How to Increase the Power Output of a 300 Watt Broadcast Amplifier?

The schematic shows that the two amplifiers on the assembly are driven 180 degrees out of phase. Note from the physical layout that the two circuits are carefully matched (length of transmission lines) so that the two outputs will 180 degrees out of phase and are then used to push/pull drive the output transmission line.

It seems that you could combine 2, 3, or 4 assemblies together, which would require making sure the inputs were in phase and the amplifiers were all matched to each other in gain and delay so that the outputs would also be in phase.

So, in theory, I believe it can be done but with careful attention to RF circuit design layout principles. The more amplifiers that are paralleled, the lower the output impedance of the combination. For optimum power transfer, the output impedance of the assembly of amplifiers should match the impedance looking into the load which will probably require some sort of matching network.

However, I would contact the vendor for a final ruling...

Example circuit:

http://www.analog.com/en/technical-articles/paralleling-amplifiers-increases-output-drive.html

I was looking at the wrong circuit....
Now for the RF one... you can't just slap extra transistors in there, it's all layout critical and unless you really really really know what you are doing, it's not going to work.
And the fact that you've asked the question shows that you won't get it to work, you'll just get a lot of blown transistors and a red hot power supply.
Sorry... but RF is a black art... even Harry Potter won't touch it
Del

Agreed. At 108 MHz, one cycle is less than 10 nanoseconds, so parallel amplifiers need to differ in delay by less than a nanosecond. You can't just slap a circuit like this together.

The limiting factor will always be the breakdown voltage and currents of the output transistors.

There are some tricky things going on in that schematic that makes me wonder how useful or stable of an amplifier design this will be. It almost looks like a stripline circuit was built using coaxial cables. Because of this, the layout of these cables in reference to the chassis (GND) will become critical.

1. Another way of increasing the apparent power would be only to transmit one of the sidebands.
2. Another way of increasing the apparent power would be to use a directional antenna at both transmit and receive locations.
3. Another way of increasing the power would be to use a "co-linear" antenna.

Radio "Hams" do this sort of stuff all the time.

<...what is the limit ?...>

There are several:

• The maximum output power of the power supply circuitry is one: clearly, if the power supply board were only capable of producing <...300 watts...>, taking more out of it by cloning more circuitry into the RF stage might cause a circuit protective device to operate to disconnect it or, in its absence, the proverbial next weakest link in this circuit might fail catastrophically before the desired result is achieved.
• The antenna and its feeder cable will have a characteristic impedance, expressed in Ω. The square of the power supply circuit output voltage divided by this impedance gives the maximum that could be expected from the equipment were the RF stage operating at 100% efficiency; given all those resistors, 100% is about as likely as discovering rocking-horse droppings on Mars.
• etc.

That is why circuits, among many other things in Engineering, undergo detailed design, and are not just bunged together from any old junk just lying around after a few words with one's mates, even if a beer is not involved.

If you apply directional antenna at both sides of amplifier that at is transmit site as well as receiver side then power output of a300 Watt Broadcast amplifier will be increased.