[AMRadio] Side band power with increased carrier/was clipperton on AM

[Gary Schafer]

Today I warmed up the HP141 spectrum analyzer and made some power
measurements of my Kenwood TS-430 in the AM mode. The TS-430 was fed into a
200 watt 30 db attenuator and another 20 db of attenuation was added to feed
the spectrum analyzer. Also used a scope to see when 100% modulation was
achieved. Also used the PEP wattmeter in the kenwood tuner. A function
generator was used to generate a 1000 Hz tone for modulation.

Max carrier output is 100 watts.
In the AM mode I set the carrier to 25 watts out. Also verified on the
spectrum analyzer to be 6 db down and the scope showed 1/2 voltage of full
carrier.

With 100% modulation I got what was expected. 100 watts PEP on the wattmeter
and the spectrum analyzer showed each side band to be down 6 Db from the
carrier. That is 6.25 watts of power in each side band. Just like any text
book says it should be.

Then I doubled the carrier power to 50 watts out. I then set the modulation
level to where the PEP was 100 watts. The spectrum analyzer showed the
carrier to be 3 Db down from full power. The side band power was
approximately 12 Db down in each side band from the carrier. This equates to
about 50% modulation. This left around 3.125 watts in each side band. I am a
little off as this comes out to a little over 100 watts PEP when you add
them up. I probably did not measure the side band power exactly. Hard to
define that close on the old analyzer. 
As you can see the side band power is around 1/2 of what it was with only 25
watts of carrier.

This shows that when doubling the carrier power the side band power must be
cut in half in order not to raise the peak envelope power of the signal.
You can equate this to the legal limit power of 1500 watts PEP and see that
raising the carrier power above the 1/4 level (375 watts) will cause the
side band power to drop if you are to maintain the legal limit of 1500 watts
PEP.

Here is how to find PEP of an AM signal:
Using my above measurements 25 watts of carrier is equal to 35.36 volts
across 50 ohms (your antenna line).
1/4 of that power, the power in each side band is 6.25 watts. That equates
to 17.68 volts across 50 ohms.

To find the peak envelope power of the AM signal you add the voltages of all
3 signals together (2 side bands and the carrier). Square the voltage and
divide by the resistance (50 ohms) and you have PEP.

The carrier is         35.36 volts.
One side band is       17.68 volts
The other side band is 17.68 volts.
The sum of those equals 70.72 volts. Squared = 5001 volts. Divide by 50 ohms
= 100 watts. That is your peak envelope power.

If you do the same exercise with the 50 watt carrier and the 3.125 watt side
bands you will get approximately the same 100 watts PEP.

At 1500 watts PEP and 375 watts carrier there are 93.75 watts in each side
band. If you do the same as above you will come out with 1500 watts PEP.

Increasing the carrier to 750 watts looks like you are running a lot of
power but you must drop your side band power in half from what you were
running with 375 watts of carrier to stay under the 1500 watt PEP limit.
750 watts carrier = 193.65 volts.
With the side band power reduced to 12 db down in each side band that gives
46.88 watts in each side band. 46.88 watts = 48.4 volts across 50 ohms. The
numbers don't come out exactly so it is probably closer to 13 Db down that
each side band needs to be.

Bottom line is if you double your carrier power you need to cut your side
band power in half to maintain the same Peak envelope power.

This is why I was saying that the 375 watt carrier power is the best power
to run a linear amp at. Reversing the audio phase to put highest peaks on
the negative side causes you to increase the carrier so as not to over
modulate on the negative side but at the same time overall modulation must
be reduced.

Probably clear as mud.

73
Gary  K4FMX