At 09:08 PM 3/21/2007, Rob Frohne wrote:
>Hi All,
>
>You guys are giving me such good advice today, that I can't resist
>another question.
>
>I'm considering trying to phase a four element quad at 40 feet on one
>tower with a four element 20 meter monoband yagi on another tower at 90
>feet. In order to do this, I need a variable delay line. Is the best
>way to do this using a set of coaxial delay lines whose lengths are
>related to one another in a binary fashion, or are there other better
>ways of doing this?
How much power are you handling?
A set of suitable DPDT relays (or coax transfer switches) and coax in
a 1,2,4,8 sort of progression might work, but you'll have to deal
with the fact that phase shift isn't proportional to length, in the
general case, particularly if you have any coupling between antennas.
The other approach is to use a controllable L network, either a
conventional tuner with stepper motors or servos turning the knobs,
or with relays and L and C in a 1,2,4,... sequence (like in an
autotuner). Off the shelf, the LDG-AT200PC has an RS-232 interface
that you can send commands to set the Ls and Cs (i.e. you use it as a
computer controlled tuning network, rather than an auto tuner) I
haven't checked, but I think one of the Palstar tuners has a remote
control interface.
Depending on the range of phase shifts you need, you might need two L
networks in series, or some relays to do phase reversing.
http://home.earthlink.net/~w6rmk/math/wyedelta.htm has some equations
Jim, W6RMK
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