This week I watched the Netflix show about the Israeli intelligence agent Eli Cohen: https://www.netflix.com/title/80178151 I thought it was pretty good. The sequences about his use of CW to communicate with Tel Aviv were really interesting. I asked AI (Gemini) for more details. See below.
Cohen's story is also interesting Here is the Wikipedia on Eli Cohen: https://en.wikipedia.org/wiki/Eli_Cohen
Here is what AI reports about him in response to my questions:
Eli Cohen’s CW (Morse code) transmissions were heavily encrypted, and the underlying plaintext language of his messages was French.
1. Encryption Method: One-Time Pad (OTP)
- The
Cipher System: Eli Cohen (operating as Mossad Agent 566 under the
cover identity Kamel Amin Tha'abet) used a One-Time Pad (OTP)
cipher system. Plaintext characters were encoded using single-use,
pre-shared key sheets that were destroyed immediately after use.
- Over-the-Air
Format: Before transmitting over High Frequency (HF) CW, the encrypted
text was converted into standardized 5-character groups (either
5-letter or 5-number blocks).
- Cryptographic
Security: The Syrian military’s radio intelligence division (Signal
Company 134) regularly intercepted his daily CW signals from his apartment
in the Abu Rummaneh district of Damascus. However, because proper OTP
encryption is mathematically unbreakable without the key pad, Syrian and
Soviet cryptanalysts were completely unable to decipher the content of his
transmissions.
2. The Underlying Language
- French
Plaintext: Prior to enciphering the messages into 5-character blocks,
Eli Cohen drafted his intelligence reports in French.
- Why
French? Raised in Alexandria, Egypt, Cohen was natively fluent in
French, Arabic, and Hebrew. During the early 1960s, Mossad’s clandestine
unit handling deep-cover foreign agents (Unit 188 / Aman
prior to Mossad’s consolidation) frequently used French as the primary
operational language for field reporting and code sheets.
- Reception
in Israel: His encrypted Morse transmissions were picked up by Israeli
receiving stations (including a unit where his own brother, Maurice Cohen,
worked as a cryptographer/radio operator decoding incoming agent
transmissions). Once received, the operator applied the corresponding
one-time pad page to decrypt the 5-character blocks back into French
plaintext, which was then translated into Hebrew for intelligence
evaluation.
How He Was Detected
Syrian authorities did not bust Cohen by breaking his
cipher. Instead, they used Soviet-supplied Direction Finding (DF) vehicles
during a forced power blackout in Damascus, locating his apartment by tracking
his active 
/ 
HF radio signal while he was physically
transmitting.
During his deep-cover mission in Damascus (1962–1965), Eli
Cohen transmitted covert High Frequency (HF) CW (Morse code) signals directly
to Mossad receiving stations in Israel over a path distance of approximately 220
kilometers (135 miles).
Frequencies & Operating Bands
Cohen used pre-arranged HF utility frequencies in the 7
MHz to 14 MHz range, selecting specific frequencies based on time-of-day
ionospheric propagation:
- 7
MHz Region (40-Meter Band Equivalent): Used primarily for early
morning and late night transmissions (e.g., his daily 07:00 AM
window). At night and early morning, lower HF frequencies provided the
ideal Near Vertical Incidence Skywave (NVIS) and short-skip regional
propagation required to span the ~220 km distance over the Anti-Lebanon mountains
into Israel.
- 11
MHz – 14 MHz Region (20-Meter / Upper HF Utility Bands): Used for daytime
scheduled bursts when higher D-layer ionospheric absorption attenuated
lower frequencies.
- Frequency
Agility: To evade routine fixed-frequency monitoring, Mossad assigned
multiple pre-set crystal-controlled frequencies. Cohen changed frequencies
according to a strict calendar matrix provided on his one-time pad signal
sheets.
Antenna Design & Disguise
In the crowded Abu Rummaneh district of Damascus, hanging an
obvious transmitting antenna would have invited immediate suspicion. Cohen
utilized a stealth end-fed thin-wire antenna:
- Rooftop
Wire Element: A thin copper wire element strung across the roof of his
top-floor apartment building.
- Visual
Disguise: The wire was masked alongside the dense maze of commercial
TV receiving antennas and broadcast radio long-wires ubiquitous on Syrian
rooftops at the time.
- Feedline
& Entry: A thin black lead-in cable ran from the rooftop
wire element down through the window frame into his bedroom, hidden inside
a wooden curtain valence/box where he concealed his Morse key and
miniature transmitter.
How the Antenna Led to His Capture
When Syrian military signals intelligence (Signal Company
134) brought in Soviet-supplied direction-finding (DF) equipment, they narrowed
the signal down to the Abu Rummaneh neighborhood but could not identify the
exact apartment.
The breakthrough occurred when investigators inspected the
building's roof and spotted an unmarked black feedline cable connected
to a rooftop wire that did not route to a standard television set or broadcast
radio. Tracing the black cable down the exterior wall directly into the window
of apartment "Kamel Amin Thabet" gave them the exact location. Syrian
security forces then cut power to his specific apartment circuit during his
scheduled 07:00 AM transmission window on January 18, 1965, confirming the
transmission ceased simultaneously before raiding the room.
Eli Cohen used a custom-built, subminiature High Frequency
(HF) CW spy radio set developed by Israeli technical intelligence (IDF Signal
Corps / Mossad).
The equipment was vacuum tube-based (using
subminiature "pencil" tubes) and operated at a QRP power level of
approximately 3 to 5 watts (with a maximum output around 10 watts peak).
Technical Breakdown
1. Tube-Type vs. Solid-State Construction
During Cohen's operational period in Damascus (1961–1965),
high-frequency power transistors capable of generating several watts of RF
output at 7 MHz and 14 MHz were not yet reliable or commercially viable for
clandestine agent sets.
The set relied on subminiature vacuum tubes (similar
to the American 1AD4, 5678, or 3B4 series originally engineered for military
proximity fuses and portable field radios):
- Rugged
& Compact: These tiny, wire-leaded glass tubes consumed very
little filament current, generated minimal heat, and could withstand
physical transport.
- AC
Mains Powered: His primary set ran off standard
wall power, utilizing a miniature
transformer and silicon/selenium rectifiers disguised inside the chassis
of household appliances.
2. Power Output & Path Dynamics
- Power
Level: Operating at 3 to 5 watts CW, the set was a classic QRP
(low power) transmitter.
- Path
Distance: The distance from his apartment in the Abu Rummaneh district
of Damascus to Mossad receiving stations in northern and central Israel
was only about 220 kilometers (135 miles).
- Propagation:
On the 7 MHz (40-meter) band during early morning/evening and 11–14 MHz
during the day, 3 to 5 watts into a simple end-fed wire provided a strong,
reliable skywave/NVIS signal across the Anti-Lebanon mountain range into
Israel without requiring a massive, power-hungry transmitter.
3. Physical Packaging & Concealment
Because custom spy sets could not be purchased off the
shelf, Mossad's technical workshops built the transmitter and receiver into
everyday consumer goods that a wealthy import-export businessman like
"Kamel Amin Tha'abet" would naturally own:
- Primary
Set: The core transmitter components and power supply were built into
the false bottom of an electric kitchen blender and an electric
razor. The razor's power cord double-buffered as part of the power
feed/ground lead.
- Backup
Set: Syrian security forces also recovered a secondary backup CW
transmitter concealed inside a green leather-bound desk set/box.
- Keying:
He used a compact, hand-held micro-key (a miniature CW hand key) that
could be quickly stowed inside a wall cavity or curtain rod assembly.
Transmitter Circuit Architecture (~3–5 Watt CW)
The transmitter was a compact, two-stage Master
Oscillator Power Amplifier (MOPA) vacuum tube circuit optimized for high
efficiency, low heat dissipation, and minimal footprint.
+-----------------------------------+
| Transmitter RF Chain |
+-----------------------------------+
+-------------+
+-------------------+
+-------------------+
| FT-243 |
| Oscillator Stage | | Power Amplifier |
| Quartz |------>| Subminiature |------>| Subminiature |------> Pi-Network
| Crystal |
| Triode/Pentode | | Beam Pentode |
Matching
+-------------+ | (e.g.,
1AD4/5678) | | (e.g., 3B4/3A5) |
(Low-Pass)
+-------------------+
+-------------------+
| | |
+-------
Cathode Key -------+ v
(Hand
Key) End-Fed
Antenna Wire
1. Crystal-Controlled Master Oscillator (MO)
- Active
Element: A single subminiature "pencil" tube (wire-leaded
glass pentode/triode such as the 1AD4 or 5678) configured in a grid-controlled
Pierce or Colpitts oscillator topology.
- Frequency
Control: Utilized miniature FT-243 or HC-6/U crystal sockets. Cohen
swapped pre-cut quartz crystals corresponding to his assigned operational
schedule in the 7 MHz (40m) and 11–14 MHz (20m) bands.
- Output:
Delivered
to 
of stable RF drive to the grid of the
power amplifier, preventing frequency pull or chirp when keying.
2. Class C Power Amplifier (PA) Stage
- Active
Element: A subminiature filamentary beam power pentode (such as the
3B4 or dual-triode 3A5 operating in parallel/push-push).
- Bias
& Efficiency: Biased strictly into Class C operation using
a grid-leak resistor/capacitor combination (
, 
). Class C provided
over 65–70% DC-to-RF conversion efficiency, critical for keeping tube heat
low inside a concealed chassis. - Anode
/ Tank Circuit: The plate circuit used a parallel LC resonant tank
tuned to the operating band via an adjustable ceramic trimmer capacitor
and a slug-tuned toroidal coil wound on a carbonyl iron powder core (e.g.,
T-50-2 / T-50-6 type).
3. Output Antenna Matching Network
- Pi-Network
(L-C-C): The PA plate was coupled through a high-voltage blocking
capacitor to a low-pass Pi-network matching filter.
- Impedance
Matching: Transformed the high plate load impedance (
) down to match the
variable reactive input impedance of his random-length, end-fed rooftop
wire antenna (
). - Harmonic
Suppression: Provided
attenuation of second and third harmonics
to prevent interference with local broadcast services in Damascus.
4. Cathode/Grid Keying & Power Supply
- Morse
Keying: Direct switching of the cathode/filament return line to ground
via a subminiature micro-key. A small
ceramic capacitor and choke across the
key contacts suppressed key clicks and sparking. - Power
Supply Circuit: Powered from standard mains. The internal power supply utilized
a compact step-down/step-up multi-tap iron core transformer with
silicon/selenium diode rectifiers:
(Plate Voltage): 
to 
, smoothed by a
subminiature multi-stage LC filter choke and electrolytic capacitor
block.
(Filament Voltage): 
to 
regulated filament supply to preserve
subminiature tube cathodes.
Receiver Circuit Architecture
The receiving set accompanying the transmitter was a
high-sensitivity, subminiature Regenerative Receiver (or 3-tube
Superheterodyne depending on the specific backup set revision):
- RF
Preamplifier Stage: A single 1AD4 subminiature tube isolated the
antenna from the detector to prevent local oscillator radiation (which
could be detected by direction-finding equipment) and improved weak-signal
selectivity.
- Regenerative
Detector: A subminiature pentode operated at the verge of oscillation
for maximum CW sensitivity. A panel-mounted potentiometer adjusted the
screen-grid voltage (
) to control
regeneration depth and beat-note frequency for Morse code reception. - Audio
Amplifier Stage: A low-frequency audio amplifier tube drove
high-impedance (
) magnetic earphone
inserts concealed in standard pens or desk items.
Mechanical & Physical Implementation
- Point-to-Point
Wiring: Components were mounted on phenolic miniature terminal strips
using point-to-point wiring to minimize stray capacitance and physical
volume.
- Concealment
Modularization: The circuit was split across functional sub-assemblies
small enough to fit inside the false bottom of a leather-bound desk
stationery set and an electric kitchen blender chassis, with the AC power
cord serving as a multi-conductor umbilical.
