LETTER TO (SANITIZED) FROM (SANITIZED)

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03330A004100030010-8
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 8
• Document Creation Date: December 22, 2016
• Document Release Date: February 14, 2012
• Sequence Number: 10
• Publication Date: February 6, 1957
• Content Type: LETTER

Declassified in Part - Sanitized Copy Approved for Release 2012/02/14: CIA-RDP78-03330A004100030010-8_X1
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Enclosed please find four copies of the report,
"Addendum to Report Titled Problems Associated With
The Use of External Transmitting Sources to Protect
RS-6 Transmitters From Direction Fin-ding".
If you require any additional information, please
let us know.
Yours very truly,
DOC C REV DIVE - Iy 9P8D By _ee r
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ADDENDUM TO REPORT TITLED "PROBLEMS ASSOCIATED
WITH THE USE OF EXTERNAL TRANSMITTING SOURCES
TO PROTECT RS-6 TRANSMITTERS FROM DIRECTION FINDING"
In, view of recent developments, it has become necessary
to add a third type of system to the two systems described in,
the original report. This new system was originally described
in. a paper titled "Con.tinuous Monitoring for Voice Communication.
Jamming" by Dr. L. W. Orr and K. S. Watkins, presented at the
Symposium on. Electronic Countermeasures held in. October, 1956.
Basically, the Continuous Monitor Jamming System, or
CMJS, is a system for jamming an, enemy's receivers while still
monitoring his transmissions. In. the CMJS, monitoring is done
through the jamming rather than. by reducing the jamming signal
to a low level at the receiver site through the use of highly
directional antennas and/or large separations between. transmitter
and receiver. As applied to the RS-6 transmissions, CMJS would
permit jamming of enemy direction. finding equipment while receiv-
ing the transmissions at a site relatively close to the jamming
transmitter location.. This would result in. a considerable simpli-
fication, over the two systems previously analyzed. However,
estimates of other limitations such as the amount of jammer power
required and the degree of protection. given. remain unchanged with
the new system.
THE CMJ SYSTEM
The CMJ system consists of an. FM jamming transmitter, the
output of which also serves as the local oscillator for a "read-
CONFIDENTIAL
Declassified in Part - Sanitized Copy Approved for Release 2012/02/14: CIA-RDP78-03330A004100030010-8
 Declassified in Part - Sanitized Copy Approved for Release 2012/02/14: CIA-RDP78-0333OA004100030010-8
through" receiver, "readthrough" being the process of reconstruct-
ing the modulation, on, the channel being jammed. The readthrough
receiver consists of an. un.tun.ed triode mixer which feeds an. IF
amplifier tuned somewhere in. the frequency range from 30 to 40
kilocycles. The jamming signal and the signal being jammed are
both fed to the mixer by the antenna. The jamming signal consists
of an. RF carrier frequency modulated by a sin.e wave of random
frequency with the deviation. (20 - 30 kc) carefully controlled so
that the beats produced in, the mixer will fall on. the slope of the
IF ban.dpass characteristic at the extremes of deviation. The output
of the IF amplifier is detected and fed to a sampling gate. The
sampling gate is operated by pulses from the FM modulator at each
extreme of deviation., taking a 50 microsecond sample from the
output waveform and storing it in. a memory circuit.
If the signal being jammed is CW, the beat frequency fed
to the IF amplifier comes to the same point on. the slope of the
IF characteristic at each extreme of deviation, and the output of
the sampling gate is constan.t. If the signal being jammed is
FM modulated, the modulation, is detected by the slope of the IF
characteristic. If the signal being jammed is AM modulated, the
amplitude of the IF output varies with the amplitude of modulation..
In. either case, the output of the sampling gate varies in. ampli-
tude and frequency in. accordance with the modulation..
The memory circuit takes each of the output samples and
PANIMUNMI,
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stores it until the arrival of a new sample at which time the old
sample is erased and the new sample stored. The output of the
memory circuit is thus a stepped waveform containing the quantized
modulation, envelope. The reconstructed modulation, can, be obtained
by passing this waveform through a low pass audio filter. The
degree to which the reconstructed modulation resembles the original
depends largely on. the sampling rate which must be high in. compari-
son. with the highest frequency to be reconstructed.
The above is a brief description, of the operation, of the
CMJ system in, the readthrough function,. The experimental system
actually contains two additional functions; "lookthrough" and
"search". In. the lookthrough function., the system output is
displayed as a vertical deflection, on an oscilloscope having a
horizontal deflection. linearly related to the frequency deviation..
This display provides a means for accurately setting the jamming
frequency on. the channel being jammed and, to some extent, of
observing the effectiveness of the jamming. In. the search mode,
the jamming power amplifier is turned off an.d the FM oscillator
signal is injected directly into the mixer to give a sensitive
display without radiation.. At the same time, the deviation. is
increased and a mechanical frequency sweep set into operation. to
search a band of frequencies. Reference should be made to the
original report for a more complete description. of the system an.d
its capabilities.
UFDUNT AI.
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RECEIVER-TRANSMITTER ISOLATION
While the CMJS does not require isolation. between. the
jamming transmitter an,d the receiver to nearly the extent required
in. a system where the jamming signal must be below the minimum
signal to be.received', there will undoubtedly be some minimum
isolation. required, particularly in. systems using high power
jammers. The actual isolation. required will depend on. the maxi-
mum oscillator injection. voltage that can, be handled by the input
stages of the readthrough receiver. An. input stage consisting of
an,un.tun.ed triode mixer such as used in the system described in
preceding paragraphs is capable of handling several volts of
oscillator in.jection.. The experimental system described by Orr
an.d Watkins used loop antennas for the transmitter and receiver
located 40 feet apart and was reported to give satisfactory opera-
tion. with 100 watts of RF jammer power.
SENSITIVITY
The sensitivity of the experimental readthrough receiver
was stated to be between, one and two microvolts. However, no
indication, was made of the minimum field strength necessary for
communications monitoring. Indications are that the CMJS sensi-
tivity can be made comparable to normal receiving systems that
are operable only in. the absence of jamming.
JAMMING EFFECTIVENESS
The type of signal which is most effective against direction.
1flNRDENTAL'-..,.
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finders is not exactly known,. Presumably, a jamming signal of a
type which will prevent the detection, aurally or by field strength
measurements of the presence of a signal would be the maximum
requirement. A jamming signal sufficient to prevent reception at
the direction. finder of CW communications would meet this require-
ment. For such communications jamming, the FM by random sine
wave used by the CMJS has been, found to be less efficient than.
FM by random noise. Further development work is being done to
remove the requirement that the frequency deviation. be of con-
stant amplitude so that FM by random noise and several other
jamming signals can. be used with the system. In, attempting to
reduce the effectiveness of direction finding, the use of CW code
permitting lower modulating frequencies is strongly indicated as
most desirable.
Some work has been, done in, developing a receiver using the
CMJS output system to give a considerable improvement in. communi-
cations through FM jamming. While techniques can, be used with
the CMJS to counteract such anti-jamming receivers, the present
CMJS would be ineffective against receivers using this output
system. Any program carried out to study the effects of jamming
signals on. direction..fin.ders should include the possibility of
using such anti-jamming techniques with the direction. finders.
COMPATABILITY WITH PRESENT SYSTEMS
The CMJS can be operated into any transmitting system
CON f ENTIAL
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presently capable of radiating an, FM signal of the required band-
width (approximately 40 kilocycles) without spurious AM, the
spurious AM resulting in. degeneration, of the output quality of
the system.. The readthrough receiver requires a directional antenna
separated sufficiently from the transmitting antenna and properly
oriented to prevent overloading of the input stages. A simple
tuned loop an.ten.na should suffice in. the frequency range from
3 to 8 megacycles. The maximum separation would be limited to
the necessity to obtain. sufficient injection, voltage into the
antenna an.d to main.tain, a stable phase relationship between the
CMJS transmitter and receiver. Lines capable of carrying the
transmitter modulating signal (audio frequencies) would be necessary,
between. transmitter and receiver sites.
CONCLUSION
The Con.tinuous Monitor Jamming System offers the possibility
of considerable protection. to RS-6 transmissions, both against
interception, of intelligence an.d direction. finding, in. areas where
sufficient jamming signal strength is present. Although the
present system is limited to jamming modulation, types that are
less than. ideal, the system is still in. the experimental stages and
future developments are expected to correct this difficulty.
Until anti-jamming measures are taken, by the en.emy, even, the present
system should work well. The system is capable of operating into
existing RF power amplifiers an.d only a minimum separation. is
ONMENTAi
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required between. receiving and transmitting sites. High gain,
directional antennas are not necessary and only audio lines are
required between. receiver-control and transmitter-modulator
equipmen.ts.
Respectfully submitted,
Byy
February 6, 1957
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'nNUMBITIAL
Declassified in Part - Sanitized Copy Approved for Release 2012/02/14: CIA-RDP78-0333OA004100030010-8