On 2/24/2020 5:46 AM, Rob Atkinson wrote:
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.
73
Rob
K5UJ
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In defense of Bob Zavrel, he actually is an RF expert.
The choice of wire gauge has nothing to do with power
handling, but rather low loss. Especially if the
line is run at a high mismatch. And it was AWG 6
not AWG 8 by the way. The economics of
aluminum wire are such that you might as well use a
large gauge and have really low losses. The weight
of aluminum is much less than copper, so that is
another reason to use a large gauge. My OWL uses
4 AWG XHHW, but it used to be 1,000 feet long;
I built a new ham shack 400 feet back and now it
is "only" 600 feet long.
Regarding the line Z: he uses 4.25" on centers
spacing, which would give 450 ohms if the wire
diameter were 0.2". This would be perfect for
bare solid 4 AWG wire. Allowing for the fact
that stranded wire is slightly larger than solid
wire, and allowing for insulation, that sounds
about right to for 6 AWG. My spacing (with 4 AWG)
is about 20% wider, and I know I have Z=450 ohms
because I measured it.
The clamps shown in the photo do sort of look like they
might be steel, but I know at least the ones I use are
100% aluminum. I don't have splices every 100 feet
like Bob does. The 1000 ft run used two 1000 foot
spools of wire. There are only clamps at the ends.
I will agree with you that the clamps will lose their
grip in a few years, "I guarantee it". In the 20 year history of this
line I have had to retighten them every few years. I can
live with this minor issue. It's not the big deal like
it would be in a 200 amp 60 Hz feeder.
I will disagree with Bob on the use of the line at 15:1 SWR.
I run my line matched with 450 ohm to 50 ohm baluns.
Bob does mention that as an option. Besides the extra loss
at 15:1 SWR, the other big disadvantage is that the
match bandwidth gets very narrow when the line is very long.
Rick N6RK
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