer to operate on 24 volt 5-10 amp power supply for 150 watt rf output? If yes then Second question , i understand that with this power supply it produce 50 to 100 watt .is it right.can anyone please throw some light?
I'd suggest
A you start a new post
B don't go changing a PSU on a RF circuit unless you understand what your doing.