Balancing shpi and chokes on ferrite tubes. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Transformers on ferrite tubes perform several functions at once: they transform the resistance, balance the currents in the antenna arms and suppress the current on the outer surface of the coaxial feeder braid. The best domestic ferrite material for broadband transformers (BCT) is grade 600NN ferrite, but tubular magnetic cores have not been made from it...

Now ferrite tubes from foreign companies with good characteristics have appeared on sale, in particular, FRR-4,5 and FRR-9,5 (Fig. 1), having dimensions dxDxL 4,5x14x27 and 9,5x17,5x35 mm, respectively. The latter tubes were used as noise suppression chokes on cables connecting computer system units with cathode ray tube monitors. Now they are being massively replaced with matrix monitors, and the old ones are thrown away along with the connecting cables.

Rice. 1. Ferrite tubes

Four ferrite tubes, stacked side by side in twos, form the equivalent of “binoculars”, on which transformer windings can be placed, covering all HF bands from 160 to 10 meters. The tubes have rounded edges, which prevents damage to the insulation of the winding wires. It is convenient to fasten them together by wrapping them with wide tape.

Of the various broadband transformer circuits, I used the simplest one, with separate windings, the turns of which have an additional connection due to the tightly twisted conductors among themselves. This makes it possible to reduce leakage inductance and thereby increase the upper limit of the operating frequency band. We will consider one turn to be a wire threaded through the holes of both “binoculars” tubes, and “half a turn” to be a wire threaded through the hole of one “binoculars” tube. The table summarizes the options for transformers that can be used on these tubes. Here N1 is the number of turns of the primary winding; N2 - number of turns of the secondary winding; TO_U - voltage transformation ratio; K_R - resistance transformation coefficient; M - resistance ratio for a source with an output impedance of 50 Ohms.

Table

N1	N2	КU	KR	М
1	1	1:1	1:1	50:50
1	1,5	1:1,5	1:2,25	50:112,5
1	2	1:2	1:4	50:200
1	2,5	1:2,5	1:6,25	50:312,5
1	3	1:3	1:9	50:450
1	3,5	1:3,5	1:12,5	50:625
2	1	1:0,5	1:0,25	50:12,5
2	1,5	1:0,75	1:0,56	50:28
2	2	1:1	1:1	50:50
2	2,5	1:1,25	1:1,56	50:78
2	3	1:1,5	1:2,25	50:112,5
2	3,5	1:1,75	1:3	50:150
2	4	1:2	1:4	50:200
2	4,5	1:2,25	1:5	50:250
2	5	1:2,5	1:6,25	50:312,5
2	5,5	1:2,75	1:7,56	50:378
2	6	1:3	1:9	50:450
2	6,5	1:3,25	1:10,56	50:528
2	7	1:3,5	1:12,5	50:625

As you can see, a very wide choice of resistance ratios is obtained. A transformer with a 1:1 ratio, like a choke, balances the currents in the antenna arms and suppresses the current on the outer surface of the power cable braid. Other transformers, in addition to this, also transform resistances. What should you consider when choosing the number of turns? All other things being equal, transformers with a single-turn primary winding have approximately four times the lower limit of the passband compared to a two-turn primary winding, but their upper passband frequency is also much higher. Therefore, for transformers used from the ranges of 160 and 80 meters, it is better to use two-turn options, and from 40 meters and above - single-turn ones. It is preferable to use integer values ​​for the number of turns if it is desirable to maintain symmetry and space the winding terminals on opposite sides of the “binoculars”.

The higher the transformation ratio, the more difficult it is to obtain a wide bandwidth, since the leakage inductance of the windings increases. It can be compensated by connecting a capacitor in parallel with the primary winding, selecting its capacitance to the minimum SWR at the upper operating frequency.

For windings, I usually use MGTF-0,5 wire or thinner if the required number of turns does not fit in the hole. I calculate the required wire length in advance and cut it with some margin. I twist the wire of the primary and secondary windings tightly until it is wound onto the magnetic circuit. If the ferrite hole is not filled with windings, it is better to thread the turns into heat-shrinkable tubes of suitable diameter, cut to the length of the “binoculars”, which, after winding is completed, are shrinked using a hair dryer. Pressing the turns of the windings tightly against each other expands the transformer bandwidth and often eliminates the compensating capacitor.

It should be borne in mind that a step-up transformer can also work as a step-down transformer, with the same transformation ratio, if it is “inverted”. Windings intended for connection to low-resistance resistances must be made of screen “braid” or several wires connected in parallel.

The transformer can be checked with an SWR meter by loading its output onto a non-inductive resistor of the appropriate value. Band boundaries are determined by the permissible SWR level (usually 1,1). The loss introduced by a transformer can be measured by measuring the attenuation introduced by two identical transformers connected in series so that the input and output of the device have a resistance of 50 ohms. Don't forget to divide the result by two.

It is somewhat more difficult to evaluate the power characteristics of a transformer. This will require an amplifier and a load equivalent that can handle the required power. The same circuit with two transformers is used. The measurement is carried out at the lower operating frequency. Gradually raising the CW power and maintaining it for about a minute, we determine the temperature of the ferrite by hand. The level at which the ferrite begins to heat up slightly per minute can be considered the maximum permissible for a given transformer. The fact is that when operating not on an equivalent load, but on a real antenna that has a certain reactive component of the input impedance, the transformer also transmits reactive power, which can saturate the magnetic circuit and cause additional heating.

In Fig. Figure 2 shows a practical design of a transformer having two outputs: 200 ohms and 300 ohms.

Rice. 2. Practical design of a transformer having two outputs

Transformers can be placed on a suitable sized board, protecting it from precipitation in any practical way.

Author: Vladislav Shcherbakov (RU3ARJ)

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