Computer Assisted Low Profile Antenna Modeling II Computer Design of a Low Profile Horizontal Loop Antenna for a Limited Space Backyard by Dr. Carol F. Milazzo, KP4MD (posted 25 September 2010, updated 18 Feb 2018) E-mail: [email protected] SUMMARY This article describes the application of computerized modeling to design and analyze the performance of a low cost, low profile horizontal loop antenna that exhibits gain over a dipole within a limited space backyard using household materials and low cost speaker wire zip cord as the transmission line. INTRODUCTION I first experimented with antenna modeling in 19971 when I moved to a residential housing development where restrictive covenants did not permit outdoor antennas.  The space available for antennas at that time was inside a peaked roof attic.  In 2006, I moved to a mobile home park with covenants that restricted antennas visible from the street.  My first antenna at this location was a remotely tuned base loaded 6 foot vertical whip antenna (High Sierra Sidekick) mounted on the roof at the rear of the home.  The aluminum siding served as its counterpoise.  Its observed performance was fair on 21 and 28 MHz, mediocre on 7 and 14 MHz with little reception except to the north, and overall quite poor on 3.5 MHz.  Seeking better performance on the lower frequencies, I later installed a 100 foot end-fed random wire antenna of 20 gauge insulated stranded wire supported by two 15 foot PVC poles at the south corners of the property.  That antenna performed marginally for contacts within 200-300 miles on 1.8 and 3.5 MHz but quite poorly on the higher frequency bands.  Additionally, it was highly susceptible to local radio frequency noise conducted from cheap electronics into the house wiring and radiated strong radio frequency fields within the home's living space that caused erratic operation of equipment and appliances. Recently, I decided to analyze the performance of these antennas with computer modeling in an attempt to design a more effective antenna system. NEC AND MININEC Most modern antenna analysis programs have their origins in a very large and complicated FORTRAN program called the Numerical Electromagnetics Code or "NEC." NEC analyzes wire antennas by dividing them into a number of segments, calculating the current in each segment and summing the results. This provides information on the radiation pattern and impedance of the antenna for any selected frequency. NEC was written in the 1970's and was composed of tens of thousands of lines of computer code requiring the use of a mainframe computer inaccessible to most radio amateurs. In 1980, the team of John Rockway and Jim Logan successfully wrote a very simplified version called MININEC that had about 500 lines of BASIC and could run on a personal computer. Since that time, MININEC has evolved through several versions and enhancements to take advantage of the increased power of modern personal computers. MININEC, NEC-2 and NEC-4 provide the basis for a large portion of the amateur radio literature concerning antenna analysis.

CONTENTS Introduction NEC and MININEC Modeling the existing antennas Designing the loop antenna model Analyzing the loop antenna model Building the loop antenna Adjusting & measuring the loop antenna On the air testing Analyzing as a top loaded vertical Antenna Feed System Improvements Optimization for 80 meters/WARC Noise levels Ladder Line Installed Loop Antenna Raised References 4nec2 input files

Since my work with the antenna modeling program NEC4WIN in 1998, several new user interfaces for MININEC and NEC have become available and lists and comparisons of their features are available elsewhere.  For this study I chose to use 4nec2 by Arie Voors due to its functionality (3-D graphics of antenna model, radiation pattern plots, graphs of impedance, VSWR, etc. vs. frequency) and its availability as a freeware download at http://www.qsl.net/4nec2/.²

MODELING THE EXISTING ANTENNAS

Fig. 1.  Aerial photo of property (north is right)	Fig. 2.  Survey map of property	Fig. 3.  2-D model	Fig. 4.  3-D model

Figures 5 through 16 are the calculated far field radiation patterns comparing the vertical antenna and the random wire antenna on 7 and 14 MHz (North is at the top of all azimuth and 3-D patterns).

Fig. 5.  Vertical 7 MHz azimuth pattern	Fig. 6.  Vertical 7 MHz elevation pattern	Fig. 7.  Vertical 7 MHz 3-D pattern
Fig. 8.  Random wire 7 MHz azimuth pattern	Fig. 9.  Random wire 7 MHz elevation pattern	Fig. 10.  Random wire 7 MHz 3-D pattern
Fig. 11.  Vertical 14 MHz azimuth pattern	Fig. 12.  Vertical 14 MHz elevation pattern	Fig. 13.  Vertical 14 MHz 3-D pattern
Fig. 14.  Random wire 14 MHz azimuth pattern	Fig. 15.  Random wire 14 MHz elevation pattern	Fig. 16.  Random wire 14 MHz 3-D pattern

Table 1 below lists the calculated major lobes of radiation, the direction of the major lobe, and overall radiation efficiency.  The radiation efficiency is a measure of the overall proportion of power that is radiated into space after losses in the structure and the ground are subtracted.

The calculated directionality of the vertical antenna corresponds with the observed performance with nulls in the south and east directions.  From 65% to 95% of the transmitter power was wasted in structure and ground losses.

DESIGNING A HORIZONTAL LOOP ANTENNA MODEL

I chose to raise the feed point to the top of the 20 foot mast that supported a VHF/UHF J-pole antenna. This additional height was needed for the loop to clear the top of the tree in the backyard. A rope and pulley would be used to raise and lower the feed point for maintenance and antenna adjustments. A new 15 foot PVC pole would be tied to a carport upright to support the fourth corner of the loop.  The resulting loop geometry would be a horizontal trapezoid with two sides sloping up to the feed point (see Figures 17 and 18). In order to have a wire segment length on the order of .05 wavelength on the highest frequency (28 MHz), 80 segments were required. To have all segments of nearly equal length, the two sides nearest the feed point were given 18 segments each, and the far sides were given 22 segments each.  By trial and error, it was found that locating a corner of the loop over the center carport upright yielded the desired resonant frequencies in the 7 and 14 MHz bands (Figures 19 and 20). Download the NEC input file.

Fig. 17.  Horizontal loop 2-D model	Fig. 18.  Horizontal loop 3-D model	Fig.19.  Reactance/resistance over 7 MHz Resonance near 7.01 MHz.	Fig.20.  Reactance/resistance over 14 MHz Resonance at 14.25 MHz.

ANALYZING THE LOOP ANTENNA MODEL

Fig. 21.  Frequency sweep of SWR over 6-29 MHz	Fig. 22.  Frequency sweep of reactance & impedance over 6-29 MHz

Figures 23 through 34 show the calculated far field radiation patterns for the horizontal loop antenna on 7, 14, 21 and 28 MHz.

Fig. 23.  Loop 7 MHz azimuth pattern	Fig. 24.  Loop 7 MHz elevation pattern	Fig. 25.  Loop 7 MHz 3-D pattern
Fig. 26.  Loop 14 MHz azimuth pattern	Fig. 27.  Loop 14 MHz elevation pattern	Fig. 28.  Loop 14 MHz 3-D pattern
Fig. 29.  Loop 21 MHz azimuth pattern	Fig. 30.  Loop 21 MHz elevation pattern	Fig. 31.  Loop 21 MHz 3-D pattern
Fig. 32.  Loop 28 MHz azimuth pattern	Fig. 33.  Loop 28 MHz elevation pattern	Fig. 34.  Loop 28 MHz 3-D pattern

Table 2 below compares the calculated major lobes of radiation, the direction of the major lobes, and overall radiation efficiency of the vertical, random wire and horizontal loop antennas.

On 7 MHz, the loop antenna offered omnidirectionality with mostly high angle radiation, 5.5 dB more maximum gain and over double the radiation efficiency of the vertical antenna, and 10 dB more maximum gain and 10 times the radiation efficiency of the random wire antenna..  On 14 MHz, compared to the other antennas the loop antenna offered near omnidirectionality, low angle of radiation, up to 3.3 dB more maximum gain and up to twice the radiation efficiency.

The random wire antenna was removed, and Table 3 compares the 4nec2 calculations for the horizontal loop antenna (SWR at the feed line characteristic impedance of 114 ohms) with the vertical antenna on all amateur radio high frequency bands.

*High standing wave ratios were expected to cause increased losses on frequencies other than 7, 14, 21 and 28 MHz.
**Replacing the random wire with the loop antenna significantly increased the maximum gain and radiation efficiency of the vertical antenna on 7 MHz where near field coupling induced significant currents in and radiation from the loop antenna.

BUILDING THE LOOP ANTENNA

The wire loop antenna is made of 140 feet of CTI-20 gauge insulated stranded wire. Cost $10.	The balanced feed line was this 40' roll of 24 gauge speaker wire.  Later replaced with 18 gauge speaker wire.	The feed point insulator is made from a paper clip and half of a ball point pen barrel.  UV exposure degraded this plastic.  Later replaced with a more durable insulator.	Five holes are drilled in the pen barrel and the clip is fashioned into an eye hook for the support rope.	The feed point is half assembled. Wire nuts splice the feed line to the ends of the loop antenna.
This tie string tension pulley is mounted atop the mast that supports the feed point with a machine screw eye bolt.	Here is the assembled feed point raised 20 feet to the top of the supporting mast.	Here is the southeast corner of the loop antenna. The screwdriver antenna base is visible on the roof of the house.	At the corner supports, the loop wire passes through a zip tie secured through a hole in the PVC pole.  Later replaced with 20 foot telescoping fiberglass poles.	The feed line is suspended away from the mast with twin lead standoff insulators.
A standoff insulator maintains the feed line (twisted along its length) distance from the porch awning.	A nylon monofilament suspends the feed line where it bends to pass through the vinyl window frame.	4:1 Ruthroff (voltage) balun matched the coax cable to the balanced load. Later replaced with 1:1 current balun.	The feed line & balun are installed.  The copper braid exits the window to the ground rod.	After the blinds are closed, the feed line and balun are out of view.

ADJUSTING AND MEASURING THE LOOP ANTENNA

Ideally, antenna impedance measurements should be taken directly at the antenna feed point.  The data in Figures 35 through 37 were collected remotely with a Bluetooth connected miniVNA Pro vector network analyzer connected directly to the antenna feed point.  Measurements with an unbalanced antenna analyzer, such as an MFJ-259, at the feed point or through the balanced transmission line must be taken through a 1:1 current balun.  I built one such balun by passing 12 turns of RG-174/U miniature 50 ohm cable through a stacked pair of FT114-43 toroid cores and another one by wrapping 30 turns of RG-174/U around a 4-3/4" x 3/8" ferrite rod salvaged from a transistor radio (Figure 38).  Either of these gave comparable results.

Horizontal Loop Antenna Parameters Measured at Feed Point
Fig. 35. Standing Wave Ratio and Reflection Loss over 3-30 MHz	Fig. 36. Resistance and reactance over 3-30 MHz
Fig. 37.  Impedance and phase over 3-30 MHz	Fig. 38.  Homemade 1:1 current baluns

Comparing these to the NEC model data in Figures 21 and 22,  the measured frequencies of SWR and impedance minima and points of zero reactance at 7, 14, 21 and 28 MHz coincided with predicted values.  Note that frequency data obtained through a transmission line will be skewed by impedance transformation.

An interesting finding was that the measurements were significantly affected when the loading coil of the vertical antenna was tuned to the same frequency at which the loop was being measured.  This effect was most prominent on 7 MHz and decreased at the higher frequencies.  This confirmed others' observations that loop performance is affected by other resonant objects within the near field of the antenna.  For this reason, the loading coil on the Sidekick vertical antenna was set to minimum inductance during all measurements of the loop antenna.
 

ON THE AIR TESTING On frequencies below 7 MHz, the loop antenna often provided clearer reception of signals with lower noise levels, although signal reports from other stations showed that the vertical antenna was the more efficient radiator.  Later, received noise levels on both antennas were quantitatively compared.  On 7 MHz received sky wave signals were predominantly up to 12 dB stronger on the loop, with few signals favored by the vertical antenna.  Above 7 MHz received and transmitted sky wave signals were predominantly stronger on the loop antenna. In the week after the loop antenna was erected on 11 September, 2010, my online log recorded solid contacts on 40 meters with stations in the eastern USA, Australia, Brazil, Guatemala, Japan and South Korea, areas that I could rarely contact with my previous antennas.  Within 12 hours on 10 November 2010, the horizontal loop antenna yielded confirmed contacts on 6 continents using 5 watts with WSPR mode on 7 and 14 MHz.  Starting in January 2011 WSPR data was used for further analysis of antenna performance.8 Comparisons between the antennas in contacts with local stations via ground wave were variable, favoring either the vertical antenna or the loop antenna depending on polarization and antenna directivity.

Fig. 39.  6 continents in 12 hours using 5 watts WSPR mode on 7 & 14 MHz.

ADDENDUM - 07 October 2010 - The Horizontal Loop as a Top Loaded Vertical Antenna

ANALYZING THE LOOP AS A TOP LOADED VERTICAL ANTENNA

Fig. 40.  Frequency sweep of SWR over 1.6-4.2 MHz	Fig. 41.  Frequency sweep of reactance & impedance over 1.6-4.2 MHz

Figures 42 through 47 show the calculated far field radiation patterns for the antenna used in this manner on 1.8 and 3.5 MHz.

Fig. 42.  Loop as vertical 1.8 MHz azimuth pattern	Fig. 43.  Loop as vertical 1.8 MHz elevation pattern	Fig. 44.  Loop as vertical 1.8 MHz 3-D pattern
Fig. 45.  Loop as vertical 3.5 MHz azimuth pattern	Fig. 46.  Loop as vertical 3.5 MHz elevation pattern	Fig. 47.  Loop as vertical 3.5 MHz 3-D pattern

Table 4 below shows the calculated low gain and overall radiation efficiency of the horizontal loop antenna used in this manner.

ON THE AIR TESTING

As 4nec2 predicted that the antenna would present non-resonant load, the transmitter was connected to it through the MFJ-949B antenna tuner without the balun.  The tuner was able to present a 50 ohm non-reactive load to the transmitter, which delivered the full rated 100 watts.  Reception was adversely affected by radio frequency interference from appliances, and a significant transmitted radio frequency field inside the station caused intermittent operation of a computer, touch lamps and television.  Despite the predicted low radiation efficiency, my initial contact on 3.5 MHz was reported as S9 signal strength at K2LMQ, 459 miles away in Kingman, AZ on 7 Oct 2010 at 0502 UTC.

Update - 04 December 2010 - Antenna Feed System Improvements

Described in detail separately in Zip Cord Transmission Lines and Baluns⁹  In December 2010, the 24 AWG transmission line was replaced with lower loss 18 AWG speaker wire (Figure 48), common mode chokes were added (Figure 49) and the 4:1 voltage balun was replaced with a 1:1 current balun (Figure 50) in order to reduce the antenna feed system noise and attenuation.

In December 2017, to accommodate an increase in transmitter power to 500 watts, I wound a new 1:1 current balun on two stacked FT240-43 ferrite cores and replaced the speaker wire feed line with 300 ohm Ladder Line. The latter also very significantly decreased the feed line dielectric losses on the WARC bands and on frequencies above 14 MHz.

Figure 60 shows the present station antenna configuration.  With the 1:1 current balun on the output of the antenna tuning unit and the total transmission line length adjusted to 1/2 electrical wavelength at 7 MHz, the impedance presented was well within the range of the tuner on all frequencies 3.5, 7, 10, 14, 18, 21, 24 and 28 MHz.

Figure 48.  Pfanstiehl 18AWG AS-18/50Z Speaker Wire	Figure 49.  1:1 choke at feed point - 11 turns on FT114-43 core	Fig. 50. 1:1 current balun made of RG-174/U  coaxial cable on two FT114-43 toroid cores	FT240-43 1:1 current balun	Ferrites on Ladder Line

Update - 28 August 2013 - 40 Meter Loop Antenna Transmission Line Optimization for 80 Meters and WARC Bands

As seen above in Figures 35 and 36, the 40 meter full wave loop antenna presents a non-reactive load on its fundamental resonant frequency and harmonics thereof, but between these frequencies high impedance presents matching problems at the transmitter end of the transmission line. In addition, the impedance transforming properties of random transmission line lengths skew the resonant frequencies seen at the transmitter end of the feed line.

The vector network analyzer measurements at Figures 51 and 52 below demonstrate this effect measured at the transmitter end of the original random length 37.9 foot zip cord transmission line. Not only were the resonant frequencies skewed outside of the desired frequency bands, but the high impedance presented to the transmitter on the 80 meter and the 30, 17 and 12 meter "WARC" bands rendered impedance matching problematic at those frequencies.

Figures 53 and 54 demonstrate the effect of lengthening the zip cord transmission line to an electrical quarter-wavelength at 3.5 MHz (51 feet). This eliminated skewing of the natural resonant frequencies of the loop antenna at its natural fundamental and harmonic frequencies. Also, the impedance transforming characteristics of this transmission line at frequencies where it is also an odd multiple of a quarter-wavelength (3.5, 10.5, 17.5, and 24.5 MHz) transformed the loop antenna's high impedance at those frequencies to a low impedance, eliminating high RF voltage on and facilitating transmitter matching with my LDG Z-11Pro II automatic antenna tuning unit with an attached 1:1 current balun on 3.5 MHz and the WARC bands.

The principle of the 7 MHz full-wave loop operation on 3.5 MHz may be conceptualized by comparison with the HO loop, or halo antenna—a half-wave open loop antenna popular on VHF frequencies. The halo antenna is essentially a horizontal half-wave dipole with its elements curved to form an open-ended circular loop to provide a nearly omnidirectional radiation pattern. The halo antenna is typically fed at its low impedance center point, while its open ends exhibit high impedance. By analogy, the 7 MHz full-wave loop operates on 3.5 MHz as a half-wave halo antenna fed across its high impedance open ends. In this case, the impedance transforming characteristic of a 1/4 wavelength transmission line assists in coupling the high impedance antenna to the low 50 ohm impedance of the transmitter.  This situation repeats at frequencies where the loop antenna is an odd multiple of a quarter-wavelength, that is, at 10.5, 17.5 and 24.5 MHz, near the "WARC" frequency bands.

The 100 and 300 ohm lines are preferable to 450 and 600 ohm lines for this application. As can be seen in Figure 37, the measured loop feed point impedance varies between 80 and 350 ohms at its resonance points and between 800 and 1800 ohms at the midpoint peaks between resonance. For a quarter wavelength transmission line, the impedance seen by the transmitter (Z) is given by Z = Z₀² / Z_ant, where Z₀ is the characteristic impedance of the transmission line and Z_ant is the antenna impedance. Figures 55 and 56 compare these impedance transforming characteristics for 100 ohm and 450 ohm feed lines. The 100 ohm feed line transforms the impedance to 6 to 125 ohms at the transmitter, within the range of most antenna tuning units. However, the higher impedance feed line renders an impedance range of 110 to 2500 ohms at the transmitter, values that exceed the operating range and safe voltage ratings of many antenna tuners.

See more graphic analysis at Google Photos.

Horizontal Loop Antenna Parameters Measured at Transmitter End of Feed Line over 3 to 30 MHz
Fig. 51. Standing Wave Ratio and Reflection Loss - Random 37.9' feed line	Fig. 52. Resistance and Reactance - Random 37.9' feed line
Fig. 53. Standing Wave Ratio and Reflection Loss - 51' feed line (0.5λ @ 7 MHz)	Fig. 54. Resistance and Reactance - 51' feed line (0.5λ @ 7 MHz)
Impedance Transforming Characteristics of 100 and 450 ohm 1/4 Wave Transmission Lines
Fig. 55. A 100 ohm feed line transforms the antenna impedance to 6 to 125 ohms at the transmitter, within the range of most antenna tuning units.	Fig. 56. A 450 ohm ladder line renders an impedance range of 110 to 2500 ohms at the transmitter, exceeding the safe operating range of most antenna tuners.

NOISE LEVELS

The received noise levels with the loop antenna and the Sidekick vertical antenna were compared using a FlexRadio 3000 receiver set to a 1 kHz passband on 3 February 2011 at 0400Z.  The MFJ-949B antenna tuner was adjusted for 1:1 standing wave ratio through the parallel line 1:1 balun for each measurement with the loop antenna, and the antenna loading coil was adjusted for minimum standing wave ratio for each measurement with the vertical antenna.  Table 5 lists the noise levels within 1 kHz and their conversion to the standard dBm/Hz values and S units within a 3 kHz passband.  These levels represent a composite of atmospheric noise and local man-made noise that vary with season and time of day.

As expected, the noise levels decreased with increasing frequency.  The noise levels were comparable on the loop antenna and the vertical antenna on 7, 14, 21 and 28 MHz.  The loop yielded lower noise and lower received signal strengths on 1.8 & 3.5 MHz and the WARC bands probably due to losses associated with high line SWR on the speaker wire there, and to the low radiation efficiency of the 40m loop on frequencies below 7 MHz. Although the vertical antenna was the more efficient radiator below 7 MHz, the loop antenna yielded an average 10 dB better signal to noise ratio on received signals below 7 MHz. This difference is likely due to: coupling of radio frequency noise generated by home electronics into the aluminum siding that serves as the counterpoise half of the vertical antenna; and to the relative insensitivity of the horizontal loop at the lower frequencies to the low elevation angles at which radio frequency noise sources predominate.

The common mode signal and noise rejection were tested by shorting both sides of the balanced feed line together and observing for quieting of the receiver. Among the baluns, common mode signal rejection was greatest in the current baluns and least in the 4:1 voltage balun.  At 3.5 MHz, the 4:1 voltage balun showed no measurable rejection of common mode noise at all.  The toroid current baluns with their increased common mode signal rejection also improved reception in the low frequency and medium frequency ranges that were previously covered by strong intermodulation products from nearby medium frequency AM broadcast stations.

UPDATE - 09 Dec 2017 - Ladder Line Installed

The 18AWG speaker wire transmission line performed acceptably at power levels up to 100 watts and on 7 and 14 MHz where the standing wave ratio was under 3:1 and the dielectric loss was acceptable. However, the dielectric loss was increasingly high above 14 MHz and with the high feed line SWR on 80m and the WARC bands.

The acquisition of a 500 watt amplifier required replacement of the 18AWG speaker wire feed line. A 17m (54 foot) length of JSC-1320 stranded copper 300 ohm Ladder Line was determined to be an electrical 0.25λ length at 3.5 MHz. Television twin lead type standoff insulators served to route the wire to the 40 meter full wave horizontal loop antenna. Since parallel transmission lines must be routed several diameters away from other conductors, the excess length of the Ladder Line was loosely coiled and suspended on a rope. Photo album

Ladder Line exhibits low dielectric loss regardless of high feed line SWR. Using a feed line of an electrical quarter wavelength for 3.5 MHz offers the benefit of transforming the loop's very high feed point impedance at 3.5, 10.1, 18, and 25 MHz to a low impedance at the transmitter end that is within the matching range of my antenna tuning unit. Although a full-sized 80m loop would be more efficient, this arrangement allows me to operate on 3.5 MHz and the WARC bands with the 40 meter loop that fits in my smaller available space. Here is an interactive chart of the SWR measured at the transmitter end of the feed line.

In order to increase its power capacity, the old FT140-43 ferrite choke balun was replaced with nine turns of 18AWG speaker wire on two stacked FT240-43 toroid cores. Here is an interactive chart of the common mode impedance of the balun.

I placed several Bluecell 13mm clip-on ferrite cores over the feed point end of the 300 ohm ladder line as a common mode choke (similar to a W2DU balun). Its purpose is to help suppress undesired common mode current on the transmission line that would be caused by the asymmetric environment of the horizontal loop antenna. Photo album

Replacing the speaker wire with 300 ohm Ladder Line yielded an average 10 dB increase in signal to noise ratio of 3.5 MHz WSPR spot reports at KR6ZY. This demonstrates the significantly lower loss that the Ladder Line exhibits under the high feed line SWR condition on 3.5 MHz. The change from speaker wire transmission line to the 300 ohm Ladder Line also yielded a significant increase in number of 80m WSPR spot reports over 1800 km distance as shown on Distance vs. Date chart after 10 DEC 2017.

				>
17m (54 feet) of 300 ohm Ladder Line	SWR at the transmitter end of the feed line	FT240-43 1:1 current balun	Clip on ferrites at feed point	Ladder Line raised SNR by 10 dB on 3.5 MHz

UPDATE - 07 June 2011 - Loop Antenna Raised

With the horizontal loop antenna at this increased height above ground, the modeling program predicted increases in radiation efficiency on all frequencies and overall increases in gain at both low and high elevation angles.  Figures 57 through 59 compare the performance of the horizontal loop antenna against the vertical screwdriver antenna.  The loop antenna yielded greater gain at the desired high radiation angles (near vertical incident skywave or NVIS) for short distance paths on frequencies below 14 MHz and at greater gain at low radiation angles as desired for long distance paths on and above 14 MHz.  The loop antenna had significantly superior radiation efficiency on all frequencies above 3.5 MHz.

CONCLUSION Computer antenna modeling was useful in significantly improving my station antenna performance.  Adding some wire and supports converted an inefficient random wire antenna into a much more efficient full wave horizontal loop antenna. The $15 spent for the loop antenna wire, feed line and balun yielded significantly superior results to the screwdriver antenna which cost $450.  Compact antennas like the Sidekick screwdriver vertical compromise performance but have their applications for their portability and when space is especially limited, as on a vehicle. NEXT PROJECT: Comparative Antenna Analysis with WSPR ACKNOWLEDGEMENTS Many thanks to Dutch engineer Arie Voors for sharing his excellent 4nec2 antenna modeling program with the radio amateur community, Magnus Beischer, Don Lucas, Matt Pyne for their freeware TinyCAD program used to draw the schematic diagram, Dan Maguire for the Transmission Line Details program, and to Larry Sutter, WD6FXR, who very graciously permitted me the use of his MFJ-269 antenna analyzer for this project. REFERENCES "Computer Assisted Low Profile Antenna Modeling I", Milazzo, C, KP4MD, 13 June 1998. 4nec2 antenna modeling software, Voors, A. "A Beginner's Guide to Modeling with NEC", Cebik, LB, W4RNL, QST, November 2000, pp. 35-38. "The Loop Skywire", Fischer, D, W0MHS, QST, November 1985, pp. 20-22. "Zip Cord Antennas - Do They Work?", Hall, J, K1TD, QST, March 1979, pp. 31-32. "Zip Cord Antennas and Feed Lines For Portable Applications", Parmley, W, KR8L, QST, March 2009, pp. 34-36. "Portable Antenna Notes", Wiesen, R, WD8PNL "Comparative Antenna Analysis with WSPR", Milazzo, C, KP4MD, 13 January 2011. "Zip Cord Transmission Lines and Baluns", Milazzo, C, KP4MD.

Figure 60.  The station configuration since 10 DEC 2017.  The radio is connected through short RG-58/U coaxial cable jumpers to an automatic antenna tuning unit and a 1:1 Guanella current balun (common mode choke), then through a 54 feet (17m) length of 300 ohm Ladder Line to the full wave 40 meter horizontal loop antenna, a 140 foot closed loop of 20 AWG PVC insulated stranded wire. The entire antenna is at DC ground potential.

APPENDIX: 4nec2 INPUT FILES

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