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)
E-mail: kp4md@arrl.net

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 added a 100 foot end-fed antenna of 20 gauge stranded wire with PVC insulation supported by two 15 foot PVC poles at the south corners of the property.  That antenna performed fairly 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 noise and radiated strong radio frequency fields within the home's living space.

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.
Loop
                  Skywire Antenna
CONTENTS
Return to KP4MD Home Page

4nec2 antenna modeling programSince 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

To start, I measured the property with its significant structures and the antennas (Figure 2). Figuring the x,y,z coordinates from the station ground point 0,0,0, these numbers were entered into the program, along with the power source, load, wire radius and wire connection data.  (This process and guidelines are explained in detail in the article "A Beginner's Guide to Modeling with NEC"3, Cebik, LB, QST, November 2000, pp. 35-38). Figures 3 and 4 show the program's representation of the input file. The x-axis was oriented west-east, the y-axis north-south and the z-axis from zenith to nadir. As the proximity of the aluminum metal siding of the house in the near field would exert a significant influence on antenna performance, these surfaces were modeled as wire frames using 4nec2's Geometry Builder utility. Trees and other non-metallic structures having a lesser effect on antenna performance were ignored in the model. The model ground was selected as "Real Ground" and "Good" quality. North is toward the right in Figures 1 through 4.
 
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.

Frequency
MHz
Vertical Antenna
Random Wire Antenna
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
Resistance
ohms
Reactance
ohms
SWR @
50 ohms
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
Resistance
ohms
Reactance
ohms
7 MHz
0.09
NNW
21.48%
16.2
0
3.08
-4.6
Omnidirectional
5.147%
182
-j87.1
14 MHz
2.14
NNW
27.28%
51.1
0
1.02
4.08
SSW
35.25%
497
+j962
Table 1. Comparison of vertical and random wire antennas on 7 and 14 MHz

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

The desirable features of the new antenna were: low visibility from the street, improved omnidirectionality, improved radiation efficiency, operability on multiple bands (at least 7, 14, 21 and 28 MHz), broader bandwidth than the screwdriver vertical antenna to allow some frequency changes within each band segment without retuning, 100 watt power capacity, reduced sensitivity to local noise, and removal of strong radio frequency fields away from the interior of the home. In 1985, Fischer described a full wave horizontal loop antenna (also known as a "Loop Skywire"4) as meeting these requirements. With additional supports, the existing random wire antenna could be extended to complete a 40 meter full wave loop, and the feed point would be elevated above the roof. The loop antenna would be fed with a balanced transmission line.  Despite its lower power capacity and higher attenuation per unit length than some coaxial cables, a short run of dual conductor speaker wire used as a parallel transmission line is lighter, less visible, more easily available and economical, and may present less loss than coaxial cable when operated at high standing wave ratios.  Speaker wire is also very easily wound on a ferrite toroid core to form a common mode choke or 1:1 current balun.  Hall5, Parmley6, and Wiesen7 have also discussed and characterized the use of zip cords as transmission lines.

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

The full range frequency sweep (Figures 21 and 22) predicted resonances within the 7, 14, 21 and 28 MHz bands.
 
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.

Frequency
MHz
Loop Antenna
Vertical Antenna
Random Wire Antenna
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
7 MHz
5.53
Omnidirectional
53.93%
0.09
NNW
21.48%
-4.6
Omnidirectional
5.147%
14 MHz
5.45
NW-NE-SW-SE
59.41%
2.14
NNW
27.28%
4.08
SSW
35.25%
Table 2.  Comparison of horizonal loop, vertical and random wire 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.

Frequency
MHz
Loop Antenna
Vertical Antenna
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
Resistance
ohms
Reactance
ohms
Impedance
ohms
SWR @
114 ohms
Maximum
gain dBi
Lobe
direction(s)
Radiation
efficiency
Resistance
ohms
Reactance
ohms
SWR @
50 ohms
1.8 MHz
-22*
Omnidirectional
0.158%
3.53
-j566
566
828*
 N/A
 N/A
 N/A
 N/A
 N/A
 N/A
3.5 MHz
-3.7*
Omnidirectional
8.561%
0.38
-j162
162
905*
-2.3
Omnidirectional
14.69%
6.17
0
8.11
5.3 MHz
3.55*
Omnidirectional
37.08%
1.13
-j47.4
47.4
118*
-0.6
NNW
18.06%
9.17
0
5.45
7 MHz
5.53
Omnidirectional
53.93%
287
+j34.1
289
2.56
3.79**
Omnidirectional
37.12%**
63.8
0
1.28
10.1 MHz
6.55*
Omnidirectional 
63.82%
2.5
+j93.2
93.2
76.1*
0.77
NNE
25.65%
29.7
0
1.68
14 MHz
5.46
NW-NE-SW-SE
59.4%
84.1
-j152
174
4.27
2.06
NNW
27.46%
51
0
1.02
18.1 MHz
7.61*
NE-SW
71.22%
11.6
-j159
159
29.0*
2.79
NW
32.48%
62.4
0
1.25
21 MHz
8.55
NW-NE-SW-SE
67.91%
110
-j13.7
111
1.14
3.3
N
37.02%
64.9
0
1.3
24.9 MHz
7.8*
NW-NE-SW-SE
72.09%
7.73
+j106
106
27.5*
3.91
N
40.05%
81.5
0
1.63
28 MHz
9.6
NW-NE-SW-SE
69%
210
+j12.1
210
1.85
4.42
NW
42.91%
78.4
0
1.57
Table 3.  4nec2 calculations for horizontal loop and vertical antennas from 1.8 through 28 MHz

*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

Here is a pictorial description of the materials used and the construction of 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.
(Replaced with 18 gauge speaker wire: see Update - 04 Dec 2010)
The feed point insulator is made from half of a ball point pen barrel and a paper clip. 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. 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: see Update below
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

Given inaccuracies in the measurements and unaccounted environmental influences on the antenna model, we expected to have to adjust the wire length. A preliminary test showed the loop's fundamental resonant frequency was 6850 kHz.   Shortening the loop by 4 feet brought the SWR minima measured with a vector network analyzer at the antenna feed point (Figure 35) close to those predicted in Figure 21.

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 at 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
Standing Wave Ratio and Reflection Loss over 3-30
                  MHz Resistance and reactance over 3-30 MHz
Fig. 35. Standing Wave Ratio and Reflection Loss over 3-30 MHz Fig. 36. Resistance and reactance over 3-30 MHz
Impedance and phase over 3-30 MHz Homemade 1:1 current
                  baluns
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.

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.
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

Others have reported using the horizontal loop antenna below the design frequency by feeding both sides of the feed line tied together against ground.  This would, in effect convert the loop and feed line into a random length top-loaded vertical antenna.  The full range frequency sweep (Figures 40 and 41) predicted resonance outside the 1.8 and 3.5 MHz bands.
 
Frequency sweep of
                    SWR over 1.6-4.2 MHz
Frequency sweep of
                    reactance & impedance over 1.6-4.2 MHz
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.

Frequency
MHz
Top Loaded Vertical Antenna
Maximum
gain
Lobe
direction(s)
Radiation
efficiency
Resistance
ohms
Reactance
ohms
1.8 MHz
-21 dBi
Omnidirectional
0.232%
152
-j68.3
3.5 MHz
-11 dBi
Omnidirectional 
2.187%
167
+j462
Table 4. Gain and radiation efficiency of loop antenna as vertical

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 Baluns.9  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.

Figure 60 shows the present station antenna configuration.  With the 1:1 current balun T3 on the output of the antenna tuning unit and the total transmission line length adjusted to 51 feet, that is 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.
Pfanstiehl
                  AS-18/50Z Speaker Wire
Figure 48.  Pfanstiehl
18AWG AS-18/50Z
Speaker Wire
1:1 current balun at feedpoint - 11 turns on
                    FT114-43 core
Figure 49.  1:1 choke at
feed point - 11 turns on
FT114-43 core
1:1 coaxial cable toroid choke balun. 
10 turns of RG-174/U on 2 FT114-43 cores
Fig. 50. 1:1 current balun made of RG-174/U 
coaxial cable on two FT114-43 toroid cores

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 resonant frequency and harmonics thereof, but its high impedance between these frequencies 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 half-wavelength at 7 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 half-wavelength, that is, at 10.5, 17.5 and 24.5 MHz, near the "WARC" frequency bands.

The 100 ohm zip cord is preferable to 300 ohm twin-lead and 450 ohm ladder line for this application. As can be seen in Figure 37, the loop 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 / Zant, where Z0 is the characteristic impedance of the transmission line and Zant 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, a 450 ohm 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 most antenna tuners.

Horizontal Loop Antenna Parameters Measured at Transmitter End of Feed Line over 3 to 30 MHz
Standing Wave Ratio and Reflection Loss - Random
                  37.9' feed line Resistance and Reactance - Random 37.9' feed
                  line
Fig. 51. Standing Wave Ratio and Reflection Loss - Random 37.9' feed line Fig. 52. Resistance and Reactance - Random 37.9' feed line
Standing Wave Ratio and Reflection Loss - 51'
                  feed line (0.5λ @ 7 MHz) Resistance and Reactance - 51' feed line (0.5λ @
                  7 MHz)
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
The 100 ohm feed line transforms the antenna
                  impedance to 6 to 125 ohms at the transmitter, within
                  the range of most antenna tuning units. A 450 ohm ladder line renders an impedance range
                  of 110 to 2500 ohms at the transmitter, values that
                  exceed the operating range and safe voltage ratings of
                  most antenna tuners.
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.
 
Frequency
MHz
Loop antenna noise
Vertical antenna noise
dBm/kHz
dBm/Hz
S units/3 kHz
dBm/kHz
dBm/Hz
S units/3 kHz
1.8 MHz
-114
-144
3
-100
-130
5.3
3.5 MHz
-113
-143
3.1
-106
-136
4.3
5.3 MHz
-112
-142
3.3
-108
-138
4
7 MHz
-108
-138
4
-108
-138
4
10.1 MHz
-115
-145
2.8
-105
-135
4.5
14 MHz
-109
-139
3.8
-113
-143
3.1
18.1 MHz
-117
-147
2.5
-112
-142
3.3
21 MHz
-116
-146
2.6
-118
-148
2.3
24.9 MHz
-126
-156
1
-120
-150
2
28 MHz
-125
-155
1.1
-121
-151
1.8
Table 5.  Noise levels at receiver

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.  Both noise and received signal levels were lower on the loop antenna on lower frequencies due to the preference of the loop for high elevation angles at those frequencies.

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 - 07 June 2011 - Loop Antenna Raised

In June 2011, the far corners of the loop antenna were raised with green Jackite telescoping fiberglass poles from 15 feet (4.6 m) to a height of 20 feet (6.1 m) above ground so that the entire loop antenna would be in the same horizontal plane as the feed point with the expectation that it would increase efficiency by decreasing signal absorption by the house structure and the ground.  Jackite poles were selected as they are lightweight, economical, adequate to support small gauge wire, and superior in rigidity and aesthetic appearance to PVC poles.   The new NEC files are listed in the appendix.

Table 6 compares the calculated maximum gain, gain at 15° elevation (low angle for long distance propagation) and at 90° elevation (high angle for NVIS short distance propagation) in the same direction, and radiation efficiency for Sidekick screwdriver vertical antenna with the loop antenna at the two different heights.  The WSPR column is an estimation of the overall effective radiated power at each frequency for the WSPR transmitter with a nominal power output of 5 watts, line attenuation of 35 feet of transmission line from Zip Cord Transmission Lines and Baluns9 (not including losses due to impedance mismatch), and radiation efficiency of the loop antenna at 20 feet, .

Total ERP(W) = P(W) × 10-(attn(dB)/10) × Rad. eff.(%)

Before 7 June 2011
After 7 June 2011
Frequency
MHz
Loop Antenna @ 15' (4.6 m)
Sidekick Screwdriver Vertical Antenna
Loop Antenna @ 20' (6.1 m)
Sidekick Screwdriver Vertical Antenna
Maximum
gain dBi
Gain-dBi
@15°elev.
Gain-dBi
@90°elev.
Radiation
eff. %
Maximum
gain dBi
Gain-dBi
@15°elev
Gain-dBi
@90°elev
Radiation
eff. %
Maximum
gain dBi
Gain-dBi
@15°elev.
Gain-dBi
@90°elev.
Radiation
eff. %
WSPR
Est.Total
ERP (W)
Maximum
gain dBi
Gain-dBi
@15°elev
Gain-dBi
@90°elev
Radiation
eff. %
1.8
-23
-28.3
-25.6
0.12




-18
-25.5
-18.5
0.39
0.02




3.5
-4.4
-14.3
-4.37
7.09
-3.3
-4.03
-14.9
10.8
-1.2
-10.6
-1.17
14.4
0.57
-3.4
-4.05
-14.2
10.7
5.3
3.69
-6.7
3.65
37.4
-1.2
-2.1
-11.1
14.6
5.13
-6.74
5.11
51.0

-1.2
-2.15
-10.1
14.7
7
5.73
-4.67
5.65
56.1
4.54
-2.59
4.31
45.6
6.62
-7.17
6.58
66.5
2.4
1.8
-6.07
1.79
34.4
10.1
6.55
-2.02
6.11
64.8
0.85
-1.62
-0.13
24.7
7.03
-3.46
6.73
72.4
2.4
0.64
-2.12
-0.455
24
14
5.59
0.77
-2.01
61.3
3.76
-4.08
2.07
31.5
5.42
1.17
-3.8
68.3
2.1
2.68
-4.0
1.21
28.8
18.1
7.7
0.91
4.22
75.2
3.49
-1.12
-0.62
32.9
7.74
3.85
4.88
80.0
2.3
3.09
-0.85
-1.1
32.4
21
8.2
4.64
4.29
69.4
4.13
-1.44
1.07
36.9
9.24
6.26
5.3
78.2
2.2
3.7
-1.51
0.25
37.1
24.9
9.02
6.98
-3.69
72.1
4.22
0.37
1.19
41.3
9.65
8.08
-2.29
79.4
2.1
4.05
0.68
1.05
42.0
28
10.1
8.47
-2.93
68.2
5.28
2.17
2.53
46.4
11.4
10.5
2.51
74.4
1.9
4.74
2.15
1.52
45.4
50
10.0
10.0
-9.88
68.8




10.2
10.2
-9.53
70.8
1.4




Table 6.  4nec2 calculations for the Sidekick screwdriver vertical antenna with the horizontal loop antenna at 15 feet and at 20 feet height above ground

NVIS Gain Loop vs.
                    Vertical
Low Angle Gain - Loop vs.
                    Vertical
Radiation Efficiency -
                    Loop vs. Vertical
Figure 57.  NVIS Gain - Horizontal Loop vs. Vertical
Figure 58.  Low Angle Gain - Horizontal Loop vs. Vertical
Figure 59.  Radiation Efficiency - Horizontal Loop vs. Vertical

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

  1. "Computer Assisted Low Profile Antenna Modeling I", Milazzo, C, KP4MD, 13 June 1998.
  2. 4nec2 antenna modeling software, Voors, A.
  3. "A Beginner's Guide to Modeling with NEC", Cebik, LB, W4RNL, QST, November 2000, pp. 35-38.
  4. "The Loop Skywire", Fischer, D, W0MHS, QST, November 1985, pp. 20-22.
  5. "Zip Cord Antennas - Do They Work?", Hall, J, K1TD, QST, March 1979, pp. 31-32.
  6. "Zip Cord Antennas and Feed Lines For Portable Applications", Parmley, W, KR8L, QST, March 2009, pp. 34-36.
  7. "Portable Antenna Notes", Wiesen, R, WD8PNL
  8. "Comparative Antenna Analysis with WSPR", Milazzo, C, KP4MD, 13 January 2011.
  9. "Zip Cord Transmission Lines and Baluns", Milazzo, C, KP4MD.
Loop Skywire Final Configuration
Figure 60.  The station configuration updated in 2013.  The radio is
connected through RG-58/U coaxial cable jumpers to an automatic antenna
tuning unit (currently an LDG Z-11 ProII) and a 1:1 Guanella current balun (T3).
A total of 4 feet (1.2 m) of the AS-18/50Z speaker wire are wound on the the toroid
cores (T1, T2 and T3) for an approximate 51 feet (15.5 m) of total transmission line length of speaker wire to the full wave 40 meter horizontal loop antenna.

APPENDIX: 4nec2 INPUT FILES

: top; text-align: center;">
Frequency
MHz
Models of Antennas on 07 June 2011*
Models of Antennas on 10 September 2010*
Models of Original Antennas*
Loop antenna at 20' (6.1 m)
Screwdriver vertical antenna**
Loop antenna at 15' (4.6 m)
Screwdriver vertical antenna
Random wire antenna
Screwdriver vertical antenna
1.8 MHz
Horizontal loop (1.8 MHz)
 
Horizontal loop (1.8 MHz)

 Horizontal loop (1.8 MHz)**

3.5 MHz
Horizontal loop (3.5 MHz)
Screwdriver vertical (3.5 MHz)
Horizontal loop (3.5 MHz)
Screwdriver vertical (3.5 MHz)
 Horizontal loop (3.5 MHz)**

5.3 MHz
Horizontal loop (5.3 MHz)
Screwdriver vertical (5.3 MHz)
 Horizontal loop (5.3 MHz)
Screwdriver vertical (5.3 MHz)
 

7 MHz
Horizontal loop (7 MHz)
Screwdriver vertical (7 MHz)
Horizontal loop (7 MHz)
Screwdriver vertical (7 MHz)
Random wire (7 MHz)
Screwdriver vertical (7 MHz)
10.1 MHz
Horizontal loop (10.1 MHz)
Screwdriver vertical (10.1 MHz)
 Horizontal loop (10.1 MHz)
Screwdriver vertical (10.1 MHz)
 

14 MHz
Horizontal loop (14 MHz)
Screwdriver vertical (14 MHz)
Horizontal loop (14 MHz)
Screwdriver vertical (14 MHz)
Random wire (14 MHz)
Screwdriver vertical (14 MHz)
18.1 MHz
Horizontal loop (18.1 MHz)
Screwdriver vertical (18.1 MHz)
 Horizontal loop (18.1 MHz)
Screwdriver vertical (18.1 MHz)
 

21 MHz
Horizontal loop (21 MHz)
Screwdriver vertical (21 MHz)
Horizontal loop (21 MHz)
Screwdriver vertical (21 MHz)
 

24.9 MHz
Horizontal loop (24.9 MHz)
Screwdriver vertical (24.9 MHz)
 Horizontal loop (24.9 MHz)
Screwdriver vertical (24.9 MHz)
 

28 MHz
Horizontal loop (28 MHz)
Screwdriver vertical (28 MHz)
Horizontal loop (28 MHz)
Screwdriver vertical (28 MHz)
 

50 MHz
Horizontal loop (50 MHz)
Horizontal loop (50 MHz)




* Simplified model of house structure decreased these calculations times to 30% of previous models and revised and validated screwdriver antenna model
** Horizontal loop antenna fed as vertical against ground on 1.8 and 3.5 MHz (07 October 2010).

Return to KP4MD Home Page

Top