Design & Performance of Compact, Centre-Loaded Dipole Antennas

Author: R.J.Edwards G4FGQ © 1st July 2000

When the overall length of an HF dipole is less than about 1/3rd wavelength radiating efficiency decreases rapidly. Additional loss occurs not in the antenna itself but in the feedline due to high SWR, and also in the tuner.

Locating tuning and matching functions at the dipole centre reduces line loss. Tuning loss is minimised by using a very large, high Q loading coil. Although centre loading significantly improves overall efficiency most loss now occurs in the coil itself unless the antenna is very near the ground when ground loss eventually predominates. Ground loss decreases rapidly with height till height exceeds dipole length. It will be seen the program compensates for ground proximity detuning by reducing the number of turns on the loading coil.

The fraction of length occupied by the coil varies widely. A 1/3-wave antenna is mostly wire with a short coil at its centre. A 1/40th-wave, 160m antenna might consist of a 2-metre long slender coil with a 1 metre rod or wire at each end. "Slinky's" have no rod or wire extensions and can be suspended on a taut line. But these are toys. A larger coil volume reduces loss for the same overall antenna length but an increase in coil length is preferable to an increase in diameter because a slightly better current distribution results.

There are Three Ways of Coupling a Feedline to a Centre-Loaded Antenna

1. Cut into the coil centre and connect a feedline. Input impedance of a short resonant antenna is always too low to match to any line but the VSWR loss on a short line may be acceptable on the lower HF bands. And a Z-matching network with another uncertain loss may still be necessary at the transmitter end.
2. Tap a feedline across a few of the centre turns. The number of turns can be adjusted for a fair match to a wide range of impedances but half turns are impossible. High Zo, balanced pair lines are preferable although, again, this may entail a simple Z-matching network and/or balun at the transmitter end.
3. Connect either a balanced-pair or coaxial line to a link winding over the centre of the loading coil. Plastic insulated wire allows the link to be wound directly over the coil winding. Link turns are easy to change and rewind, and fractional turns can be simulated by spreading out or closing the link turns. Or by increasing the diameter of the link relative to the loading coil.

The antenna is a large tuned circuit. The capacitances looking into the inner ends of the rods or wires are in parallel with the coil. Wire, ground and radiation losses have three equivalent lumped resistance values located in the middle of the coil. Precise tuning can be done by pruning rod or wire lengths.

In confined spaces such as attics the proximity to structural materials will have effects similar to being near to the ground - inefficiency and detuning.

Tight coupling between coil and link windings can be achieved by winding the link over or in between the coil turns. But very tight coupling is not always necessary. Use plastic-insulated wire for the link. Wire diameter need be no greater than used in the feedline itself.

Since much loss occurs in the loading coil a physically large coil allows a larger wire diameter, increases Q, improves heat dissipating capability and improves radiating efficiency. Prefer (wire diameter)/(winding pitch) ratios between 0.5 to 0.75 which will reduce weight relative to close-wound coils and may moderately enhance Q.

The centre-loaded dipole is a single band antenna most useful when there is insufficient space to erect a full size 1/2-wave dipole so most likely it will be used on the 40m and lower frequency bands. Power efficiency can approach that of a magloop if a sufficiently heavy gauge of wire is used and the antenna is erected well above ground and not near to buildings.

Design philosophy: For a given overall length make the coil physically large, giving some preference to coil length. But don't make it too slender.

Program Operating Notes
Values of five program input parameters can be swept, up or down, over wide ranges in fine increments, by holding down either one or the other of pairs of adjacent numerical keys. See menu on line 24 of the data display screen. This "what-if ?" facility is available after a complete set of data has been entered and the results displayed.

The program automatically restricts the ratio of coil wire diameter/pitch to 0.95 to allow for insulation. For a close-wound coil just enter ratio = 1 but close-wound enamel-insulated wire is recommended only for its convenience.

Warning messages may appear on the screen when impractical or conflicting values of input data have been entered. Some computed values may be in error when one of these messages is being displayed.

As overall antenna length approaches 1/2-wave the number of turns on the coil becomes small and computed values of wire size and of coil tap turns may be impractical. Other computed values may also be suspicious. However, a dipole near to a 1/2-wave in length does not need a loading coil.

Carefully observe measurement units when entering and reading data

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