Dear Will,
Some months ago Rich Measures was talking about ordering some special
resistors
from Ohmite or the-like to fill a need for a project he was doing.
In that time I was bench testing the Emtron DX-3 and was using the MFJ
oil-filled Cantenna clone, and that got really hot after 10 seconds and
the red plug
popped off and my hotel room smelled like the french-fry concession at
Yankee Stadium in the Bronx, so I had to upgrade.
When the Palstar 5KW fan-cooled dummy load arrived I took it apart to
see if UPS had incurred there usual shipping damage on anything fragile
going through their system and noticed the carbon-pile type cylindrical
resistors being used: 100.34 ohms, nominal,l about 12 inches long, 2
inches
in diameter, sintered gold plating on the ends, clips holding them in and
some HV caps at the hot ends to counter the reactance that the case
and resistors probably conjured up. This device seems to work
very well, because the fans kick in and stay lit until the pile cools
down.
I took apart the Cantenna and measured the smaller but similar carbon
pile
resistor in that unit. It was 65.7 ohms and MFJ sent me out a free
replacement
resistor, which was 50.75 ohms, well within their 10 percent tolerance.
The whole point of this is if it's possible to obtain carbon pile type
resistors
of sufficient dissipation to construct a 20KW resistive load. Mr.
Measures
uses the garden hose to cool down the resistor body in a carefully
engineered
conical assembly, but what about those experimenters who just want to
have
an air-cooled deal that will fit in the shack? Power-dividing and using
the resistor
elements from Cantennae suspended in the bathtub is a practical solution,
but that
gets real old when the YL spots it.
Sorry for the long wind, messieurs. The recent spate of posts about dummy
loads
catches my attention.
Hal Mandel
W4HBM
> I seen something about an alloy 807? It had, if I recall, about 0.87
> ohms per foot. 57' would be required for 50 ohms. I have some
> drawings here in an engineering book about winding non-inductive
resistors.
> One method is to use mirrored loops which are out of phase with each
> other. I think was the way they had that one. Anyhow, it was supposed
to
> cancel out any inductance. Any small amounts left can be taken care of
with
> some capacitance like in the old Collins dummyloads. Also, theres
> some alloy tubing by Inco who are here. They make inconel, monel, etc.
> (nickel alloys) and tubing. It would look to me like an alloy tubing
> Will Matney
>
> Harold B. Mandel wrote:
>
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