Question About Power Draw From Mobile

[UnHumanReactions]

I have a Kenwood TM-281A mobile. The manual says it requires less than 1 amp to RX and at least 14 amps to TX at 65 watts. I thought that seemed like a lot for 65 watts, so I used RapidTables calculator and it turns out 65 watts of DC power at 13.8 volts only uses about 4.8 to 5 amps. So I'm not sure where Kenwood got their 14 amps from, maybe they over estimate the power draw just to be safe? I would think the radio is using about 7-8 amps while transmitting on high power, 5 amps for the 65 watts and 1-2 amps for other radio functions. What do you guys think? Maybe someone can explain it to me better?

 

[SteveSimpkin]

The RF power amplifier used to deliver 65 watts to the antenna is less than 50% efficient. Therefore it requires more than twice the power in as it puts out. This is where most of the input current they estimate (14A) is used while transmitting at full output power. They probably also add a few amps to the estimated current draw just to be on the safe side. The radio probably draws closer to 11-12 amps maximum.

 

[UnHumanReactions]

The RF power amplifier used to deliver 65 watts to the antenna is less than 50% efficient. Therefore it requires more than twice the power in as it puts out. This is where most of the input current they estimate (14A) is used while transmitting at full output power. They probably also add a few amps to the estimated current draw just to be on the safe side. The radio probably draws closer to 11-12 amps maximum.

Ahh, I see. That makes sense.

 

[teufler]

Power = Amps x Volts your actual readings are pretty close to calculated figures. The manual states,

In order to use this transceiver for fixed station operation,
you will need a separate 13.8 V DC power supply (not
included). The recommended current capacity of your
power supply is 14 A or more.

I think what Kenwood is saying, a power supply that is higher that required, produced a potential cleaner power, or it loafs along. Think of it as a car engine. A car engine running a full bore doesn't last as long as a car engine that has excess power available. While you could get by with say a 7 amp power supply, at 65 watts level, you will hear a buzzing sound from the supply or maybe the radio will be slow to climb to full output. The power supply will fail quicker than say a larger power supply. A 65 watt radio, I would just get a 20 amp supply like an Astron 20A . A 12 amp supply would do but the difference in the price, get the 20

 

[SteveSimpkin]

According to the ARRL test report for the Kenwood TM-281A, it draws around 9.55A @13.8V while transmitting at full power. Be sure to use a power supply rated at least 14A as recommended by Kenwood.
https://forums.radioreference.com/a...321629-ft-2900r-vs-tm-281a-2.html#post2464538

 

[FKimble]

Well I just keyed up my TM-281A and the AMP meter on the PS read approximately 9 amps on high and 6 amps on low at 13.8 VDC. Hope this helps.

Frank KK4YTM

 

[RogueSteward]

Often, RF transmitters will cause a temporary surge in current draw at the beginning of the transmission known as a spike and is likely the reason the current requirement is 14 amps. A fuse can blow or the wire can break if not able to handle the power surge even though the nominal current draw may be around 9-10 amps.

 

[k6cpo]

I have a Kenwood TM-281A mobile. The manual says it requires less than 1 amp to RX and at least 14 amps to TX at 65 watts. I thought that seemed like a lot for 65 watts, so I used RapidTables calculator and it turns out 65 watts of DC power at 13.8 volts only uses about 4.8 to 5 amps. So I'm not sure where Kenwood got their 14 amps from, maybe they over estimate the power draw just to be safe? I would think the radio is using about 7-8 amps while transmitting on high power, 5 amps for the 65 watts and 1-2 amps for other radio functions. What do you guys think? Maybe someone can explain it to me better?

You're confusing RF power with DC power. They are two separate ratings used to indicate two different things. RF power is what the radio sends to the antenna—in an ideal world of a perfect antenna match and no feed line loss—and DC power is how much electricity the radio draws when in operation. The confusion comes in because both are measured in Watts.


I think what Kenwood has done is give the minimum capacity, in amps, of a power supply to be used with the radio rather than an actual current draw. The 9 to 9.55A mentioned previously is more realistic. (My 50W RF power Yaesu 2m/440 radio draws 8A DC power on full power transmit.) I think this is a mistake on the part of Kenwood because it can lead to confusion and miscalculation. (My iCom HF draws 12-14A on 100W full power transmit.)

 

[UnHumanReactions]

You're confusing RF power with DC power. They are two separate ratings used to indicate two different things. RF power is what the radio sends to the antenna—in an ideal world of a perfect antenna match and no feed line loss—and DC power is how much electricity the radio draws when in operation. The confusion comes in because both are measured in Watts.


I think what Kenwood has done is give the minimum capacity, in amps, of a power supply to be used with the radio rather than an actual current draw. The 9 to 9.55A mentioned previously is more realistic. (My 50W RF power Yaesu 2m/440 radio draws 8A DC power on full power transmit.) I think this is a mistake on the part of Kenwood because it can lead to confusion and miscalculation. (My iCom HF draws 12-14A on 100W full power transmit.)

I see, thanks for the info.