On 6/12/2013 11:58 AM, Tom W8JI wrote:
Christman phasing is at 71 degrees? I was thinking 90 degree phasing.
Whats the reasoning for this?
Optimum phasing of two verticals 1/4 wave apart is never 90 degrees,
unless the user for some reason wants a single-point zero-angle null
and less than maximum gain. Optimum element current delay for Ham use
is always more than 90 degrees, and generally around 110-120 degrees,
with 1/4 wave spacing.
Making things more complex, phase shift in a delay line is never the
line length unless the line is either 1/4 wave long or a multiple of
1/4 wave, or the line has a reasonably well-matched termination.
With that in mind, a 71 degree long line might produce considerably
different phase shift than the electrical line length, and the user
probably wants phase to be some other value than 90 degrees (if the
user understands arrays and patterns).
Right. There's some discussion of this in the ON4UN book, where I found
the Christman matching. As published there, it's 84 degrees of 50 ohm
line in each element, plus 71 degrees of 50 ohm line in the element
facing the desired forward direction,change directions by switching the
71 degree section.
73, Jim K9YC
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
|