Universal Matching Device - An ATU Plus Artificial Ground
Is your antenna unmatched? If so, you most likely need an Antenna Tuning Unit (ATU)! Does your transmitter’s case “sting”? Then, you need in an artificial ground! But, really, the two functions may be provided in only one box. Below we will examine a universal matching device with such capabilities.
T-Network: The Classic ATU Schematic
The classic ATU schematic is shown in Figure 1. The basis of the schematic is a T-network. Many ATUs use a T-network. Despite its simplicity, this scheme allows one to match a transmitter final stage with an impedance between 50 and 75-Ohms with a load having purely resistive values from 15 to 1000 Ohms.
Figure 1: A Classic ATU Schematic

It is necessary to explain what a purely resistive load is for a transmitter and explain some foundations for antenna matching. Then, we shall again turn to the T-network schematic.
Antenna Matching Foundations
Certainly, any transmitter intended for radio communication does not work into a purely resistive load. The transmitter works on an antenna, which is usually connected to it through a feedline.
The most commonly used feedlines are a coaxial cable with a characteristic impedance of 50 or 75 Ohms, or two-wire ladder line using one of the standard characteristic impedances in the 300- to 600-Ohm range. The antenna that we connect to the feedline has an input impedance. Ideally, we try to set the antenna input impedance close to the characteristic impedance of the feedline (or we choose a feedline with an impedance close to that of the antenna). Modern transmitters have an output impedance close to 50 Ohms and they require the use of a coaxial cable with characteristic impedance of 50 Ohms. Two-wire ladder line comes with a characteristic impedance of 300 or 450 Ohms. If the impedance at the transmitter end of the line is purely resistive, then to match with a transmitter final stage we must use an RF transformer with a ratio 1:6 or 1:9 accordingly.
Because there will be some disparity between the antenna input impedance and the characteristic impedance of the feedline, there will be a standing waves on the feedline. The standing wave ratio will be higher as a result of the greater the difference between an antenna input impedance and the characteristic impedance of a feedline.
Usually RF transformers are designed for matching only purely resistive loads. For the transmitter, the immediate load is the impedance at the terminal ends of the transmission line. If there are standing waves on the line, then such a transformer may only work well when the load it sees is purely resistive and that only occurs at specific places along the line. If the antenna also shows some reactance, then the standing waves will be displaced along the transmission line. Predicting just where the load will be resistive is–for most amateur operations–somewhat arduous, especially for antennas used on several bands.
An output stage of a transmitter is designed for activity for a load of definite resistance. The hook up to a transmitter of a load of some other resistance may decrease the efficiency of the transmitter final stage.
To eliminate these troubles, we normally insert an antenna tuning unit (ATU) between a transmitter final stage and antenna system port (that is, the feedline connection to the antenna) (Figure 2). The ATU matches transmitter final stage to the impedance of the feedline with the antenna connected to the other end of the line. The ATU does not eliminate SWR in a coaxial cable or other feedline. It only ensures the safe working of final stage of the transmitter. The ATU ensures a 1:1 SWR between a transmitter and the ATU.
Figure 2: Antenna system matching

Many radio amateurs often adjust the ATU by reading the transmitter’s SWR meter to obtain a minimum SWR. But, this minimum SWR between transmitter and ATU does not mean that the power necessarily all goes into the antenna! There are misleading adjustments in which the SWR between a transmitter and an ATU is 1:1, but little of the RF power goes into the antenna.
Therefore always it is necessary to adjust the system—a transmitter, an ATU and an antenna—on two parameters.
- The SWR between a transmitter and an ATU should be as low as possible. This condition implies the transfer of maximum RF energy from the transmitter in to ATU.
- A current to the antenna should be as high as possible. This condition means a maximum transfer of RF energy from the ATU into the antenna feedline and then to the antenna.
Do an experiment with your own ATU, and you will be convinced that is quite possible to tune an ATU so that the SWR will be 1:1 but the current in your antenna will be practically zero! One must pay careful attention that the ATU’s RF ammeter is inserted only into the antenna wire! Figure 3 shows the correct and incorrect insertion of the RF ammeter.
Figure 3 Correct and incorrect position of RF ammeter


What load values should an ATU see at its feedline terminals for proper operation? For most antennas designed initially for use with coaxial cable, the SWR seldom exceeds 3:1 once we have correctly built or assembled the antenna. For 50-Ohm coaxial cable, if the impedance is purely resistive, the impedance at the feedline terminals may range from 16.6 up to 150 Ohms (Figure 4).
Therefore, ATU should ensure the matching of loads having this range of antenna feedline impedances. The ATU exhibited in Figure 1, perfectly well-manages such loads!
Often, we hook up an ATU to a load having a much higher impedance than 150 Ohms. It can be random length a wire (Figure 5a), or an antenna hooked up through a two-wire line (Figure 5b). The impedance of the load may be extremely high, close to 1000 Ohms.


The ATU, exhibited in Figure 1, will manage to match such high load, if its input impedance is purely resistive, i.e., if the reactance is close to zero. In the event that there is a considerable reactive component in the impedance of the load at the ATU feedline terminals, the ATU’s overall performance may fall, even if one obtains a 1:1 SWR on the transmitter side of the ATU. Some loads just will not match because the reactance at the ATU terminals is too high. In other cases, we may obtain false adjustments that give a perfect match but do not send current to the antenna. (It circulates on the network components instead.)
T- network: Practical construction
For any real antenna and transmitter final stage, the parameters for the parts of this ATU can be calculated precisely using specific equations. (The equations are in reference [1]). In real life, almost nobody calculates the values for capacitors and coil used in real ATUs. Variable capacitors and variable coils are normally used in such ATUs. Antenna matching with the transmitter final stage will be reached by the practical selection of capacitors and coil values. Even with only a little experience, it is possible to match quickly any antenna with the usual transmitter final stage. But remember about false tuning; use an RF ammeter in antenna circuit!
Variable coil L1, having an inductance from 0.5 to 50 uH and variable capacitors C1 and C2, having a capacitance of 10 to 250 pF, allow the ATU to work in the frequency range from 3.5 to 30 MHz. To expand the ATU lower edge of operation frequency down to 1.8 MHz, C1 and C2 capacitors should have maximum capacitance of 500 pF.
The ATU has some disadvantages. We need two high-quality air variable capacitors, along with a coil with a slide control for the inductance. These are very costly components. Moreover, the frames of C1 and C2 must be isolated from the ATU case, complicating ATU construction.
Transforming L to C and Vice Versa
When we analyze the ATU equations that describe the ATU schematic in Figure 1, we discover that it is quite possible to change the capacitor to a coil and the coil to a capacitor. We will get an ATU schematic having only one grounded capacitor and two variable coils, as it is shown in Figure 6. Such an ATU schematic has advantages over the ATU shown in Figure 1. Since the capacitor is grounded, the ATU design is very simple. Variable coils can be isolated from ATU case very easy.
Figure 6: Transformation ATU

But is such ATU in use? Advanced Electronic Application (USA) produces such an ATU known as the AT-300. For further simplicity, the ATU uses switched coils instead of variable coils. Using an 18-position switching coil is almost equal to having a fine variable coil. The schematic for the AT-300 matching unit is shown in Figure 7. The ATU works well in the 1.9-30 MHz range.
Figure 7: Schematic for matching unit for AT- 300

The Need for an Artificial Ground
Any amateur knows that matching the transmitter final stage with the antenna feeder is only one part of the problem for having a well-matched antenna system. In some instances of improper matching, the transmitter case will start to produce an unwanted “sting”. It may be on one or more of the amateur ranges. Often it happens on the upper amateur HF ranges from 20 to 10 meters, especially when using random antennas lengths. There are many different ways to eliminate “sting.” (See for example, reference [2].) One of the ways is to use an artificial ground device.
The schematic diagram of an artificial ground is shown in Figure 8 used as follows:
- Connect the artificial ground to the transmitter case.
- Connect a piece of copper wire (the greater length the better) to the artificial ground.
- Tune C1 and L1 for a maximum RF current in the counterpoise. Meter M1 monitors the RF current.
Once reaching the correct adjustment of the artificial ground, we are likely able to eliminate the “sting” from the case.
Figure 8: “Artificial ground”

A Universal Matching Device: ATU plus artificial ground
It is desirable for operation to have both an ATU and an artificial ground. From Figures 1 and 6, 7, 8, it is clear that an ATU has parts identical to an artificial ground and the artificial ground parts are identical to those in an ATU. So, by proper switching, an ATU can serve as artificial ground and vice versa. The schematic for such a universal device is shown in Figure 9.
Figure 9: ATU – Artificial Ground

How it works:
Let’s first examine the unit as only an ATU. For this function, we connect the transmitter output to the J1 jack and the antenna to the J2 jack. Switch S3 is set in position “2”. Now the unit is a classic ATU (see Figure 1). If Switch S3 is set at position “1” (see Figure 10A) or “3” (see Figure 10B), it is a classic matching L-circuit. Next, let’s examine how it works as an artificial ground. The body of the “hot” transmitter is connected to the J2 jack and a counterpoise or ground system is connected to the case of this device. Switch S3 is set in position 2 or 3, depending on operating range. So, now it is an artificial ground.
Figure 10: Classic matching L- circuit

The full schematic diagram for this universal matching device is shown in Figure 11. In the schematic shown in Figure 11, the mode switch (S1) is added. Compare this schematic to the schematic shown in Figure 9. With the help of the mode switch for the Universal ATU, we can select the type of operation that we need. In position 1, we select the bypass mode: the Universal ATU is not used together with a transmitter. In position 2, we chose the ATU mode: the Universal ATU is used together with a transmitter. Position 3 is the Load mode, when a dummy load is connected to transmitter final stage. The load mode is necessary for transmitter tuning. The dummy load is connected to the J2 jack. It is provided to use the load for other purposes. For instance, it is possible to connect the load to J3 jack in position 2. It needed to control tuning of the ATU. Also, it is possible to meter RF voltage on the load and to find the power level.
Figure 11: Universal matching device


Antenna (for the ATU mode) or counterpoise (for the artificial ground mode) adjustment is accomplished with the help of an RF ammeter.
The RF ammeter contains transformer T1, diode D1, current limiting resistor R2 and meter M1.
The current transformer is made on a ceramic ring with OD of 22 mm. I used a ceramic ring from an old tube socket. It is also possible to use any plastic ring with an OD of 15 to 25 mm.
The thickness of the ring can be within the limits from 5 to 15 mm. The first winding of the transformer contained 2 turns and the second has 20 turns. The first winding uses the wire going from L2 coil to antenna jack J3.
The second winding uses wire with a diameter of 0.3-mm #29 AWG. The current transformer is shown in Figure 12.
A dummy load made from ten 500 Ohms/2W resistors is connected in a bridge. Such a load safely works at 20W transmitter power levels for a considerable time. For short periods (several minutes), the load can work at 80W. LI and L2 coils were spooled on a form with an OD of 55-mm. Each coil contains 30 turns of 1.5 mm diameter copper wire and the length of winding is 40 mm. Taps are made in this way: the first 3 taps through 1.5 turns, then 3 taps through 2 turns, all others through 4 turns. The coils must be placed as far from each other as possible to prevent mutual capacitive or inductive coupling. C2 capacitor is an air variable capacitor from old tube receiver. A view of the Universal ATU design is shown in Figure 13 (13a + 13b).
Figure 13: The Universal ATU design


When the Universal ATU works in the ATU mode, the antenna is connected to jack J3 and the transmitter to jack J1. The electrical ground is connected to screw J5 (see Figure 14). The selected position of S4 switch (1, 2 or 3) sets the ATU tuning. When the Universal ATU works in the artificial ground mode, the transmitter’s case is connected to central core J3. The electrical ground is connected to screw J5; the counterpoise is connected to screw J4 (see Figure 15). The antenna is connected to the transmitter antenna socket. The selected position of S4 switch (2 or 3) sets the artificial ground tuning.
Figure 14 Connection of the universal ATU to a transceiver in “ATU” mode

Figure 15 Connection of the universal ATU to a transceiver in “artificial ground” mode

Note: When Universal ATU works as artificial ground, do not connect the transmitter and artificial ground case in any way other than as shown in Figure 15.
For both cases, when Universal ATU works as an ATU or as an artificial ground, the best matching will be found at the maximum RF current to meter M1.
References
- Rothammels ANTENNENBUCH, edited by Alois Krishke. 11 edition, Franckh – Kosmos, Verlags – GmbH@Co., Stuttgart, 1995.
- Grigorov, I.: “Solving RF Feedback – Part-1” ( www.antennex.com , Archive V), “Solving RF Feedback – Part-2” ( www.antennex.com , Archive V).
Originally posted on the AntennaX Online Magazine by . Igor Grigorov, RK3ZK
Last Updated : 4th January 2025