[AMRadio] increase carrier or modulation

[Gary Schafer]

Good question Jim!

Here is how to figure it out. Really the same as we discussed the other day.

Let's assume that 400 watts PEP is all the transmitter is capable of.

100 watts carrier = 70.7 volts across 50 ohms.
To modulate that 100% requires 70.7 peak audio volts.
This will produce 400 watts PEP out of the transmitter. 

The total voltage will be the sum of the carrier (70.7 volts) plus the
modulation voltage of (70.7 volts). This is 141.4 volts. Square that =
1999.96. divide by 50 ohms = 400 watts PEP.

If you increase the carrier to 125 watts that will be 79 volts across 50
ohms for the carrier. 

Since our transmitter is limited to 400 watts PEP, the sum of the carrier
voltage plus the audio voltage must not exceed 141.4 volts.

If we subtract the 79 volts of carrier from the 141.4 volts maximum that
leaves 62.4 volts for the audio.

If we want to know the total side band power we square the audio voltage and
divide by 50 ohms. (62.4 x 62.4 = 3893.76 divided by 50) = 77.87 peak watts
of side band power.

Note that with the 100 watts of carrier we had 100 watts of peak side band
power. In both cases the maximum PEP output is 400 watts.

Note2 that we are showing peak audio power in these calculations. Average
audio power (what some call RMS power) is 1/2 of peak power.

To find RMS voltage from peak you multiply peak voltage by .707. 
In our 100 watt case 70.7 volts times .707 = 49.98 RMS volts.
To find average power from RMS volts we square the voltage 50x50 = 2500
divide by 50 ohms gives us 50 watts average audio power.

To make a long answer short, the more power you have in the side bands the
more copy able the signal is when conditions are marginal. All of the
information is in the side bands with none in the carrier. The carrier only
serves as a reference to demodulate the side bands in the receiver.

73
Gary  K4FMX


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jim WB5OXQ inb Waco, TX
> Sent: Sunday, April 24, 2011 9:35 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: [AMRadio] increase carrier or modulation
> 
> If operating at maximum carrier to provide 100% modulation,  which would
> provide a better more readable signal in the fringe area?
> carrier with 100% modulation, or
> 25% more carrier with 25% less modulation?
> Will you be heard better with louder audio or a stronger carrier if you
> cannot have both?
> If the transmitter can only be modulated to 100% with 100 watts of
> carrier, or 125 watts at 75% carrier.
> Which will provide a better more readable signal at the fringe?
> wb5oxq
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