# US 4,397,034 A · Low probability of intercept transmitting apparatus

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- Number: US 4,397,034 A
- Document ID: US4397034A
- Status: Granted
- Kind: A
- Application number: 06/247,686
- Filed: 1981-03-26
- Granted: 1983-08-02
- Earliest claimed: 1981-03-26
- Assignees: Sperry Corporation
- Inventors: Benjamin V. Cox; Billie M. Spencer; John W. Zscheile, Jr.
- Web page: https://patentgrep.com/patent/US4397034A

## CPC

- H04K 1/00: Secret communication
- H04K: SECRET COMMUNICATION; JAMMING OF COMMUNICATION

## Abstract

A system for transmitting desired information is provided which has a very low probability of being intercepted. The system combines a well known narrow bandwidth directional antenna for limiting the interception of the main beam information signal with a novel random noise generated signal which guarantees that the sidelobe signals which also contain the desired information cannot be distinguished as information signals.

## Claims

### Claim 1

A low probability of intercept transmitting apparatus, comprising:
a source of data information,
a local oscillator adapted to provide a carrier frequency,
a first modulator coupled to said local oscillator and to said data information for providing a main modulated data signal,
a directional antenna coupled to said main modulated data signal for transmitting main data signals and producing a desirable main beam signal and undesirable sidelobe signals,
a source of random noise signals having the same statistical waveform characteristics as said data information,
a second modulator coupled to said local oscillator and to said random noise signals for providing a random modulated masking signal, and
an omni-directional antenna colocated with said directional antenna for transmitting said masking signal and for masking said sidelobe signals.

### Claim 2 (depends on 1)

A low probability of intercept transmitting apparatus as set forth in claim 1 which further comprises:
a first amplifier coupled between said first modulator and said directional antenna,
a second amplifier coupled between said second modulator and said omni-directional antenna,
said second amplifier providing an output signal on said omni-directional antenna which is greater than the maximum sidelobe signal, whereby the signal to noise ratio of said sidelobe signal to said masking signal is less than zero decibels.

### Claim 3 (depends on 2)

A low probability of intercept transmitting apparatus as set forth in claim 2 wherein the output from said first amplifier multiplied by the gain of said main beam signal from said directional antenna provides adequate signal strength to accommodate a remote receiver.

### Claim 4 (depends on 2)

A low probability of intercept transmitting apparatus as set forth in claim 2 wherein the output from said second amplifier multiplied by the gain of said omni-directional antenna is less than the main beam signal and greater than the maximum undesirable sidelobe signal.

### Claim 5 (depends on 1)

A low probability of intercept transmitting apparatus as set forth in claim 1 wherein said data source provides coded digital signals and said random signal source provides means for independently generating random signals which emulate said coded digital signals.

### Claim 6 (depends on 1)

A low probability of intercept transmitting apparatus as set forth in claim 1 wherein said omni-directional antenna includes a plurality of omni-directional antennas.

### Claim 7 (depends on 6)

A low probability of intercept transmitting apparatus as set forth in claim 6 wherein said plurality of omni-directional antennas are placed in the near proximity to said directional antenna to serve as decoy sources.