Tuner + Coax Line + Balun + Balanced Line + DipoleAuthor: R.J.Edwards G4FGQ © 2nd February 2002
Ahigh dipole, centre-fed via a balanced-pair transmission line, is an efficient multi-band radiating system at all frequencies above that at which the end-to-end dipole length is about 0.35 wavelengths. But site topography and accommodation often require a coaxial cable between the balanced line and the transmitter. At the junction of the two lines there should be a balanced-to-unbalanced transformer of a suitable ratio. Immediately at the transmitter output is usually a variable L & C network (tuner) to transform the coaxial line input, Z = R+jX, to a purely resistive load for the transmitter, typically 50 ohms.
The program accepts a brief description of each part of the antenna system and computes the input impedance and transmission efficiency of each part. Loss in the soil under a low antenna is included in the antenna assessment. Values of matching network components and network efficiency are also computed. Overall radiating efficiency is then the product of the four individual efficiencies.
The program enables the performance of a variety of antenna + feedline arrangements to be assessed at any operating frequency in the range 1 to 60 MHz.
By setting the length of the other line to zero the whole feeder may be of the coax or balanced-pair type. Usually both will be used with the balun located at their junction. Note: when a line length is set to zero please enter sensible values of Zo, velocity factor and conductor diameter.
Conductor diameter is needed to calculate line loss. Halving diameter roughly doubles line attenuation in decibels and has a significant effect on input Z. Copper or aluminium conductors are assumed. Exact classical transmission line formulae, no approximations, are used by the program for all calculations.
Velocity factors are as important as line lengths. Fortunately at HF VF is not difficult to measure. A manufacturer's stated value of Zo of twin line can be 10% or more in error. This program can be used to investigate the effects of such discrepancies and inaccuracies on expected performance.
Enter a full set of data for a G5RV antenna by hitting "G" when G(5RV) appears on the bottom line menu. This popular antenna has a dipole length of 1.5 wave-lengths at 14.15 MHz and its balanced feedline is 1/2-wavelength long.
A ready-made set of input data for a low dipole, useable on the 160m band, can be entered by hitting O(ne sixty) when it appears on the bottom line menu.
Program output data
Line lengths in wavelengths are with respect to their own velocities and not to free-space velocity. The antenna is treated as a balanced-twin line open-circuit at its far end, with an electrical length equal to half of its actual dipole length, plus "end" effect, plus the effects of ground proximity. Power efficiency = Pout/Pin *100%. This includes an increase in line loss due to non-uniform current distribution versus length due to standing waves.
Line attenuation in dB/100 feet is the value when the line is Zo-matched. It may be useful when the conductor diameter is not known but the dB/100 feet IS known. dB/100 feet = dB/30 metres. HF attenuation on all lines is proportional to Sqrt(F). Vary wire diameter for agreement with the known attenuation.
Multiband antennas - Searching for minimum SWR and resonance frequencies
When adjusting antenna and feedline lengths for multiband operation interest lies in the resonant frequencies at which line Zin is resistive. Frequencies are not harmonically related. To assist with a search Xin is highlighted with a change in colour when its sign changes. SWR and Rin are also highlighted. SWR on a line is always at a minimum as Xin changes from -ve to +ve as frequency increases. Antenna and line lengths can also be continuously varied by using adjacent up/down keys on the keyboard. See the menu near the bottom of the data screen.
All types of balun should use HF-grade ferrite cores. For multi-band operation a choke balun is most appropriate with a 1:1 impedance ratio. This consists of a ferrite ring approximately 2" mean diameter wound with twin figure-of-eight flexible cable. Or, for lower powers, it may be wound with small gauge coaxial cable. The length of line wound on the core should not exceed 1/8-wave at its own velocity at the highest operating frequency. At the lowest frequency the winding's inductive reactance should not be less than 4 or 5 times line Zo.
The electrical length of cable wound on any type of balun should be added to the length of coaxial or balanced line on either side of it according to the form of construction. The small loss and the effect of balun winding length on system input impedance will then be approximately included in the analysis.
Storage facility for program input data
Any set of input data for an antenna + line configuration, as displayed on the data screen, can be placed in store by using the '[' key and can be recalled at a later time by using ']'. The data screen then becomes available for other antenna configurations. When a set of data is recalled the current screen data is overwritten and lost. The stored data is available until it is itself over-written by a newly stored set.
The Tuner - an L-Match Network
Any pair of complex impedances can be Z-matched with an L-network. Tuner loss is at a minimum. But inconvenient L and/or C values may occur. It is necessary also to arrange for the L and C circuit positions to be interchangeable.
In this program the tuner is located immediately at the transmitter output. It transforms the input impedance of the coaxial line to 50 ohms. If that line is of negligible length the network transforms the balun input impedance to 50 ohms. If the balun has a 1:1 ratio then Zin equals Zin of the balanced line.
L-network component settings, uH and pF, are computed and displayed. Either or both components may be either L or C. If the shunt component is at the antenna end of the network it is marked "Ant end". If at the transmitter end it is marked "Tx end".
Network power efficiency is computed on the assumption that all loss occurs in inductors and none in capacitors. Inductors are assumed to have an unloaded Q of 220 at the operating frequency. If both components are C's, loss is zero. If coil Q is doubled then, for the same through power, power loss is halved. Before accepting a system as satisfactory do NOT forget to check tuner loss. Note: The loss in an equivalent T-network is always greater than in the L.
Uncertainties in performance predictions due to program and environment
Transformation of the antenna input impedance to 50 ohms is as accurate as the data input. When the length of line wound on a 1:1 choke balun is added to the length of the coaxial or twin line residual error due to balun imperfections will be negligible.
When the balun has a ratio other than 1:1 one of the balun windings will be in shunt with the coaxial line which will shift the balun's input impedance away from computed values by an amount depending on the transformer's inductance. It will be noticed only on the lowest frequency band for which the balun was designed. The practical effect will be a shift in computed tuner L and C settings. Balun loss, ignored by the program, in a good HF balun will be less than 0.1 dB
Greatest uncertainty occurs due to poor knowledge of the soil and the effects of structures within the near field. At low heights radiation resistance falls and ground loss increases. At extremely low heights where an antenna would not normally be used, an antenna is reduced to a very lossy 'ground radial'.
At heights exceeding 1/3 or 1/2 wavelengths only a very asymmetrical feedline relative to the antenna wire might have a noticeable effect on input impedance but none on radiating efficiency except that a distorted radiation pattern may be considered in some rare circumstances to be equivalent to a loss of power.
Hints and Tips
Set a parameter to be varied somewhere in the desired range before using the designated up/down sweep keys. Sweep speeds depend on computer clock rate.
Sweep frequency to find system resonances. These occur when input reactances change signs. At resonance input resistance may be either low or extremely high. Only the low values will be of interest. Frequency can be swept up/down either fast or slow. When in the vicinity of resonance go slow. Also vary frequency to find SWR-bandwidth limits. It is usual to make the coaxial line Zo the same impedance as the load required by the transmitter. But this is not essential.
To place resonances in required bands, vary either line lengths or line Zo in addition to antenna length. Also to force resonance at a particular frequency and so obviate the need for a tuner. Or to avoid an awkward L or C value, or an inconvenient circuit arrangement inside the tuner.
Repeat any of the foregoing with another balun transformer ratio. Does this result in a lower average coaxial SWR taking one band with another? Does it increase the number of bands for which a tuner is unnecessary? In most cases a 1:1 choke or current balun will be the best choice because it is a true longitudinal choke and because its high-frequency response is better than an N-to-1 transformer. If there are Zin problems, changing open-wire length is the best philosophy.
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 Dipole3 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.
Discuss, debate and ask questions about dipole antennas in the Ham Radio Technical Topics Forum.
Search other ham radio sites with Ham Radio Search