Specials - Horizontal loop antenna designer
Design a horizontal loop antenna consisting of three or more straight wire sections suspended at the same height above ground. Suitable for home or Field Day installations, it can provide effective multiband transmitting and receiving performance, with predominantly horizontal polarisation that may help reduce vertically polarised local interference. For multiband operation, balanced ladder-line (parallel-wire) feeders are generally preferred over coaxial cable, as they offer significantly lower losses under the high standing-wave ratios and widely varying feedpoint impedances typical of horizontal loop antennas. For this reason, this designer supports balanced feeders only; coaxial feed is not offered.
The loop need not be exactly one wavelength in circumference at any particular frequency, although a full-wave loop is often a useful starting point for single-band operation. Many operators use a semi-random loop length dependent on the space available to them. At relatively low heights (in wavelengths), high-angle radiation generally favours NVIS operation, while greater heights and higher frequencies can produce stronger low-angle lobes suitable for DX. Feedpoint impedance varies considerably with frequency, geometry and height, so multiband operation generally requires a suitable matching arrangement or antenna tuner.
Horizontal loop antenna tips and design guide » » »
Horizontal loop antenna - some useful facts
The horizontal loop antenna can be a useful antenna both for NVIS and DX operations, depending on the operating band.
Listed below are some of the most important points to consider when planning, designing and building such an antenna:
- Loop dimensions: A horizontal loop is a closed wire antenna, usually with a total circumference of approximately one wavelength at its lowest intended operating frequency. Typically-quoted circumference lengths approximate to around 306 / MHz in meters, or 1005 / MHz in feet (about 1.02 * wavelength); larger diameter wires can require a slightly greater circumference for resonance, although the actual resonant length depends also on geometry and installation height. If the space available to you prevents you from building a full-wavelength antenna, then just make the largest loop you can.
- Loop shape: The loop may be triangular, square, rectangular or irregular in shape. Its geometry will influence the feedpoint impedance and radiation pattern.
- Height above ground: The height of the loop has a considerable influence on its radiation characteristics, particularly the elevation angles of maximum radiation.
- Fundamental frequency: At a suitable height, a full-wave horizontal loop radiates predominantly upwards, making it useful for NVIS (Near Vertical Incidence Skywave) communication.
- Higher frequencies: On higher bands, the radiation pattern becomes increasingly complex, developing multiple lobes and nulls, often with useful radiation at lower elevation angles for DX communication, when conditions permit.
- Polarisation: Radiation is predominantly horizontally polarised in many configurations, which may help reduce reception of locally generated, vertically polarised electrical noise.
- Feedpoint impedance: Impedance varies considerably with frequency, loop geometry, height and feedpoint position in the loop. It may range from around a hundred ohms to several thousand ohms, and can contain a substantial reactive component.
- Coaxial feeder: Coaxial feed-lines are not supported for the Horizontal Loop antenna, since very high feed-point impedances at some frequencies can lead to very large mismatch values, substantial losses, and high voltages or currents in the coax line, potentially damaging the cable.
- Ladder-line feeder: Balanced ladder-line feeders generally have very low attenuation, dependent on type and dimensions; these low losses will increase gradually with frequency, feeder length and high standing-wave ratios. However, useful attenuation data for most commercially-available ladder-line types is unavailable (the sole exception being for DXE LL300 line); consequently, this designer assumes a low generic feedline loss appropriate to the type used, so the calculated tuner-end impedance values are close approximations only. Actual feeder losses - although small - can alter both the resistive and reactive components of the impedance presented to the tuner, particularly under high-SWR conditions.
- Multiband operation: A suitable antenna tuner and matching arrangement will generally be required. Balanced feeder line is often preferred because of its relatively low losses under high-VSWR conditions.
- Balun / impedance transformer: You can choose to use a balanced tuner with your horizontal loop antenna and balanced feedline: no balun will be required. Alternatively, you may use a more conventional, unbalanced tuner: in this case, a current balun will be needed between the end of the balanced feedline and the tuner.
- VSWR and impedance curves: For multiband horizontal loops fed with ladder-line, the VSWR curves are generally less important than the impedance curves. Pay particular attention to the red and orange curves, showing the resistive (real) and reactive (imaginary) components. These values are also displayed in the chart tooltip. Your antenna tuner must be capable of matching the impedance presented at the tuner end of the ladder-line to your 50 Ω system.
- Tuner-end impedances: In the VSWR chart, you can view impedances and VSWR curves for either the feed-point or the tuner end of the ladder-line. As a bonus, the program also calculates these values at the center frequencies of the HF bands: click the "Show bands impedances" button to view these.
Designing your horizontal loop antenna
This designer page enables you to specify the number, and dimensions, of the separate wires making up your loop antenna. You can choose
to do either (or even both!) of the following:
- specify wire lengths and compass angles, step by step, to define the sides of an irregular polygon;
- specify a total loop length and the required number of sides, to completely define a regular polygon.
Designing an irregular polygon loop
Open the "Graphical design interface" dialog and choose the "User-defined dimensions" tab in the
right-hand side of the dialog.
When designing a new loop, the dialog initially displays a collection of controls which define the minimum three wire sections required to define a
loop polygon. These controls will hold some default length and compass angle values to help you design a minimal loop
antenna; you will overwrite these default values with others pertinent to the physical antenna you will build.
In order to add new sections to the loop, click on the "Add section" button: this will insert a new row
into the list of rows already present. Enter here the section length and compass angle for this new section.
Note: there will always be a single row, titled
"Last", at the bottom of
this list, which represents the final section in the loop, and which closes the loop at the position of the loop's first section.
You cannot remove this row, and you do not need to specify this section's dimensions - they are calculated automatically.
Continue as needed to add new antenna sections, and/or alter dimensions of existing sections, until you have completely specified
the antenna you already have, or the antenna you wish to build. The maximum number of sections permissible by this designer
is fixed at 8.
Note: To ensure reliable modelling, each loop section must be at least 1/50 (0.020) of the
wavelength at the principal frequency, and the angle between adjacent sections must be at least 15°.
If you alter values in an existing row, you will need to click the
"Update graphic" button to update the design: both the graphic in the dialog, as well as the graphic in the
main designer window, will be updated to display the changes.
You can delete any of the new sections which you have added to the list. Each new row is furnished with a Delete button
which you can use to delete the entire row. Upon deletion, the total length of the remaining sections will be re-calculated.
When finished, you would ideally aim to have the total length of wire sections in your antenna roughly equal to the principal
wavelength. This would ensure you have a usable antenna to start with when testing the antenna in the field. However,
if the space available to you is less than ideal, you will aim to have the total loop length as large as possible.
Designing a regular polygon loop
Open the "Graphical design interface" dialog and choose the "Regular, calculated" tab in the
right-hand side of the dialog.
In this tab, you can specify:
- the total length of wire used in the polygon: aim to have this roughly equal to the principal wavelength; this would ensure you have a usable antenna to start with when testing the antenna in the field. However, if the space available to you is less than ideal, you will aim to have the total loop length as large as possible.
- the number of sections, in the polygon; the polygon can have any number of sections between a minimum of 3 and a maximum of 8.
Changing the number of sections in the polygon will automatically update the graphics in both the dialog and in the main designer window.
However, changing the total length of wire used in the loop will not result in an automatic update - in this case, you must click
the "Update graphic" button to propagate the changes to the graphics areas.
Specify a simple ground area (GA)
In order to help you to better visualize the antenna with respect to its surroundings, you can specify a simple rectangular
ground area: this will be shown in light green in each of the graphics.
You can specify the width of the ground area (GA) in both the X and Y directions, as well as the offset of the antenna feed-point
within the ground area: use the controls at the top of the right-hand half of the "Graphical design interface" dialog.
Set the total X-width of the GA to a value sufficiently large to also include the X feed-point offset.
Set the total Y-width of the GA to a value sufficiently large to also include the Y feed-point offset.
Get started » » »
- Use the controls on the left-hand side to configure your antenna - set the design frequency, antenna element lengths and angles, wire diameter, wire insulation if required, support height, etc.
- When you are happy with your configuration, press the "Show antenna" button to display your antenna in interactive 3D graphics mode in the area on the right-hand side. Each time you change your configuration, you should press the "Show antenna" button to register the changes to the app.
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After you have configured your antenna, you can then use the controls further down in the page to generate charts and diagrams to evaluate
the antenna's performance - you will need to
scroll down in the page
to access these controls.
You can choose to view any combination of the available charts and diagrams: these include four radiation pattern types: azimuth, elevation, 3D and polarization patterns; other options include VSWR charts, an antenna currents diagram, and a Smith chart.
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Antenna performance
Here you can generate plots of
radiation patterns,
VSWR chart, antenna currents diagram and Smith chart for your antenna over a
choice of ground types. By changing the physical dimensions of the antenna, and refreshing these plots, you can learn a lot about
how such antennas will perform in the field.
Please note that
radiation patterns are here modelled over flat ground -
when the antenna is erected on a hill-top or mountain summit, the surrounding sloping ground will generally cause angles in
the elevation radiation pattern to be lowered in the direction of the
slope: the steeper the slope, the lower the effective elevation take-off angle will be, and the better the chances of making
long-distance contacts.
Read more...
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Choose ground type of
land in the vicinity of the antenna: |
Conductivity:
S/m
Dielectric const.:
F/m
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| View radiation patterns: |
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| NEW Choose type: | |
| Set ref. elevation angle: |
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| Set ref. azimuth angle: |
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| Radiation patterns for: |
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| Impedance transformer: | |
| View VSWR chart: | Check on: From: To: |
| View currents diagram: | |
| View Smith chart: |
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Antenna gains at 0° elevation: |
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| Feed-point impedance: | |
| Estimated bandwidth: | |
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