[TowerTalk] Dipole Question

[Jim Reid]

Aloha Eric,

And thank you so much for your modeling and explanations!!
This goes a long way toward adding to my understanding
of antennas vs. the various configurations we amateurs
have tried,  with the same elements:  as horizontal
dipoles,  verticals,  ground planes,  and half-wave
vertical antennas.  All that informative data from a very
simple model!!  I find it fascinating that the feed point
impedance essentially returns to the 70 ohm area for
both horizontal and vertical half wave dipoles,  and that
the simple, "one radial"  vertical drops the feed point
R about in half,  as one expects a properly set up
vertical to have a feed R of about 36 ohms.  I bet adding
more horizontal "radials"  would drop the R from the 
model 40 ohms to the 36 ohm area.

Also very interesting to see the change in angle of
max radiation from these dipole elements as the
orientation is varied.  Appears that for DX the half-wave
vertical  and the "ground plane" simulation,45 degree 
sloped radial  lower the radiation peak toward
the horizon to about half the height of the 1/4 wave
vertical simulation.

However,  a rotating 1/2 wave dipole is clearly the best
gain  performer but with double the radiation angle
above the horizon to the pair above.

Again,  thanks so much for the time you took to provide
this information,  Eric.  Hope my interpretations of your
efforts are correct,  hi!

73,  Mahalo Jim,  KH7M 

(Note new "call name" for this old dog;
given to me inadvertently by a correspondent,
seems appropriate for an isolated island dweller wearing
a straw hat and shorts around all the time!)

In response to my dipole question,  Eric wrote the following:

1.  >Suppose a 20 meter, 1/2 wave long dipole is also 1/2 wave above
> >the ground.  

> The model claims that this configuration has a feedpoint
> impedance of 68.77 +j0 ohms.  The Azimuth pattern is expected.
> The gain is 7.4 dBi at an elevation angle of 28 degrees.  The
> elevation lobe structure is simple.  There is one broad lobe
> centered on 28 degrees.


2. >Now, one side of this dipole is now pulled straight up.  So we
> >have a center fed L, with 1/4 wave wire going toward the zenith
> >at a height of 0.75 waves, the other leg is still horizontal
> >with the ground, with the horizontal leg and the feed point
> >still 1/2 wave above the ground.  

> The model shows a feedpoint impdance of 40 -j24 ohms for this
> configuration.  Elevation angle of the main lobe(s) is 29
> degrees.  The gain is 4.33 dBi at azimuth angles of 98 and 262
> degrees, or about 8 degrees south of due east / west.  Elevation
> lobe structure is still simple.  the gain to the north is 9 dB
> down from the main lobe.  The gain to the south is 3.4 dB down
> from the main lobe.


3.  >Next, lower the horizontal wire 45 degrees.  Now the far end of
> >that wire is about 0.32 wavelength above the ground, but the
> >feed point remains 1/2 wave high.  What is the feedpoint
> >impedance now, and what does the pattern appear to be.
> 
> The model shows the feedpoint impedance for this configuration to
> be 64 +j3 ohms.  The gain for this configuration is 1.99 dBi.
> This occurs at an elevation angle of 47 degrees and an azimuth
> angle of 180.  The elevation lobe is a very broad fan shape with
> two peaks on it (looks like butterfly wing).  One at an elevation
> angle of 16 degrees and the other at 47 degrees.  The upper peak
> is the strongest by one or two dB.  The azimuth pattern is nearly
> circular.  But it does favor the south by 1.03 dB.

> >In either of  cases 1 or 2  with one leg going straight up, is
> >radiation favored in the direction of the horizontal, or lowered
> >leg, by much?
> 
> Yes.  See above descriptions.
> 

3.  >Drop the lowered wire straight down now; the lower end from the
> >feed point is now 0.25 wave length above ground, while the upper
> >end is now 0.75 wavelength above ground.  What is the feedpoint
> >Z, and what is the radiation pattern?
> 
> Feedpoint impedance models to 71 +j10 ohms.  Azimuth pattern is
> circular.  Gain is 0.95 dBi at an elevation angle of 14 degrees.
> the elevation pattern has a secondary lobe at 50 degrees which is
> 1.8 dB down from the main lobe.



--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm