HOME Schematics Discover Forum Hobby Corner Dave's Circuits Electronic Resources Contact Info


  Circuit Diagnosis

  Previous Issues

Inferior 50 Ohm Power Load Resistor

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. 

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. 
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.
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.
 
  
Cantenna Dummy Load  Wire Wound Power Resistor
 
  Metal Foil Resistors Carbon Film Power Resistors
 
Wire Wound Power Resistor Home Made Low Inductance Resistor (40 Watts)
 
Home Made Dummy Load (100 Watts)  Resistor Winding Methods

4th Quarter -  2012    

Index Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
What the World
needs Now
Wily Widget


HOME Schematics Index Hobby Corner Dave's Circuits Electronic Resources Contact Info
Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches


 About Us   |  Advertise on DiscoverCircuits.com   |   Report Broken Links  |    Link to DiscoverCircuits.com  |    Privacy Policy

Copyright  2002 - 2015 David A. Johnson & Associates.  All Rights reserved. 
 Linking is ALLOWED but COPYING any content or graphics to your web site is EXPRESSLY PROHIBITED.