[Amps] Non-inductive resistors
craxd
craxd1 at ezwv.com
Mon Sep 27 13:20:28 EDT 2004
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
More information about the Amps
mailing list