a balun to feed a balanced feed line from an un-balanced T-Match
tuner, a 1:4 Guanella Current
balun design using two L15
ferrite toroid cores was selected among other balun types.
An impedance transformation
balun may be required due the variations in impedances often encounter with
multi-band balanced antenna system. The balun may be required to
sep up or down the feed impedance presented at the T-Match tuner to
improve the matching range, it
is for this reason that I chose to not include the balun as an
integral feature of the T-Match tuner, opting for the flexibility of
an outboard balun and the ability to trial various baluns subject to
the antenna system and impedances presented.
balun is a low loss, broadband balun that will ideally choke off common mode
currents entering the radio room and importantly provide a
transition from the un-balanced output of the T-Match tuner to the
balanced antenna system feed line.
using the balun to choke off common mode currents is best achieved at
the antenna end of the feed line, this is not a practical arrangement
for a balanced feed line system.
1:4 current balun is derived from two 1:1 current baluns with each consisting
of a close double bifilar winding of
8 turns wound evenly
spaced around the L15
ferrite toroid core. The toroidal cores are rapped in an overlapping layer pink
heavy duty Teflon plumbers tape to protect the enamelled copper
wire from insulation puncture from abrasion with the toroid core.
Figure 1 Schematic of the 1:4 Guanella Current
Figure 2 Wiring of the 1:4 Guanella Current
this drawing shows winding connections and not the number of turns
required. See article for details.
ferrite toroid core. Jaycar
heavy duty Teflon plumbers tape.
x 600mm of 1.25mm Enamelled copper wire.
Banana Socket Binding Post - Black. Jaycar
UHF chassis mount connector
Sealed Polycarbonate Enclosures 82 x 80 x 55mm
No. HB-6230. See
Fig 3 for details
Figure 3 Sealed Polycarbonate Enclosures 82 x 80 x 55mm
to IP65 of IEC 529 and NEMA 4
core windings assembled.
current balun individual
core windings stack assembled.
The evaluation of the efficiency
of the balun over the desired bandwidth (1.8 - 30MHz) was carried out
by testing the impedance that could be seen from
unbalanced side to a resistive load applied to the balanced side using
an antenna analyser. The efficiency is shown to be relatively flat from below 1.8MHz
to above 30MHz. The below antenna analyser
plot viewing a 200ohm resistive load attached to the balanced
side of the balun and measured at a nominal impedance of 50ohms
presented as anticipated an approximate 50ohm load to the analyser and
ideally produced about a
1:1 SWR. The results are more or less what was expected and demonstrates
that the balun's 1:4 current transformation occurs efficiently from
well below 1.8
to well above 30MHz
4AIM 4170C antenna analyser
plot viewing a 200ohm resistive load through the Guanellacurrent
Note the 200ohm resistor appears as 50ohms due to the 1:4 balun ratio
resulting in an ideal SWR of 1:1. This plot shows an SWR of
almost exactly 1:1 with no reactance at a frequency of 500kHz with consistent
flat SWR throughout the HF spectrum with almost no obvious reactance at 30MHz.
Figure 5AIM 4170C antenna analyser
plot viewing a 100ohm resistive load through the Guanellacurrent
Note the 100ohm resistor appears as 25ohms due to the 1:4 balun ratio
resulting in an ideal SWR of 2:1. This plot shows an SWR of approximately
2:1 from 500kHz through to 30MHz and with modest inductive reactance
towards the upper frequencies.
Figure 6AIM 4170C antenna analyser
plot viewing a 450ohm resistive load through the Guanellacurrent
Note the 450ohm resistor appears as 112ohms due to the 1:4 balun ratio
resulting in an ideal SWR of 2.2:1. This plot shows an SWR of approximately
2.5:1 at 500kHz through to 30MHz and with
significant capacitive reactance towards the mid and upper
AIM 4170C antenna analyser explanation;
Standing Wave Ratio.
Resistive component of the total impedance
Phase angle between voltage and current.
+ indicates inductive reactance while - indicates capacitive
Video of the practical
construction and background of a higher powered version of this balun.
This YouTube video was the inspiration for my version of the 1:4
current balun and provides a great construction example.