On 2/24/20 5:46 AM, Rob Atkinson wrote:
I have a few problems with this wire. My experience with aluminum is
that it is not worth the trouble integrating it into the rest of the
antenna system when it comes to bonding it to other metals. I
appreciate the light weight and lower cost but aluminum has another
undesirable property: It cold flows so clamps on it eventually come
loose and then you have resistance and heat. This is why aluminum
house wire which was tried in the 1960s came to a quick end--too many
cold flows, heat and fires. I hope it isn't coming back for that
purpose.
The lugs in your panel box are aluminum as well as the wire, so there's
no CTE mismatch, so no loosening problem. For that matter, the circuit
breakers engage with the aluminum bus bar. Utilities have been using
aluminum wire for a very long time with no problems. It was when they
started to go to smaller sizes AND the use of the "wrap around the
screw" type termination.
The problem with aluminum house wire isn't cold flow (mostly) - it's
that about that same time they changed the material of the screw
terminals on receptacles and switches from brass to steel. Steel and
Aluminum have significantly different CTE, so with temperature cycling,
the connection would loosen, and then you'd have the heating/fire
issues. This is aggravated by aluminum rapidly forming an oxide layer
Today, with push in spring loaded connections, that isn't an issue -
the spring makes a gas-tight seal with the aluminum. Modern aluminum
wire is a different alloy than that used pre-1970s and has a different
CTE (and slightly different creep, but apparently that's not
significant) If you do need to connect to aluminum wire, they have
wire-nuts specifically designed to join copper and aluminum.
Airplanes have used aluminum wiring for large conductors for a long
time, without many problems. They also use crimp connectors designed for
aluminum wire. I understand that the A380 uses a lot of aluminum for
smaller wires, but it's some exotic plated stuff with multiple layers.
You're not going to be getting it for a ham antenna in a surplus sale
any time soon.
I have my 200 amp service drop to my home using aluminum. I don't
like it but I can't make the power company use copper. Every few
years I check the lugs on the main breaker to make sure they are still
tight. This is one area where an IR camera would come in handy. It
WILL cold flow eventually and you'll know it when your lights flicker
or dim briefly when a big load like A/C comes on.
Well, you probably don't check the lugs under the meter, and those are
aluminum too. You can get an IR camera and check, but I suspect you'll
not find any problems. Don't forget to put a piece of tape on the lug so
the emissivity is what the camera is expecting, otherwise the temp reads
partly what's reflected.
Light flicker is more likely due to insufficient distribution
transformer capacity, or a failure of the grounded (neutral) conductor.
We used to have the former because when the tract was originally laid
out it had half the number of houses it was finally built with, so all
the pad mount transformers wound up with twice as many houses as
expected. A co-worker had the neutral failure (on infrastructure
installed in the 1920s and 1930s in South Pasadena) - changing the loads
would change the voltages on the two "phases". They replaced it with,
of course, aluminum.
I was dismayed to see that QST ran an article by some sort of RF
expert on how to construct open wire feedline using aluminum AWG 8 or
maybe it was 6, stranded wire. Ridiculous overkill for ham power, and
he spliced line sections together using what appeared to be steel
clamps. Those clamps will lose their grip in a few years and he'll
have problems, besides the use of steel for RF. None of this came up
in the article. Then there was the line Z due to the spacing and
diameter of AWG 8 cable but that's outside the focus here.
yes, proper crimps with a proper tool would be a better way to connect
it. Picking the right alloy is important.
As far as loss goes, I suspect that for open wire line (already low
loss) the larger aluminum compared to a smaller copper might actually
come out ahead. Lots of people homebrew open wire line with AWG 14, and
AWG 8 is twice the diameter. If we assume that skin depth is small
compared to diameter (which is worst case here), then the fact that
aluminum is half the conductivity of hard drawn copper would be made up
for by having twice the diameter. If the skin depth is bigger, then the
fact that the AWG 8 has 4 times the cross sectional area makes it even
better.
Stranded wire has it's share of RF problems, but sometimes, you use what
you have.
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