Library, S.W. Eldg

FEB 2 8 1966

^ecknical <v2ete 92o. 331

A 100 KW 2-25 Mc/s DISTRIBUTED AMPLIFIER,

DESIGNED FOR USE WITH 10 KW IONOSPHERIC

SOUNDERS

W. B. HARDING, M. W. WOODWARD, AND J. C. CARROLL

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS

technical ^lote 33/

ISSUED February 15, 1966

A 100 KW 2-25 Mc/s DISTRIBUTED AMPLIFIER,

DESIGNED FOR USE WITH 10 KW IONOSPHERIC SOUNDERS

W. B. Harding, M. W. Woodward, and J. C. Carroll

Institute for Telecommunication Sciences and Aeronomy*

Environmental Science Services Administration

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

"'Formerly the Central Radio Propagation Laboratory of the National Bureau of Standards;

CRPL was transferred to ESSA in October 1965, but will temporarily use various NBS

publication series pending inauguration of their ESSA counterparts.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington D.C. 20402

Price: 20*

Contents

Page

Abstract iii

1. Introduction 1

2. Design and Construction 1

2. 1 Theoretical Design Considerations 1

2. 2 Some Construction Details 5

3. Tests and Results 6

4. Conclusions V

5. References 8

in
A 100 kW 2-2 5 Mc/s Distributed Amplifier,

Designed for use with 10 kW Ionospheric Sounders

W. B. Harding, M. W. Woodward, and J. C. Carroll

Abstract

The principles of distributed amplification have been known

for many years and have been applied to fast rise time pulse and

oscilloscope amplifiers for small-signal applications [Percival,

1935-37; Pettit and McWhorter, 196l], This paper describes the

design, construction, and testing of an r-f distributed amplifier

intended for ionospheric sounding and backs catter applications. The

amplifier has a power gain of 10 dB and a rated output of 100 kW

peak pulse power, over a range of 2 to 25 Mc/s. A feature of this

amplifier is the elimination of input and output transformers.

Key Words: Amplifier, balanced, bandwidth, ionospheric, power

gain, sounder, sweep frequency.

IV
1. Introduction

Sweep frequency sounding techniques for ionospheric investigations yield much valuable information. However, for some applications, presently available ionospheric sounding devices are power

limited. To increase the power output of a sweep frequency transmitter, a wide -band amplifier is required. Since a distributed

amplifier is a reasonably simple device, it was decided to design one

for use at fairly high r-f power levels.

2. Design and Construction

2. 1 Theoretical Design Considerations

Distributed amplifier design in the past has been toward unbalanced configurations and in the case of small signal devices has

resulted in non-standard unbalanced input and output impedances.

High powered designs have appeared using a broadband transformer

to convert the output impedance to 50 ohms unbalanced. The use of

broadband transformers for high peak pulse powers leads to a high

probability of a voltage breakdown within the transformer. Designing

a distributed amplifier to match a standard open wire line reduces the

probability of destructive voltage breakdowns. This is one of the

unique features of this particular amplifier.

For balanced input and output configuration, two side-by-side

amplifiers were designed and constructed. Figure 1 is a system block

diagram. Figure 2 is the distributed amplifier schematic diagram.

Figure 3 is the schematic diagram of the control circuitry.

Very briefly described, a distributed amplifier [Percival,

1935-37; Pettit and McWhorter, 1961 ] consists of a series of tubes

with the grids connected at intervals along a delay line. The plates of
these tubes are connected at corresponding time intervals along another

delay line. A signal entering the input travels down the grid line,

exciting successive grids, and is finally absorbed by the grid line

terminating resistor. Similarly, the signal in the plate line travels

down the line, being augmented at each tube until it reaches the output.

Backward component voltages are absorbed by the terminating resistor

at the reverse end of the plate line. The power dissipated in the line

terminating resistors results in a lower efficiency, but this is a price

which must be paid for the wide bandwidth obtained. Thus amplification

is achieved over a wide frequency range without tuning.

The distributed amplifier under discussion was designed to be

driven by the C-3 ionospheric sounder. The C-3 equipment has a nominal peak pulse power capability of 1 kW and a sweep frequency

range of 1 to 2 5 Mc/s. Consideration was given to the desire to extend

the frequency range of the present equipment to 60 Mc/s, and a cut-off

frequency of 70 Mc/s was inserted into the design requirements for the

subject amplifier. Seventy Mc/s was used in order to maintain as much

output as possible thru 60 Mc/s; however, lack of a suitable driving

power source has prevented testing above 25 Mc/s.

In conventional broadband amplifiers, if the tubes are paralleled

to obtain the necessary output power, the tube capacitances add, thus

lowering the input impedance excessively. On the other hand, the distributed amplifier provides a method of effectively paralleling tubes so

that the transconductances add while the input and output capacitances

remain separate and are used as parts of the delay line constants.

Therefore, the input and output capacitances of the tubes selected are

the first design consideration.

The nominal output impedance of the C-3 ionospheric sounder

is 600 ohms, balanced, but tests were conducted which indicated that
the C-3 could be operated into a balanced load of 3 00 ohms. A design

value of 300 ohms input impedance permitted use of readily available

tubes. The output impedance selected was 600 ohms, balanced.

The tube most nearly approaching the requirements for this

project was the 4-1000A. The input capacitance of this tube varies

from 23. 8 to 32. 4 pf. Tubes are selected to have as nearly as is

possible the same input capacitance. This value limits the input

impedance to 150 ohms (for one side) or lower for a cut-off frequency

of 70 Mc/s. The output capacitance of the 4-1000A will vary from

6.8 to 9. 4 pf. This value is small enough to allow for trimming of

the plate line, if necessary, with small values of capacitance to

equalize the delay in the input and output lines.

The following design values, assuming an unbalanced half of

the circuit, are fixed:

Z. (input impedance) = 150 ohms

in

Z (output impedance) = 300 ohms

out

f (cut-off frequency) = 7X10 c/s.

The design values to be calculated are as follows:

C = grid line capacitance per section

g

C = plate line capacitance per section

P

L = grid line inductance per section

g

L. = plate line inductance per section.

P

The design equations were obtained from references on smallsignal distributed amplifiers [Ginzton, Hewlett, Jasberg, and Noe,

1948; Rudenberg and Kennedy, 1949; Horton, Jasberg, and Noe, 1950;

Cormack, 1952; Stockman, 1956],

The values of C and C were determined by the following

g P

equation:
1 c =

TTf Z

c

The values of L and L were determined by the following

g P

equation:

L = Z

2 C.

The reverse termination for the grid and plate lines consists of

non-inductive resistors equal to the line impedance.

The plate and grid lines of each unbalanced half of the amplifier

are essentially low pass filter sections of the constant K type. The

delay in the plate and grid lines must be precisely the same in order

for the signal in the plate line to be reinforced at the plates of

successive tubes. When this condition exists, it results in gain from

each tube and a relative phase shift thru each tube which will vary from

zero at zero frequency to 180° at the cut-off frequency. Low frequency

response of the amplifier is limited by the necessity for d-c voltage

blocking. The phase relationship at the output terminals of the

amplifier is dependent upon the phase relationship at the input

terminals. In considering the signal at the input terminals, it must

be realized that an amplifier of this type will amplify all of the signals

present within its bandwidth. The amplifier is designed to operate as

a linear device and is operated in class AB to minimize harmonic

generation.

The filament of the 4-1000A is of the thoriated tungsten type.

With thoriated tungsten filaments operating at the rated voltage in

these tubes, the available emission is approximately 80 milliamperes

per watt of filament power. By raising the filament voltage 10%, this

figure can be approximately doubled. This is recommended by the
tube manufacturer when using these tubes in pulse service. Tests were

made with normal and normal plus 10% filament voltages. With normal

voltage, the amplifier was capable of a power output of 60 kW. Byincreasing the filament voltages 10%, 100 kW was easily obtained. The

filaments of the subject distributed amplifier are operated at 10% above

nominal voltage.

2. 2 Some Construction Details

The distributed amplifier is constructed with the grid and plate

lines mounted vertically in a cabinet. The tubes, two to a chassis,

are placed between the input and output lines and at the proper intervals

along the lines. The cathode of each tube is metered from the front

panel with an adjustable range meter to compensate for different duty

factors. The blowers, one for each tube, are mounted on the front

panels at the level of each chassis. Filament transformers for each

tube are mounted on each chassis and controlled by means of a variable transformer in the bottom of the cabinet. One transformer

is continuously metered as a means of adjusting the filament voltage.

Each filament transformer secondary can be measured at the terminal

strips on the side of each chassis. All d-c power supplies are external,

and all voltages are adjustable. Voltages and currents are metered on

the power supply panels. Bias and air interlocks are built into the

amplifier cabinet. All voltages are present on the tube elements at all

times. Figures 4, 5a and 5b show views of the complete distributed

amplifier and power supplies.

The amplifier will operate with a plate potential of 1 5 kV. However, for dependable long term operation, this should be reduced to

12 kV, as flash-over will occur at times between the plate caps and

the next chassis when operating with the higher voltage.
The plexiglass forms on which the plate inductors are wound

are 1 inch in diameter and extend a minimum of 4 inches beyond the

end of each coil. Initially these sections were fastened to a plexiglass

support with 1/4 inch brass screws. Large corona discharge appeared

at these screws when operation was attempted, and they were replaced

with nylon screws. No metal should be used in the plate line supports,

even though they are floating with reference to ground.

The present packaging, while nearly ideal mechanically and

electrically, presents some shielding problems. The sides of each

compartment must be removable in order to replace tubes. This

requires a rigid frame with r-f gaskets and some type of speed

fastener. It is recommended that, in future amplifiers, consideration

be given to the mounting of all eight tubes on one large chassis in a

horizontal plane. The power supplies could be mounted beneath the

chassis, resulting in a self-contained unit, and any tube could be

replaced from one opening.

3. Tests and Results

Although the amplifier was designed for operation to 60 Mc/s,

it has been tested thus far only to 25 Mc/s because of lack of adequate

drive for the higher range.

The distributed amplifier has been tested and is presently being

used within the limits of the C-3 driver. No attempt has been made to

select tubes or to trim the plate lines. There has been no indication of

instability with AB operation. Figure 6 is the response with reference

to 100 kW over the tested frequency range. The measurements were

made with open wire line into a load of 600 ohms, constructed of eight

100 watt, 300 ohm, non-inductive resistors connected in series-parallel.

Figure 7 shows the measured phase shift of the distributed amplifier.
These measurements were made at low power by comparing the signal

at the input terminals with the signal at the output terminals with an

oscilloscope.

Figures 8a and 8b show sweep frequency backscatter records

obtained using (a) C-3 only and (b) C-3 driving the distributed amplifier.

These records were made on February 25, 1963, using a rotatable log

periodic antenna directed toward 150° E of N. The pulse repetition

rate is 12. 5 pps and the pulse length is 250 [is. Each record is a complete sweep from 7 Mc/s to 25 Mc/s. The vertical markers indicate

frequency and are spaced at 1 Mc/s. The horizontal markers indicate

range and are spaced at 500 km.

Improvement in the ionograms as a result of adding the distributed amplifier is evidenced by the filling-in of the F layer signature,

and the appearance of E layer returns not seen in the ionogram using

the C-3 alone.

4. Conclusions

The results of this project confirm that distributed amplification

is useful for sweep frequency applications. It has also demonstrated

the possibility of designing high powered distributed amplifiers without

the use of critical input and output transformers. There are presently

available tubes with which it seems practical to attain an additional

10 dB of power gain, although the costs would have to be balanced

against the value of the possible increase in information achieved with

such a system.

Acknowle dgment

The authors wish to express their thanks to Dr. W. V. Tilston

for his interest in this project and his suggestions in the preparation of

this report.
5. References

Cormack, A. (April 1952), Distributed amplification, Electronic

Engineering, pp. 144-147.

Ginzton, E. L. , W. B. Hewlett, J. H. Jasberg, and J. D. Noe,

(August 1948), Distributed amplification, Proc. IRE 36,

No. 8, pp. 956-969.

Horton, W. H. , J. H. Jasberg, and J. D. Noe (July 1950),

Distributed amplifiers; practical considerations and

experimental results, Proc. IRE 38, No. 7, pp. 748-753.

Percival, W. S. (1935-37), British Patent 460562.

Pettit, J. M. , and M. M. McWhorter (1961), Electronic amplifier

circuits, McGraw Hill, New York.

Rudenberg, H. G. , and F. Kennedy (December 1949), 200 Mc

traveling-wave chain amplifier, Electronics, pp. 106-109.

Stockman, H. (1956), Introduction to distributed amplification,

published by SER Company, Waltham, Massachusetts.
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