[AMRadio] Side band power with increased carrier/was clippertononAM

[Gary Schafer]

Hi Jim,

It doesn't matter about the 50 ohm antenna line impedance. You could use any
impedance that you want as long as you use the same impedance for all the
calculations. This just gives a comparison. John used 4000 ohms plate
impedance for his calculations. That of course gives different voltages than
those that I showed but as long as everything is referenced to that same
impedance it doesn't matter. All the relations work out the same. Just ohms
law at work.

The modulation waveform shape is not too important when looking at maximum
modulation on negative peaks and positive peaks that determine Peak envelope
power. In the real world with voice modulation all we need to do is use a
scope to see all of this. As an example at 375 watts carrier set the scope
to read 4 divisions on just carrier. When modulation peaks hit 8 divisions
on the scope that is 1500 watts PEP (2x the voltage). When negative peaks
hit the base line that is 100% modulation in the negative direction. All you
really need to know except that the more high peaks the audio has the lower
the average audio power in the side bands will be.

A sin wave is just easier to look at and do calculations with than a voice
wave form is for understanding what is happening.

Further you can look at the number of divisions on the scope that the audio
occupies. The more divisions the greater the audio power on the signal. If
you raise the carrier above the 375 watt level you will not hit the 100%
mark on negative peaks as soon but you will sooner hit the 1500 watt PEP
mark and you will have less total divisions of audio on the scope. This
means less audio power transmitted.

73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jim Wilhite
> Sent: Wednesday, April 13, 2011 8:36 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Side band power with increased carrier/was
> clippertononAM
> 
> Nice explanation of the test Gary.  Maybe this will clear up some of the
> confusion.  It is too bad that test equipment like spectrum analyzers
> are so expensive.  I have wanted one for a very long time, but just
> don't have the extra cash.
> 
> Everyone remember this description as you work toward maximum PEP.
> There are two flies in the ointment.  Your voice isn't like a 1000 cycle
> symmetrical tone and your antenna may not be 50 ohms resistive.
> Explaining these two variables will make it as muddy as water in the
> Mississippi at New Orleans.
> 
> Almost all of us must relay on meters.  If you expect any form of
> accuracy then the meters must be capable of .25 sec time response.  They
> also must be in good repair and when the book says that you will have
> .300 ma. for full power, that means that you meter will go only to .300
> ma on those infrequent excursions up to that point.  The meter will not
> be there very many times as you talk.
> 
> If you follow good engineering practice, as the FCC like to specify,
> then you signal will be clear and very understandable.
> 
> Jim/W5JO
> 
> 
> 
> ----- Original Message -----
> 
> 
> > 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.
> >
> >
> > 73
> > Gary  K4FMX
> 
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