Spies On The Air? Israel's Spy in Syria: Eli Cohen and HF CW in the 1960s -- Cohen's Antenna and Thermatron QRP Transmitter

 

Eli Cohen in 1959

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.