[TowerTalk] RE: Stacking C-31's and C-51

[Dan Levin]

Interesting thread - a couple of thoughts to add.

1) Keep in mind that even if you are trying to maximize stacking again, you
can be out of phase by 20-30 degrees and still get acceptable results.  You
can see this in the table on the article on my web site (also on the Array
Solutions side) re: stacking dis-similar antennae.  The price for being 30
degrees out is around .3-.4 db - high, but not unacceptable.  I mention this
only to discourage you from sweating it too much :-)  Plus or minus 5-10
degrees is plenty close enough, and 10 degrees at 10 meters is 10 inches or
so (assuming .84 vf cable).

2) Since the C-31 and the C-51 are identical on 10, 15 and 20 meters, and
use identical feed systems, the only thing you really need to worry about is
the horizontal displacement of the driven elements, as you are doing.  If
you can, it is best to start with a good guess (as you are), and then trim
using a fast dual trace 'scope, and mentioned by other posters and on the
Array Solutions web site.  Remember, if you can get within a foot, that is
plenty close enough even on 10 meters.  Remember also that the electrical
length of the driven element offset will be different on 20 meters than it
is on 10 meters (even though the mechanical length is the same), and thus
that the phase shift caused by the offset will be roughly twice as much on
10 meters as it is on 20 meters.  You can't do anything about that, but you
might want to take it into account.  In other words, you can't make the
phase the same on all three bands - so you might want to make the error
purposefully negative on 20 meters so that it will be less positive on 10
meters - shoot for the middle, as it were.

3) As Tom mentioned, you can't just remove "1 wavelength" of cable on the
lower antenna.  That is because "1 wavelength" is different on 10, 15 and 20
meters.  Use feedlines that are the same length (after taking into account
the phase shift on the C-51), even though it wastes cable - the phasing
network assumes equal lengths (assuming that you are using a StackMatch).
Coil the extra at the base of the tower.

4) Don't worry about mixing different types of coax, as long as they are 50
ohms, and as long as the coax that you use between the phasing network and
the yagi's is all the same.  If you can, use coax from just one spool for
the phasing lines, or trim them with a 'scope.  Velocity factor can differ
quite a bit from batch to batch - so use just one spool if at all possible
to eliminate that factor.

GL es 73,

			***dan, K6IF

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