07/15/2016 08:40

Hartley on Air


Here what the connections look like for the first tests. The first tests were successful: power output was from 5W to 8W at plate voltages ranging from 250V and 400V, on frequencies around 3.55 Mhz. I was heard 200 miles away. The tone was very clean, but the frequency was unstable due to the antenna wires oscillations in the wind: there was a thunderstorm outside at that time.

Lessons learned.

1) The antenna link is too close to the main coil. So, the capacitive coupling makes the set sensitive to antenna oscillations. If the linke were a bit more apart, then the magnetic coupling would have been the one transferring energy to the radiating system, and not the capacitive one.

2) It seems that a thick copper tube as main coil is not that needed. The only reason to have it is to have relatively high efficiency for the production of oscillations. This would result in a relatively high current circulating between the coil and the main capacitor. In this way it is possible to couple the RF power to the antenna by use mainly of the magnetic field, and so by convenient use of a link, which can be, in turn, installed a bit apart from the main coil. High 'Q' in the resonating circuit, then, means that a loose coupling can be afforded so to reduce the influences on the frequency stability due to unpredictable load variations, like an antenna oscillating in the wind, and transferred mainly by the capacitve portion of the coupling.

3) The oscillations initiation is critical along frequency. For higher tuning capacitor values, the oscillations are hard to start. So, the feedback tap (see the wire within the coil) should be moved towards the plate connected end of the coil. This topic, though, is to be investigated deeper on next days ...



As said, the tone quality was very good. No chirp and no AC noise. The first Hartley was built some 30 years ago, when I was a teen. That experience was a blast to me. I was even able to apply AM modulation on the feedback tap at the main coil, by use of a carbon mike and a voltage elevator transformer, without using any audio amplification.

Now, after 30 years, I'm apparently returning on my footsteps due to some nostalgia. A kind of ... even though, today, I am bitten by the curiosity and by the desire of understanding deeply the whole mechanism by which it is possible to have such a 'ridicolous' circuit working properly as a decent CW transmitter. For staying in touch on the Shortwaves.


Radioman, July 2016

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