antenna load

[Bob]

LS,

In our local radio club we had a presentation about the Grimeston radio station at 17,2 kHz..
Questions did come up about the alternator load by the antenna.
As expected the antenna is very short for the wavelength of 17,44 km. and so being tuned by a very high-Q network and antenna transformer (Q ca 350). This implying de-tuning by a small amount already means considerable dropping of power. Therefore the alternator is to be kept at a very accurate speed.
We understood the antenna is coupled to the alternator by a special transformer in the matching network, which in turn is getting de-tuned by a magnetic amplifier in the keying circuit, to modulate the antenna current.
To get a stable frequency also for keeping within the optimal antenna tuning range, the alternator should see a constant load, to keep a stable speed. Now de-tuning by keying means alternator load is constantly changing, which at 80 kW. is generating considerable force to the generator and also to the mechanics to and in the drive motor. We presume this to be avoided.
Can you tell us if, and if so by what means the alternator load is being kept constant during this keying action.
Regards,
Bob, ON9CVD.

Topic

[AlexanderSAQ]

Dear Bob,
We cannot hear any sound from the transmitter set that can be connected to the keying. The transmitter set is equipped with two Bartlett Hayward flexible couplings between drive motor/gearbox and gearbox/high frequency generator. According to the RCA description of the transmitter, a special lubricant is needed to these couplings ”to absorb the chock in telegraph service”.

However, the keying is easily heard in the transmitter hall, as four huge relay-switches are operating for the speed compensation. Moreover, these relay-switches are cooled with compressed air in order to blow out arcs and cool the contact surfaces. The sound from the cooling air is very aggressive and varies with the keying, so use of ear protections is recommended.

Ola Hernvall / AlexanderSAQ

[Bob]

Thanks Ola for your quick and informed reaction.
Your answer also is largely in line with the discussions at our radio club.
This mechanism also is implying almost all station power is ‘felt’ in
the axel and gearbox between the drive motor and the alternator.
We speculate this to be distinctly audible during modulation and
also to require solid construction details to avoid excessive wear.

Bob, ON9CVD.

[AlexanderSAQ]

Dear Bob,
Thanks for the question.
No, the alternator load is not kept constant, the load variations are handled by the power network.

In order to keep the frequency constant at keying, additional torque is given by the drive motor at key-down to match the increased power to the antenna. The drive motor is an induction motor. The supply lines to the motor are equipped by transductors, i.e. choke coils with variable inductance, so the voltage to the drive motor can be adjusted between 1500 and 2300 V. The rotor winding of the drive motor is connected to variable liquid resistors, so the rotor resistance can be varied over a wide range. At key-down the inductance of the transductors is reduced and also the rotor resistance is reduced, both measures contributing to increased torque. The additional torque can easily be adjusted by the liquid resistors to match the increased load. A German radio amateur has analyzed the frequency variation at keying and found it to be within +- 10 Hz.
Ola Hernvall /AlexanderSAQ

[Author]

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