Book Excerpt: Sevick’s Transmission Line
Transformers, Chapter 9, Part 3
Steve Taranovich - August 12, 2014
Chapter 9: Baluns, Part 3
This chapter excerpted from Sevick’s Transmission Line Transformers describes the balun as a
subset of transmission line transformers with an in-depth treatment of most used types. Schematics
and design /build information details are shown and are well described. There are three parts to this
excerpt due to its length. Here is part three.
Published by Scitech Publishing, an imprint of the IET.
Copyright © 2001, 2014 by Scitech Publishing, Edison, NJ. All rights reserved.
Fifth edition 2014
ISBN 978-1-89112-197-5
The book, authored by Raymond A. Mack and Jerry Sevick has been reviewed on EDN as well and
that review can be found here.
Editor’s note: Part 1 of this three-part book excerpted article discussed the 1:1 Balun. Part 2 of this
three-part book excerpt article discussed the 1:4 Balun.
9.4 The 1:9 Balun
When matching a 50 Ω coax down to a balanced load of 5.56 Ω or up to a balanced load of 450 Ω, the
Guanella balun is the transformer of choice. There is little doubt that these baluns offer the widest
bandwidths under these two very different conditions. Further, this modular concept (i.e., adding
transmission lines in parallel-series arrangements) offers the highest efficiency at high impedance
levels. Experiments have shown that efficiency, at least with the Ruthroff unun, decreases as the
impedance level increases. With Guanella’s approach, each transmission line shares a portion of the
load; therefore, his transmission lines can work at lower impedance levels. Also, the longitudinal
gradients are less with his transformers.
The balun in Figure 9-15 is designed to match 50 to 5.56 Ω. This transformer could be used to match
50 Ω coax cable directly to short-boom, four-element Yagi beams with resonant impedances of about
6 Ω. This low impedance 1:9 balun has turns of low impedance coax (Z0 = 13 Ω) on each of the three
ferrite rods (μ = 125). The rods have a diameter of 1/2 in and a length of 4 in.
Figure 9-16 is a photograph of a 1:9 balun capable of broadband operation from 450 to 600 Ω. Each
of the three 2.4 in OD, no. 64 toroids (μ = 250) has 11 bifilar turns of 300 W TV ribbon. Due to the
proximity of the turns on the inside diameters of the cores, the characteristic impedance is lowered
to 205 Ω. At the 68.33:615 Ω, level, which is optimum, the response is flat from 5 to 40 MHz. By
using a larger core, one to two more turns would be possible, thereby extending the frequency range
to 3.5 to 40 MHz. At the 66.67:600 Ω impedance level, the high frequency response of these
transformers is still above 30 MHz. The power rating is at least 500 W continuous power. Further,
by adding a 1:1.36 unun and a 1:1 balun in series (and this can be done with one core; see Chapters
7 and 8), this compound arrangement becomes an excellent unun for matching 50 Ω (unbalanced) to
600 Ω (unbalanced). Sevick found it necessary to use a hole punch to remove material between the
wires to allow the twin lead to bend appropriately. Figure 9-16 was created using twin lead from a
GQ brand FM dipole antenna, and the cable was flexible enough to easily wind around the cores.
Flexibility will depend on the wire gauge of the twin lead and the thickness of the insulation.
Figure 9-15 Photo shows a 1:9 low impedance transformer designed for the 50:5.56 Ω range.
Figure 9-16 Photo illustrates a 66.7:600 Ω transformer constructed using 300 Ω twin lead.
The high frequency and low frequency models of the low impedance transformer shown in Figure 915 are presented in Figure 9-17a and Figure 9-17b, respectively. The inner conductors are no. 12
wire with two layers of 3M no. 92 tape. The outer braids (unwrapped) are from RG-122/U cable (or
equivalent). At the 5.56:50 Ω level, the impedance ratio (with the load floating) is constant from 1.5
MHz to over 30 MHz. The power rating is in excess of 2 kW continuous power. With ML or H
Imideze wire, the voltage breakdown is in excess of 3000 V. Although more awkward to construct,
four turns of the same coax on toroids with permeabilities of 250 to 300 would yield a 1:9 balun with
much greater bandwidth. Finally, a broadband 1:16 balun could be constructed with four coax
cables using no. 10 wire with one layer of 3M no. 92 tape for the inner conductor. The characteristic
impedance of this coax would be about 9 Ω. This balun would match 3.125 Ω to 50 Ω.
Figure 9-17 (a) High frequency model of the Guanella 1:9 balun. (b) Low frequency model. It is
assumed that Z0 = RL/3.
9.5 Baluns for Yagi, Quad and Rhombic Antennas
9.5 Baluns for Yagi, Quad and Rhombic Antennas
A very popular balun for antenna use has been the 1:1 (50:50 Ω, nominally) trifilar design by
Ruthroff. It has been used successfully in matching 50 Ω coax to Yagi beams after shuntfed methods
were employed to raise the input impedance. It has also found success in matching 50 Ω coax
directly to 1/2 l dipoles at heights of 0.15 to 0.2 l, where the resonant impedances are 50 to 70 Ω,
respectively (the resonant impedance reaches a peak of about 98 Ω at a height of 0.34 l). Outside of
these two cases, baluns have found very little use in matching 50 Ω coax cable to resonant
impedances far removed from the ‘‘nominal’’ 50 Ω. For the experimenter, baluns for the following
antennas are offered.
9.5.1 Yagi Beams
Sections 9.3 and 9.4 described Guanella baluns with ratios of 1:4 and 1:9 that can match 50 Ω coax
directly to Yagi beams with balanced and floating impedances of about 9 to 15 Ω and 5 to 8 Ω,
respectively. Sevick designed two other baluns capable of matching 50 Ω coax directly to higher
impedance Yagi antennas. One balun is designed to match 50 Ω coax to a balanced (and floating)
impedance of about 20 Ω. Its useful impedance range is probably from 16 to 25 Ω. The schematic is
shown in Figure 9-18. It is a compound transformer consisting of a step-down (50:22.22 Ω) Ruthrofftype unun in series with a low impedance coax cable 1:1 (22:22 Ω) Guanella balun. The core is a 2 in
OD, no. 61 toroid (μ = 125), and both transformers are wound on the same core. The unun has five
trifilar turns of no. 14 wire. The 1:1 coax cable balun also has five turns. The coax cable uses no. 12
wire with two layers of 3M no. 92 tape for the inner conductor. The outer braid, which is left
untaped, is from RG-122/U. At the 50:22.22 Ω level, the response is flat from 3.5 to well beyond 30
MHz. The power rating is in excess of 1 kW continuous power. If 160 meter operation is desired,
then a core with a permeability of 250 to 300 is recommended.
Figure 9-18 Schematic shows a fractional ratio balun designed to match 50 Ω cable to a balanced
floating load near 20 Ω. The 1:1 balun on the right uses 22 Ω coax cable.
The other balun is designed to match 50 Ω coax to a balanced and floating impedance of about 30 Ω.
Its useful impedance range is probably from 25 to 35 Ω. It is also a compound transformer using a
step-down (50:28.13 Ω) Ruthroff-type unun in series with a low impedance coax cable 1:1 (30:30 Ω)
Guanella balun (Figure 9-19). The common core is a 2.4 in OD, no. 64 toroid (μ = 250). The unun,
which has an impedance ratio of 1.78:1, has six quadrifilar turns. Winding 5-6 in Figure 9-19 is no.
14 wire, and the other three are no. 16 wire. The 1:1 coax cable balun also has six turns. The inner
conductor of the coax cable is no. 14 wire with two layers of 3M no. 92 tape, followed with two
layers of 3M no. 27 glass tape. The outer braid (untaped) is from RG-122/U cable. At the 50:28.13 Ω
level, the response is flat from 1.5 to 50 MHz. The power rating is in excess of 1 kW of continuous
power. This transformer, as well as the previous one, could also have been constructed with two
separate cores.
9.5.2 Quad Antennas
9.5.2 Quad Antennas
The quad antenna generally has a balanced (and floating) resonant impedance in the range of 100 to
120 Ω. This antenna also lends itself readily to a compound balun. Several approaches can be used;
for example, a 1:2 step-up unun (50:100 Ω) followed by a 1:1 balun (100:100 Ω) or a 2:1 step-down
unun (50:25 Ω) followed by a 1:4 step-up balun (25:100 Ω). Sevick tried the latter approach, and it
was implemented as a compound balun using a single 2.4 in OD, no. 61 toroid (μ = 125) (Figure 920). It uses a tapped trifilar step-down unun in series with a Ruthroff 1:4 balun. The Guanella 1:4
balun, although possessing a better high frequency response, was not used since it did not lend itself
as readily to a single core. If a much wider bandwidth is required, then two separate cores, with the
1:4 balun using Guanella’s approach, is recommended. The unun in Figure 9-20 has six trifilar turns.
Figure 9-19 Schematic shows a fractional ratio balun designed to match 50 Ω cable to a balanced
floating load near 30 Ω. The 1:1 balun on the right uses 30 Ω coax cable.
Figure 9-20 Schematic shows the connections for a compound 1:2 step-up balun. With the input at
terminal B, the impedance ratio is 1:2 (50:100 Ω). With the input at terminal A, the impedance ratio
is 1:1.78 (50:90 Ω).
Winding 3–4 is no. 14 wire and is tapped at one turn from terminal 3. The other two windings are no.
16 wire. With the input connection to the tap, the impedance ratio is 2:1. The Ruthroff 1:4 balun
uses 10 bifilar turns of no. 14 wire. This compound balun, 50 Ω coax to 100 Ω (balanced), is flat from
3.5 to 30 MHz. The response is quite the same in matching 60 Ω (unbalanced) to 120 Ω (balanced).
With the input connection directly to terminal 3, a similar response is obtained at the 50:90 Ω
impedance level. If 160 m operation is also desired, then a toroid with a permeability of 250 to 290 is
recommended. The power rating is 1 kW continuous power.
9.5.3 Rhombic Antennas
Compound baluns also lend themselves to matching 50 Ω coax to the balanced (and floating)
resonant impedances of V and rhombic antennas. These impedances are generally in the range of
500 to 700 Ω. Section 9.4 described some compound baluns using ununs in series with Guanella
baluns, yielding wideband responses over this impedance range. This subsection presents one of
Sevick’s earlier approaches for a 1:12 balun using only two toroidal transformers. One is a tapped
bifilar Ruthroff unun with a ratio of 1:3, and the other is a Ruthroff 1:4 balun. Figure 9-21 shows the
performance when matching to a balanced (and floating) load of 600 Ω. In this case, the input
impedance is measured as a function of frequency. As shown, a constant ratio is obtained in the
frequency range of 7 to 30 MHz. Figure 9-22 shows the schematic for the two-series transformers.
The transformer on the left has eight bifilar turns of no. 14 wire on a 2.4 in OD, Q1 toroid (μ = 125).
The wire is covered with 17 mil wall Teflon tubing. The top winding in Figure 9-22 is tapped six
turns from terminal 3, giving a 1:3 step-up ratio. The transformer on the right has 11 bifilar turns of
no. 16 wire on a 2.4 in OD, Q2 toroid (μ = 40). The wire is also covered with 17 mil wall Teflon
tubing. This is a 1:4 Ruthroff balun. Although Q2 material has a lower permeability than Q1 material,
it was chosen because it has a lower core loss at these high impedance levels.
Figure 9-21 Plots show the performance of a 1:12 balun designed to match 50 Ω coax cable to a
balanced load of 600 Ω.
Figure 9-22 Schematic shows the connections of the 1:12 combination balun measured in Figure 921.
This combination of transformers allows for broadband operation at the high impedance levels of
500 to 700 Ω because of the canceling effects they have in a series configuration. Since the
characteristic impedance (Z0) of the 1:4 balun on the right is only about 130 Ω (it should be 300 Ω
for optimum response), the input impedance as seen at its terminals 1–3 is capacitive and the real
part is less than RL/4. Since the characteristic impedance (Z0) of the tapped transformer on the left is
about 115 Ω and is greater than would be normally used to match 50 to 150 Ω, it has the opposite
effect on its load. It causes the load to look inductive; the reactance of the right-hand transformer is
effectively canceled over a large portion of the band. The resistive component is not altered when
the characteristic impedance is greater than would normally be used.
Even though the compound baluns described in section 9.4 (using Guanella’s approach) have the
potential for much wider bandwidths, the two octaves achievable using the simple schematic in
Figure 9-22 should prove to be quite useful.