OCFD antenna designer
Design an Off-Center-Fed Dipole (OCFD) antenna, in either flat-top or inverted-Vee form, using a single main support. This type of antenna is designed for one principal amateur HF band, usually 80 meters or 40 meters, but typically displays VSWR minima on at least two higher bands, and can therefore be a good multi-band antenna. The impedance at the feed-point of this antenna is often stated to be in the range of 200 ohms to 250 ohms, so it is most often matched using a 4:1 unun. Read more...
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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Set antenna color: |
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OCFD (Off-Center-Fed Dipole) antenna designer
portable-antennas.com
Antenna feed-point: ⬥
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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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| Set ref. elevation angle: |
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| Set ref. azimuth angle: |
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| Radiation patterns for: |
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Patterns hover info:
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| Impedance transformer: |
unun
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| View VSWR chart: |
Check on:
From:
To:
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| Effect of coax on VSWR: |
Coax length:
Type:
Effect of coax- and system-losses on VSWRWhen a coaxial cable is used to connect the antenna to a transmitter, the cable introduces signal loss, the magnitude of which can vary strongly with both frequency and antenna feed-point impedance. The main blue VSWR curve for this antenna is calculated at the antenna feed-point. The optional green curve represents the VSWR "seen" at the transmitter after losses in the coaxial cable are taken into account. At frequencies of antenna resonance, the blue and green curves usually coincide closely, since the coax contributes little additional attenuation at those frequencies. At frequencies away from resonance, however — often those corresponding to WARC bands — the feed-point impedance may become highly mismatched, producing very high VSWR values in the blue curve while the green curve shows substantially lower values. This reduction in VSWR at the transmitter occurs because signal losses in the coaxial cable attenuate both the forward and reflected waves. As a result, the reflected signal returning to the transmitter is weaker, causing the transmitter-end VSWR to appear lower than the actual feed-point VSWR. This effect can make some bands appear more "usable" from the transmitter's perspective, even though the antenna system may still be operating inefficiently at those frequencies. The green curve represents the "flattening" of the VSWR caused by the coax. Additional losses elsewhere in the system can further reduce the transmitter-end VSWR - these may include:
The magnitude of such losses, and their effect on actual radiated RF power, can only be estimated reliably through careful analysis of a particular antenna setup. In every-day portable operations, such losses can only be roughly estimated; however, comparisons can be made between commercially-produced EFHW and OCFD antennas for amateur radio use, and carefully-configured NEC models of those same antennas. Useful estimates can then be made of the efficiency of the commercial products by comparing their published VSWR curves, and the VSWR curves produced from the models. The dashed red curve (VSWR-FIELD) represents what might be measured in the field by an antenna analyzer connected to the transmitter end of the coax when this antenna is erected "in the field," i.e. either when operating portable, or at the home QTH. This curve approximates the effect on VSWR of all the sources of signal loss mentioned above, and illustrates how real-world losses can further flatten the measured VSWR, either with a home-brew, or a commercial, EFHW antenna. Improved transmitter-end VSWR therefore does not necessarily imply improved radiation efficiency: in some cases, a significant proportion of the transmitted power may simply be lost: dissipated as heat within the antenna system. Read more... |
| 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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