The Windom AntennaAuthor: R.J.Edwards G4FGQ © 21st May 2002
The Windom is a horizontal dipole fed off-centre by a single-wire non-resonant transmission line. It is named after L.G.Windom, W8GZ, who described it in September 1929 QST.
A feedline can be attached to any point along the dipole. When the dipole is resonant the input impedance at any point is always very nearly a pure resistance. Zin varies from a low value resistance at the dipole centre to a high value resistance at its ends.
A single wire, horizontal, sloping or vertical, suspended well above ground can be used as a low-loss transmission line. Its impedance Zo depend on its height, wire diameter and length. Ordinarily, Zo will be in the range 400 to 650 ohms. It is always possible to connect such a line to a resonant dipole at a point where the line is terminated with Zo and SWR = 1:1. However, although there may be no standing waves on the single-wire line it will still radiate. But since the dipole it is attached to has nulls in its radiating pattern any radiation from the feedline may be welcomed. Taking one vertical angle and frequency with another, for practical purposes the Windom is omnidirectional.
The Windom is also an Efficient, All-Round, Multi-Band Radiator
In the early days of radio when a transmitter worked at only one frequency the antenna would be used on the dipole's fundamental resonance and the feedline would be Zo matched by the dipole's input impedance as just described. However, as modeled by this program, the Windom is an efficient radiator at all frequencies at which the height of the vertical element plus the longest horizontal section is greater than about 1/6th wavelength, i.e., >26 metres at 1.9 MHz.
When the vertical wire is connected to one end of the dipole the antenna then becomes an inverted-L. When the dipole is physically very short the antenna reduces to a simple vertical. When the vertical wire is connected at or near the the centre of the dipole we have a T-antenna. It will not require much effort to lower the dipole and tap the vertical wire into it at a different position. This will greatly change the feedpoint impedances. It will also randomly modify the radiation patterns at different frequencies.
Because the feedpoint resistance tends to be high, high efficiency is obtained with a modest ground electrode system. Eight shallow-buried wires under the antenna, no longer than about 1/10th wavelength, in soil of average resistivity of about 150 ohm-metres, will have a loss resistance of the order of 10 ohms. In high resistance soils double the number of wires rather than increase their length. NOTE: Efficiency calculation includes loss in the L-match. Coil Q = 200.
Buried Radials: Crude Estimates of Ground Connection Loss Resistance
- This table applies to a soil resistivity of 100 ohm-metres = 10 milli-Siemens.
- N is the number of uniformly radially distributed wires in a set.
- Lengths are in metres. It is assumed length is less than 1/8-wavelength.
- Wire diameter is 14 gauge, depth is 200mm, both are very non-critical.
- For other soils loss resistance is proportional to soil resistivity.
Run this Program from the Web or Download and Run it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link Windom2 then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.
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