[TowerTalk] 3000' feedline...can I make it work?

[Jim Lux]

----- Original Message -----
From: "Paul" <radioboy at telus.net>
To: <towertalk at contesting.com>
Sent: Sunday, May 02, 2004 10:27 AM
Subject: RE: [TowerTalk] 3000' feedline...can I make it work?


> Hi Group,
>
> Thank you for all of the replies. Lot's of good ideas were offered. I had
> another look around the property, and found that if I go the other
> direction, ie away from the mountain down into the meadow below, I should
be
> able to get about 25 deg takeoff angle. Not great, but not bad either, and
> only 600' of feedline.  I still may try running the 3000' up the mountain
> but the meadow looks much easier to start with.
>
> I also found out that the aluminum wire I have is ACSR/AW 4ga, 6 strands
of
> aluminum around a galvanized steel core. Has anyone had any good or bad
> experiences with this material? Will it work for RF as well as say 8 or
10ga
> copper? It has a very tough outer plastic jacket.

AWG 4 is twice the diameter of AWG 10 (approximately), so if all things were
equal (they are NOT, but bear with me), the surface area would be twice, so
the resistance would be half.  Surface area is what we're worried about
because of skin effect.  Not much RF winds up in the middle of the wire,
which is why they can put a steel core in. (and also why copperclad steel
wire, i.e. Copperweld(r), is used for antennas to good effect, strong as
steel, conductive as copper, as far as RF goes)

Aluminum is about 60% the conductivity of copper, so overall, you'd make up
the loss going from copper to aluminum because of the increased surface
area.  The skin depth varies too (less conductive > more skin depth), but in
either case, the current is all on the surface at HF frequencies.

The real bugaboo is the stranded aluminum vs the solid copper.  The current
will flow not only along the strands, but between strands, and that's
through a semiconductive aluminum oxide layer.  Aluminum oxide is actually a
good insulator (and very low loss for RF), but the surface of a wire is
probably a sort of composite of aluminum, aluminum oxide, and other stuff,
and quite resistively lossy.

So..... what condition is the wire in?  Can you do a quick test?  DC
resistance won't tell you much, but if you were to rig up a 20-30 foot test
section of open wire line, you could measure the VSWR of a known mismatch
and estimate the line loss (meter to balun to open wire line, to mismatch,
then do same test with the mismatch at a different place, balun effects
should be the same at both measurements)



Handy way to compare wire gauges, since they are based on a log scale:

Think of gauge as dBarea:
10 times the area is 10 gauges
100 times the area is 20 gauges
10 times the diameter is 100 times the area, so 20 gauges.
AWG10 is about 0.1 inches in diameter, so AWG30 is 0.01 (i.e. one tenth)

So, that's the tens... now think in terms of halves and 3dB..
3 gauges is half (twice) the area (3dBarea)
6 gauges is 1/4 (or 4 times) the area, or half (twice) the diameter