[ARC5] AM modulating an "ARC-5" transmitter...

Sat Oct 10 16:10:21 EDT 2015

    AM can be really confusing.  The PEAK power is four times carrier 
but average is 1.5 times carrier. PEP works for AM but the FCC 
definition does not make sense. In the old days the power of amateur 
transmitters was regulated by _input_ power, I think because measuring 
RF output can be difficult. It also applied to the carrier only.  So, 
the output would depend on the efficiency of the transmitter. Likely 
carrier output of a good plate modulated transmitter with Class-C RF 
output stage would be on the order of 700 watts carrier.  The FCC by 
memory refers to the "average" PEP.  So average of a sine wave is about 
0.637 of peak. Now, does the FCC want the _average_ of a 1KW total at 
100% modulation or what?  If they specified peak power at the crest of a 
100% modulated wave as no greater than 1KW the carrier could not be more 
than 250 Watts. If they are specifying the _average of the peak power_ 
then we have a maximum of 1.57 times this value or about 392 Watts 
carrier or about 1.57KW for 100% modulation. Given our assumption of a 
70% efficiency of a Class-C plate modulated stage this would be on the 
order of 600 watts input for the carrier.
     I am not at all sure this is correct but think the FCC could 
certainly write the spec in a less confusing way.  They should also have 
grandfathered all the 1KW input AM rigs which existed at the time the 
new regulations were instituted.


On 10/10/2015 12:40 PM, Kenneth G. Gordon wrote:
> On 10 Oct 2015 at 14:56, Mark K3MSB wrote:
>
>>>> Does that 60 W PEP mean 30 W fixed carrier and 15 W per sideband max when
>>>> modulating?
>> Wouldn't that be for a pure sinusoidal modulating frequency?
> Most probably...
>
>> I don't think PEP is relevant to AM.
> Well, yes it is. At least the FCC sure thinks it does.... !!! This is the reason
> the power limit for AM now is 275 watts. This figures to around 1100 watts
> (275 X 4, roughly) PEP according to the FCC's reckoning. Many AM
> affciandos vehemently disagree with that though.
>
>>     When I do AM I simply state the carrier
>> power I'm running.
> But with controlled carrier AM, the carrier can be as small was zero watts
> output during pauses in speech. So what would you state in this case? The
> most accurate measure of output power in the case of CC modulation IS
> PEP. There are a number of PEP wattmeters available these days, (the
> Heathkit IP-2140 for instance) which fairly accurately measure PEP.
>
> In any case, the maximum power output one can reach with CC modulation
> is the maximum plate dissipation rating of the tubes being modulated. Oh,
> sure, you could force it over that, but that would limit the life of your tubes,
> and besides, there really is no good reason to do that.
>
>>     If anything else, I'd state the modulation percentage
>> too, but my modulation meter is not working.  I'm currently using just an
>> SB-610 monitor scope.
> Modulation percentage with a good CC design is 90%, on average, but can
> be either higher or lower depending on how it is set up. FYI, to properly set
> up a CC modulation system, one really should use a 'scope and a single or
> two-tone audio input.
>
>> I just got a T-17, so I hope to try my BC-456 out soon on AM.   Assuming the
>> T-17 still works, which I gravely doubt.
> I have not seen one of those within the last 30 years that still works. Back in
> the late 1950s, when I was first licensed, there were many that worked...and
> many that didn't too.
>
>>       I think there's a way to use a
>> dynamic mic in a circuit meant for carbon mics -- I thought I saw a thread on
>> that some years ago.
> Yes. In fact, one can remove the old carbon element and replace it with
> some modern element which will allow it to work properly in the old
> carbon-mic circuits. However, I don't have that info at my fingertips.
>
> You might also remember that the T-17 (and others like it) were purposely
> designed so that the operator must shout into it in order to get enough audio.
> This was due to its primarily being used where external noise was very
> high...like on a battlefield.
>
> We used to remove the front cover from the T-17 and drill many more holes
> in it in place of the three holes which were usually there. That made a big
> difference.
>
> We would also routinely replace the original element with one from a
> telephone....but you can't find those any more either.
>
> Sometimes we could get a recalcitrant T-17 element to work, at least for a
> while, by beating the crap out of it to loosen up the granules. That didn't
> often work though.
>
> vy 73,
>
> Ken W7EKB
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL