Download Free ferrite from TV sets in BALUN use

Free ferrite from TV sets in BALUN use
JK De Marco, PY2WM
18/jan/2006, revised on 2/April/2009
After an article by Ian White, G3SEK, in RadCom magazine, suggesting the use of ferrite removed
from deflection coil (“yoke”) found in TV and PC monitor sets for balun construction, and having
already used the ferrite core obtained from horizontal transformers (“fly-back”), I decided to
measure some baluns made with these cores and compare them to one made over a conventional
FT240-61 toroidal core.
Fig. 1
Figure 1 shows the schematic drawing of a balun. It consists of a single winding of bifilar wire or
coaxial cable. It is known as “current balun” or Guanella balun, one of the first to describe its
functioning. Note that it is placed in series with the transmission line. Traditionally we’ve known
the “voltage balun” or Ruthroff’s, connected in parallel like a conventional transformer. This has
been shown to be inferior (a commercial example of voltage balun is the W2AU balun, not to be
confused with W2DU's which is a current balun).
Photo 1
As an image is worth a thousand words, let's look at some photos. In photo 1 we see two baluns
wound with bifilar wire. On the left 14 bifilar turns of #16 enamelled wire (kept together with
pieces of heat-shrink tube). On the right 7 turns over a ferrite core from a TV deflection core, the
halves were glued with cyanocrylate instant glue.
Photo 2
For maximum bandwidth it is interesting to use coaxial cable for the winding as shown in photo 2
(beware, it is wired as a transformer). Here I used RG-58 with the outer jacket removed in order to
make winding easier. It should be well protected from weather effects. Sevick does not recommend
RG-8X cable since the insulation, foamed polyethylene, would not prevent migration of the inner
conductor (and hence shorting) due to the small radius of curvature.
I also included a balun using a fly-back core, photo 3.
And now for the measuring results. My set-up consisted of a Yaesu FT840 MF-HF xcvr, a
commercial/modified and calibrated SWR and power meter, a laboratory 50Ω DC-8 GHz
Weinschel Corp M1426 load, an oscilloscope Tektronix 453, an antenna analyser Autek Research
RF-1 and a homebuilt LC meter (described in my website).
1 - INDUCTANCE
TABLE 1 - number of turns and inductance measured in LF (about 700 kHz) with the LC meter
Yoke, bifilar Fly-back, bifilar
FT240-61 Yoke, RG-58
wire
wire
# turns
14
6
7
10
Inductance
31,6 uH
14 uH
18 uH
99 uH
In table 1 we can see that the fly-back ferrite has much more permeability than the others. I
correlated these cores with those found on Fair-Rite catalogue (they are the manufacturers of the
ferrite cores sold by Amidon in North America). I compared models with similar transversal area.
This is a valuable way to infer the actual permeability.
The yoke core has an intermediate permeability between ferrite materials 43 and 61, exactly the
most used for transmission line transformers in MF, HF and VHF, this is good news!
The fly-back core looks similar to ferrite 77, this is a high permeability but potentially lossy
material.
2 – REACTANCE AND LOSS INTRODUCED WITH WINDING IN PARALLEL TO THE
LOAD
Fig 2
This measurement, shown in Figure 2 with results in table 2, simulates a balun in the worst
situation, when asymmetries in the antenna/transmission line induce currents on the external part of
the transmission line (in case of a coaxial cable). These currents are called “common mode” and
travel on the outer surface of the coaxial cable sheath. The impedance presented by the balun has to
be high enough to impede this current. In this measurement, if the impedance is low the indicated
SWR will be high. All the models did well in this test.
TABLE 2 – SWR measuring with windings in parallel to the load
Yoke, bifilar Fly-back, bifilar
FT240-61 Yoke, RG-58
wire
wire
Frequency
SWR
SWR
SWR
SWR
1,8
1,05
1,15
<1,05
1
3,5
1
1,05
1
1
7
1
1
1
1
21
1
1
1
1
29
<1,05
1
<1,05
1,05
3 – USE AS CONVENTIONAL VOLTAGE TRANSFORMER
Fig 3
This measurement, shown in Figure 3 with results on table 3, evaluated the performance as a
conventional transformer. The blank cells in the table mean no measurements were taken.
TABLE 3 - balun used as conventional voltage transformer
FT240-61 Yoke, RG-58 Yoke, bifilar Fly-back, bifilar
Frequency
SWR
SWR
SWR
SWR
1,8
1,15
1,4
1,2
<1,05
3,5
1,2
1,3
1,25
7
1,45
1,5
1,4
1,2
21
29
2,5
The yoke core is similar to the FT240-61, and the fly-back core seems to be better on the low
frequencies but losses were not evaluated.
The fly-back core heats up with 100W on frequencies above 14MHz. It is the only situation where
heating occurred. Please note that as a current balun it will never be subjected to this much power as
long as there is a suitably matched antenna.
A conventional transformer like this is found in interstage coupling in transmitters and receivers,
not in antenna and high power applications. The reason is loss in the core. For example a loss of 0.5
dB may be insignificant on an HF receiver or transmitter signal chain but at the output of a kilowatt
amplifier it equals 108 Watts, this is what the core will have to dissipate as heat!
4 – CALCULATED REACTANCE AND MEASURED IMPEDANCE
TABLE 4 - Calculated reactance
FT240-61 Yoke, RG-58 Yoke, bifilar Fly-back, bifilar
Frequency
Ohms
Ohms
Ohms
Ohms
1,8
357
158
203
1119
3,5
714
316
406
2238
7
1428
633
813
4476
21
4284
1899
2439
13428
29
5715
2532
3255
17900
TABLE 5 - Impedance, measured
FT240-61 Yoke, RG-58 Yoke, bifilar
Frequency Ohms
Ohms
Ohms
1,8
370
170
3,5
880
346
7
>2000
830
21
614
880
29
380
490
Fly-back, bifilar
Ohms
1330
1080
630
286
216
I calculated the reactance using the inductance value obtained with the LC meter, table 4. Table 5
shows the impedance measured with an antenna analyser. A simple rule of thumb says the reactance
must be at least 4 or 5 times the impedance of the load/source. Therefore 200 or 250 ohms in a 50Ω
system. The yoke core balun would need 1 or 2 more turns to operate on the 160m band. I also took
this balun and made a frequency sweep with the RF-1 antenna analyser while observing the
impedance and noted a very broad peak of 1440 ohms when the frequency was 13.3 MHz. This is
where a resonance should occur with 10 pF of stray capacitance.
The fly-back balun has too many turns. The impedance decreases with frequency (even though the
inductive reactance increases) because we are past the resonance frequency and now the impedance
is capacitive. Permeability decreases while losses increase with frequency, complicating
characterization.
The yoke core is a good substitute for a large toroid core, as used in baluns for antennas and antenna
tuners/couplers. With permeability between that of material 43 and 61 it is a perfect choice.
5 – POWER HANDLING
According to measurements made by Sevick, different cores with similar permeability have their
other characteristics also similar. So we can use this attribute to determine power handling capacity
of unknown cores, comparing to a known type with the same cross sectional area. A good indicator
of power handling capacity is heating. If a balun under test suffers a temperature rise then it is
unsuitable.
Fly-back core: this is a high permeability core with high expected dissipative loss. This is only a
problem with high power. Heating was observed with 100W at 14MHz in conventional transformer
use. A current balun will be subjected to much less current through its winding provided a
reasonably matched load exists. Losses increase with frequency.
For conventional transformer to adapt impedances this is a good choice as it requires less turns for a
given inductance. One example would be a Beverage antenna for 160m and/or 80m. Losses must be
evaluated though.
Another good use for this type of core is in common mode filters for switching power supplies. In
such a case the loss may come as an asset.
Yoke core: it is expected that a balun with such a core will handle the full legal power. It is
recommended to wind with a suitable coaxial cable capable of this much power. For powers up to
100W a good candidate is RG-174.
Stacking is always a good option to increase power handling and even the bandwidth as fewer turns
are needed for the same inductance, spurious capacitance is also lower. The inductance is
proportional to the number of cores and to the square of the number of turns. The W2DU balun
consisting of a string of ferrite beads works on this principle, it is a single "turn" of coax cable
through a long core.
6 – SOME ADDITIONAL POINTERS
What happens when the core breaks? One of the fly-back cores broke while I was dismantling it. I
simply glued the parts together with cyanocrylate glue. I compared it to a similar, intact unit which
measured 1.5 uH with a single pass of wire, the broken core measured 1 uH. This is the result of the
gap introduced by the glue thickness. I also measured a core with the original gap as used in the flyback TV transformer, there is a very thin (around 0.2 to 0.4 mm) plastic piece between the two
halves, it measured 0.3 uH. See photos 4, 5 and 6.
Photos 7, 8 and 9 show the destruction necessary to reach the core! I also used a Dremel tool to
make some cuts in some places; The fly-back transformers are generally filled with a though epoxy
material.
On a later date I managed to discover more information regarding ferrites employed in TV sets. An
interesting aspect is the resistivity of the materials. High permeability is associated with metallic
MnZn mixtures having low resistivity so the core acts as many paralleled resistively shorted turns.
Fly-back core resistivity is only 3 ohm/m. Resistivity is strongly frequency-dependent and
decreases rapidly with frequency increase. Resistivity is expressed in ohms per metre units or ohms
per centimetre units, in table 6 I have converted all that I have collected into ohms/metre units to
allow direct comparison.
Table 6 – Common ferrite materials, their permeability and resistivity
Material
Permeability
Resistivity
43
800
1x10exp3 ohm/m
61
125
1x10exp6 ohm/m
77
2000
Yoke
350
Fly-back
1800
1 ohm/m
1x10exp5 ohm/m
3 ohm/m
Sabin, W0IYH, reported his measurements of three W2DU current baluns, one with 100 beads of
77 material and inductance 100 uH, other with 100 beads of 43 material, inductance 82 uH and
another with 50 larger beads of 43 material for an inductance of 105 uH. He concluded that the high
permeability is good for low HF bands but the other two are better overall and tolerate more power.
He indicates the need of a balun also at the equipment side as a way to reduce pick-up of household
noise.
7 – CONCLUSION
The yoke core is well suited to HF broadband use in baluns and transformers. The fly-back core is a
high permeability material better suited to low HF. These cores can be found for free in repair
shops.
There's much more to discover as one delves into this subject. For the time being I'm very happy to
report these findings and I thank Ian, G3SEK, for his enlightening RadCom note.
References: Below is a list of what I consider the most informative texts. Some older publications
contain incorrect information based on postulates that were demonstrated to be wrong.
"Transmission line transformers", Jerry Sevick, W2FMI, ARRL, 2001.
"Some aspects of the balun problem", Walter Maxwell, W2DU, QST Mar 1983.
"Some additional aspects of the balun problem", Roehm, W2OBJ, Antenna Compendium 2, ARRL,
1989.
"Baluns: what they do and how they do it", Roy Lewallen, W7EL, Antenna Compendium 1, ARRL,
1985.
"Exploring the 1:1 Current (Choke) Balun", Sabin, William E., W0IYH, QEX July 1997.
"Designing wideband transformers for HF and VHF Power Amplifiers", Chris Trask, N7ZWY,
QEX Mar/Apr 2005.
"The 1:1 Current Balun", Roy Lewallen, W7EL: http://eznec.com/misc/ibalun.txt
"Balun Balance Quality Test", Tom Rauch, W8JI: http://www.w8ji.com/Baluns/balun_test.htm
Fair-Rite Technical Information: http://www.fair-rite.com/newfair/pdf/Broadband.pdf
http://www.fdk.co.jp/cyber-e/pi_fer_tv.htm
http://www.magtek.com.cn/SFC.html
Resistivity: http://www.earthsci.unimelb.edu.au/ES304/MODULES/RES/NOTES/resistivity.html