10 Meter AM Transmiter #2

[W9LCE]

Solid State AM Transmitter for 10 Meters of about 100 watt output

What would be the best way to AM modulate a medium power Solid State Transmitter for 10 Meters?

As previously noted, I picked up the RF amplifier board from a Heathkit HW 5400 (includes the heat sink).  The final amp uses a pair of MRF 454 transistors (or equivalents) in push-pull, which are rated at 80w each (?Class AB).  They have a peak voltage (CEO) of 25v and 20 Amps each (100w each).    Dissipation is at 250w each (500w).

To operate the board as Class C, the final transistors (and probably the second driver stage, a pair of MRF 479 transistors, also in push-pull) could be modified by removing the base resistor and replacing it with some form of RFC (a couple ferrite beads?).  (The first stage amplifier - pair of MRF 476 pp - likely could be left Class A or AB.)

The AM modulation transformer approach (previous) is one approach -

An alternative is series modulation.  As I understand, there are two methods of doing this, both using transistors in the collector lead.  One seems simply to be the varying of the input voltage according to an applied audio signal using a power transistor as a variable resistor.  What I have seen indicates that the transistor output is set for half of the input voltage.  This is applied to the input of the Final.  If the MRF479 transistors are driven at 3.5w, the Final output (pair of MRF454s) is about 70w each (or pp - 140w), that means the 5400 Final is running at just under 12A (so a single 2N3055 – rated 15A each - could be the modulating transistor).   That would mean the modulator would have an input Supply Voltage of 24v with 12v output, through an RFC to the Final transistors.   Is this correct?

The other is PWM (Pulse Width Modulation) which achieves the same thing by square wave switching the incoming current on and off, just varying the lengths of the on-off wave.  I could still use the single 2N3055, using the alternate PWM exciter.  On this approach, the question I still have is, do you use the CEO (25v) for the transmitter final? 

[M0VRF]

Yes, as PWM is a series modulator you wont get the voltage doubling action of the missing transformer!

SO...... you must start with twice the voltage (or more)

Forget the mod tranny, fine for low power stuff and the Big Valve stuff but for moderate to high power and sensible voltages stick to MOSFET PWM. (as per, check the class E Website, it's ALL on there!)

I was building and selling PCB's for a board barely bigger than and inch square that would pass 500W.

You have the ability the set the carrier to peak to ANY level so you could set a 1W carrier and 500W peak but as this would cause distortion in most rx's so it's best to set to 150% (so a duty cycle of 40:60)

Totally linear at ALL frequencies

Alas I have run out of PCB's but can supply the design details.

BTW you must use a Low Rdson Mosfet, forget the 3055, I used a pair of IRF640's but the CREE SiC family are gaining popularity and there's the usual 11N series from UTC too.

Regards from the U.K.

JB.