The antenna's physical length is determined by one half of the wavelength at the fundamental frequency. As the voltage ratios of tow quarter wave sections have a self-cancelling effect on each other, only one quarter wave section is shown in the above illustration. The antenna centre is represented by a vertical dotted line. On the 10 horizontal lines all quarter wave length harmonics may be seen. The bold lines indicate the lambda/4 sections that are divided at a ratio of 1:2 around the vertical balun line. The matching conditions at this point are equal and are suitable for a 1:6 balun. For the 10, 21 and 32 MHz harmonics this is not the case; the intersection of the balun line is at the end of a half wave and coincides with a voltage antinode at approximately 3,600 ohms. By means of current or voltage coupling, all other antenna sections as well as the lambda/4 sections indicated above are effective.

FD antennas are coax fed versions of the Windom antenna. FD4's are distinguished by their wide resonant ranges. Between 3...30 MHz 4.7MHz are available if the high frequency is directly coupled out and about 15 MHz if a matching device is used (see SWR window section below).

The antenna's length of 41.5m has been chosen so that the FD4 may be used not only for the amateur service, but also other radio services. Apart from the balun in the feed point for multiband operation, these antennas have no other accessories. For this reason, they are light yet robust and weather resistant.

The highlight of this antenna is the balun, which provides both symmetry and transformation. The FD4's feed point lies at one third of its total length and here the impedances for 6 rangers are almost the same at about 300 ohms. If 50 ohm coaxial line is used, the impedance must be transformed down at 1:6. The AMA and COM models have a built-in block against the radiation from the coax outer sheath on 80m.

There are three output classes for the FD4. The length of the antenna remains the same in all three and they differ only in the maximum power handling capacity of the baluns.

The diagram shows a 2kW FD4 as it is delivered. The 5kW FD4 looks similar, but has COM printed on the balun sticker. The 500W model of this group is distinguishable by the smaller balun casing.

Manufacture of the FD antennas began in 1970 with the 500W model and was extended in 1983 to include the 2kW and 5kW models. This multiband antenna is used by more than 20,000 radio stations with great success.

Measurement Set-Up

• Vector Analyzer ZPV
• Signal Generator SMS2
• Process Controller PCA5 from Rohde & Schwarz

Measurement Procedure

• Directional coupler measurement with compensation lien at the antenna feed point

Measurement position of antenna

h1 free height feed point above ground:	12m
h2, end insulators' free height above ground:	3m, 6m
g water table below ground:	-2m
B buildings within spanning radius:	25%
buildings, average height:	5m
s spacing of end insulators:	36m
β angle of "inverted V":	150°
Coax down lead, running at same angle to both sides of antenna:	10m

Reproduction

Data concerning the resonant range, feed point impedance, SWR and SWR bandwidth are only valid for the given antenna position. Variations to the height, the V-angle, the straight wire configuration and local buildings result in other values. Guaranteed data cannot be given for dipoles under 10 MHz over lossy ground – please regard results as approximate values.

History of Windom Antenna

This type of antenna was originated by Loren Windom, W8GZ, who energised a half wave radiator with a single wire feed line using ground as a counterpoise. VS1AA varied the wire thickness between the feeder and radiator and achieved a better match to the transmitter by including a Collins filter. DL1BU described a duo-band Windom using a matched 300 ohm line. In his antenna book, DM2ABK mentions a four band Windom variation using a 300 ohm feeder matched to the transmitter by means of a double Collins filter. DJ2XH introduced in 1970 a Windom variant fed with coaxial cable, achieving the necessary impedance with a ring-core 1:6 balun. This transformer was modified in 1983 to increase RF power and reduce RF radiation from the coaxial cables' outer shield.