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Wire Balun (published in
Electron #4, 2007) General 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. Introduction 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 -
Measurements to the 6 - turn wire balun Transmission 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. Reflection 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. System power 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) Imax. = 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, on9cvd@veron.nl
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