Interesting stuff, Will. Like I say, I am sure that these
techniques work, but to what extent is unclear. I gave
it some more thought, and it occured to me that if you
could control the characteristic impedance of the
transmission line formed by the two wires in the bifilar
winding so that it was a constant 50 ohms, then you
would have what amounts to a 50 ohm transmission
line terminated in a 50 ohm resistive load, except that
in this case instead of being a lumped 50 ohm resistive
load at the end, the resistive load would be distributed
along the length of the two-wire transmission line because
you are using resistance wire. In principal, that should
work. The key to getting good frequency response would
be to control the characteristic impedance of the two-wire
transmission line. If you had spiral grooves cut in the
form that kept the characteristic impedance of the bifilar
winding fairly constant at 50 ohms, then you might have
it. Of course, I would argue that buying a Globar is
easier and it is guaranteed to work :)
73 de Mike,
W4EF................................................................
----- Original Message -----
From: "craxd" <craxd1@ezwv.com>
To: <amps@contesting.com>
Sent: Monday, September 27, 2004 10:20 AM
Subject: Re: [Amps] Non-inductive resistors
> 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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>
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