Wire Dipole Antenna with Adjustable Ladder Feed Line

Last crawled date: 2 years, 8 months ago

This is a model of a Half-Wavelength Wire Dipole Antenna with Adjustable Ladder Feed Line that is used to experiment with various wire feedline gaps that provide different feedline input impedances by changing the wire gap in 1/2 inch increments as shown in the table. The wire gauges are from #16 AWG (0.051" / 1.25mm) thru #8 AWG (0.128" / 3.25mm) and may need to be glued in place once the antenna is fully tuned. (Silver plated stranded wire with a protective insulation is best as long as the exposed areas are tinned with solder.) Non-metallic cable ties are used to secure the 3D printed plates together with (3) holes in the top of the plates for non-metallic ropes to hoist and secure the antenna.

The adjustment of the feedline impedance is NOT linear since the impedance is affected by the square of the radial distance between the wires. Coarse adjustment will happen as the wires get closer, and will have finer impedance changes as the wires are separated. Changing to thicker wire gauges (smaller #numbers) will lower the impedance roughly ~ 28 ohms. The most significant change is the resistance of the wire, but also wire diameter affects the wire-to-wire capacitance as larger wires will have smaller resistance but yet larger capacitance, so there will be a frequency shift and a necessary adjustment in inductive length for a specific center frequency. Again capacitance is dependent on the square of the radial distance between the wires, so widely spaced wires will lower the capacitance.

Optimal conditions are low resistance to high reactance = high Q which also determines the useable bandwidth for operation and reception. So tuning for bandwidth is probably a important first step. Next high impedance will lower current and maximize voltage for the electro-static field, but high currents will generate a higher magnetic field. Inductance is typically maximized for straight line wires and capacitance is typically used to center tune the frequency, but inductance length is limited to the wavelength and is typically shorter due to the velocity of propagation = velocity factor.

Note that from the magnetic field moments that the antenna feedline is directly inside the NEAR-FIELD radiation pattern and for this reason a differential pair wire transmission feedline is best since the effects on the feedline will be cancelled !! This is NOT true for coaxial cables and there will be currents on the outside of the coax that will cause undesired EMC/EMI/RFI radiation problems as well as VSWR and other antenna tuning problems, not to mention radiation power losses.

What is not show in the model is the antenna tuner that would be the next connection typically at the ground level where the coax will attach or preferably (if possible) to the tuner that is very close to the transmitter/receiver; possibly right outside a widow.

The problem with a ladder line is that it is VERY SENSITIVE to other metallic objects, so very careful considerations must be made just as if the feedline was the antenna ! So careful understanding of any nearby metallic objects is important (power lines, telephone lines, gutters, metal roofs, other antennas, ...etc) : https://www.rfcafe.com/references/electronics-world/loss-figures-300-ohm-twin-lead-january-1965-electronics-world.htm

The use of this experimenter is best explored in RECEIVING mode only; NOT in any transmitting modes and can be explored with a simple antenna tuner, antenna analyzer, or Vector Network Analyzer (VNA).

SAFETY NOTES : Antennas of this type can handle 10's of thousands of Volts and hundreds of Amps of current when used with high power transmitters typical in the Amateur Radio (Ham) bands. Therefore NEVER tune an antenna when it is live ! ! ! - - - Also DO NOT stand in the radiation pattern (NEAR FIELD) when it is operating either as there may be serious health effects. A good rule of thumb is to stay away at least a 1/4 wavelength (typically the distance the antenna is raised in height above Earth Ground) when it is operating. Also stay away from the mid-point of the antenna which is directly under the feed line when it is operating.