- functions of the ATU

For correct operation, antennas at HF and below are often dependent on antenna tuning units or ATUs or other matching devices such as transformers and baluns. ATUs make it possible to tune over a wide frequency band and provide two necessary functions. Firstly, because a range of frequencies is normally required and in each case the antenna needs to look like a different length electrically, the antenna must be tuned or made resonant at the frequency required. Secondly, the feed line impedance needs to be matched to that of the transmitter, usually 50 ohms...

While calculating the input impedance of a transmission line being determined by frequency, characteristic impedance, physical length, velocity factor, matched line loss and the impedance of the load (antenna), essentially what we actually have is a load impedance of resistance and reactance that is transformed from one value at the input and another at the load. How much it is transformed relates to the electrical length of the line, its characteristic impedance and the inherent losses in the line.

If the impedance at the input of the transmission line connected to the transmitter differs appreciably from the load resistance into which the transmitter output circuit is designed to operate, an impedance matching network must be inserted between the line input terminals and the transmitter. Otherwise mismatch will occur.

The function of the antenna tuner is to transform the impedance at the input end of the transmission line to the 50 ohms needed to keep the transmitter loaded properly. It does not alter the SWR on the transmission line going to the antenna. It only ensures that the transmitter sees the 50 ohm load for which it was designed.

The input impedance of a transmission line is mainly resistive at all line lengths when the SWR is low, but can have a relatively high reactive component when it is high. This can be shown by a series of resistances and reactances (forms of resistance to the flow of current), usually denoted as the load ±jX, the j indicating that the two values cannot be added directly together and X representing the value: “+ jX” denotes inductive reactance, “- jX” denotes capacitive reactance. resistance and reactance combine to form impedance.

Resistance is less than the Characteristic Impedance

The current and voltage are exactly in phase at points that are multiples of 90° (1/4 wavelength) from the load, showing pure resistance. In the case of a transmission line coupled to an antenna designed to work at a single spot frequency, the resonant frequency, the load can be fairly close to being a pure resistance. In the drawing above E = Voltage and I = Current.

However, with an antenna that operates across a range of frequencies, this will only be true at the resonant frequency. Elsewhere the current is either ahead or behind the voltage. This means that at all other frequencies, there will be a certain amount of reactance as well as resistance , which will result in a higher SWR. Where the current is behind the voltage, the input impedance of the line has inductive reactance (+jX), where it is ahead, it has capacitive reactance(-jX).

Where the load resistance is LESS than the characteristic impedance, the reactance is inductive in every uneven 90° of the line from the load to the power source and capacitive in every even 90° section towards the power source. (E = Voltage I = Current)

Where the load resistance is HIGHER, the reverse occurs.

In the example below you can see where the Moonraker 8 metre 80 Series antenna is self resonant in the different modes and closest to 50 ohms.

At 90 degrees from the antenna load the antenna will be in quarter wave mode, at 180 degrees half wave mode and at 270 degrees from the load three quarter wave mode. In the case of the 80 Series antenna is in quarter wave mode 8-10 MHz, half wave mode 16-20 MHz, and three quarter wave mode 26-28 MHz.

In matching the transmission line/antenna to the transmitter the ATU is required to provide the best match for each individual frequency. In the case of a vertical whip or a wire antenna which is to be operated over the HF frequency range from 2 to 30 MHz, (or below) the required frequency lengths will vary widely, while the antenna length remains constant.

Using the formula below it is possible to determine on what frequency an antenna of a given length will be operating in different modes.

As in the example below a 5.5m antenna in 5/8 wave mode will be naturally resonant at 32.4 MHz, whereas a 10m antenna in the same mode will be resonant at 17.8 MHz.

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Reversing the formula to

will give the antenna length required in a given mode at a given frequency.

Similarly,

gives the mode factor.

An ATU calculates the closest, most efficient mode factor and makes the antenna look the required length electrically. Sometimes it will load it down to ¼ wave, sometimes up to 5/8 wave. If the antenna is too short, inductance is added; if it is too long, series capacitance is added to arrive at the correct electrical length.

In optimum matching circuit design, the reactance to resistance ratio in the input circuit, that is, the Q of the impedance at line input, is low. A system with a high Q resonates with a greater amplitude (at the resonant frequency) than one with a low Q factor, and its response falls off more rapidly as the frequency moves away from resonance. However, a radio receiver with a high Q would be more difficult to tune with the necessary precision, but would do a better job of filtering out signals from other stations that lay nearby on the spectrum.

Efficient tuning is especially important on MF frequencies and below where antennas are normally physically short for frequency in order to avoid losses, frequency drift and interference from adverse weather conditions. At these frequencies antennas are often operating at very low efficiencies. The very short 10 metre Moonraker 100MF and ATU, despite being designed for minimal losses, only has around 1.5% efficiency at 300kHz with a system bandwidth of 1.125 kHz, assuming an earth loss resistance of 1 ohm (good ground) and an ATU working Q of 300. .This rises to approximately 4.9% at 500 kHz and 25% at 1.6 MHz.

The impedance of a thick antenna changes slowly over a comparatively wide band of frequencies, the Q is low and the reactance is small, changing rather slowly as the frequency is varied on either side of resonance. In the case of a thin antenna such as thin wire conductors in a fibreglass whip, however, the reverse occurs, resulting in a faster change in impedance.

In L Networks the series connected capacitor (C1) and inductor (L1) count as one. In this type of network the load resistance (resistive part of a series impedance) must be below 50 ohms, preferably below 30 ohms. While L Networks are often used for single band operation, multi band types with switched or variable coil taps do exist. In this type the ratio of the series reactance to the series resistance is defined as the network.

Where a wide range of frequencies are required, as in HF and below, Pi Networks offer more flexibility, especially if thin wire antennas are used as these do not respond well to L Networks. Pi Networks can respond better to the rapidly changing impedances and can match low to high or high to low impedances, such as 50 ohms to several thousand ohms or to 1 ohm.

With Pi and L Networks, large capacitances are often required at lower frequencies to match to 50 ohms. In addition, the range of capacitances is often wide, especially with broadband single wires, as the output impedance varies radically with frequency.

High pass T networks are able to match a wide range of load impedances, but can be very lossy in comparison with Pi and L Networks. This is especially so on low frequencies with low load resistance. Large losses can result where maximum capacitance of the output capacitor is low. Low pass T Networks, however, can have very high Qs with large losses. (to be continued)

Installation wise, the required antenna feed at HF is high voltage silicone cable. The actual antenna feed point is at the insulator on the ATU case. For this reason ATUs should be located as close to the antenna feed point as possible to keep losses to a minimum. Extended antenna feed lines form part of the antenna and will to radiate RF. This reduces the strength of the radiated signal, and alters the intended radiation characteristics. Therefore, it is generally unwise to exceed recommended lead lengths.

Frequently manufacturers of ATUs will specify a minimum antenna length of 2.4m (8ft) for operation on 3-30 MHz and 7m (23ft) for operation on 1.8-30 MHz. Having a short antenna, therefore, will make it difficult to tune on the lower frequencies. This is more important in fibreglass whip antennas which only have thin wire radiators, compared with metal rods which have large radiating surfaces. Lack of antenna length can be compensated for by increasing the antenna feed line slightly, although this may affect antenna radiation characteristics.

Best performance is achieved when an antenna is naturally resonant at the frequency you are using. An ATU requires power to tune and the greater the amount of adjustment it has to make, the more power is used up. An antenna naturally resonant at around 20 MHz has to be adjusted electrically from about 3.65m (12ft) to 26m (118ft) to tuned to 2 MHz. This results in power lost to radiation of the signal. You can use a tuned antenna but you will still need an ATU for impedance matching or there will be considerable losses on the feed line due to standing waves.

In the case of antennas that are physically short for the frequencies to be used, it is possible to minimise losses in the ATU by incorporating centre loading coils and capacity top hats to make antennas look longer electrically than they actually are, thus maximising the power available for radiation. When an antenna is to be used over a range of frequencies, choosing the highest of regularly used frequencies for loading can make a significant difference to performance on the lower frequencies. This, of course, works in the receive direction as well.

Earths must be sufficiently large and connections suitably bonded or problems will arise. When an earth system is too small, the earth mat may radiate instead of the antenna and you may get electric shocks from touching metal objects connected to the ground. Where the antenna and the earth system are made of different metals, you should be aware that they have different resistances. Aluminium, for example, has a significantly higher internal resistance than copper, so aluminium earth systems used with copper wire antennas should be larger than copper types. At frequencies where the earth system is small enough to be near resonance, it can be difficult for the ATU to decide whether the earth or the antenna should radiate, creating a loop. If you are connecting to an existing ground source, it is important to make sure that it is suitable and does not have poor connections.

Lastly, ATUs can provide a certain amount of filtering to eliminate interference on frequencies higher and lower than the one you are using due to their inherent selectivity, and this can be very useful. So, if you are faced with interference within the band, you should consider using an ATU and whip rather than a broadband antenna.