[TowerTalk] stack help needed

[alsopb]

Guys,

Need some advice.  On Friday before the 10M contest, I added a second
3 element 10 Meter yagi stacked 18' below the 80' one.  On paper it
looked like a good combo.  

It didn't work as expected.  Currently only the "stack" configuration
is available due to delay in getting control cable.

Here are the particulars.

1) Both yagi's are CC TEN-10 beams with gamma matches.  Low level
(25') SWR checks on each antenna show them to be working correctly.
2) The top yagi is mounted 10' above a 402CD.  They rotate together.
3) The bottom yagi is mounted 8' below a 402CE.  It is fixed on EU.
4) Both gamma matches on the same side of the boom.
5) WX0B type stack match used.  It connects the stacks together T
fashion through a 2.25:1 unun in the stacked configuration. 
Measurements with resistors substituted for the antennas at the output
ports show the correct impedance transformation.
6) Equal lengths of RG213 cut from the same 60' piece used.
7) No baluns or beads used at the antenna feedpoints.

The SWR of the stack is about as expected (under 1.4:1 at resonance). 
However, the signal with the beams aimed the same way is 30db (!)
below the 4 element quad (350' away).  Both the quad and stack have
their main elevation lobes at about 8 degrees.  Rotating the stack so
that the top and bottom antennas are skewed brings the signals in the
two skewed directions back to expected.  Many stations were worked
this past weekend in this configuration. Comparisons with the quad
seemed proper.

It appears like some 180 degree phase shift is being introduced in the
feed somewhere.  Darned if I can find it.

One note: The SWR of the "stack" does vary as the two beams are
rotated over to separate directions.  What is your experience?

Here are some steps I intend to take next week to troubleshoot.

1) Re-inspect the antennas to make sure I didn't do something dumb
like mounting the reflector on the director location.  Ground checks
indicate no such problem.
2) For the heck of it, reverse the direction of one gamma match. See
if it plays.
3) Take down both beams and set the gamma matches for equal impedances
with the antenna pointing straight up.  Right now both gamma matches
are set identically -- based upon the tuning done for the top one.  
4) Get the switching going so that A/B/stack comparisons can be made.

Don't have any bead baluns to try and eliminate the possibility of
feedline radiation messing things up.  The effect seems way too large
to be this.

One real concern is possible interaction of the yagis with the 402CD
in the middle.  I could only model the 402cd as a full sized 2 element
beam on the same 24' boom in EZNEC.  It didn't show any effect.  The
measured resonances on the 402CD beam are on 40 meters (thankfully)
and at 19MHz.  I see no dips indicating resonace anywhere near the 10M
bands.

Any advice on resolving this.  Everything but the results say it
should work.

73 de Brian/K3KO

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