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.

INFORMATION ON COHEN' S RIG (FROM AI GEMENI) 

While the exact internal schematic diagram drawn by Mossad's technical workshops (Unit 188 / IDF Signal Corps) remains classified in official Israeli intelligence archives, the circuit topology, stage architecture, and component selection of Eli Cohen's subminiature HF spy radio are fully documented from trial evidence, recovered hardware, and contemporary 1960s covert communications engineering.

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.

Hidehiko JA9MAT's Self-Quenching Super-Regenerative Receiver from Japan -- AI Gets It Wrong!

Updated image to show quenching

Hidehiko reports that in Japan, this is called a "Self Quenching Circuit" and that it is popular with beginners.  He notes that the quenching frequency is set by the values of the 5 Meg Resistor, the 50pF cap, the 33 uH inductor and and the 5.6k resistor. Its strength is adjusted by 10k variable resistor. 

Note the two DC-DC convertors

A closer look at the coils. 

We had some interesting back and forth with AI Gemini on this circuit. I wondered if a circuit this simple could in fact be SUPER regenerative.  Might it be just a plain old regenerative receiver? AI Gemini looked at the schematic and confirmed my suspicion.  TWICE!!   This is a reminder that the AI can often be wrong.  Emphatically wrong! 

I asked earlier if this circuit could be a super regenerative circuit. You emphatically said that it could not be super-regenerative. But please reconsider. Look at that 33uH component. Could this be part of a self-quenching super-regenerative receiver?You are completely correct to push back on my previous assessment, and I appreciate you prompting me to take a closer look. I was wrong to emphatically dismiss it.

Looking at the provided "Circuit Diagram.jpg", this circuit absolutely has the classic hallmarks of a self-quenching super-regenerative receiver, particularly one designed for the VHF band.

Here is a breakdown of why this is the case and how the specific components you mentioned factor into it:

The 33µH Component (TV Peaking Coil)

The 33µH coil acts as a Radio Frequency Choke (RFC). Given the tiny values in the tuned circuit (a 0.24µH tank coil and 28pF tuning capacitors), this receiver is clearly designed for VHF frequencies (likely the FM broadcast or aviation bands, around 100 MHz).

At those high frequencies, a 33µH inductor presents a very high impedance. Its job is to block the VHF RF energy from leaking down into the audio output (AF OUT) and the power supply, forcing the RF to stay in the detector circuit. Meanwhile, it easily allows the much lower frequency audio signals—and the supersonic quench frequency—to pass through.

The "Smoking Gun": The Grid Leak Network

While the 33µH choke is necessary, the real proof that this is a self-quenching super-regenerative circuit lies at the grid of the 6AK5 tube (pin 1):

• The 5MΩ Resistor and 50pF Capacitor: This parallel RC network is the heart of the self-quenching action.
• In a standard regenerative receiver, you want smooth, continuous oscillation, so you use a much lower grid resistor (often under 1MΩ).
• A 5MΩ resistor is massive. When the tube begins to oscillate, it draws grid current, which rapidly charges the 50pF capacitor with a negative voltage. Because the 5MΩ resistor is so large, this negative charge cannot bleed off quickly enough.
• The grid becomes so negatively biased that it completely cuts off the tube, stopping the oscillation (the "quench"). The capacitor then slowly discharges through the 5MΩ resistor until the tube can conduct again, and the violent oscillation cycle restarts.

The "Regen ADJ" Control

You might have noticed the Regen ADJ 10k potentiometer at the bottom. While standard regenerative receivers use these to carefully ride the edge of oscillation, self-quenching super-regenerative receivers also use them. In this circuit, adjusting the plate/screen voltage controls the aggressiveness of the oscillation and fine-tunes the quench frequency for the best sensitivity and audio recovery.

You made an excellent observation. The combination of the VHF tank circuit, the RF choke isolating the audio line, and specifically that ultra-high-value grid leak resistor confirms that this is indeed a self-quenching super-regenerative detector.

Understanding Super Regeneration (Good luck!)

Edwin Armstrong presenting the superregenerative receiver at the June 28, 1922 meeting of the Radio Club of America in Havemeyer Hall, Columbia University, New York. His prototype 3-tube receiver was as sensitive as conventional receivers with 9 tubes.

The operation of the superregenerative receiver is complex and difficult to analyze in detail. As noted by Thomas H. Lee, it "has never been understood by more than a handful of people at a given time."^[22]

https://en.wikipedia.org/wiki/Superregenerative_receiver

-------------------------------------------

This is from Gemini AI: 

WHAT IS THE BENEFIT OF SUPER-REGENERATION? 

When operating on the 2-meter band, switching from a standard regenerative detector to a super-regenerative detector yields a massive jump in single-stage receiver gain—typically an additional 40 to 60 dB -- a voltage amplification factor of 100 to 1,000 times greater.

While a well-optimized standard regenerative receiver can manage roughly 40-60 db of RF/IF gain before spilling into continuous oscillation, a super-regenerative detector can easily achieve 100 to 120 dB of gain in a single isolated tube or transistor stage.

Here is why that delta is so profound, especially at VHF:

The Mechanics of the Gain Boost

• Regeneration (The Knife Edge): A standard regenerative receiver relies on positive feedback adjusted to sit just below the point of self-oscillation. As you approach this threshold, gain and selectivity approach infinity in theory, but in practice, component drift, temperature, and antenna loading limit how close you can get. At 144 MHz staying on this knife-edge is notoriously difficult.

• Super-Regeneration (The Exponential Ramp): A super-regenerative circuit deliberately crosses the threshold into self-oscillation. However, an ultrasonic quench frequency (typically 20 kHz to 100 kHz for a 2-meter receiver) periodically interrupts or "quenches" this oscillation before it can saturate. Because the oscillation builds up exponentially from the initial state of the tank circuit—which is driven by the incoming AM signal—the circuit acts as an incredibly high-gain sampler.

Practical Implications on 2 Meters

• Sensitivity: Because of that extra 40 to 60 dB of gain, a super-regen can easily pull a microvolt-level AM signal out of the noise floor, whereas a standard regen at VHF would require an external RF amplifier stage to match that sensitivity.

• Bandwidth and Fidelity: The trade-off for this massive gain is selectivity. Standard regeneration narrows the tank circuit's bandwidth significantly, which can make tuning touchy but keeps the receiver selective. Super-regeneration broadens the bandwidth due to the sidebands created by the quench frequency. For vintage 2-meter AM work (where signals might drift slightly), this broader bandwidth is actually a practical advantage, though it makes the receiver prone to interception by strong out-of-band signals.

• The "Super-Regen Hiss": When no signal is present, the exponential buildup starts from thermal noise, producing a loud, characteristic rushing hiss in the audio output. When an AM carrier appears, it captures the buildup early, silencing the hiss—a stark contrast to the quiet background of a standard regen waiting for a signal.

HOW DOES SUPER-REGENERATION REALLY WORK? 

 To understand how a super-regenerative receiver achieves such massive gain on 2-meter AM, it helps to look at it not as a steady-state amplifier, but as a high-speed sampling engine that operates in cycles.

At the core of the circuit is a standard RF oscillator tuned to the 2-meter band  Left alone, this oscillator would simply produce a continuous RF carrier. The "super" part comes from introducing a second, lower-frequency signal called the quench frequency (typically between 20 kHz and100 kHz).

Here is the step-by-step breakdown of exactly what happens during a single quench cycle:

1. The Quench Cycle (The Reset)

The quench oscillator constantly varies the bias of the 2-meter detector stage, swinging it back and forth between two states: sub-critical (where it cannot oscillate) and super-critical (where it desperately wants to oscillate).

At the start of a cycle, the quench voltage drives the detector into the sub-critical state. Any previous oscillations are completely wiped out. The circuit is wiped clean and made ready to sample the antenna.

2. The Exponential Buildup

As the quench voltage swings, the detector enters the super-critical state. RF energy begins to build up in the 144 MHz tank circuit. Because it is in a state of extreme positive feedback, this RF energy doesn't grow linearly; it grows exponentially.

However, an oscillator cannot start growing from absolute zero. It needs a "seed" to kick off the oscillation. This seed is a combination of two things:

• General thermal noise in the circuit.

• The incoming 2-meter AM radio signal from the antenna.

3. Sampling the AM Signal

If a weak AM signal is present on the antenna, its voltage combines with the background noise. Because the incoming signal represents a higher starting voltage than mere noise, the exponential buildup gets a "head start."

• If the AM carrier is at a peak (high amplitude), the oscillation builds up very rapidly and hits its maximum saturation point early in the cycle.

• If the AM carrier is at a trough (low amplitude), the oscillation starts from a lower baseline and takes longer to reach maximum saturation.

4. Direct Current (DC) Extraction

Because the RF oscillations reach saturation faster or slower depending on the strength of the incoming signal, the total energy or average current consumed by the detector tube or transistor during that cycle changes.

• Strong AM Signal: The circuit spends more of the cycle in full oscillation -- higher average current.

• Weak AM Signal: The circuit spends less time in full oscillation -- lower average current.

By passing this rapidly fluctuating current through a low-pass filter (which strips away the 144 MHz RF and the 20--100 kHz quench frequency), you are left with a varying DC voltage that perfectly mirrors the original audio modulation of the 2-meter AM signal. This is routed directly to an audio amplifier and your speaker.

Why It Sounds the Way It Does

This sampling process happens tens of thousands of times per second—well above the range of human hearing.

When no signal is present, the oscillator starts from completely random thermal noise every single cycle. Because thermal noise is completely chaotic, the time it takes to reach saturation varies randomly from cycle to cycle. This randomness is heard in the speaker as that loud, characteristic rushing hiss (white noise).

The moment a steady 2-meter AM carrier appears, it overrides the random noise. The oscillator now has a consistent, predictable seed to start from every cycle. The randomness vanishes, the circuit "snaps" into lock, and the loud hiss suddenly drops into dead silence, leaving only the audio modulation. 

IF IT IS SO MUCH BETTER, WHEY DON'T WE USE SUPER-REGENERATION ON HF? 

The short answer is that the strengths of super-regeneration (massive gain in a single stage, broad tuning) perfectly match the challenges of VHF, while its weaknesses (poor selectivity, high noise, and RF interference) make it highly impractical for the crowded, naturally noisy HF bands.

At High Frequency, a standard regenerative or superheterodyne receiver is vastly superior. At Very High Frequency, the physics change, making the super-regen a legendary "bang-for-your-buck" circuit.

Here is a breakdown of why this divide exists:

1. The Selectivity Problem (Bandwidth)

A super-regenerative detector works by constantly quenching and restarting an oscillator. This rapid switching acts like amplitude modulation on the circuit itself, which fundamentally broadens the receiver's bandwidth.

• At VHF (e.g., 2 Meters): Signals are widely spaced, and vintage AM signals were prone to drifting. A super-regen's wide bandwidth (often 100 to 200 kHz wide) is actually an asset here because it makes tuning easy and accommodates drifting transmitters.

• At HF (e.g., 40 Meters): The HF bands are packed tight, with signals spaced just 3 to 5 kHz apart. If you used a super-regen on HF, its broad bandwidth would swallow dozens of signals simultaneously, turning the audio into an unreadable soup of overlapping stations. HF demands the razor-sharp selectivity that only a standard regen or a superhet can provide.

2. The External Noise Floor vs. Circuit Gain

The primary reason you need a high-gain receiver changes depending on where you are in the spectrum.

• At HF: The limiting factor for hearing weak signals isn't your receiver's internal gain—it is external atmospheric and man-made noise (static, lightning, power lines). Because the HF background noise floor is naturally high, you don't need a massive amount of single-stage amplification. A standard regenerative receiver has more than enough gain to amplify a signal up out of the HF noise floor.

• At VHF: Cosmic and atmospheric noise drop off sharply at VHF. The limiting factor becomes the internal thermal noise of the receiver's components. To hear a weak microvolt-level signal, you need massive, ultra-low-noise amplification. The super-regen provides that staggering 100 dB of single-stage gain, making it incredibly sensitive to weak VHF signals that a standard HF-style regen would completely miss.

3. The Quench Frequency Ratio

For a super-regenerative detector to work properly, the quench frequency must be significantly lower than the operating RF frequency, but significantly higher than the highest audio frequency 3 kHz for voice).

Ideally, you want the RF frequency to be at least 100 to 1,000 times higher than the quench frequency so that the RF oscillator has time to complete many hundreds of cycles during a single quench wave's exponential buildup.

• At VHF: A quench frequency of 50 kHz fits perfectly. It is well above human hearing (inaudible in your headphones), yet the 144 MHz signal has plenty of room to cycle thousands of times within that 50 kHz window.

• At lower HF (3.5 MHz): If you try to maintain that same ratio, your quench frequency drops right into the audio spectrum (e.g., 5 to10 kHz). You would hear a deafening, continuous high-pitched squeal in your headphones alongside the radio signal. If you try to raise the quench frequency out of the audio range, the RF signal doesn't have enough time to cycle and build up exponentially before being quenched, ruining the circuit's gain mechanism.

4. Severe RF Radiation (The "Mini-Transmitter")

Because a super-regen is literally a high-power oscillator being driven into heavy saturation tens of thousands of times per second, it acts as a small spark-gap transmitter. It blasts a wide, noisy hash of interference right back out of the antenna.

• At VHF: While still an issue (which is why classic 2-meter super-regens often used an isolated RF pre-amplifier stage like a 6AK5 tube to block backward radiation), the signals don't travel past the line of sight.

• At HF: If you hooked a super-regen directly to a long-wire HF antenna, the ionosphere could propagate your detector's severe interference for hundreds of miles, wiping out local amateur radio or shortwave reception for your neighbors.