(published in Electron #4, 2007)
This chapter is the second in a series of articles on baluns for antenna applications. The first article is an introduction with some background on balun types. It is advisable to read the articles in the above order especially since each next chapter is building on information and formula's already explained earlier and referencing to this.
At many radio-amateur sites one may observe the coaxial feed-cable to be tied up into a coil with a few number of turns next to the antenna. To the casual observes this may look like some spare wire, to be used later-on when constructing the antenna at a higher position. At closer examination however this 'spare wire' is a carefully constructed inductor to keep antenna current away from the (outside of the) feed-line. The construction is hereby acting as a sleeve choke or 1 : 1 current balun.
To obtain some feeling for numbers we will calculate this current balun for a system impedance of 50 Ohm, that may be found in a tuned dipole antenna, depending on antenna height and ground type. From the introductory article we already know the sleeve impedance to be at least four times system impedance to be effective at 'adverse' operating conditions. This impedance should be calculated at the lowest of operating frequencies since impedance will go up with frequency. At a lower frequency of 2 MHz. and a sleeve impedance of 4 x 50 Ohm = 200 Ohm, we calculate:
L = XL / w = 200/ (2.p. 2.106) = 16 mH
Note: The sleeve impedance has been calculated for a system impedance of 50 Ohm, to accomodate the example dipole and the antenna feed-line. Outside resonance, the antenna impedance will be much higher, so also the sleeve impedance should, to still be effective. In a practical situation therefore, sleeve impedance should be calculated at all operating frequencies together with the antenna impedance. At the most demanding frequency (highest antenna impedance) the sleeve impedance should still be at least four times the antenna impedance to be effective. We should be aware of this situation whenever operating an antenna outside resonance.
General formula for calculating an 'air' inductance is:
L = n2 . μ . Q / l, (H)
with 'n' for the number of turns, 'Q' for the
area of one turn and 'l' for the inductor length. This (fundamental) formula
is accurate for all inductors with a length to diameter ratio of three or
more. A bunched coil as in the antenna situation does not comply to this
ratio, so has to be approached in a more practical way. In the ARRL handbooks
it is advised for the frequency range 3 - 30 MHz. to construct this sleeve
choke with 6 - 10 turns of transmission-line, with a diameter of 10 -
The balun as in figure 1 has been tested for current balancing properties by measuring transmission and reflection when connected into 50 Ohm. For each measurement, the balun was to operate at maximum unbalance, i.e. with the center conductor grounded. Transfer qualities have been depicted in figure 2.
Figure 2 depicts the transfer characteristic (insertion loss) of the wire balun of figure 1.
Insertion loss is lower than 1 dB starting at 2 MHz. and stays that way to over 50 MHz. Low frequency behavior is dictated by the coil inductance, that effectively is measured in parallel to the input. At the high frequency side, the parasitic parallel capacity is limiting sleeve impedance. This parasitic capacitance is an incidental value, that may easily be higher or lower depending on the way the coil is 'bunched up'. The returning of the curve above 100 MHz. is related to this particular sleeve inductor only and will be different (value and position) for a different 'bunch'.
This transfer function is quite acceptable although one may be tempted to enhance performance at the low frequency side by increasing the number of turns. When increasing, the parasitic capacitance will also increase, lowering the cut-off frequency at the high frequency side. Although some room may for play is available, margins are small.
Equally important is the behavior of this balun as seen from the input position. We therefore measured input reflection which is depicted in figure 3 as a SWR graph.
In figure 3 reflection is measured for the wire balun of figure 1, when terminated into 50 Ohm and in a maximum unbalancing situation. We find SWR below 1,5 between 4 - 50 MHz. which is acceptable for most HF applications.
Comparing figure 3 with figure 2, we find SWR to go up where transmission is going down, at the low frequency side as well as at the high frequencies. Also the 'return' of the curve around 150 MHz. is visible again.
Taking both graphs together this certainly is an acceptable component that will serve its purpose in a 50 Ohm system environment for a wide number of HF amateur bands. This is even more so when looking at the maximum allowable system power; at this component entirely determined by the characteristics of the transmission-line. When applying RG 58 :
Vmax. = 1900 V. (600 V. in the foamed RG58 variation)
According to the manufacturer of 'standard' RG58 cable, this type may be operated up to 350 Watt at 30 MHz., to de-rate with increasing frequency
Around the end of last century Steve
Steltzer, WF3T, has performed a series of measurements using an automated
test bench with a HP vector voltmeter. He started-of by constructing his wire
baluns on drainage pipes with a diameter of 4 1/4 " (
In figure 4 all baluns have been constructed at
the drainage pipes of the indicated diameter except the graph marked 'bunch'.
This last balun is the balun on
As we have noticed before, the balun should have an impedance that is at least four times the system impedance. In figure 4 this is depicted by the red line at 200 Ohm, implicitly comparing baluns for application in a 50 Ohm system. Taking this line it is clear only 8 wdg or more will qualify with the tested diameters for frequencies starting at 3 MHz.
Next interesting part is the resonance peak for all of the constructions. At these frequencies the inductor is resonating with the parasitic parallel capacitance making the choke impedance really high indeed. Taking the same diameter, but more turns, total impedance at the low frequency side will go up with resonance frequency to go down, making this higher-turns choke even less interesting at the high frequency side (compare blue and green curve).
After resonance, the impedance will still be high for some time, later to drop off sharply when this parasitic capacitance is the dominant circuit reactance. The only chokes to still qualify at 30 MHz. are those that do not qualify at 3 MHz. for too low impedance (lower number of turns).
The 'bunch' graph is showing nicely what is happening when windings are close together. When comparing the red and orange curves, it is clear the bunch type is showing higher impedance at the low frequency side since inter-winding coupling is much better with close-winding. The parasitic capacitance is higher because in the bunched-up situation first and last winding, carrying highest potential difference, are much closer together now although the exact extent of the phenomenon is highly dependant on the accidental way of constructing the bunch. On the other hand, this effect may be applied to good use when a choke balun should exhibit a particularly high impedance at a desired frequency; the bunch may be 'fondled' until the desired effect has been reached.
Because of the higher parallel capacitance, parallel resonance is occurring at a lower frequency, making this choke fall-of earlier at high frequencies too. Steve notes this effect to also showing at the other choke baluns.
From the above series of measurements it may be concluded wire baluns are useful over a number of HF frequencies and at the specific resonance frequency even very good. In general we may safely notice that wire baluns may be applied over a frequency range of about 1 : 3, but it will be hard to construct a wire balun that will effectively operate over the entire 3 - 30 MHz. field of frequencies, let alone 1,8 - 30 MHz.
Fortunately other solutions exist to solve this problem and may even be applied over a much higher bandwidth as may be seen in the next chapter.
Please contact me for your remarks and questions at:
Bob J. van Donselaar