Thought it felt a bit dubious, but I hadn't got the core loss information
available so couldn't justify the feeling. Anyway, isn't 2 mix - the red
coloured one - the wrong material for this frequency ? I thought that was
meant for the 2 - 20 MHz region.
The tank coil on my 700W PA, wound with 110 turns of 2.5mm Litz in three layers
on a 60mm diameter former just runs warm to touch, so I would guestimate it is
dissipating about 10 - 15 Watts and consequently contributing about 0.1dB loss.
Possibly about the same loss again in the aluminium case, with three of the
surfaces less than a coil's diameter from the tank. The detuning effect of the
case proximity caused about 5% reduction in inductance.
Before I realised the error of my ways, the first test build of this
transmitter had the steel top of the case quite close to the tank coil. This
got extremally hot ! Magnetic materials can have very high losses at RF.
After several hours of test transmitting the steel was too hot to touch, with
all the rest of the PA just comfortably warm. Completely cured by fixing a
pure aluminium plate under the steel.
A ETD-44 core of 3C85 material forming the output transformer with B max at
0.1T runs at around 50 deg so it looks as if this has the higher loss. Just
never got round to making this transformer with a bigger core.
Andy G4JNT
-----Original Message-----
From: James Moritz [SMTP:[email protected]]
Sent: 2002/06/11 13:07
To: [email protected]
Subject: RE: LF: DJ
Mal is using the big thick one. The area of this
is 2.32cm^2, using Andy's figures, the flux density works out to 50mT or
500 gauss. At 136k this results in core loss of 0.44W/cm^3, and with a
volume of 30cm^3 the total core loss is about 13W. The thinner core is
about double this. By itself, the 13W will result in a temperature rise of
about 50 or so degrees C - the resistive loss in the windings will add at
least a few watts and another 20deg C or so to this. By the time you have
added some more temperature rise due to being in a box and the insulating
effect of the PCB, the actual operating temperature will be way over 100deg C.
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