> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Ian White GM3SEK
> Sent: Monday, October 16, 2006 4:30 AM
> To: amps@contesting.com
> Subject: Re: [Amps] PEP power..
>
> Peter Chadwick wrote:
> >Actually, the definition of PEP quoted by SM2EKM is the definition in
> >ITU-R Recommendation SM328. That definition has been around since the
> >1950's.
>
> It is also how "PEP" is legally defined in the US and UK licences, and
> probably in many others.
>
> Note that the word "peak" applies to the modulation envelope - NOT the
> RF power! Your RF power is defined as the average over one complete RF
> cycle, so your PEP is simply the power of the largest RF cycle you ever
> transmit.
>
> The audio modulating frequency is much lower than the RF frequency, so
> a modulation "peak" will contain at least several hundred RF cycles, all
> at the same power level within a fraction of a percent. That gives a
> power meter plenty of time to make a measurement... but many so-called
> "peak reading" power meters still fail to grab the true peak level of
> the modulation.
>
>
> --
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek
To add a little more clarification to Ian's nice explanation;
The AVERAGE power that Ian is describing is what many (incorrectly) refer to
as RMS power of the RF.
Peak envelope power has nothing to do with "peak power". It is all to do
with average rf power.
Think about the number of RF cycles contained in the time it takes for one
audio cycle, even a high audio frequency such as 3 KHz. On 80 meters (say
3900) in order to transmit that 3 KHz audio you have to transmit 1300 RF
cycles (3.9 Mhz/3Khz) for each cycle of audio. So even though the FCC says
that the pep measurement needs to be over 1 RF cycle you can see that many
RF cycles are present in one audio cycle transmitted. Of course not all the
RF cycles will be at the same level over that one audio cycle as the audio
cycle is varying or constantly changing level. But you can see that with
that many rf cycles contained in one audio cycle that even at the CREST or
peak part of the audio cycle there will still be many rf cycles that occur
in that time. I suppose the reason that they specify "over one rf cycle" is
to preclude any arguments as to how many actual rf cycles there are at the
actual crest of the audio cycle. (this may raise it's head with different
types of modulation).
With lower audio frequencies and or higher RF frequencies the number of
cycles of RF at that same crest of the audio cycle will increase in number.
You can think about peak envelope power in relationship to a cw signal. If a
transmitter is tuned to produce 1000 watts cw output that is the same as
1000 watts pep. (In this case the PEAK of the modulation ENVELOPE is maximum
and infinitely long)*. With ssb modulating that carrier (raising and
lowering its level at a voice rate) the highest power that can ever come out
of the transmitter with full modulation is 1000 watts, same as the cw
power.**
In other words at the highest level of an audio cycle (crest or peak) the
transmitter is passing the maximum level or amplitude of the many RF cycles
that occupy the time that it takes while that audio cycle is at its maximum.
Thus the name "peak envelope power". It is the average power of the RF (same
as cw power) at the highest level that the modulation audio allows the
average RF power to reach.
* Keying a cw transmitter on and off produces a modulation envelope.
** Carrier is not transmitted by itself in ssb but instead the would be
carrier is converted to frequencies that are offset by the frequency of the
audio signal(s). So the result is the same as varying the amplitude of a
single carrier as far as power level is concerned in the above explanation.
73
Gary K4FMX
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