The FD3 antenna is the little brother of the large FD4. It has only half the span length of the large antenna, i.e. about 20m, and can cover three ranges of operation. Between 6...30MHz a 5MHz bandwidth is available with ˂2:1 SWR in three ranges. By using an antenna coupler, a bandwidth of up to 14 MHz may be operated (please see SWR window that follows).

The diagram shows an FD3 with an 83 series balun as it is delivered. The AMA model for 2kW and COM model for 5kW look alike.

The line drawing above shows the matching ratios for the FD3 in 5 ranges. The total length of 20.2m is divided to the right of the antenna centre (dotted vertical line) into quarter-wave sections. The left side mirrors that shown. The interface for the 1:6 balun must divide a quarter wave by the ratio 1 to 2. This condition applies for the FD3 on 7.1/14.2/28.4/35.5 MHz, but not on 21 MHz. The feed point quarter-waves are shown by bold lines. With the indicated lambda/4 antenna lengths, all other sections are connected using current or voltage coupling and are active as follows: 0.5 lambda for 7.1, 1 lambda for 14.2, 2 lambda for 28.4, and 2.5 lambda for 35.5 MHz.

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:	9m
h2, end insulators' free height above ground:	5.5m, 7m
g water table below ground:	-2m
B buildings within spanning radius:	25%
buildings, average height:	5m
s spacing of end insulators:	19.5m
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.