Modeling Commercial Amateur Radio Antennas Estimating Commercial Antenna Performance Through
NEC Modeling
by Dr. Carol F. Milazzo, KP4MD (posted 06 August 2011) 




Arrow OSJ 146/440 JPole AntennaThe Arrow OSJ 146/440 JPole Antenna is a vertically
polarized omnidirectional antenna. The manufacturer's
specifications^{1} state a VSWR of less than
1.5:1 over 143149 MHz and 430450 MHz and has been tested
to 1,000 watts. As this antenna is often mounted on metallic mast, it was
modeled both with and without the mast to demonstrate the
effect on the radiation pattern. When using the
dimensions provided in the manufacturer's literature,
4nec2's calculations of resonant frequency were several
MHz below the specification. Crothers^{2}
also noted this discrepancy and postulated its cause as
the irregular shape and disparate effective diameter of
the horizontal shunt section. Therefore, the element
lengths in this NEC model were shortened for the 4nec2
model to render a frequency response comparable to the
observed and published data. The following files and
elevation radiation patterns represent the 4nec2 antenna
model mounted at 80 inches (1 wavelength at 146 MHz) above
simulated good ground on a 1.2 inch diameter metal mast.

Arrow OSJ 146/440 NEC Model 



Cushcraft R7 Vertical AntennaThe Cushcraft R7 is a multiband vertical antenna rated at
1800 watts peak power on the 7 through 28 MHz amateur
radio bands. Not owning one of these antennas, I
designed the NEC model with the 4nec2
modeling program using the length and radius
dimensions in inches from the online
manual and the trap frequencies posted by Tait on
his web page Cushcraft
R5/R7 Maintenance and Repair. All segments
were between 5 to 6 inches in length. Although the
radial elements are stainless steel, for simplicity the
entire antenna was modeled as aluminum. A Q factor
of 250 was selected as representative of typical multiband
antenna traps. The inductance values were determined
using the 4nec2 optimizer function to obtain resonances in
the center of each frequency band, starting with 29 MHz
and progressing down towards 7.15 MHz. Standing wave
ratios were calculated for a characteristic impedance of
200 ohms due to the 4:1 transformer in the matching
assembly at the base. In the elevation radiation
patterns, 0° and 180° represent the horizon.
The azimuth radiation patterns are not shown as they are
circular. The patterns for a height of 5 feet over
ground were chosen as specified in the manual. The
accompanying model files and radiation patterns represent
the antenna in free space and at a height of 5 feet above
ground as specified in the manual. For other
heights, one needs to change the height in feet (hft, the
first variable in the file). SY hft=5 'Input height above ground in feet (default=5) Cushcraft R7 NEC Model SWR vs. Frequency 
Cushcraft R7 NEC Model 

Cushcraft R7 NEC Model Calculations

Cushcraft R7 Dimensions 

Cushcraft R7 NEC Model Files


Diamond D130J (Icom AH7000)
Discone Antenna (A Universal Discone NEC Model)

D130J
Discone Antenna SWR (501300 MHz) 
D130J Discone Antenna Impedance (501300 MHz) 

Discone Antenna without Top SWR (501300 MHz)  Discone Antenna without Top Impedance (501300 MHz) 
Variable
definitions 
Variable 
Default 

Input feed point
height (in.) 
hgh 
120 
Input disc radial
length (in.) 
dsc 
11.5 
Input cone radial length (in.)  rdl 
34 
Input segment length (in.)  s  1 
Input # segments in source wire 
sn 
2 
Input radial element radius (in.)  rad 
0.1 
Input number of spokes 
sp 
8 
Input cone angle
(90º=horiz) 
ang 
34 
Input design
frequency (MHz) 
f  222 
Input top section
length (in.) 
v 
30 
Input top section radius (in.) 
rad2 
0.05 
Input top section
inductance (H) 
L 
1.07e6 
High Sierra SidekickThe High Sierra Sidekick is a shortened electrical
1/4wave vertical monopole antenna with a remotely
controlled continuously adjustable base loading
inductor. This style of antenna is commonly called a
"screwdriver antenna." The Sidekick antenna is rated at
200 watts and tunes from 3.5 MHz to 60 MHz with the
included 0.1 inch diameter 3 foot long stainless steel
whip. The measured tuning range is from 2.6 MHz to
32 MHz when using the optional 6 foot (69.3 inch) long
whip that tapers from 0.2 to 0.1 inch diameter. The
base section of the radiator consists of a 15.25 inch long
2 inch outer diameter aluminum tube that contains the
motor and the loading coil. The coil consists of
approximately 144 turns of 24 AWG solid copper wire wound
over 6 inches length (24 turns/inch) on a 1.77 inch (45
mm) diameter phenolic form. While tuning the antenna
for lowest standing wave ratio, the motor controls the
amount of the coil that protrudes from the top of the
aluminum tube. This adjustment varies the length of
the base section, and the direct current resistance and
the inductance that are in series with the stainless steel
whip top section. The following table lists the
measurements of the base section with the coil fully
retracted and fully extended. The minimum inductance
and maximum parasitic capacitance values were calculated
by physically measuring the resonant frequencies of the
antenna with an attached 6 foot whip.
These NEC models have been validated as closely
approximating the dimensions and measured performance of
the physical antenna, calculating the loading coil
resistance, inductance and parasitic capacitance as
functions of the entered number of coil turns. Number of turns 0 < n <
144 : Coil length (inches) len=
n/24 : Coil diameter
(inches) d=1.79 The stainless steel whip is modeled with a 0.4 inch diameter lower end that tapers down to 0.1 inch diameter at the upper end because the NEC program constraints require that adjacent segment diameter ratios not exceed 5:1. As this antenna requires a counterpoise, the NEC file represents the antenna mounted on a perfect ground. The antenna model may be relocated onto a suitable modeled structure or vehicle by adjusting the variables x, y, and z1. 
Sidekick Photo and Model



High Sierra Sidekick NEC Model
Files
The variables have been selected so that the NEC files may
be customized for other similar antennas by entering the
corresponding measurements. 
Measuring the Sidekick Coil 
Sidekick with 3 foot whip 3.5 MHz Performance  Sidekick with 3 foot whip 3.5 MHz Elevation Pattern 

Sidekick with 6 foot whip 3.5 MHz Performance  Sidekick with 6 foot whip 3.5 MHz Elevation Pattern 
The Workman WHF40 is a 7 MHz mobile vertical antenna rated at 250
watts peak power. It consists of a 50.7 inch long by 0.4
inch diameter fiberglass section with a 320 turn 40 µH coil
wound on it and an adjustable 45 inch long stainless steel whip on
top. For this model, two of these antennas were mounted
horizontally to form what is known as a Hamstick dipole.
The antenna also tuned to 10 MHz by substituting shorter 10.5 inch
long wires for the end sections. The dimensions for these
models were measured with the antenna mounted on a tripod 5 feet
above the ground and tuned for 7.035 MHz and 10.135 MHz
respectively. On both frequencies the azimuth radiation
patterns were omnidirectional and the elevation radiation patterns
were directed toward the zenith. These NEC models have been
validated as closely approximating the dimensions and measured
performance of the physical antenna. Some variation in the
values of loading inductance and end section length are expected
due to manufacturing tolerances and the narrow bandwidth of this
antenna.
Workman WHF40 Dipole SWR vs. Frequency  Workman WHF40 7 MHz Dipole NEC Model 

WHF40 Dipole 7 MHz Performance at 5 feet  WHF40 Dipole 7 MHz Elevation Pattern at 5 feet 

WHF40 Dipole 10 MHz Performance at 5 feet  WHF40 Dipole 10 MHz Elevation Pattern at 5 feet 