Modeling Commercial Amateur Radio Antennas

Estimating Commercial Antenna Performance Through NEC Modeling
Cushcraft R7, High Sierra Sidekick, and Workman WHF40

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

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 pattern 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 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 NEC Model Calculations

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.

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.

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 adapted 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

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.

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

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

Return to KP4MD Home Page

Top