Thermal rise in an RF coil

Discussion in 'General Electronics Chat' started by grizzly1, Sep 11, 2012.

1. grizzly1 Thread Starter New Member

Jun 24, 2008
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After much searching, I am wondering if anyone out there has come up with an accurate way to predict the temperature rise in an air core inductor being used at high frequency (100 - 500 MHz) in shunt with a high power transmission line (100 watts or more) as a bias coil carying 100 mA of DC current. Chalenging problem -- Detail on the inductor would be a wire diameter of .0031, ID of .085, about 16 turns which would be around 513 nH. So how do I predict how hot will it get @25 C ambient in air ? Stumps me !

2. Papabravo Expert

Feb 24, 2006
11,779
2,495
Twinkle Twinkle little star,
Power's equal I-squared R

There are two aspects to the problem.
1. Whatever resistance the coil has will cause power in watts equal I-squared R to raise the temperature of the wire. The ambient surroundings are a pretty good heat sink.
2. It is possible that something in the vicinity of the coil will respond to the field or the frequency of the magnetic field in the vicinity of the air coil and this is more difficult to quantify. If it resonates it will get hot.

Last edited: Sep 12, 2012
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3. Dodgydave AAC Fanatic!

Jun 22, 2012
7,652
1,271
The only way to measure temperature is with a "thermometer" !

Work out the reactance of the coil at the lowest frequency, this will give the lowest resistance.

so XL= 2*PI *F* L

6.284*100mhz*513nH= 322 Ohms , W=I*I*R= 0.1* 0.1*322= 3.22Watts

so at 500mhz this will be 3.22W *5= 16.1Watts