Hacker Chronicles 2/HACKER2.BIN/299.PLOTCOMP.DOC

Text File | 1988-11-15 | 6KB | 143 lines

                Plot Comparisons


    Here are some antennas whose patterns are interesting to compare using
the instant plot compare feature of the PLOT program:


2ELYAGI - 3ELYAGI

    This compares a 2 element Yagi optimized for highest gain with a 3
element Yagi optimized for smallest sidelobes.  It is surprising that both
antennas have essentially the same forward lobe shape, with the differences all
in the rear half of the plane.  The gain difference between the two antennas is
about 1 dB.


204BA - 3ELYAGI

    The above 3 element Yagi is compared with the classic Hygain 204BA 4
element 20 meter beam, widely regarded as a very good short-boom antenna.  At
the center of the band the gains are the same and the rear patterns are very
similar, but the boom length of the smaller Yagi is only 21 feet, whereas the
204BA has a 26 foot boom and an extra element.  When examined over the whole
band the 204BA shows a little better SWR and F/B, but not by much.  This
example illustrates the savings possible using computer-aided antenna design
compared with the traditional empirical approach.


2ELYAGI1 - 2PHASED

    A conventional small Yagi is compared with a similar antenna having
both elements driven.  The improved backlobe performance is remarkable.  The
Yagi director was adjusted to provide the same gain as that of the phased
array.  Both designs have a forward gain about 0.5 dB below that obtainable
from an all-out max gain design which ignores sidelobe level.


BEVERAGE - BEVERAG1

    You can see immediately where the longer Beverage gets its gain, by
noticing that it drops nulls right on top of the sidelobe peaks of the shorter
antenna.


N6BV - K1FO

    It is interesting to compare two highly optimized Yagis, one for HF and
the other for UHF.  If you have ever wondered what the pattern of your big HF
array would look like if you could somehow add another 6 dB of gain, here it
is.


VDIPOLE - SLOPER45 - SLOPER60

    Use the N key to normalize the actual gain difference between the
antennas into the plots.  Both slopers have higher gain than the vertical
dipole, but it is all at high angles and thus not very useful for DX.  However,
the 60 degree sloper does not sacrifice any gain to the dipole at angles below
15 degrees.  Thus it can be used to reject high angles signals off the rear of
the antenna for receiving, while preserving good low angle forward gain for
transmitting.  The 45 degree sloper has better high angle directivity than the
60 degree job, but loses about 1 dB of gain to the vertical at angles below 10
degrees.


3ELQUAD - 3ELYAGI

    Both of these antennas were optimized for low sidelobes.  The Yagi lost
about 1.5 dB forward gain in so doing, but the quad only a few tenths of a dB.
The quad has about 2 dB more gain than the Yagi, but about 10 db worse sidelobe
performance.  (The quad really needs more segments for best accuracy, and in
addition, it could be optimized further for better pattern.)


QUADGND - YAGIGND

    Each antenna uses 3 elements on a 0.3 wave boom, and the boom of each
is located 0.5 wave above ground.  This plot comparison is designed to test the
widely-held notion that quads have a lower angle of radiation than Yagis when
mounted low to the ground.  This and the previous comparison are good to have
at hand during your next Quad vs. Yagi argument (no matter which side you
take).


3ELYAGI - 3ELYAGI2

    Here we have a max gain Yagi vs. one optimized for a small rear
pattern.  The max gain antenna really isn't a very practical design, as the
impedance bandwidth is too narrow to be of much use, except on spot frequencies
or the narrow 12 meter band.  But note how the forward lobe of the max gain
design is significantly narrower than that of the other antenna.  This
illustrates how the use of a single F/B number to describe an antenna's
directivity characteristics can be quite misleading.  If you are a W6 trying to
work Europe on the short path, and want to reject loud East Coast stations as
much as possible, the max gain Yagi would be a better choice than the antenna
having the high F/B.


3ELYAGI - 3OVER3

    A single Yagi vs. a stacked pair of the same antennas.  The single Yagi
was optimized in free space for best pattern.  Note how the presence of a
second antenna, 44 feet away, degrades the pattern considerably.  The stacked
pair should be reoptimized as a set.


DIPOLE - VERTICAL

    This is perhaps the most surprising plot comparison of the lot.  Note
how the horizontal antenna has several dB advantage over the vertical right
where the vertical is supposed to be king, at low wave angles.  At the lobe
peaks, around 30 deg, the dipole is 7.5 dB better.  Average earth
characteristics were used for both antennas.  Try the comparison with other
earth characteristics.


TURNSTIL - SQUALO

    Two more or less omnidirectional horizontal antennas.  The differences
in omnidirectionality are immediately visible using the plot compare feature.


BROADSID - W8JK

    Two simple wire arrays yielding about the same gain, but with very
different azimuth and elevation patterns.  One uses broadside elements, and the
other collinear and endfire elements.


LAZYH - BISQUARE

    Each has a pair of broadside and a pair of collinear elements, but the
Bisquare elements are not parallel, costing about 3 dB gain.  Note how the
nulls in the Lazy H pattern are filled in for the Bisquare.


2INPHASE - EXTDZEPP

    Note how different the azimuth pattern becomes just by lengthening the
wire 28%.