Explain picture (Transmitter related room)

[60 oldjohn]

This picture is from Wireless World Dec 1947 Page 455. Described as "Modulated Amplifier Tank" Can anyone expand on this term? I use a picture of this room in public displays. Maybe a couple of lines that the man or woman in the street can understand.

TIA John.

[russell_w_b]

It's a 'high-level' modulation transmitter. The MODULATING amplifier is a high-power audio amp, the output load of which is the primary side of a modulation transformer.

The MODULATED amplifier is the RF amplifier, the d.c. HT supply of which has the output from the secondary of the modulation transformer superimposed upon it, so modulating the carrier wave. The tank circuit is the parallel-resonant RF load into which the antenna is coupled, the 'Q' of which depends upon the antenna load and the degree of coupling into it.

This is a very basic explanation.

[russell_w_b]

Here's a picture of one half of the old Droitwich BBC R4 LF (formerly R2) output stages: one of the old 200kW ex-Ottringham transmitters.

[60 oldjohn]

So it amplifies the output of the transmitter?

John.

[HamishBoxer]

Power amp stage?

[Cobaltblue]

The picture shows the Tank circuit.

The Tuned circuit for the output stage of the transmitter.

And this explanation might help straight off the web when I was struggling to put it into words.

Why is L-C circuit is called as Tank circuit?
Because stored energy moves to and fro between the capacitor and the inductor, like water sloshing side to side in a tank.

Hope that's of a bit of help

Cheers

Mike T

[Radio Wrangler]

Originally Posted by 60 oldjohn

So it amplifies the output of the transmitter?
John.

Not really, it makes the output of the transmitter.
The tank circuit is a tuned circuit, made from an inductor (clearly visible) and a group of capacitors.

The big RF power valve of the transmitter acts as a switch turning on and off its power supply into the tank. The tank acts as a filter to turn the switching into a smooth sinewave to prevent interference with other stations.

The power of the transmission can be varied by changing the supply voltage, so the output of a large audio amplifier is added to the DC from the transmitter's main power supply. The audio amplifier thus varies the voltage to the transmitter above and below its nominal value. This 'Modulates' the amplitude of the transmission, following the audio signal. Hence Amplitude Modulation.

This transmitter puts on the AM right at the very end, in the RF power amp itself. This is quite an efficient way of doing it.

A few different explanations from different viewpoints usually helps make things clearer. I've actually designed and put into production a couple of AM transmitters in the 21st century! They are high-level modulation types, just an awful lot smaller than the pictures. Air-ground speech radio is still AM.

David

[60 oldjohn]

Thanks, these are my two of my pictures taken through glass so not the best quality. They give an idea of the size of the equipment. I think the first photo is the Modulator Amplifier Tank. Any idea what the second picture is? Some of the coils look like they are made from copper pipes, are they water pipes, water cooled?

John.

[60 oldjohn]

Originally Posted by Radio Wrangler

[/I]

A few different explanations from different viewpoints usually helps make things clearer.

David

Exactly, I have never had anything to do with transmitters let alone 800Kw.
(4x 200Kw)

John.

[russell_w_b]

Originally Posted by 60 oldjohn

Any idea what the second picture is? Some of the coils look like they are made from copper pipes, are they water pipes, water cooled?

Water pipe is often used as there's no need to use a solid conductor because of 'skin-effect' at RF: it just flows on the outside. Certain parts of a system might be water cooled, air-blown, or neither.

That pic looks like a big RF switch with feeders going to it and a motor mechanism at the base for rotating the blades. They'd generally be a large surface-area, rounded, to prevent the onset of corona (no... not that corona).

[russell_w_b]

The first picture looks like part of the antenna matching unit / tuning house with the tuning inductors shown, and the feeder bushing going through the wall to the antenna base connection.