--- On Fri, 6/14/13, Jim Lux wrote:
> One thing to bear in mind on any of these approaches is that
> phasing errors don't change forward gain very much, but a
> small phase error turns a -20dB null into a -10 or -3dB
> null. (this is probably the source of the 4square
> complaints about installed performance..)
This topic happens to be discussed in one of the examples in the AutoEZ User
Guide. The illustration linked below shows how the pattern is degraded when
the elements of a 4 square are incorrectly fed.
http://ac6la.com/ae4sqfeed3a.png
The blue trace shows the correct feed, the red trace shows what happens when
one assumes (incorrectly) that because a 90 deg phase shift is desired between
the leading and side elements a 90 deg difference in transmission lines will do
the trick. Notice that the forward gain doesn't change much but the F/B goes
from (roughly) 24 dB to 9 dB.
Another illustration highlights the change in the back lobe with just a small
change in line lengths. Here one TL of a 40 meter 4 square is being varied
from 48 ft to 52 ft in steps of 0.25 ft. The pattern has been "zoomed" to
emphasize the back lobe.
http://ac6la.com/ae4sqfeedani1.gif
The complete example with several other illustrations may be found in the
AutoEZ User Guide ...
http://ac6la.com/autoez.html
... section "Transmission Lines and Tuners", subsection "Phasing Lines for a 4
Square Array".
Dan, AC6LA
http://ac6la.com/
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