I was listening to the Win System “tech net” on Friday evening (4/20/2012) while building a J-Pole in the garage. A gentleman came on with a question to the net. The question was: “I have a Yaesu FT-7X00 radio, and when I transmit, the back part of the radio near the antenna connector gets hot – should I be concerned about that?”. Net control’s advice was nothing short of idiotic: “Your antenna is mismatched and RF power is reflecting back into your radio and heating it up. You should cease transmissions at once and repair your antenna.” WOW! A technical ham radio network spreading bullshit across the world! Shame on you net control!
First, every transmitter dissipates energy in heat. FM power amplifiers operate using Class C final stages. Those are typically about 35%-45% efficient. So if the RF power output is set to 10W output the radio will be dissipating about 25W in heat. Let’s assume the mentioned radio is a Yaesu FT7800R. Per the specifications, current draw is as follows: TX=8.5A, RX=0.5A at 13.8V. So lets do the math. If the radio is transmitting at 50W RF, the transmitter will draw 8.5-0.5 = 8A. At 13.8V that’s 110W, which means that 60W must be dissipated in heat! You know where the power amplifier resides in a transmitter? Right next to the antenna connector! Do you think 60W won’t heat up the radio and the antenna connector? Thing again!
Second, the comment about RF power reflecting, heating up the radio and damaging it is plain wrong. Power amplifiers are not resistive – they don’t dissipate much power. Their reverse impedance is NOT 50Ohm. Reflected power from a non-resonant antenna comes into the radio and gets re-reflected back into the antenna where some or most of it will be radiated. The only thing that can damage a radio is excessive voltage which causes component breakdown. The excessive voltage is a result of a inductive (i.e. non resonant antenna). It is important to understand that the power amplifier fails not because of heat dissipation – but due to voltage breakdown.
It’s time hams stop perpetuating false information!