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August 14th 03, 02:46 AM

(Art Unwin KB9MZ) wrote in message om...
(Brian Kelly) wrote in message . com...

"Can motor oil be used " and the correct answer is a
qualified YES

What API spec crankcase oils do you have in mind?

If you want to leave a KILOWATT active overnight then I
would certainly say NO.
I certaily would recommend the Teflon coated cans since
can seams can sometimes 'weep'when thin oils are used.

In the case of a paint can being used for the housing for a dummy load
the outer surface of the can needs to be able to radiate as much power
as possible. A Teflon liner between the oil and inner surface of the
metal can is a thermal insulator and will degrade the ability of the
can and the dummy load to transfer the BTUs in the oil to the air
surrounding the can.

Simply degrease the can thoroughly and resolder the seams. Still won't
get rid of all the oil forever because the relief valves burp and/or
leak here and there.

If viscosity is your main concern then additional
circulation methods will alleviate that in a way far
superior to that using the best known transformer oil
( ARRL shows a method of achieving this)

I once wound a double-layer full length coil of 1/4" copper tubing
which was immersed in the mineral oil and soldered into the lid of a
Cantenna. I ran faucet water thru the coil. The dummy load ran quite
cool under all conditions when we were key-down smoke testing 2kW
output HF amps for an hour at a crack.

If resisters are your main worry
then use a couple of wire resisters with the windings
oriented in opposite fashion since absolute accuracy is
intended for real dummies.
( Note resistors change value
over time even when not in hostile environments.)
Check the values on an old tube/valve reciever for yourself)

Dummy loads are simply places where one can dump RF power without
cluttering up the bands. And I certainly wouldn't get involved with
winding any inductors when non-inductive 50 Ohm deposited-film
resistors are freely available.

As a point of interest, have you considered what creates
the first point of failure,

No, I've never had a dummy load fail.

and why transformers housings
are built so mechanically strong ?

Because they're the stuctural supports and housings for particulaly
heavy AC power machinery often located outdoors and subject to all
sorts of physical abuse. Plus the shells need to be of heavy enough
plate to reliably facilitate leakproof welds. I fail to understand
what this topic has to do with ham radio dummy load mechanicals.

Can you tell me at what
time the 'flash point ' comes into play when considering
it.s use for a dummy load ?

No, but if the flash point ever becomes a concern I'll look up the
temp and take it from there.

These questions go to the very
point that I discussed i.e using industry methods as
pointers BUT with due reference to the different environnments
and duty cycles required with respect to cost and availability.

.. . OK.

Regards
Art

w3rv