On 9/1/13 10:06 AM, Mark Beckwith wrote:
So my question at this moment is - which is the
proper method for me to calculate the proper
delay line length to properly phase the two
dissimilar SteppIR yagis?
The calculations are decidedly non trivial. The two antennas will
interact, small changes in the lengths of the elements (SteppIR, after
all) or their relative orientation (wind) will change the feed point
impedances, particularly the reactive component. That won't change the
"match" (in terms of VSWR) but will change the relative phasing.
This is not meant to be a snarky or otherwise "cute" response. Just
more like a reality check. The answer is:
Feed them with equal length feedlines and see how they play. If you
like the way they work, then you're done. I suspect you will like the
way that they work and I also suspect changing one feedline length by as
much as 8 feet would not affect your overall conclusion.
<snip>
Get on the air and work guys. I bet you work more guys with two antennas
at you disposal than just one antenna, and I bet when you do
top-bottom-bip-bop* tests you will find some times when some stations
are lounder under some of those four conditions. I suspect this would
be the same conclusion whether your coaxes are the same length or
different lengths by 4 feet or 8 feet.
Exactly this. You can have a few jumpers of some shortish length (1/4
wavelength ish) and try substituting them in and see what happens.
Here's the deal..
For phased arrays you can have fairly large phasing errors and the
*gain* doesn't change much. What changes is the depth and position of
the nulls. So the whole deal on the BIPBOPTOPBOT thing is that you're
moving the nulls around to suppress the stations you don't want to hear,
while slightly changing the gain in the directions of the stations you
DO want to hear. (for that matter, most likely, what you're doing is
moving the null OFF of the desired station).
So, you could model and measure and carefully calibrate your system (you
*are* allowing for the thermal expansion effects, aren't you <grin>),
and maybe not have any better effect than you would by essentially doing
the "try it and see".
For what it's worth, having two SteppIRs makes for a very interesting
scheme. You could feed with fixed lines, and just bump the driven
element a bit shorter or longer to shift the phasing. I don't recall
the exact numbers, but I seem to recall that a 1% change in length off
resonance is about 5 degrees in phase. 10% is about 30 degrees.
There will be all sorts of interactions among the elements, so you
really need to model it, but it's an interesting approach.
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