Modeling Commercial Amateur Radio Antennas

Estimating Commercial Antenna Performance Through NEC Modeling
Arrow OSJ 146/440 J-Pole, Cushcraft R7 HF Vertical, Diamond D130J (Icom AH7000) Discone, High Sierra Sidekick, and Workman WHF40

by Dr. Carol F. Milazzo, KP4MD (posted 06 August 2011)
E-mail: kp4md@arrl.net

Arrow OSJ 146/440 J-Pole Antenna

The Arrow OSJ 146/440 J-Pole Antenna is a vertically polarized omnidirectional antenna.  The manufacturer's specifications1 state a VSWR of less than 1.5:1 over 143-149 MHz and 430-450 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.  Crothers2 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 Performance on 146 MHz Arrow OSJ
                        146/440 Performance on 430 MHz
Arrow OSJ 146/440 Performance on 146 MHz Arrow OSJ 146/440 Performance on 430 MHz
Arrow
                        OSJ 146/440 Pattern @ 80" on 146 MHz Arrow
                        OSJ 146/440 Pattern @ 80" on 430 MHz
Arrow OSJ 146/440 Pattern @ 80" on 146 MHz Arrow OSJ 146/440 Elev. Pattern @ 80" on 430 MHz
Arrow OSJ 146/440 NEC Model
                        SWR 140-150 MHz Arrow OSJ 146/440 NEC Model
                        SWR 430-450 MHz
Arrow OSJ 146/440 NEC Model SWR 140-150 MHz Arrow OSJ 146/440 NEC Model SWR 430-450 MHz


Arrow OSJ 146/440 NEC Model Files
In free space   80 inches above ground   80 inches above ground on metal mast
Arrow OSJ 146/440 J-Pole Videos

References:

1 Arrow OSJ 146/440 J-Pole Antenna, http://www.arrowantennas.com/inst/OSJ146440.pdf
2 "The J-Pole Antenna", Crothers C Bronson, AA1ZB
3 "The J-Pole Revisited", Richardson D, K6MHE, CQ, March 1998, 34-41

OSJ 146/440
              NEC Model

Arrow OSJ 146/440 NEC Model

Arrow OSJ
              146/440 J-Pole Antenna


Cushcraft R7 Vertical Antenna

The 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 SWR vs. Frequency

Cushcraft R7 Model

Cushcraft R7 NEC Model

Cushcraft R7 Free Space Performance on 14.175
                      MHz Cushcraft R7
                      Free Space Pattern on 14.175 MHz
Cushcraft R7 Free Space Performance on 14.175 MHz Cushcraft R7 Free Space Pattern on 14.175 MHz
Cushcraft R7
                      Performance @ 5 feet on 14.175 MHz Cushcraft R7 Pattern @
                      5 feet on 14.175 MHz
Cushcraft R7 Performance @ 5 feet on 14.175 MHz Cushcraft R7 Pattern @ 5 feet on 14.175 MHz

Cushcraft R7 NEC Model Calculations

Cushcraft R7 Dimensions

Cushcraft R7 Dimensions

Cushcraft R7 NEC Model Files
In free space    at 5 feet above good ground   
Cushcraft R7 Videos

Feedback: Cushcraft R5 NEC Model

On 3 March 2012, Peter van Latum PA0PVL from the Netherlands wrote and sent his 4nec2 model and pictures for the Cushcraft R5 vertical antenna (similar to the R7 but without the 7 and 10 MHz bands).
Cushcraft R5 NEC Model File by PA0PVL Cushcraft R5 4nec2 Screen Capture Files by PA0PVL
FHV Geerligs PA0FRI has also posted a detailed description of the construction of the Cushcraft R5 vertical antenna at http://pa0fri.home.xs4all.nl/Ant/R5/onhr5eng.htm.


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

The Diamond D130J (also marketed as the Icom AH7000) is an ultra wide-band omnidirectional vertically polarized antenna designed to receive over the 25-1300 MHz frequency range.  It can also be used for transmitting up to 100 watts (50 watts PEP on 6 meters) on the 50, 144, 222, 430, 900 and 1200 MHz amateur bands.  The top vertical whip and loading coil may be removed if 25-50 MHz is not required.  These NEC models are for both configurations.

Rather than specifying fixed coordinates, this NEC model was constructed using variables for the element dimensions and the GM NEC card to copy the elements around the vertical axis (a useful technique for antennas with radial symmetry).  This facilitates the use of the 4nec2 optimizer function to customize the antenna design and to observe the effects of varying the antenna configuration and dimensions upon expected performance.  The 4nec2 program automatically calculates the segmentation and the endpoint coordinates of all the antenna elements.

In his article "Notes on HF Discone Antennas"1 Cebik described NEC-2 code limitations that also affect VHF discone models.  The specified default values of the variables yield models that pass all validation tests.  However, for the 0.1" default element radius of this model, the 4nec2 program reports segment length validation errors for s values less than 0.6" or greater than 3" depending on frequency, segment angle errors for s values less than 1", and Average Gain Test (AGT) failures for disc to cone apex separations less than 1" or greater than approximately 6" depending on frequency.  Therefore, the source wire length and the disc-to-cone separation of these models has been adjusted to yield the best AGT scores and therefore the most reliable model values as measured internally to the model.

Discone
                        Antenna SWR (50-1300 MHz) Discone
                        Antenna Impedance (50-1300 MHz)
D130J Discone Antenna SWR (50-1300 MHz)
D130J Discone Antenna Impedance (50-1300 MHz)
Discone
                        Antenna without Top SWR (50-1300 MHz) Discone
                        Antenna without Top Impedance (50-1300 MHz)
Discone Antenna without Top SWR (50-1300 MHz) Discone Antenna without Top Impedance (50-1300 MHz)

Diamond D130J (Icom AH7000) Discone NEC Model Files
With Top Section (25-1300 MHz)
Without Top Section (100-1300 MHz)
Discone Antenna Videos

References:

  1. "Notes on HF Discone Antennas", Cebik LB, W4RNL
  2. "A Homebrew 100-1300 MHz Discone Antenna for $15", Milazzo CF, KP4MD
  3. 4nec2 file for Homebrew 100-1300 MHz Discone Antenna
Diamond D130J NEC
              Model

Diamond D130J Discone NEC Model


Diamond D130J Discone Antenna
Diamond D130J Discone Antenna


Discone Antenna NEC Model Variable Definitions
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
1.07e-6


High Sierra Sidekick

The High Sierra Sidekick is a shortened electrical 1/4-wave 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.

High Sierra Sidekick Base Section Measurements
Coil Position
Base Section
Length (inches)
Base Section
Resistance (ohms)
Coil Inductance
(microhenries)
Coil Parasitic
Capacitance (pF)
Fully retracted
15.25
0.2
0.096
0
Fully extended
21.25
2.0
245.0
35.3

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
R (ohms) = (2-0.2)×len/6+0.2    :    L (µH) = (d×n)2/(18×d+40×len)+0.096    :    C (pF) = 35.3×(len/6)

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.

High Sierra Sidekick PhotoHigh Sierra Sidekick
              Model
Sidekick Photo and Model

High Sierra Sidekick NEC Model Files
With 3 foot whip on perfect ground  tuned to 3.5 MHz    tuned to 50 MHz  
With 6 foot whip on perfect ground  tuned to 3.5 MHz    tuned to 28 MHz
With 6 foot whip on a six foot steel shed  tuned to 7 MHz    tuned to 28 MHz
Screwdriver Antenna Video

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
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 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
Sidekick with 6 foot whip 3.5 MHz Performance Sidekick with 6 foot whip 3.5 MHz Elevation Pattern

Workman WHF40 Dipole (Hamstick®-type dipole)

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.

WHF40
              Dipole SWR WHF40 Dipole Model
Workman WHF40 Dipole SWR vs. Frequency Workman WHF40 7 MHz Dipole NEC Model


WHF40 Dipole 7 MHz Performance at 5
              feet WHF40 Dipole Elevation Pattern at 5
              feet
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
WHF40 Dipole 10 MHz Performance at 5 feet WHF40 Dipole 10 MHz Elevation Pattern at 5 feet

Workman WHF40 Dipole NEC Model Files
At 5 feet above good ground tuned to 7.035 MHz    tuned to 10.135 MHz
Hamstick Dipole Videos



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