-----Original Message-----
Since the KX1 is power limited I would like to build a dummy load and attach
it to the side of the unit. If I take a short piece of coax and use a 5 watt
wire wound 50 ohm resistor dip the end of the coax and the resistor in
plastic vinyl the kind used for insulation tools handles, would the close
proximity of the dummy load cause undue heat or other problems.
Mike
MC -- KB7DPC
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Mike, in earlier posts you mentioned that you have the antenna tuner in your
KX1. In that case it will load up a 50 ohm wire-wound resistor fine, but the
50 ohm wire-wound resistor will NOT present a 50 ohm load to the KX1. That's
because it is a "wire wound", meaning the resistive element is a resistance
wire that is wound into a helix on a form, so it is also an inductor. Such a
resistance presents an impedance quite different from 50 ohms, and the value
of the impedance is dependent upon the frequency. A better choice for a
dummy load is a metal film resistor, such as Elecraft uses in their DL1
dummy load. A metal film resistor has very low inductance, so it's impedance
at RF is very close to the 50 ohm value and varies very little with changes
in frequency.
As far as dipping the resistor in a coating, keep in mind that resistors
convert the power they absorb into heat. Their power ratings assume they
have free-air circulation around them. When you coat a resistor with some
material, you must de-rate it. How much depends upon the thermal properties
of the material you want to cover it with. That's why dummy loads typically
have the resistors on a pc board sitting in free space (like the DL1) or, as
in the case of high power dummy loads, either the resistance element is
inside an enclosure with lots of ventilating holes for free air movement or
the resistive element immersed in a thermally conductive oil that will
absorb the heat and quickly conduct it away from the resistance element.
Ron AC7AC
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