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For way too long did I think being a radio architect meant shutting off old designs and letting software defined digital radio push forward. But now I am realizing I should have never let the simplest radio be forgotten! By not letting it be, we are still moving forward not only in our hobbies but also in our lives!

So, as I always do, I started playing with the radio, but in this case, the simplest radio product of my own design, minimum parts count transceiver Hejkal, that must have been the top-line of the world, in its category.

The initial and very basic goal is to keep the part count of the complete satisfactorily* working transceiver below 50! No ICs, only components with no more than three legs!

*for those not being spoiled or high-fed


Hejkal's schematic: 


and with CW Reverse functionality



This is the first Hejkal prototype ever built, TNX OK1BJH, OK1HCD and OK1DXK - I took this opportunity to thank our team for pushing me forward!


A picture taken while Hejkal's input IP3 is being measured:


Hejkal's local oscillator phase noise:


Hejkal 20-meter band:

  • Noise Floor: -110dBm* 
  • 100kHz blocking: >120dB
  • Sensitivity: 2uV or so
  • LO Noise: -136dBc/Hz @1kHz, -142dBc/Hz @10kHz, -144dBc/Hz @50kHz
  • Input IP3:  >15.5dBm, spacing 20kHz (wanted at 14.060MHz, unwanted 14.080MHz and 14.100MHz)
  • Dynamic Range Wide Spaced (20kHz): 87dB
  • Tuning Range: 14.050MHz to 14.070MHz (adapted for SOTA)
  • XIT range: cca 800Hz @14.050MHz to cca 1.8kHz @14.070MHz
  • Selectivity (-6dB): 1.3kHz
  • AM breakthrough suppression (fmod 1kHz, AM Depth 30%): >60dB
  • Maximum Audio Output: 50mW @16ohms
  • Transmitter Power Output: 1W (30dBm)
  • Local Oscillator Radiation (conductive measurement): 2.5mW (4dBm)**
  • II. harmonic suppression: 26dBc***
  • III. harmonic suppression: 44dBc***
  • Current consumption: RX: 110mA @12.6V, TX(1W): 340mA @12.6V
  • Weight (TBD, approx): 0.1111111 kg (0.2449580 lb)


*In direct-conversion receivers both sidebands are not only frequency translated but also utilized for the wanted signal. Consequently, if we receive a single-sideband signal (SSB, CW), all the noise from the second "empty" sideband degrades the resulting signal-to-noise ratio by at least 3dB!

**Local oscillator radiation is the big problem of all minimum parts count DC receivers. Moreover, Hejkal's original mixer design utilizes a clone of the unbalanced resistive mixer architecture suffering from LO to RF port isolation. On the other hand, being stable and very clear makes Hejkal's local oscillator much less annoying then other simplest designs like regenerative or superregenerative receivers. 

Be cautious not to bother other hams around when you listen. BUT to allow this to happen means very remarkable stochastical coincidence: sharing exactly the same frequency, being in the same cw narrow radio channel at the same time at the same area.

***I am about to add two more coils and one capacitor (those marked "optional").

Published on  August 19th, 2019

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