REPORT ON A PRELIMINARY INVESTIGATION OF FACTORS INFLUENCING THE DESIGN OF A LIGHTWEIGHT PARATROOP BEACON

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03300A001100080003-7
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 11
• Document Creation Date: December 22, 2016
• Document Release Date: May 24, 2011
• Sequence Number: 3
• Publication Date: January 15, 1951
• Content Type: REPORT

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'. IMAMN I IAL
REPORT ON A PRELIMINAFtI INVESTIGATION OF FACTORS
INFLUENCING THE DESIGN OF A LIGHTWEIGHT PARATROOP BEACON
751 25X1
INTRODUCTION
This report will discuss briefly the findings of a two months'
study of the technical factors influencing the design of an ultra-
portable paratroop beacon of special characteristics.
The problem of designing of a paratroop beacon to meet the tentative
specifications resolves itself into the following considerations:
1. Choice of operating frequencies.
a. Operation with existing radars.
b. Availability of a suitable low-drain transmitting tube.
c. Suitability with respect to position resolution,
d. Security from known enemy radars.
e. Rigging of antennas by inexperienced personnel.
2. Choice of power supply.
a. Suitability for long life in the expected climatic conditions.,
b. Multi-input type,
c. Circuit arrangements to conserve power.
3. Choice of reliable design for minimum power drain.
a. General system
b. Receiver
c. Decoder
d. Switching circuits
e. Coder
f. Transmitter
g. Antenna
h. Test equipment
14. Packaging
CON Flbtvi r rAL
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a. Security
b. Ease of handling
c. Dropability
d. Protection from weather
The findings reported here are subject to revision due to the fact
that an investigation of other possibly system and component designs will
continue during the development of the first models.
The remainder of the report consists of Section I, Recommendations
for First Model Design; Section II, Discussion of Factors Influencing
the Recommendations; and Section III, Conclusions.
RECOMMENDATIONS FOR FIRST MOTEL DESIGN
The following is a brief list of recommendation design features for
a first model beacon.
1. Operating frequencies - X -band for both receiver and transmitter.
2. Power supply - Separate unit construction is recommended to allow
for various weights of batteries under various weather conditions.
Such construction will also allow use of available a-c power.
LeClanchh low-temperature batteries are temporarily recommended
until low-temperature tests can be performed on the Yardney
"Silvercel."
3. Reliable design for minimum power drain.
a. Receiver - A crystal-video receiver using five low-drain
subminiature pentodes and a biased crystal detector is.
recommended.
b. Decoder - a Two-tube pulse-repetition-frequency decoder is
recommended. The same tube types as in the video amplifier
should be used.
c. Switching circuits - A one-tube relay circuit for actuating
the transmitter section is recommended. The same tube type
as used in video amplifier is recommended.
d. Coder - It is recommended that there be no coding of the
reply.
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e. Transmitter - the QK299 is recommended as the transmitting
oscillator. The 2D21 is temporarily recommended as the
modulator switch tube pending the outcome of tests on the
subminiature 2D29.
.f. Antenna - It is recommended that an existing omnidirectional
beacon antenna be used.
g. Test Equipment - It is recommended that a resonant buzzer
test-signal generator be used.
1. Packaging
a. The use of camouflage should be considered, but no recom-
mendation can be made. Means for self-destruction should
be included.
b. It is recommended that the package consist of two cases, the
transmitter receiver case and the power supply case. Both
should be designed to be handled simultaneously by one man.
c. It is recommended that to minimize bulk and weight, remov-
able dropping containers be used when needed.
d. Hermetic sealing of the entire transmitter-receiver case is
recommended.
SECTION II
DISCUSSION OF FACTORS INFLUENCING THE RECOMMENDATIONS
1. Choice of Operating Frequencies
a. The operating frequencies of both the receiver and the trans-
mitter may be chosen freely for optimum beacon operation. However, it is
desirable for the first model to be designed to operate with existing air-
borne bombing and navigation radars with as few modifications to the radars
as possible. This consideration makes X-band the logical choice for both
the receiver and the transmitter.
Very little increase in sensitivity can be gained by operat-
ing a crystal-video receiver at lower frequency. It is, therefore, not
likely that further study will affect the choice of receiver frequency
except insofar as overall system choice may dictate.
b. The choice of transmitting frequency is mainly a matter of
finding a low-drain oscillator tube of sufficient power regardless of
operating frequency. This is true because, for line-of-sight operation
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with a fixed antenna intercept area, the power required for a given
range is independent of frequency except for atmospheric absorption.
Such a consideration indicates that a low frequency is the
logical choice for transmission because of the higher plate efficiencies
obtainable from low-frequency oscillators. For the type of operation
contemplated, however, filament power is the primary consideration. For
a reliable range of 100 miles using antennas of reasonable size, a peak
power of the order of 100 watts is required. Of the tubes capable of
this order of peak power in the 100 to 10,000 Mc range, few require less
heater power than 0.6 ampere at 6.3 volts.
This is the filament power required by the recently developed
QK299, tunable X-band magnetron. This tube is capable of 300 to 1000 watts
peak power at 30 to !i5 percent plate efficiency. No other microwave
oscillators were found to compare favorably with these characteristics,
although a number of low-frequency tubes are available that require about
as little as one-fifth as much filament power. It is believed that the
desirability of using X-band in the first model is sufficient to outweigh
the slightly longer battery life that would be available if a low-frequency
transmitter were used.
c. One of the important systems considerations that makes X-band
desirable for transmission is the high resolution obtainable in azimuth
with available high-gain X-band radar antennas.
To obtain the available resolution, it is necessary to main-
tain the gain of the X-band receiver at such a level that minor-lobe
response is not evident. Two alternatives for accomplishing this result
are available. Automatic gain control can be used in the beacon receiver,
or either manual gain control or automatic gain control can be used in the
radar receiver. In either case it is necessary that the gain-controlled
receiver be at X-band or at a frequency at which the required directivity
can be obtained.
Due to the added complexity and hence power-drain, it is
undesirable to add automatic gain control to the beacon receiver. There-
fore, it is desirable to use an X-band beacon transmitter.
d. Added security for the beacon can be obtained by operation
of its receiver at frequencies outside the much-used radar and beacon
bands. Such operation would require the use of a specially designed inter-
rogating radar transmitter of power comparable with that of the usual Xi-
band radars. Such a design is probably not feasible at microwaves where
new components would have to be designed.
Cross-band operation would also add to the security of the
beacon. It is believed, however, that prf coding will provide sufficient
security without the necessity for cross-band operation.
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e. One of the more difficult operational problems is the in-
stallation of the antenna in a satisfactory location. It is probably true
that, the higher the operating frequency is, the more difficult it will
be for untrained personnel to install the antenna satisfactorily.
From this consideration low-frequencies would be more de-
sirable than microwave frequencies. It is intended that this aspect of
system design will be studied further.
2. Choice of Power Supply
a. The primary consideration in the choice of batteries is their
suitability for long life at low drain under the expected climatic con-
ditions. There are two types of batteries that are suitable for this
application from the standpoint of watt-hours-per-pound.
The LeClanchg low-temperature dry cell exhibits a watt-hours-
per-pound characteristic which increases with decreasing current drain.
A ten-to-one reduction in current drain approximately doubles the watt-
hours-per-pound capacity. At room temperature the LeClanchd cell has a
capacity of between 50 and 100 watt-hours per pound for the current
drains contemplated for the beacon. The capacity is reduced almost
linearly to a value of about 15 percent at -60?F. Figure 1 shows
LeClanche battery weight required as a function of temperature.
The room temperature capacity of the Yardney "Silvercel"
actually exceeds that of the LeClanche dry cell and in addition can be
recharged a large number of times.
The low-temperature characteristics are still not known
definitely. Conflicting unofficial reports have been received. One re-
port indicated that satisfactory operation could be obtained at -20?F
with derating to only 50 percent of room-temperature capacity. Another
report indicated that drastic reduction to less than 10 percent of room-
temperature capacity occurs at 0?F and below. This discrepancy will be
cleared up by cold-chamber tests.
If the rechargeable feature is found essential, the Yardney
"Silvercel" is the logical choice provided the cold-chamber tests prove
it to be satisfactory. However, if the "Silvercel" proves unsatisfactory
at low temperatures, the low temperature, lead-acid cell is the next best
choice. Its derating at low temperatures is much less than that for the
LeClanch6 cell, and its capacity approaches that of the LeClanchg cell at
-60?F.
The other cell types considered were the nickel-cadmium, the
zinc-mercury, and the non-rechargeable silver cell. These types are best
suited for short-life, high-rate discharge and exhibit poor low-temperature
characteristics.
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characteristics. There are also several "single-shot" short-life bat-
teries available, but their characteristics were not considered applicable
to this problem,
b. The possibility of providing multiple inputs was considered,
but it is believed that best use can be made of the allowed weight and
volume by concentrating on one efficient type of power source.
A logical alternative to the multiple input power supply is
the use of a separate replaceable power supply package. A number of
different power supplies could then be designed for particular weather
conditions and locations.
c. Several unusual techniques for reducing the duty ratio of
the batteries were considered.
The high power requirements of suitable modulator and trans-
mitter tubes make it essential that they draw current only during an
active transmission. Relay circuits must be provided to operate from a
signal accepted by the decoder. The operation should be such that modu-
lator and transmitter filaments will be turned on by the first accepted
pulse packet, plate voltage will be applied 10 to 15 seconds later, and
both will be held on for as suitable time after the last interrogation.
The possibility of using sunlight switching, clockwork
switching, and electronic switching to reduce the "on" period of the
beacon by 50 percent or more was considered.
Sunlight switching is obviously useful only when operation
will be expected only at night or only in the daytime. It is believed
that such a restriction of operating time more than balances the increased
life expectancy of the beacon. It is, therefore, not recommended.
Clockworks of satisfactory characteristics have not been
found for study.
Electronic switching could possibly be used for switching
at a recurrence period of several seconds. However, any of the schemes
for reducing the battery duty ratio at the same time reduce the probability
of receiving a correct interrogation. It is therefore believed that only
the on-off relay for the transmitter and modulator should be incorporated
in a design.
3. Choice of a Reliable Design for Minimum Power Drain
a. The requirements of reliable, long-life operation and low
power drain are antagonistic. However, it has been found that an X-band
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beacon can be built to weight approximately 44 pounds and have a life of
3 weeks to 6 months depending upon temperature conditions and the total
length of interrogation time.
The suggested beacon design consists of an X-band crystal-
video receiver followed by a decoder and a transmitter eon-off" switch-
ing circuit which turns on the modulator and transmitter tube after re-
ceiving an interrogation,
b. The crystal-video, the super-regenerative, and the super-
heterodyne circuits were considered for the receiver design.
For X-band the superregenerative receiver cannot be used
because of the lack of a suitable tube.
A superheterodyne requires the use of a high-drain local-
oscillator tube and is hence unsuitable. Besides these short-comings
both the superregenerative and the superheterodyne radiate r-f energy
which would reveal the location of the beacon.
The crystal-video receiver, although of marginal sensi-
tivity, is by far more economical of battery power than the super-
heterodyne.
To eliminate the filament drain of the video amplifier,
transister amplifiers were considered; however, their inherent non-
uniformity and instability, especially with respect to temperature varia-
tions, make their use inadvisable. Suitable low-drain subminiature pen-
todes are available for the video amplifier.
A recent investigation has shown that the sensitivity of a
crystal-video receiver, can be increased from -45 dbm to about -60 dbn
for a tangential signal by the use of 10 microamperes of forward bias.
The use of such bias also stabilizes the crystal impedance and current
sensitivity factor against variations due to temperature changes. The
resistance of a typical unbiased crystal varies from about 3000 ohms at
4 150?F to about II00,000 ohms at -60?F. The resistance of the same crystal
varies from about 2500 ohms to about 9000 ohms when biased with a constant
current of 10 microamperes. Current sensitivity factor for the unbiased
crystal changes about 3 or 4 to 1, but stays vitually constant with constant-
current bias. A constant-voltage bias allows approximately the same order
of variation of characteristics as zero bias. For unattended operation at
peak performance, it is essential that the impedance (and hence noise
voltage) and current sensitivity be as nearly constant as possible.
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The reliable range on interrogation can be calculated from the
range equation
GRGT x PT
RRel = A
PR
RL1 = reliable range in the same units as the wavelength
A _ wavelength
GR _ receiver antenna gain (at 3 db points)
GT _ transmitter antenna gain (at 3 db points)
PT _ transmitted power
PR _ receiver tangential signal power
Typical values for a low-powered X-band airborne radar would be
A = 3.2 x10-2 meter = 2 x 10-5 mile
GR =0.5x3.0=1.5
GT =0.5x103
PT = 8 x 103 watts
PR = 4.0 x 10-9 watts (allowing 6 db safety factor)
giving
RRel = 61 miles
Maximum range at antenna gain peaks and optimum receiver performance would be
RMax = 4 x 61 = 244 miles
For interrogation on 2.2 microsecond pulses a band-width of
240 kilocycles per stage is sufficient to give pulse reproduction which
reaches about 30 percent peak amplitude. The choice of such a narrow
bandwidth is necessary because of the low gain-bandwidth product of ~ow
drain subminiature pentodes. With this bandwidth a gain of about 10 is
required to bring a tangential signal to the 1 volt level. Five sub-
miniature CK544DX tubes are sufficient for this gain and bandwidth at
a drain per tube of 13 milliwatts filament power and 6 milliwatts plate
and screen power. Total video amplifier power drain should then be
about 0.1 watt.
6. The
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The added complication of screen bypassing when using pen-
todes is offset by the fact that no suitable low-drain triode is avail-
able. The subminiature capacitor art has reached such a state that this
problem is no longer a severe design limitation.
c. The decoder security requirement is such that the life
expectancy of the beacon should not be seriously impaired by the use of
a large number of tubes in a multiple-pulse circuit, and at the same time
the probability of accidental triggering by known models of existing
radars should be very small. Another consideration in decoder design is
the desirability of requiring as little modification of the airborne
radar as possible.
A prf decoder using an LC ringing circuit operated by a
pentode, the pulse input of which is amplitude-limited, was the most
promising type investigated. Such a decoder can be designed to have a
reasonably fast response and narrow bandwidth simultaneously because of
the bandwidth-narrowing effect derived from the amplitude-limited input.
Two CK544DX pentodes are sufficient for the decoder. The decoder power
drain should then be 0.01 watt.
It should be possible to make such a decoder to reject any
prf differing by more than - 0.7 percent from the correct frequency.
The radar prf can be accurately controlled to a frequency chosen to be
as far removed from known radar repetition frequencies as possible.
Probably 330 to 360 ape is a useful range to which most 400 pps radars
could be easily modified with a slight reduction in transmitted energy.
It is believed that high traffic capacity will not be a
problem of the decoder for this beacon.
d. After the decoder has accepted an interrogation, it is
necessary that the modulator and transmitter filaments be turned on and
that there be a delay of at least 10 seconds before plate power is
turned on. It is also desirable to have a delay following the last inter-
rogation to prevent the occurrence of dead times due to fades.
Comparison of mechanical and electronic delay switching
schemes showed the electronic method to be the more economical of power
and sufficiently reliable. A single CK544DX tube using 0.02 watt is
sufficient for both switching operations.
A large number of contacts is required in this operation
because of the fact that the 30 or 145 volt batteries normally paralleled
for receiver operation must be placed in series to give about 450 volts
for transmitter operation. A large number of 6-milliwatt SPDT switches
can perform such an operation at less power drain than fewer multicontact
switches. Approximately 20 such switches will be required.
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e. It is believed that identification coding will not be re-
quired in this application and, therefore, no coder should be required.
If, however, it is decided to use coding, there is avail-
able a simple scheme of 1"ulse coding that requires no more active
circuits than the non-coded reply. The scheme consists of using a
pulse-forming line which generates a video modulating pulse with 100-
percent "ringing" modulation superimposed. The "ringing" frequency can
be set for each beacon to give a predetermined number of equally-spaced
pulses per reply.
Coding requiring the use of several additional tubes to
generate the codes should not be used because of the added battery drain.
f. The choice of a transmitting tube was eased by the recent
development of a low-drain X-band magnetron, the QK299. Its power capa-
bilities range from 300 watts at 30-percent efficiency at 2500 volts to
1000 watts at 45'percent efficiency at 3500 volts. Its filament power
drain is 3.8 watts and average plate power drain about 0.5 to 1.0 watt.
There are no other known X-band tubes of comparable characteristics.
However, for frequencies from 1000 to 3000 megacycles the
RCA pencil triode and the GE L29 ceramic triode are capable of about 100
watts peak at considerably less power drain.
As a modulator the 2D21 miniature thyratron has satisfactory
characteristics. Its heater power is 3.8 watts and its plate power loss
should be about 5 percent of the modulating pulse power.
The subminiature 2D29 is being studied as a possible short-
life substitute for the 2D21. The tube is designed for use in long-range
missiles and is, therefore, not a long-life type. If its life promises
to be greater than 100 hours, it should be used. Its filament power is
only 0.13 watt.
The reliable reply range available using the QK299 can be
calculated from the expression
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71 FTGR PT 1
7W E_
PR gx 0
where
LS - scanning loss
LO : loss due to off-band operation
Other symbols same as in (1)
(2)
Using
UPI T
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CONFIDEiv i iKL
Using PT 300 watts, PR = 4 x 8 x 10-13 watts (allowing 6 db safety
factor), L - 6 6-degree beamwidth and
S
- (for ), = 1.0 (for radar-
360
band operation) the reliable reply range is
RRel - 152 miles.
There is a margin of 8 db in power to allow for slight off-band reception
in the airborne receiver.
The maximum "searchlight" range using no safety factors
Rmax = 1700 miles.
g. There are X--band antenna designs available that are suitable
for this application. One in particular consists of an omnidirectional
horizontally polarized triple tripole antenna with a gain of 3. Single-
wire transmission lines are being studied in an effort to facilitate re-
mote placement of the antenna.
Directional antennas obviously should be avoided unless the
interrogation bearing is known. This is an unlikely situation.
The proper placement of the antenna will be one of the most
difficult operational tasks. To obtain omnidirectional characteristics
there must be no shadowing objects such as trees, hills, buildings, etc.
The acuteness of this problem strongly suggests that an operating fre-
quency below the U.H.F. range be used. In such a case it would be neces-
sary to use direction-finding techniques to obtain azimuth information.
However, considerably less care could be exercised in the placement of the
antenna at these frequencies than is required at X-band.
If a parallel design were to be considered because of antenna
difficulties, the operating frequency should probably be between 2 and 20
megacycles. Such a system would require complete development of an inter-
rogator including its direction-finding components as well as the beacon.
A large saving in transmitter power consumption could be obtained at these
frequencies, but receiver power would not be changed.
h. For testing, only the simplest of test gear were considered.
Since the decoding is done on the prf, a buzzer type noise generator
resonant at the proper prf should give sufficient signal of proper char-
acteristics to actuate the beacon.
!&. Packaging
a. One of the foremost problems of packaging is one of security.
Camouflage and self-destruction techniques should be used, but no recom-
mendations can be made. The various phases of this aspect will be studied
further.
CONFIDENTIAL
be
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