Performance of a T-Antenna Tuned Against a System of Buried Radial WiresAuthor: R.J.Edwards G4FGQ © 23rd August 1998
A dipole, centre-fed via a twin feeder, can be used as a T-Antenna at LF and VLF, but a good ground system and a physically large tuning coil are necessary to obtain a useful radiating efficiency. This program assists with design by predicting the power lost in the various parts for a given power input.
When a station's effective radiated power is limited by license conditions this program will estimate the maximum power allowed from the transmitter power amplifier. Once a station has been established, the actual power radiated can be found by measuring antenna input current and using the value of radiation resistance given by this program: E.R.Power = Current-squared times Radiation resistance, in watts. In practice, radiation can be assumed to be entirely vertically polarised.
The program requires ground electrode details to be entered as a set of shallow-buried radial wires. Other electrodes, such as rods and plates, can be accommodated by entering equivalent dimensions. E.g., the length of a plate is its diagonal and the equivalent diameter is half the width of its shorter side.
The program covers a frequency range from ELF to the natural resonant frequency at which the required coil inductance falls to zero. A warning is displayed.
Radiating efficiency depends on soil conductivity in the antenna vicinity. It is not necessary to enter conductivity figures into the program. The greater the moisture content and the greater the amount of dissolved salts the greater the soil conductivity will be. The program user is asked to select from five different types of soil for which conductivity is related to soil fertility:
|Very Good||Moist, dark, highly fertile, compacted agricultural loam.|
|Good||Drier and lighter, but flower gardens, hedges, lawns, trees.|
|Average||Some clay or sand content, nevertheless nice lawns, gardens.|
|Poor||Mixed soil and sand, fairly dry, but flowers, weeds, grasses.|
|Very Poor||Drained, sloping, sandy clay, rocky, hardy weeds, peat.|
For simplicity the length of the connection from the bottom of the coil to the ground electrode has been omitted. It may be included with twin feeder length without serious error. If disregarded the program will over-estimate the number of coil turns required. The height of the horizontal top section and feedline length are entered separately which also allows for a sloping or bent feedline. Ideally, the coil should be located just above or near to the radials' common radiating point which need not be directly underneath the HF dipole.
When the coil has many turns, fine tuning may be accomplished by changing taps at top of the coil. This entails switching off the Tx when readjusting. But a small trimming capacitor across the coil needs a very high peak voltage rating.
A larger capacitor of a lower voltage rating can be tapped across just part of the coil to have practically the same effect. 1/Nth of the turns will give one Nth of the voltage and the capacitance needed will be N-squared times greater.
To reduce power loss in the coil the size of a tuning capacitor should be minimised by increasing coil inductance. Coil circulating current will be smaller.
Fine tuning is also possible using a roller-inductor at the LV end of the coil. Or by rotating extra turns of smaller diameter inside one end of the main coil.
Exceptionally high RF volts may require the feeder to be re-routed further away from other structures. Insulators at each end of the dipole may need replacing.
To isolate the transmitter's own ground from heavy RF antenna current, instead of tapping the Tx into the coil use a coupling link around the low voltage end. A link may need one or two more turns than the tap. If adjustable relative to the end of the coil, the link will allow an exact Z-match to the transmitter.
If a coaxial line is used to feed the HF dipole centre, enter sheath diameter into the program in place of twin-feeder wire spacing. If only a single wire is used in place of a feeder then enter the wire diameter instead of spacing.
The program selects coil wire diameter such that winding pitch is always 1.41 times diameter. Close-wound, enamel insulated wire must not be used for high power levels. For wire diameters greater than 2.5 mm annealed copper tube may be preferred.
A suitable coil former length/diameter ratio is from 1.0 to 2.5. Enter exact former external diameter - the program automatically allows for an increase in effective coil diameter due to wire thickness.
As natural resonant frequency is approached and only a few turns are needed on the coil, the computed 50-ohm tapping point becomes unreliable. In any case, a conventional matching network will be more appropriate at the relatively high frequency. As a matter of interest a G5RV antenna without the bottom-end coax has a natural resonant T frequency in the vicinity of the 160-metre band.
Antenna-ground capacitance is the value at the operating frequency, not at DC. Ground loss and radiation resistances are referred to the antenna feedpoint.
To avoid undue complexity a simplifying assumption is that the transmitting site is horizontal, without tall buildings or much vegetation. But see below:
When part of an antenna is above a house, green-houses, dense bushes or trees, enter in the program the average height of the top above these structures or vegetation, taking one short length of the top with another. Do likewise when the ground is sloping, the top is sloping or is an inverted-V. When in doubt about average height give the advantage to the antenna because a reduction in height not only increases antenna capacitance to ground - it also reduces the radiation resistance.
When the vertical wire or feeder runs through foliage of a tree which supports the antenna, its capacitance will increase considerably and the number of turns required on the coil will be fewer than the computed value. The program is not able to allow for this. But more important, this additional capacitance will be very lossy - there will be a loss resistance of which the program will not be aware and radiating efficiency will be considerably over-estimated. The program user can partially compensate by selecting a soil with lower conductivity. The success in such compensation can be judged by comparing the computed coil tap turns with the actual number of turns needed to match to a 50-ohm transmitter.
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