Antenna System
General
The antenna system comprises two half wave horizontal dipoles oriented in parallel, in phase, with a separation of half a wavelength and one fifth of a wavelength above ground. Each dipole is connected via a balun to an electrical half wave length of coaxial feeder. These two feeders are coupled, using a combiner, to the main feeder which connects to the receiver.
This configuration of dipoles gives a vertical beam width of approximately sixty degrees at the half power points.
Antenna
- Length of dipoles . . . . . : 0.5 wavelengths (4.832m)
- Number of dipoles . . . . . : 2
- Orientation . . . . . . . . . : in phase
- Spacing . . . . . . . . . . . : 0.5 wavelengths (5.340m)
- Height above ground . . . : 0.2 wavelengths (2.05m)
- Feed point impedance . . : 75 Ohms (nominal)
- Resonant frequency . . . : 28.250 MHz
- Beam width . . . . . . . . . : 60 degrees (-3dB)
- Gain . . . . . . . . . . . . . . : 7 dBi (approximate)
The dimensions given (in metres) are for this particular antenna and will vary slightly for other antennas, depending on local conditions.
Baluns
- Type . . . . . . . . . . : Current mode
- Toroid material . . . : 43 mix
- Toroid size . . . . . . : 16x10x4.5mm (OD x ID x H)
- Wire . . . . . . . . . . : enamelled 0.63mm
- Number of turns . . : 7 bifilar wound
Feeders
- Coaxial cable type . . . : RG6 Quad Shield
- Impedance . . . . . . . . : 75 Ohm nominal (82-j8 Ohms measured)
- Velocity factor . . . . . : 0.876 (measured)
- Loss @ 28MHz . . . . . : 0.088dB per metre (measured)
Dipole to combiner
- Length . . . . . . . . . : 0.5 wavelengths electrical (4.694m)
Combiner to receiver
- Length . . . . . . . . . : 1.5 wavelengths electrical (14.072m)
Combiner
Modified Dick Smith Electronics “2-way “F” Connector Splitter” part number L4221.
- Insertion loss . . . . . . . . : 0.5 dB
- Isolation between inputs . : 26.5 dB
The un-modified splitter can be used as a combiner but with a slight loss of performance.
Picture
The following picture shows the author's antenna located above the swimming pool, the only available space in a relatively small suburban block.
