On Mon, 10 May 2004 03:12:05 GMT, "Jerry Martes"
wrote:

I'm condidering building a slotted line for measuring impedance at 137
MHz. I find no referances to home made lines thru my quick Google search.
Does anyone know of any publication that show how someone has already worked
out the problems of contructing one?

Jerry

Hi Jerry,

It is not all that hard to do in fact. HP solved that problem long
ago by thinking outside of the box by thinking box. As with other
test equipment issues, it is simply a matter of planning and testing
with very simple methods. For those whose greatest physical effort in
Ham radio is sliding a credit card across the showroom display
counter, this is called "work."

Basically you construct your coaxial line with the usual interior
line, but unlike the conventional expectations, you do not try to
emulate the outer portion as a cylinder. You construct the outer
portion as two parallel conductive planes (appropriately shorted to
the connector shells at each end (hence the allusion to box):

view HP 805C Slotted Line picture on ebay at:
http://web.ask.com/redir?bpg=http%3a...html&qte=0&o=0

Where the two planes stand apart, you insert a probe to measure the
potential along the line. The depth of the line within and in between
the two parallel surfaces insures the line isolation (no leakage) as
well as preserving the line characteristic Z.

However, anyway that you look at it (even the lecher line suffers from
this) you run the risk of over coupling and throwing the measurement
into confusion (very simple to make errors). The problem is the probe
will introduce its own SWR and gum up the works if it lacks
sensitivity. I won't bother too much with dimensions here, but
instead offer a formula for such a structu
Zc = (138/sqrt(e))·log(4h/pi·d)
where
e: dielectric constant (= 1 for air)
d: interior line diameter
h: wall separation

You will want to build it long enough to be more than a wavelength of
course.

You will also need to calibrate it to determine the residual SWR it
presents to the system (this will reveal construction errors). Off
hand, I would suggest that the walls be roughly a 2 to 4 cm apart and
at least a 20 cm wide (larger wouldn't hurt).

Build one quick and dirty to get your gross mistakes out of the way
without spending too much time on them. I can guarantee no one here
could build it right the first time (including yours truly). Once
you've got the first pass attempt on the bench, then we can talk about
how to use it right. ;-)

73's
Richard Clark, KB7QHC