Wire Dipole Antenna with Adjustable "Open-Wire" Ladder Feed Line

Last crawled date: 2 years ago

This is a model of a Half-Wavelength Wire Dipole Antenna with Adjustable "Open-Wire" 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. Note : This type of feedline is only good from 0.5 MHz (600 meters) to 50 MHz (6 meters).

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 shown in the model is the BalUn to coax/antenna tuner that would be the next connection typically close to the ground level (but not at - with at least a BalUn transition at least a foot above Earth ground). This is where the coax will attach or preferably (if possible) to the alUn to coax/antenna tuner that is very close to the transmitter/receiver; but it could also be outside a widow making sure that the entire length of the open wire feed line is a few feet away from metallic objects.

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.

UPDATE (10/3/2021) : There is a good section in the ARRL's 'Antenna Physics' book that describes the experimentation of open wire transmission lines using a free simulation program called EZNEC that can warn and optimize wire length and wire spacing based upon frequency, wire diameter, and insulation types. I recommend using this as part of the experimentation to help achieve optimal results. Additional source material is in chapter 23 of the ARRL 'Antenna Book' as well.

UPDATE (1/6/2022) : Hollowing out (cone drill) the tuning plates where at all the wire entry and exits out will break sharp edges that will cause the wire to kink and break over time from wind vibrations. Also it is recommended to keep the wires taut to restrict the vibrational motion of the antenna under wind gusts, which will shift the wires back and forth creating stress fractures in the wire, even with stranded wire. Adding some silicon adhesive at each of the cone wire entry and cone wire exit points will create a soft grommet to reduce the stress of fracturing the wire.