The Simple Tee AntennaAuthor: R.J.Edwards G4FGQ © 1st August 2005
A vertical or sloping wire is connected to the approximate centre of a straight horizontal top wire to form a T-Antenna. The vertical wire may be curved inwards towards the shack and is fed at its bottom end via a series L or C. The top wire provides a capacitance load on the vertical and increases the vertical's radiation resistance but does not itself contribute much to the radiation.
The most interesting performance characteristics are feedpoint input impedances of the top and vertical sections, and radiating efficiency which takes ground loss, wire loss, and loading coil loss into account.
Behaviour of the vertical section alone is obtained by setting length of top to zero. When length of vertical is very short the computed input impedance of a long top becomes inaccurate because the program ignores capacitance to ground. It will be noticed however, when the length of the vertical is set to zero, its input impedance is identical to that of the top as it should be.
Results are useful but fall to ballpark accuracy when wire lengths exceed 0.4 wavelengths. Or when long top lengths are associated with very short verticals.
An ordinary centre-fed dipole can be converted into a T-antenna by strapping together the twin feedline wires at the bottom end and using them as one wire. Assume the effective wire diameter to be 1/3 of feedline wire spacing.
The radiation pattern of a T-antenna is that of a simple vertical of the same height, i.e., omni-directional in the horizontal plane.
If the centre tap of the T is moved off-centre by approximately 10 percent of overall top length the antenna becomes a Windom when top length is 1/2-wavelength.
An L-network tuner may be used to replace the bottom loading coil or capacitor. The program estimates the additional loss in the tuner when a tuner is used.
Behaviour is insensitive to wire diameters. Most sensitive to vertical length.
Computed input resistances and reactances are series components.
Observe how base tuning component changes from coil to capacitor when the input reactance of the vertical changes sign.
Antenna Height and Operating Frequency
Use of the top horizontal wire as a load on the vertical to draw current into the top end is most useful when the vertical is short, its radiation resistance is small, and its radiating efficiency is poor.
The resulting more linear distribution of current along the wire can increase a low radiation resistance of the vertical by as much as 3 or 4 times with a corresponding improvement in radiating efficiency.
However, as the height of the vertical increases towards 1/4-wavelength and its radiation resistance and efficiency are already fairly high, the top wire has a less beneficial effect. When height is 5/8 wavelengths or more and the ground loss resistance is low, efficiency can be 90 percent or better and the top wire becomes redundant. Efficiency cannot be increased to greater than 100 percent.
Of course, the most efficient way of using extra wire at the end of a vertical is to extend it upwards. The T-antenna comes into its own only when height is restricted. It always is restricted at some low operating frequency when, ideally, one would wish to erect a 1/4-wave or higher antenna. So, T-antennas are generally used only on the three lowest HF bands or when height is restricted for some reason to about 1/4-wavelength or less.
Top Length _________________________________ Height @ Ground !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The most uncertain impedance is Zin of the top wire. Uncertainty increases as height decreases and length of the top increases. It is difficult to model or or measure Zin to the top with great accuracy because the impedance between top and ground bypasses the vertical wire. On the other hand, Zin(top) is used only to calculate the input impedance of the vertical wire.
As the length of the top decreases to zero the accuracy of computed input impedance of the vertical wire is high and is very easy to measure. The program computes the performance of a simple vertical antenna with high accuracy.
Run this Program from the Web or Download and Run
it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link TAntenna then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.
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