Tuner Design for Half-Wave Vertical & Similar-Length End-Fed AntennasAuthor: R.J.Edwards G4FGQ © 1st June 2005
Sometimes it is necessary to match the high feed-point impedance of an end-fed antenna to a low impedance such as 50 ohms. This antenna tuner will match end-fed half-wave vertical or inverted-L wire antennas to low impedance transceivers or coaxial cables with high power-transfer efficiency.
It consists of a parallel-tuned circuit between the antenna and ground with the coil being tapped. There is a capacitor in series with the antenna which may or may not be needed, depending on the antenna's reactance at the operating frequency. If it is not needed, set C1 to a very high value such as 999,999 pF in the accompanying program.
A disadvantage of this circuit is the range of tapped inductances needed to cover all the HF amateur bands. However, end-fed wire antennas usually are used only the lower HF bands, which reduces that problem.
How the Program Operates
The values of C1 and C2 are chosen after entering antenna dimensions. C1 often will not be needed. C1 is intended only to tune out unusual values of antenna inductive input reactance. So initially enter a very large value for C1. C2 is the main tuning capacitor. Enter a typical midrange value to which C2 is expected to be set in normal operation. (Do NOT enter a C2 maximum value.) A small C2 value results in more turns on the coil and higher radiating efficiency. Try 30 pF.
The program then calculates the value of inductance needed to bring the whole circuit, including the antenna, into resonance. It also calculates the required number of coil turns. The final operation is to enter the transmitter load resistance or impedance of the coax line from the transceiver to the coil tap. The number of tap turns is then computed. If the results are not satisfactory the user then can change the antenna dimensions.
- The computed coil wire diameter assumes the (wire-diameter)/(winding-pitch) ratio = 0.7.
- That ratio maximizes the coil Q, but is very non-critical. Coils may be close wound.
- Close-wound enameled wire limits the maximum volts-per-turn and hence limits the power handling capability of the tuner.
- The diameter/pitch ratio can be between 0.6 and 0.95.
- Lower values allow use at higher power.
- The coil Q approximately doubles when all coil dimensions including the wire diameter are doubled.
- The Q increases approximately proportional to Sqrt(Frequency) for given coil dimensions.
When the warning "An impedance match is not possible" appears on the screen the computed antenna input impedance values are still valid.
When C1 is very small, say less than 10 or 15 pF, stray, uncertain, antenna and wiring capacitance may cause calculating errors.
End-fed antennas near to 1/2-wavelength and 1-wavelength long do not need very good ground connections to obtain high radiating efficiencies, because of their high feed impedances.
Antenna lengths that are an odd number of 1/4-wavelengths may be more easily tuned with L and T networks which allow the use of roller-inductors.
Ideally, the circuit working Q should not be less than 5 or 6. Otherwise, the tuning operation, in practice, may appear very flat and ineffective and the efficiency may be poor.
There may be several different sets of component settings which give similar tuning results. This program allows users to become familiar with the range of component values which are most appropriate for given antenna lengths.
The program calculates coil dimensions and turns to assist with winding home-made coils.
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 TuneHalf 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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