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Inferior 50 Ohm Power Load Resistor |
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I received a call from
a ham radio operator. He was testing some older vacuum tube type radio transmitters,
using a classic 100W 50 ohm power resistor, immersed in mineral oil. This “cantenna” dummy
load allows testing without actually transmitting RF out into the ether.
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The caller said that at 1.6MHz, the
160 meter band, the load seemed to work about right but when he went to higher
frequencies, he could no longer get the transmitter to tune properly. In theory, the
load resistor should have looked to the transmitter as a perfect 50 ohm impedance. I
asked him about the nature of the resistors he used to build his load bank. At first
it seemed reasonable but then I asked an important question. I asked him if the
resistors were the “non-inductive” type. He didn’t know. I suspected that they
were not. |
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Typical power resistors are wire wound
type. They consist of a resistive wire wound around a ceramic core. As anyone
should know, a coil of wire will have both a DC resistance and an AC inductive reactance.
The inductance of typical wire type resistors could be high enough that at progressively
higher frequencies, the reactance would exceed the DC resistance. That would explain why
he could not tune the transmitter properly. |
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Non-inductive resistors are available
but are not always marked as such. They can still use wire but the wires are wound
so that the current flows in opposite directions. A bifilar winding forces the
magnetic fields to cancel, producing a pure resistance with little inductance.
Carbon composition, carbon film, metal film and metal oxide power resistors could also be
used to make a RF dummy load, when wired in a parallel configuration. Flat foil
power resistors have some of the best characteristics but they must be mounted to a heat
sink before immersed in the oil. I suggested that the guy find some suitable
non-inductive resistors and rebuild his load bank. |
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Cantenna Dummy Load |
Wire Wound Power Resistor |
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Metal Foil Resistors |
Carbon Film Power Resistors |
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Wire Wound Power Resistor |
Home Made Low Inductance Resistor (40 Watts) |
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Home Made Dummy Load (100 Watts) |
Resistor Winding Methods |
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