Re: [Amps] Non-inductive resistors

[craxd <craxd1@ezwv.com>]

I went back and decided to post some texts quoted out of the Radio 
Engineers Handbook. I read on and also found a section termed, "Load 
Resistors for Absorbing Radio-frequency Power". I'll start first with 
the uses text for the wound resistors then on to the load portion.

"The mica-card, reversed-loop, and figure-eight types of resistors can 
be made to have very low phase angles and are the types used in 
radio-frequency attenuators. The mica-card, fish-line, and woven-tape 
types of construction are commonly used in decade resistance boxes 
designed to have a low phase angle at radio frequencies, particularly 
for the high-resistance units. The Ayrton-Perry winding is also suitable 
for use at radio frequencies, particularly for resistances up to several 
thousand ohms. The simple bifilar winding is suitable at radio 
frequencies only for resistances so low that capacity effects are of no 
importance. The slotted type of construction is used in very high 
resistance units where only moderately low phase angle is essential, as 
in the case of voltmeter multipliers".

"Load Resistors for Absorbing Radio-frequency Power.-Resistors used as 
radio-frequency loads, i.e., as dummy antennas, present a particularly 
difficult problem, since here one desires a nonreactive unit capable of 
dissipating appreciable wattage. When the power is in the order of fifty 
watts or less, several satisfactory arrangements are available. One 
consists of a bifilar resistance element supported on mica and mounted 
in a glass bulb filled with inert gas, preferably hydrogen". Sounds like 
a bomb to me..LOL (WM). "Another arrangement consists of a mica-card 
type of unit mounted between two large aluminum castings that are for 
the purpose of conducting away the heat. Both these arrangements give 
excellent phase-angle characteristics".

"When larger amounts of power are to be handled, various expedients are 
used. In most of these, the reactance is eliminated by tuning, and the 
dissipated power is evaluated by a calorimetric or photometric method, 
or by measuring the equivelant circuit resistance at the frequency 
involved. Another possibility is to use a metalized type of resistor 
immersed in cooling water (1). In this way, the rating can be increased 
to 50 to 80 times that for air, and is of the order of 100 watts per 
square inch of surface. The power being dissipated can be determined 
from the rate of flow and temperature rise of the cooling water. By 
making the resistor the central conductor of a concentric line shorted 
at the receiving end, the reactance at the input terminals can, by 
suitable design proportions, be made zero".

(1) See G.H. Brown and J. W. Conklin, Water-cooled Resistors for 
Ultrahigh Frequencies, Electronics. Vol. 14, p. 24, April, 1941.

"The bifilar  winding has negligible inductance, but the capacity is 
relatively large, because the beginning and end of the resistance are 
close together. This capacity effect can be minimized to some extent by 
sub-dividing the total resistance into several bifilar sections, as 
shown in Fig. 10h (Bifilar-series)".

Will Matney
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