+++++++++++++++++++++ Date: Wed, 14 Nov 2001 14:23:14 -0800 From: Phil Wheeler Subject: Re: [Elecraft] QRP Power levels penzo at juno.com wrote: > > I plan on using my K2 during the phone SS contest this weekend and I have > a question about the legal power limits. I always thought QRP power > levels were 5 watts or less for CW and 10 watts or less for phone. Is > this true? Can I run 10 watts during phone SS and still be considered > QRP or must I run 5 watts? > Check the ARRL website, Mike. But I think 5 W PEP is the ARRL QRP limit. Phil +++++++++++++++++++++ Date: Wed, 14 Nov 2001 21:03:23 -0500 From: Radioham Subject: Re: [Elecraft] QRP Power levels Mike and All- ARRL rules are 5 watts out - SSB or CW. QRP ARCI is 5 watts CW, 10 watts SSB. Why the difference - I don't know, but them's the rules. Steve, N4EUK Reston, VA +++++++++++++++++++++ Date: Fri, 16 Nov 2001 19:41:07 -0600 From: "George, W5YR" Subject: Re: [Elecraft] QRP Power levels (long) The relationship between peak and average power in a modulated signal depends upon the waveform of the modulated output. For a constant carrier, the average and PEP power readings from a wattmeter would be the same. For CW, each dit or dah causes the transmitter to output its maximum PEP during that code interval, but the average power depends upon the nature of the text or whatever is being transmitted. A string of perfect dots would result in an average power that is 50% of PEP or maximum key-down power. Sending random text results in a lower average power determined by the relative number of dit and dahs and spaces, etc. With SSB, the average power depends upon the modulating waveform as with the CW case. For the special case of a two-tone modulating signal, found in PSK31 as the IDLE signal, for example, the average power is 50% of the PEP reading. That property makes the two-tone waveform a very convenient test signal to use with a scope. The output r-f envelope under keydown conditions will show by the height of the unmodulated trace the relative PEP output of the transmitter. If the modulating levels are properly adjusted, the two-tone signal peaks will just coincide with the PEP levels showing that the transmitter is not being overdriven. If the peaks flatten off, this shows that the transmitter is overdriven and that the distorted peaks are generating splatter. For voice, a properly operated SSB transmitter will output its PEP maximum power only on voice modulation peaks. The average power associated with that PEP reading will depend upon the modulating waveform and the linearity of the transmitter, all the way from the mic to the antenna. Keep in mind that the scope waveforms are instantaneous while meters have a time constant that prevents them from reading accurately on anything except a steady-state waveform. That is why it is difficult to set up a SSB transmitter with nothing to go by but meter readings. Sadly, it is not true in all, or even most, cases that "PEP equals twice average power." For some voices, it might work out that way, but for the "typical" voice without any speech processing, the average is much lower than PEP. So, we have an ambiguity of meaning when we say the QRP SSB limit is 5 watts. If we adjust the transmitter for a 5 watt PEP output and then drive modulation peaks to that limit, then our PEP is 5 watts but our average power may be as low as only 0.5 watts. That is due to the peak-to-average power ratio of typical speech which is in the order of 14 db. And that is why speech processing is used: to modify the ratio to obtain a higher average power and better utilize the average-power capability of the transmitter. A transmitter is peak-power limited by its linearity specifications and average-power limited by how much heat it can dissipate without burning something up. Now, if you say that 5 watts is the average power output limit for SSB, then we really have a bag of worms unless we can find a way to measure average output accurately with a time-varying speech waveform. Usually, there is no attempt to do this, but it is relatively easy to measure the PEP output. Most meters for this are typical average power meters with long time-constant circuits added to integrate the meter indications and show an estimate of the PEP being attained on modulation peaks. It is not uncommon, again depending upon a person's unique voice, for one person to modulate a SSB rig with the average power kicking up to 1/3 to 1/2 or even more of the key-down power limit. Another person might hit the same PEP limit, but the average power might be only 10% of max PEP. Thus, for the 5 watt average power limit the PEP could reach as high as 50 watts if the transmitter were capable of that level of undistorted output. The accurate way to operate a SSB transmitter and really know what is happening is to monitor the r-f output envelope with a scope. I use a 20 Mhz dual-trace scope to monitor (a) the audio output of the receiver or transmitter and (b) the r-f output envelope. For PSK31 and the digital modes, a scope is almost a necessity to derive the best performance from a transmitter. The r-f output indication is useful up through 10 meters although individual r-f cycles cannot be resolved up there. But, it is only the envelope that we are interest in. Sorry this got so long, but if we are gonna kick around these terms, seems like we should all be on the same page. All the above has been in the Handbook for many years, but perhaps it will be helpful to some of the newcomers. 72/73, George W5YR - the Yellow Rose of Texas QRP-L 1373 NETXQRP 6 Fairview, TX 30 mi NE of Dallas in Collin county EM13qe SOC 262 Amateur Radio W5YR, in the 56th year and it just keeps getting better! Icom IC-756PRO #02121 Kachina #91900556 IC-765 #02437 penzo at juno.com wrote: > > I have received a number of responses to my original question and most > agree that for ARRL contesting, 5 watts is the limit but I was reminded > about the difference between 5 watts (RMS?) and 5 watts PEP. > > It's been a while since my college days but if I remember correctly: (5w > * 1.414)*2 = 14.14 watts peak to peak (PEP? I can't remember if PEP is > same as peak to peak thus the * 2). > > The 2001 ARRL November Sweepstakes Rules in the October issue of QST > (page 114) state that QRP is 5 watts of output or less. They do not say > 5 watts PEP of output or less. Would it be a stretch to consider 10 > watts PEP equal to about 5 watts average power? +++++++++++++++++++ Date: Fri, 16 Nov 2001 19:44:43 -0700 From: Larry East Subject: Re: [Elecraft] QRP Power levels At 06:56 PM 11/16/01 -0500, Mike KT4FJ wrote: >I have received a number of responses to my original question and most >agree that for ARRL contesting, 5 watts is the limit but I was reminded >about the difference between 5 watts (RMS?) and 5 watts PEP. > >It's been a while since my college days but if I remember correctly: (5w >* 1.414)*2 = 14.14 watts peak to peak (PEP? I can't remember if PEP is >same as peak to peak thus the * 2). You are confusing Peak Envelope Power (PEP) with Peak to Peak (PP) voltage or current. PEP is the peak power in the RF waveform -- which is the same as "carrier power" for a CW signal. It's all rather confusing, but maybe the following from a recent ARRL Handbook will clarify it a bit: "PEP is the average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope, taken under normal operating conditions. Since calculation of PEP requires the average power of the cycle, multiply the maximum or peak value of the RF voltage achieved (Peak Envelope Voltage, or PEV) by 0.707 to obtain the RMS value. Then calculate power by using the square of the RMS voltage divided by the load resistance: PEP = (PEV x 0.707)^2 / R " (end of quote) As you can see from the above, the PEP value for a CW (non-varying in amplitude) signal is just what we call the "carrier power". Hope this helps... Larry W1HUE PS -- I have a mod for the OHR WM-1/WM-2 Wattmeter that will allow it to read PEP (to a very good approximation) of SSB signals. It can be found on the ARCI web site under "Technical/Projects". ++++++++++++++++