At 04:04 PM 2008-06-01, Tom Osborne W7WHY wrote:
>If 90 degrees is 360/4 (1/4 wave) then is 72 degrees 360/5 (1/5 wave)?
>Would that make a 72 degree phasing line 23.4 feet (133/5 * .88)?
>Then an 80 degree phasing line would be 26.1 feet (133/4.5 * .88)?
>
>I am actually trying to figure 80 and 71 degrees lines for 40 meters using
>RG8X.
>
>If this is true, how can I find which frequencies these would be a 1/4 wave
>at so I could use the analyzer to cut them to length?
Tom,
You're doing pretty well with your high school math.
To make it general, just divide the desired (matched) phase shift by
360, then multiply this by the length for one wavelength.
So for 72 degrees, it's (72 / 360) = 0.2 and then multiply by
(983.571 / 7.15) = 137.562 to get 27.512 feet. Of course, you then
multiply by the VF (0.88) to get 24.211 feet.
I got 26.9 feet for the 80 degree line. (Your answers may reflect a
different design frequency or an approximation for the calculation of
a free space 1/4 wl.)
To get the frequency where the line is 1/4 wavelength, just multiply
your design frequency (I assumed 7.15 MHz) by (90 / 72) to get 8.938
MHz. In the same way, the 80 degree line would be 1/4 wl at
(90/80)*7.15 = 8.044 MHz.
have fun,
Terry N6RY
PS - The comments about phase shifts being a function of load Z and
SWR are true, but in your case, I'm pretty sure that has already been
factored in by the array designer, which is why the lengths aren't
some nice number like 90 degrees. One thing to note is that with
this type of design, how good the back nulls are will be dependent on
whether the equivalent ground losses in your radial system are close
to what the original designer assumed.
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