# Means for generating ultra-wideb and radio-frequency emissions with semiconductor field generators

### Abstract

#### For citation:

Lebedev E.F., Ostashev V.E., Ulyanov A.V. Means for generating ultra-wideb and radio-frequency emissions with semiconductor field generators. *Journal of «Almaz – Antey» Air and Space Defence Corporation*. 2018;(1):35-42.
https://doi.org/10.38013/2542-0542-2018-1-35-42

## Introduction

In recent times, the semiconductor technology for generating subnanosecond electrical pulses has been developing intensively. The peak power of small-size generators may reach hundreds of megawatts. At such power the continuous pulse repetition frequency amounts to approximately 10^{3} pulse/s.

Short electrical pulses with voltage change rate of (1…2)×10^{15}V/s are effective for excitation of ultra-wideband (UWB) emissions.

Subnanosecond electrical pulses generated by semiconductor devices can be stably synchronised to the accuracy of pulse random time jitter value, which makes it possible to build synchronous active antenna arrays (SAAA) with the use of those devices as modulators.

Radiators of such type can be considered as illumination sources in the tasks of radio-frequency location of various objects [1, 2], as a means of powerful influence in studying resistance to pulsed radiation of various electronic engineering products, and as a means of suppression and setback of facilities in electronic warfare tasks [3–5].

## Powerful semiconductor subnanosecond pulse generators

In Russia, successful developments of powerful and fast semiconductor switches have been underway in the Ioffe Physical Technical Institute of the Russian Academy of Sciences (St. Petersburg), Institute of Electrophysics of the Ural Division of the Russian Academy of Sciences (Ekaterinburg), National Research Tomsk Polytechnic University (Tomsk), Saint Petersburg Electrotechnical University “LETI” (St. Petersburg) [6–11].

A series of powerful sub- and nanosecond pulse voltage generators (PVG) has been developed in PK FID-Tekhnika, CJSC (St. Petersburg) [12, 13] and raised to the level of commercial product. In particular, the PVG product line covers the ranges of pulse repetition frequencies of 10^{3}…10^{6} pulse/s and amplitudes of 1…10^{0} kV. PVG pulse rise time reaches 50 ps, and voltage change rate on a matched electric load – (1…2)×10^{15} V/s.

A signature waveform of unipolar nanosecond pulses of semiconductor PVGs is given at the web-site of PK FID-Tekhnika, CJSC [12]. As an example, Fig. 1 features a photograph of a generator of the PVG-100 kV family (а) and the parameters of voltage pulse on its feeder (b).

**. PVG and its generated voltage pulse recorded in the measurement path with impedance of 50 Ω and transient response rise time of 70 ps**

Fig. 1

Fig. 1

The considered PVG (see Fig. 1) is fed with voltage of 300 V and started by an external pulse with amplitude of 15 V, rise time of 1 ns and duration of approx. 100 ns. The peak power of PVG pulses on the load of 50 Ω is equal to P_{ГИН} ≈ 200 MW, and the average power, at pulse repetition frequency of 10^{3} pulse/s, is approx. 50 W. The efficiency of primary energy conversion in PVG is in this case 25 %. Generator specific power as per average power at the output feeder is 50…70 kg/kW.

It is worth mentioning the positive experience in developing systems of automatic synchronisation of the pulses of such generators, implemented in active antenna arrays of radiators. For example, paper [14] describes an 8-element synchronous active antenna array (SAAA) with the amplitude of element excitation pulses equal to 40 kV.

## Emission of UWB electromagnetic pulses

The problems of generating and studying UWB emissions are dealt with in monographs [15–19], in which the results of some 20 years of research are summarised.

Let us estimate the amplitude of pulse E (V/m), radiated by a flat uniformly excited synchronous array aperture along its axis, at a distance R in the far radiation zone. Since E ~ R^{-1}, we shall estimate the maximum value of product ER , referred to as the radiator electrodynamic potential.

We shall use the following designations:

U_{A} – aperture excitation voltage;

I_{A} – current excited by this voltage;

c – free-space electromagnetic wave propagation velocity;

µ_{0} – dielectric constant;

Z_{0} – free-space characteristic resistance;

Z_{0} = μ_{0}c= 120π Ω;

S – aperture area.

Then, following [20], we put down:

Voltage U_{A} will be determined from the balance of excitation generator pulse energy U_{g}, feeder losses, and energy delivered to the radiating aperture:

Here, η – characteristic of feeder ohmic losses (if there are no losses, η = 1);

Z_{g} – antenna-feeder system (AFS) input impedance;

Z_{A} – AFS output impedance;

4ηΖ_{0} Z_{a} /(Z_{0} + Z_{a} )^{2 }– efficiency of energy transfer from PVG to aperture (feeder efficiency factor).

With array aperture being excited with a pulse having tilt of 10^{15} V/s, and with array aperture of 0.3×0.3 m (aperture wave impedance ≈180 Ω), radiator potential is ER ≈ 0,4 МV.

Let us associate potential ER with effective peak radiator power, i.e. equivalent peak power of isotropic radiation. The radiation energy-flux density at distance R from the aperture, along its axis, is n = E^{2} / Z_{0}(W/m^{2}). Then

where Ку(м) – AFS peak power gain factor;

Ку(м) = P_{эф} /Р_{ГИН} .

In this way, at ER ≈ 0,4 МV, the effective peak radiation power is P_{эф} ≈ 5 GW. With pulse ratio in a train being equal to (0.5…1.0)×10^{7}, the effective average power of this radiation lies within the range of 0.5…1.0 kW. The local parameter values in a set observation point are computed as per effective radiation parameters.

The AFS peak power gain factor

and energy gain factor – Ку(э)=Ку(м) х (τ_{изл} /τ_{ГИН}).

Here, Z_{g} = 50 Ом;

U_{g} = 100 кВ;

τ_{изл}, τ_{гин} – characteristic durations of radiation and excitation pulses determining their energy.

To check and update the radiation parameters, we shall use a computational model in which the aperture is represented by an aggregate of unidirectional radiating Huygens elements [20] excited from a point on the aperture axis, due to which the aperture is excited nonsynchronously and non-uniformly. The model was verified on examples of analytical solutions, it satisfies the reciprocity theorem and does not contradict the principle of energy conservation during its conversion. The model satisfactorily defines radiation of an experimental mockup having an aperture antenna based on the flaring TEM horn (aperture 0.3×0.3 m), excited by one of the generators developed by PK FID-Tekhnika, CJSC (Fig. 2).

It is obvious that at great angles of deflection from the axis of a real antenna, its radiation parameters are worse reproduced in a flat-aperture model.

## UWB radiator module

Based of the computational model, we shall estimate the limit parameters of SAAA equipped with excitation generator of PVG-100 kV type. An electric energy pulse with voltage amplitude of ≈100 kV and effective duration of ≈0.5 ns can be entered in AFS without resorting to special measures for ensuring electrical strength of the system (oil, pressurised gas, etc.). It simplifies the radiator design and its operation.

Given a specified SAAA aperture area, the radiator power will be increasing with the increase of synchronous power of antenna excitation, or, which is the same, the number of generators arranged within the limits of this aperture. Each generator must be loaded to its AFS, matched both to the generator and the radiation propagation space. Matching was implemented in a passive antenna array of four connected TEM horns in 2×2 format. Each horn has input impedance of 200 Ω and output impedance of 280…300 Ω. The parallel connection of horn inputs to the PVG feeder was made using 4 flexible two-wire waveguide lines with 200 Ω impedance.

The time of powerful voltage pulses appearance on the PVG output feeder has a random spread characteristic, or jitter. It is associated with time instability of the multi-stage sequential process of output subnanosecond pulse formation, with the total duration of this process equalling Т_{форм} ≈ 100 ns.

The pulse jitter of a single PVG, as a random process, is characterised by the time of rootmean-square deviation σ_{1} . During joint operation of N single-type generators, the jitter of pulses generated by them is reduced in accordance with the law of large numbers

Non-synchronism of the output pulses of generators is conditioned not only by the jitter, but also by the relatively slow process of the change of pulse-forming duration Т_{форм} in generators that are single-type but not identical. Moreover, if jitter, as a random process, cannot be excluded, systematic time error Т_{форм} can be minimised by a system of Т_{форм} average value automatic stabilisation in the course of periodic correction of each generator external start time.

In digital acquisition of PVG-100 kV pulses with repetition frequency of 10^{3} pulse/s for ≈10 s, σ_{1} ≈ 20 ps is recorded. Since all the pulses fall within time interval ±3σ_{1} ≈ 60 ps, then, with generator pulse rise time of ≈100 ps, jitter must be taken into account in estimation of SAAA parameters.

Let us consider an SAAA element (module) with antenna aperture of 0.3×0.3 m and three excitation PVGs arranged within the limits of this aperture (the PVG-100 kV dimensions allow this). Let us estimate radiation parameters of such module, taking into account non-synchronism and non-uniformity of antenna aperture excitation, transient behaviour of AFS element of the module, and generators’ pulse jitter (limiting probable spread to the interval of ±2σ). The parameter calculation result averaged for 200 radiation pulses is given in Fig. 3.

Module electrodynamic potential ER ≈ 0,52 МV (≈5.2 kV/m at a distance of 100 m from the module along its radiation axis). A corresponding value of the radiation pulse effective peak power is ≈9 GW, and effective pulse energy – 1.15 J. This energy is concentrated in a frequency range of 1.6…4 GHz with characteristic density ≈ 0.2 mJ/MHz. At pulse repetition frequency of 10^{3} pulse/s, effective average radiation power is ≈1.1 kW, and average power spectral density is 0.2 W/MHz. The AFS of the module is characterised by gain factor Ку(м)≈17.8 in peak power and Ку(э) ≈ 8 in energy.

Fig. 4 shows the computational data defining antenna radiation pattern (ARP) of the module.

**. Module ARP in energy density (1), peak power (2), frequency spectrum width (3), and mean value of frequency spectrum density (4), as well as radiation energy share within specified solid angle (5)**

Fig. 4

Fig. 4

The full divergence angle of module radiation with respect to the pulse energy density is ≈18°. The energy coefficient of module antenna directional radiation D_{Э} = 34. In accordance with the cited data, energy efficiency of an AFS with antenna aperture of 0.3×0.3 m, excited by a specified unipolar pulse, is approximately 24 % (Ку(э)/О_{Э}).

The ARP width with respect to peak radiation power (line 2) is somewhat smaller than its energy-density width (line 1). It is important to note that with an increase of the angle of deflection from the radiator axis, the radiation frequency spectrum width (line 3) is reduced as a result of radiation high-frequency boundary shifting towards low frequencies. The ARP width with respect to this parameter and the mean value of frequency spectrum density (line 4) is approximately twice as small as the ARP width in radiation pulse energy and power.

One of the specific features of directional antenna UWB radiation is that the radiation energy within the ARP main lobe limits is noticeably lower than for sinusoidal signal [21]. Energy efficiency of the main lobe of the considered module’s ARP is approximately 15 % (see line 5 in Fig. 4). However, it does not mean that UWB radiation energy flux on the whole cannot be compressed in physical space. It can be possible with an increase of antenna aperture area (Ку ~ S). In that case a relative portion of UWB radiation energy concentrated in the ARP main lobe is still relatively small.

## Synchronous active antenna array with UWB radiators

Let us estimate SAAA radiation parameters drawing on the example of an array with 6×6 elements. As an array element, we shall use a module with the aperture of 0.3×0.3 m described above.

Assuming that the array is synchronous with the accuracy to the jitter of each active element, we obtain the value of radiator electrodynamic potential at the level of 19 МV (≈2 kV/m at the distance of 10 km from the radiator). A corresponding value of the radiator effective peak power will be equal to P_{пик} ≈ 12 ТW.

The effective array radiation pulse energy is ε_{эф} ≈ 15 kJ under effective pulse duration of τ_{эф} = ε_{эф} / P_{пик} ≈ 130 ps. At repetition frequency of these pulses equal to 103 pulse/s, effective average radiation power will amount to 1.5 МW.

Distribution of pulse energy in the frequency region basically corresponds to its distribution in a single module. In that case the maximum value of radiation energy spectrum effective density will amount to approx. 0.4 J/МHz in the frequency interval of 1.5…3.5 GHz (see Fig. 3, b).

With array aperture of 1.8×1.8 m, the gain factor of its AFS in peak power is equal to Ку(м) ≈ 640, and in energy -Ку(э) ≈ 280.The ARP main lobe width is equal to ≈3°, and its energy efficiency amounts to approx. 6 % under a given (reduced) excitation pulse form. The latter can be explained by the fact that the ARP main lobe is formed by the HF portion of radiation pulse, and the share of that portion in the UWB pulse total energy is relatively small.

Antenna array process variables are determined mainly by specific parameters of the excitation generator. The weight of 108 generators is ≈380 kg. The mean-value specific effective radiation power will amount to approximately 0.25 kg/kW (380 kg/1.5 МW).

The synchronous peak power of antenna excitation, with account of generators’ jitter, is equal to approximately 18.4 GW, and average power at pulse repetition frequency of 10^{3} pulse/s – 5.4 kW. The average supply power of all excitation generators will be equal to 20…25 kW.

The generators’ duty is as follows: units of minutes without cooling (with heat absorption by the structure) or long-term, with powerful switching elements cooled in the generator circuit.

## Conclusion

The development of technology for generating powerful UWB emissions is conditioned by the practical interest towards the use of such signals. Lying in the core of this technology is fast progress in creating powerful semiconductor devices generating pulses with subnanosecond rise time.

The semiconductor technology products are appealing due to their compact size, reliability, possibility to generate powerful pulses with high repetition frequency, and possibility to ensure stable synchronisation of subnanosecond pulses within the limits of a random time jitter. The limiting value of the electrodynamic potential of an SAAA built on the base of the considered technology lies within the range of 5…6 МV per 1 m^{2 }of its aperture. A corresponding value of the effective peak radiation power is about 1 TW. The SAAA radiation power is proportional to the square of the number of its elements.

Using the considered technology, it can be possible to build synchronised radiators with antenna excitation peak power of tens of gigawatts, effective peak radiation power of tens of terawatts under effective average radiation power of the megawatt level and the power consumption of modulators of tens of kilowatts. The characteristic frequency spectrum width of such emissions within the ARP main lobe limits is 3…5 GHz.

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### About the Authors

**E. F. Lebedev**Russian Federation

**V. E. Ostashev**Russian Federation

**A. V. Ulyanov**Russian Federation

### Review

#### For citation:

Lebedev E.F., Ostashev V.E., Ulyanov A.V. Means for generating ultra-wideb and radio-frequency emissions with semiconductor field generators. *Journal of «Almaz – Antey» Air and Space Defence Corporation*. 2018;(1):35-42.
https://doi.org/10.38013/2542-0542-2018-1-35-42