Performance of a Dipole, Feedline and Balun followed by a TunerAuthor: R.J.Edwards G4FGQ © 30th May 2006
The most efficient, general purpose, multi-band antenna is a simple dipole of no particular length fed via an open-wire feedline of no particular length or impedance. A choke-balun and tuner is used at the transmitter end of the line.
Performance begins to decline when dipole length is less than about 1/3-wavelengths at the lowest operating frequency. Radiation resistance falls at LF.
This program computes dipole feedpoint impedance, feedline input impedance, balun input impedance, and antenna and feedline losses. Loss in the balun is very small and tuner losses are covered by other programs.
The antenna input impedance varies widely vs frequency from one band to another. The antenna terminates the transmission line. Consequently, the input impedance of the line also varies widely. The line transforms antenna input impedance to another set of values which are presented via the balun to the tuner, finally to be transformed to 50 ohms. It is the inability of a tuner to transform ALL line input impedances to 50 ohms which constitutes the main drawback of the system. Two alternative, different, tuners may be needed.
Manually operated tuners can deal with a greater range of line impedances than automatic tuners. They are physically larger and so are their component values. Automatic tuners, often built inside the transmitter, are restricted in size. Switched changes in L and C circuit configurations are usually absent. Limiting factors are parasitic circuit reactances and minimum component settings.
If a frequency at which the tuner has difficulty lies in an amateur band then it can be shifted to another frequency just by changing the length of the feedline. It can also be shifted or removed by changing line impedance Zo but this is very inconvenient. And it might be undesirable to change an antenna length which otherwise has been found satisfactory. The question arises whether the line should be lengthened or shortened and by how much? And will the problem just be shifted to another favourite band or perhaps multiplied to several bands?
This program will assist in deciding what ought to be done for the best. It calculates line/balun input impedance vs frequency for a given line and antenna. After making changes, sweep the frequency over all HF bands to check that a problem has been cured and investigate where it may have reappeared.
But first it is necessary to find, or at least have a guess at, the values of line input impedance between which the tuner will have no difficulty in transforming to 50 ohms. This needs a knowledge of how tuners work plus experience.
Practice Using the Program with the Famous G5RV
The program can examine the behaviour of a G5RV. Enter a dipole length of 102 feet = 31 metres, and a feedline length which is 1/2-wavelength long at a frequency of 14.15 MHz. G5RV never specified line impedance so enter Zo = 400 ohms for an open-wire line with a velocity factor of 0.99 and an attenuation of 0.15 decibels per 100 feet at 10 MHz. (100 feet = 30 metres).
The G5RV dipole is 1.5 waves long at 14.15 MHz and resonates in the middle of of G5RV's favourite 20 metre band. Make the wire diameter 1.64 mm = 14 AWG. If the feedline is not 1/2-wave long at the same frequency then it is NOT a G5RV.
Then vary frequency, fast and slow, and observe how the input impedance of the transmission line changes. You will be most interested in values of R+jX which occur in the amateur bands. With experience you may be able to detect values with which a tuner might have difficulty in transforming to 50 ohms.
A well-designed choke-balun between the end of the line and the tuner does not greatly affect the magnitude of impedances seen by the tuner in the frequency range from 1.8 to 30 MHz. Impedances seen by the tuner tend to remain in the right ball park. A balun is not necessary if the tuner is balanced. Balanced tuners behave differently to the much more common unbalanced variety.
There are several means of making an HF choke balun.
A choke balun consists of a very short length of transmission line wound on a ferrite core. It provides a means of connecting a balanced circuit to an unbalanced circuit with small loss and without a great deal of interference with the operation of the two circuits.
The presence of the ferrite core has only a small effect on normal twin-line transmission through the balun. Balun line length is less than 1 metre.
The balun used in this program consists of 13 turns of 19 AWG twin, stranded, speaker wire wound on a 50mm, 2" outside diameter ferrite ring, inside diameter 30mm. Width = 20mm. Core material permeability is in the range 250-400. Balun line characteristic impedance Zo is in the region of 125 ohms.
Useable operating frequencies are from 1.8 to 30 MHz. Power handling capability is at least 100 watts.
Examine balun behaviour by terminating it with a 5000 metre length of the main transmission line (to obtain high attenuation) and varying line Zo and frequency.
A dipole is resonant when its input reactance is very near to, or is at zero ohms. The reactance changes sign as frequency is varied on either side of resonance. Resonance occurs when dipole length is any number of electrical 1/2-wavelengths long.
When length is an odd number of electrical 1/2-waves the input resistance is low. When length is an even number of electrical 1/2-waves the input resistance is high.
A distinction is made between physical length and electrical length. The electrical, resonant length of a thin-wire 1/2-wave dipole is about 2.5 percent shorter than its physical length. That is, to make it resonant, its physical length has to be slightly shortened. This is known as the antenna "End Effect".
The program approximately models End Effect.
The effects of dipole height above ground are not modeled.
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