[TowerTalk] Dissimilar Yagi Stack

[Pete Raymond]

     In an effort to improve my 15meter capabilities I have been thinking of
mounting a 5el 15meter yagi above my 5el 20meter yagi.
One problem is that I only have about six feet of mast left above the 20m
yagi.  Not wanting to replace the mast at this point I decided to
model the effects using AO 6.01.
     As far as I am amble to tell the only major effects are that the gain
of the 15meter beam is down by about a 1/2DB and the F/B is down
by a little over 2db.  These figures were obtained while modeling over
real ground.  I then modeled this configuration in free space and the
effects are more pronounced.  The 15meter yagi pattern appears to loose
most of its F/B.  Which model represents the most accurate results?
     It appears that if the 20meter yagi has a long boom as compared to the
15meter yagi the effects are not so pronounced, I guess its because
the 20meter directors are not as close to the 15m yagi.
     My 20meter yagi has a 46foot boom and the 15meter yagi I want to put up
has a 26foot boom.  I forgot to mention that the drive impedance of the 15
meter yagi only changed slightly and that of the 20meter yagi did not
change.  The 15 meter yagi does not appear to change the performance of the
20meter yagi.
     Since I am showing such a big difference in results I'm not sure which
model is the most accurate.  I'd been interested in hearing from anyone who
has been there tried it and knows which model is most accurate.  Pete N4KW

p.s. both of the above models were also tested with and without booms.
seems like a waste of time to put the boom in as there were no difference in
any of the results.  





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