Antennas, simulations,
antennas related projects and information. |
Peter a VM user sent this project that does not work at all. He
asked:
I TRIED THIS EXAMPLE TO MODEL ZL SPECIAL
BUT IT DID NOT WORK PROPERLY. WHAT I HAVE I DONE WRONG?
CM ZL Special for 145 MHz
CM
CE
Free Space
UNITS Feet
Height 60.000
Boundary Circular
F 145.000
GW 1 10 -255.000 482.500 0.000 -255.000 -482.500 0.000 12.700
GW 2 4 -255.000 -482.500 0.000 -255.000 -482.500 -25.000 12.700
GW 3 10 -255.000 -482.500 -25.000 -255.000 482.500 -25.000 12.700
GW 4 4 -255.000 482.500 -25.000 -255.000 482.500 0.000 12.700
GW 5 10 0.000 463.500 0.000 0.000 -463.500 0.000 12.700
GW 6 4 0.000 -463.500 0.000 0.000 -463.500 -25.000 12.700
GW 7 10 0.000 -463.500 -25.000 0.000 463.500 -25.000 12.700
GW 8 4 0.000 463.500 -25.000 0.000 463.500 0.000 12.700
S 1 5 100 135 I
S 2 45 100 0 I
Coax 35
A folded dipole is difficult to model because of
the parallel lines but we can try and see what we can do with this.
A quick look at the 3D View showed immediately
many problems
Segmentation is not good. Too many segments on
verticals not enough on horizontals. Segment length ratio at corners
is way off the chart. Always remember to keep segment ratio between
adjacent wires in a maximum 1 to 2 ratio.
First thing to do is increase segmentation on
horizontals (x4) and reduce segments on vertical to one to have
segments approximately of same length. Increasing segments means that you must use the VM version if you go
beyond the 50 segments limit found in the standard version.
Next, the source on the director is off-centered
to the left by two segments (red dot). We recenter the source. This
is how it looks now:
A quick impedance calculation now...
Antenna in Free Space
Frequency : 145.000 MHz
Wave Length : 2.068 m (6.783 ft)
IMPEDANCE = 381.31 + j 758.51 Ohms at Source 1
SWR = 54.08
Voltage = -79092.34 - j 19913.58 at Pulse 20
Current = -62.80 + j 72.70 Amps
Power = 1759677.63 WATTS
Antenna in Free Space
Frequency : 145.000 MHz
Wave Length : 2.068 m (6.783 ft)
IMPEDANCE = -7.42 + j 3.34 Ohms at Source 2
SWR = Undefined
Voltage = 100.00 + j 0.00 at Pulse 102
Current = -11.20 - j 5.05 Amps
Power = -560.11 WATTS
Something seems wrong here!
We use the 3D view to see if wires are connected correctly.
Everything seems ok.
Display currents by clicking on the 1st button in
3D View. Wow bad! Look like antenna is multi-wavelength with all
these peeks.
A quick check of the
project in the geometry grid is done by clicking multiple times on
the unit button until wavelength is displayed. Then click on a wire
and you get the wire length in wavelengths. Normally it should be
around .5 wlnght long. Look what we have here!
Hey look at the project source:
CM ZL Special for 145 MHz
CM
CE
Free Space
UNITS Feet <--------
we have feet
here and wire is 482 to -482 feet long!
Height 60.000
Boundary Circular
F 145.000
GW 1 10 -255.000 482.500 0.000 -255.000 -482.500 0.000 12.700
GW 2 4 -255.000 -482.500 0.000 -255.000 -482.500 -25.000 12.700
GW 3 10 -255.000 -482.500 -25.000 -25
........
OK Seems that the unit is millimeters
and user entered all lengths in mm but forgot to set units as
feet. How can we correct that quickly?
We open the file with WordPad and
replace feet by mm on the UNITS line, then reload the
project in NEC4WIN/VM. Quick check in geometry grid:
Impedance calculation:
Antenna in Free Space
Frequency : 145.000 MHz
Wave Length : 2.068 m (6.783 ft)
IMPEDANCE = 92.26 - j 110.73 Ohms at Source 1
SWR = 3.53
Voltage = 1138.51 + j 14234.87 at Pulse 20
Current = -70.82 + j 69.29 Amps
Power = 452829.94 WATTS
Antenna in Free Space
Frequency : 145.000 MHz
Wave Length : 2.068 m (6.783 ft)
IMPEDANCE = 118.42 + j 159.08 Ohms at Source 2
SWR = 4.86
Voltage = 11947.54 + j 15534.35 at Pulse 102
Current = 98.81 - j 1.55 Amps
Power = 578174.06 WATTS
SWR is still high because coax impedance is 35 Ohms
and should be Higher. We still have a high reactance. We will have
some fine tuning to do on this model to get better results.
Lets take a look at the patterns
We have a 6.32 dBi forward gain and
30 dB Front to back! This seems a bit optimistic and in the real
world it will be lower but these ZL are known for good front to
back.
Remember that the model is still not
well tuned and need more fine tuning. A quick look at the 3D Far
Field and we understand the horizontal pattern shape. We have
"holes" at the director location and a hole in the back.
That why we have this very high front/back. The Azimuth pattern
above is a horizontal slice of this pattern in the antenna elements
plane. The vertical pattern is a vertical slice centered on the
elements center.
Note:
All graphs and view produced on NEC4WIN/PRO our 'next generation' VM
version with improved graphics. NEC4WIN/PRO is still in development
and not released yet.
