S-MATCH.docx

(PA0FRI's "S-MATCH ATU" published in RSGB's RadCom March 2003)

 06-03-2011 The sequence of toroid's windings.

How the idea arose.

Micro Metals/Amidon toroids suitable for this ATU.

This idea has not yet been fully tested.

A 1500 W S-Match.

S-MATCH©

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Several possibilities of the S Match design.

I was regularly asked whether my system can be applied as an anode circuit for a RF amplifier. It is also possible, but due to lack of time I was not able to test the circuit with high power to fully support the idea. That is a good opportunity for you to try it out.

After an extensive research and experimenting I achieved the design shown in the above drawings of the Balanced Universal ATU System. To make it distinctive from other types of ATU's, I call my design S-Match©. The system is intended to accommodate balanced antenna systems transmitting on a very wide frequency spectrum, ranging from 160 m band up to 10 m band. Likewise, it is able to take proper care of impedances and associated reactances ranging from less than 20 Ω and up to 3000 Ω. Built with the proper components, it is able to transfer loads of more than 1000 Watts. This balanced antenna-tuning unit has only three components: a capacitor, a roller inductor and a balun or RF transformer. The input circuits are isolated from the output circuits, resulting in a system, which is suitable also for single-wire antennas and coax fed antennas.

The efficiency of mechanical and electrical a well-built S-Match is equivalent to other ATU's in this (fig») table. See my Test Symmetrical ATU's.

In the design of one of my ATU’s, the balun using T200A-2 toroid is mounted between two PCB disks and located in between the 50 µH roller-inductor and the 12 – 1200 pF vacuum capacitor.

An antenna like my present antenna system is matched using only 200 pF on all nine HF bands, still my S-Match is built with a 1200 pF vacuum capacitor. My ATU was able to match easily the antennas of a couple of friends. In one case it was barely adequate to match another friend's 4 × 40m loop antenna on 160 m. I had to add an extra switchable 1200 pF capacitor in parallel with the roller inductor to handle any impedance. (see further below)

CAPACITANCE

With some antenna systems a better efficiency may be achieved if the antenna is connected to the capacitor as shown in the drawing by the dotted line. It is also possible to use two separate toroids. I didn’t notice any difference between an ATU with two cores and an ATU built with a single toroid.

TRANSFORMER

Transformers made with Amidon toroids. The T200A-2 toroid is a nearly exact substitute of two T200-2 units sandwiched together.

The transformer contributes a lot to the final result, which is my conclusion after I tested many types of windings. A couple of ham friends successfully tested a number of my own ideas for a suitable HF transformer/balun. The above chosen type of transformation ensures that input and output connections are galvanic isolated. The experiments show that an inductance of about 3 µH is a good compromise for all HF bands. Sometimes the minimum inductance (coil + balun) is too large for some 10-30 m antenna systems. Subsequently, extending or shortening the feeding line was necessary to change the antenna system impedance to cope with the tuner, or build a separate ATU with fewer turns e.g. less than 3 µH on the toroid.

The transformer/balun is part of the tuning system. Most ferrite toroids are not suitable for high power or high RF current while the indicated Amidon iron powder core is.

The sequence of the windings. Left side: blue = input, black & red = capacitor. Right side: black & red = inductor.

The windings should provide high-voltage insulation by using Teflon covered wire. Eventually RG58 coax cable with the outer isolation and screen stripped off may be used. Do not twist the wires of the primary windings with the wires of the secondary windings. In the picture (see fig») the light blue with the blue wires.  

"PIG NOSE" BALUN

RZ3AE's S-Match ATU

RZ3AE has sent me an email with attachment: "Your idea for a balanced tuner allows for a very interesting design. A translated part of his Russian article, he wrote:" With the unconventional design and the use of special components a small symmetrical tuner may be created.

The balun in the original design was replaced with two stacked ferrite tubes of high permeability and the roller inductor by a T200-2 toroidal coil with 15 taps. Taps 1 through 3 one turn, 4 to 6 two turns, 7 and 8 three turns, and 9 to 15 four turns evenly distributed over the rest of the toroid.

The balun is suitable for an output less than or equal to 100 W and the wire is insulated with Teflon.

The various amateur bands may be adjusted to 600 Ohm with VSWR <1.1 and 1000 ohms with SWR to 1.3. For impedance less than 50 ohms, the antenna must be connected to the variable capacitor

DL7RU has personally experienced that it is not easy to wind Teflon covered wire on a toroid and keep them firmly in place. DL7RU: I have now found a method to do the job much easier. First of all, I start with a couple of tight pulled cable ties on each side of the toroid. Under these ties I put smaller and thinner ones. This fixes the beginning and end of the windings so that it is easier to settle the wires.

With some antenna systems a better efficiency can be obtained when the antenna is connected as shown by the dotted line. It is also possible to use two separate toroids and I have not noticed any difference between an ATU with two cores or with a single core.

IMBALANCE

If any unbalance occurs, try switching to the other connection or interchange the connections to the roller inductor.

My experience shows that some unbalance in an ATU occurs due to the construction and the location of the components. For example, a variable capacitor has frame and movable plates, which are not in any balance with respect to the chassis or system ground. The basic mechanical construction of roller coils is also not symmetrical.

Always begin with matching the system for SWR = 1, to obtain maximum balance. Unbalance is usually the result of inaccurate tuning. With the usual baluns it may look as if the feeding line is symmetrical, but the centre taps of the bifilar and trifilar wound toroids force the balance with respect to "ground" or chassis (fig.a).

Some Internet sites state that in comparison tests the S-Match failed regarding balance. The authors brought no evidence whether the balance of the "reference" ATU was tested properly….

You can conduct proper tests if your feeder line is in balance with real ground. Use the S-match or any other unbalanced tuner for the test. Connect one of the feeder lines to the ATU (fig. c), match for lowest SWR and measure the RF current. Repeat the test with the other end of the feeder line (fig. d). Check and mark whether both currents are equal or not. As a "ground" you can use a single wire in the backyard or a wire to your central heating system.

INDUCTORS WITH SWITCHED TAPS

The use of a roller inductor enables tuning only by turning knobs. Some hams don’t like roller coils, not realising that a switched inductance with many taps has a lot more self-capacitance and self-inductance. Most of the time you can’t achieve a proper adjustment because the needed point is somewhere in between the taps. Still, I provide in this present application a switched inductance of 25 µH, which will be adequate for most of the antenna systems. An extra variable capacitor of 470 pF from a vintage AM receiver can be added in series with the input to help reducing the VSWR.  

 REPEAT TESTING

The earlier made notes had been lost and it was decided to repeat the testing of the different designs to determine which one was the best solution. At the same time it was investigated to see if there was even a better solution than those known to-date. A temporary set-up, shown below, was used.

There was hardly any difference in the efficiency of any of the four designs. Afterwards the symmetry became the focus. The settings of the S-Match need to be adjusted for a SWR = 1 between the transmitter and the ATU in order to achieve the best possible balance. When this was done, design figure 2 showed to be the better one for use on the frequencies between 1.8 MHz and 10.150 MHz. Design figure 3 showed to have the best symmetry for the frequencies between 14 MHz and 30 MHz. No sound explanation has been discovered for this difference. Design fig.2 requires slightly more inductance than design fig.3 (which has the antenna parallel to the capacitor). Between 14 MHz and 30 MHz the higher inductance requirement of the roller inductor is advantageous. Please refer to the below mentioned comments of a German HAM on using the S-Match on the 10 meter band.

The capacitor can be a split stator with a grounded stator. If the parts are built in a metal or screened cabinet, they should be assembled on at least 5 cm distance of all metal parts to minimise the asymmetry of the used components.

All pictures are just illustrative examples of this S-Match design.

This simple ATU with variable capacitors (from a vintage AM receiver) and a T200-2 toroid is suitable for 400 Watts (fig»). The matching range can be increased with an extra-switched 100–220 pF/5 kV fixed capacitor and used when the antenna impedance is below 50 Ω. With my inverted dipole it was necessary to switch-in the fixed capacitor in parallel with the roller inductor only on the 80 m band.

CONVERSION

A T-match unit can easily be converted to an S-match by removing the second capacitor or bypassing the first capacitor. The free hole in the front panel of a converted commercial unit can be used for a switch for paralleling fixed capacitors to the tuning capacitor to provide additional capacitance.

PAØLL

As an encouragement for home brewing see in alphabetical and numerical order examples of S-Match tuners. Not everyone is just as skilful but it is most important that it works well.

Kees, PAØLL who is famous for his LL-tuners, sent me photographs of his "LL S-Match". There was much demand for a small portable antenna system including a small ATU. For this reason his intention was to build and sell a handy ATU inclusive the antenna in a carry box. The final product is built with components, which are made especially for the tuner. C = 11 - 470 pF (knob with 1 :_6 reduction), L = 34 µH. The ferrite toroid for the balun was specially made by Amidon for this ATU. I expect that all his handcrafts made ATU's ends as "collector items"!

PE1KQP's and PE1ADY's S-MATCH

4 × PE2B's S-MATCH

PAØMJM

The version of PAØMJM's S-Match. The graphics were made with a miniVNA (www.miniradiosolutions.com) program.

COMMENTARY

Since I published this design, may hams successfully copy this simple ATU.

Some of the selected emails are presented: 

DJ9XG:

Ich habe wieder einmal etwas von deiner ufb homepage nachgebaut, eine S-Match ATU. Ich habe hier 3 verschiedenen ATU's und diese auch alle 3 getestet.
1. Drake MN2700 mit externem 1:4 Balun
2. Parallelkreis Koppler ähnlich Johnson oder Annecke
3. S-Match nach PA0FRI

Als Antenne verwende ich hier eine 2 × 20 m Dipol symmetrisch mit 240 Ohm Schlauchkabel (Antennenkabel für UHF-Fernsehantennen aus den 70er Jahren) gespeist. Antenne hängt zwischen 15 m und 9 m über Grund.

Mit allen 3 Antennen erreiche ich von 160-10 m 1:1 SWR... die Ergebnisse vieler Tests sind folgende Reihenfolge:

1. S-Match auf allen Bändern Nummer 1
2. Parallkreis nach Annecke Nummer 2
3. Drake Tuner Nummer 3

Du siehst dein S-Match ist der beste ATU, hinzu kommt noch die komfortable Bedienung. Ich habe den hier mit einer riesigen kommerziellen Rollspule 14 µH und einem 20-200 pF Sendedrehko aufgebaut. Als Ringkern verwende ich im Moment noch 1 Amidon T200-2, werde den aber gegen eine 2 Ringkernlösung mit 2 x T200-2 ersetzen, da der eine Kern bei 1 kW Output doch etwas warm wird.

Nun zu meinen Beobachtungen mit der S-Match. Bei 160 m benötige ich nur ca 7 µH der Rollspule und ca 400 pF für den Drehkondensator (ich hatte eigentlich damit gerechnet dass die Rollspule zu wenig Induktivität auf 160 m hat). Bei den höheren Bändern wird das...