"No Spam" wrote in message
...
I have dremmel-tooled open a used dried-out water filter cartridge from
my fridge that contained a hollow carbon shaft about 10" long and 3" in
diameter with a 3/4 inch hollow inside diameter. The end to end
resistance was about 600ohms. The carbon shaft is a bit soft and
crumbles
under the pressure of tools. This was how I destroyed the first unit I
played with in trying to make it 480ohms by trimming the ends.

Using another filter, I encapsulated the carbon shaft inside a sewer
tube. (Denser PVC and hopfully more impervious to heat) I attached
conductors to the ends using flat washers with springs and left vent
holes in the PVC end caps when I attached them to push it all together.
I
did not attempt to trim the resistance by shaving the length. The
modules
finished resistance measured ~590ohms.

Since this WAS a used water filter, I ran some HV through it to heat it
up a bit to see if it would vent gas or smoke. The resistance decreased
a
bit as it heated but returned to near 600ohms when it cooled. I kept
the
power to less then 100w using my bench HV power supply. After about 1
minute, the PVC started to feel warm but no smoke was seen though it
did
stink a bit.

The whole PVC module I made sweeps out flat from 1Mhz (bottom of my
test
gear) to ~40Mhz where I think my wiring started having some effects. I
had to use springs on the ends to push the contact washers up against
the
carbon shaft and suspect I need to come up with something better here.
Also, the pressure of the springs and end caps varied the resistance
about 20 ohms.

I have run about 400w avg for several minutes into the load under voice
and so-far the damn thing is still hanging on the T2FD antenna and
loads
just fine. In usage, the resistance varied about 50ohms upward in the
last several months of tracking it. I hope to remove the resistor
shortly
and inspect if there is any chemical reaction with the steel washers
and
the carbon rod ends. I need some better way to make this contact and
also
some way to increase the surface area of the contact to stabilize and
perhaps reduce the resistance. I'm thinking copper pipe endcaps heated
up
and cooled over the ends either on the outer diameter or on the inside
hollow part of the carbon shaft.

/****/
Another idea I just had is perhaps having someone with a ceramics oven
paint on the endcaps with ceramic glaze. It would look like a giant
ceramic resistor! But then again, I would guess that as the resistor
heated up, it would crack. Never mind...
/****/

I now have another filter module to dissect. I hope to come up with a
better way to mount contacts to the carbon shaft and some way to assure
a
nice airflow around and hopefully through it to allow for higher power.

In retrospect, with the amount of time and work I've put into this
resistor project, I would have been better off purchasing the buxcomm
resistors for $25. (I have one) But then again, I had fun playing
with
this too! If anything, ideally, I hope to come up with something that
weighs half the weight of the buxcomm resistor so my T2FD antenna looks
more graceful under the burden of the resistor. :-)

The water cleaner cartridge is activated charcoal with a small amount of
organic binder. It is not graphite as graphite will not function to
clean the water. Activated charcoal has an extremely large surface area
which adsorbs contaminants. Depending on how it is produced, a gram of
activated charcoal can have from 100 to 1000 square meters of surface
area.

Your idea of a metallic ceramic glaze will not work as the glaze is a low
temperature melting glass (usually a lead glass) with fine metallic
particles in it. Once fired, the glass melts, but the metallic particles
are still insulated from each other.

You can buy conductive copper, nickel, and silver based paints for
repairing printed circuit boards. GC Electronics sells small bottles of
silver print paint (Part No. 22-024 1 troy oz. Bottle) which has a
surface resistivity of 0.1 ohms per square, and nickel print paint (Part
No. 22-207 2 fl. oz. Bottle) which has a surface resistivity of 5 to 6
ohms per square. Be extremely careful when diluting these paints. Use
only chemically pure solvents as any oil or other such material in the
solvent will serve to insulate the particles from each other. GC Part
No. 10-4102 Polystyrene
Q-dope thinner (toluene) might be suitable. I would suggest the nickel
print paint as the silver print is quite expensive. Heed the warnings in
the respective MSDS's for these products.

Another alternative is to have the ends of the activated charcoal rod
electroplated. A thin copper flash film applied to each end, followed by
a thicker conventional electroplate would give you contacts that could be
soldered to. If there is someone in your area that does electroless
nickel plating, this would also be suitable.

An entirely different approach would be to use a conductive liquid for
the resistance. An old QST article in June, 1965, entitled "Aqueous
Dummy Loads" by Alexander Marion, W2CUE (now a silent key, I believe),
should give you some ideas.

73, Dr. Barry L. Ornitz WA4VZQ