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Author Topic: When did FCC change PEP?  (Read 8711 times)
WX2S
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« on: July 15, 2012, 04:49:07 AM »

Something I've been curious about... Back in the day, the power limits were 1000 watts PEP for CW, 2000 watts PEP for SSB. Somewhere in the last 30 years, it appears to have changed to 1500 watts PEP for either. What happened?

73,
- Steve (WX2S.)
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
AD4U
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« Reply #1 on: July 15, 2012, 04:53:05 AM »

"It" happened at least 25-30 years ago.  I guess PEP INPUT was too ambiguous and nobody could accurately measure it.  With the advent of accurate and affordable wattmeters (PEP) and otherwise, measuring power output became much more accurate than measuting power input, especially PEP.

Dick  AD4U
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W4VR
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« Reply #2 on: July 15, 2012, 08:34:15 AM »

Those old power limits were based on input power (plate current times plate voltage).  Now it's based on power output.  The only ones that were severely impacted by this change were the AM'ers; they are limited to 375 watts output unmodulated carrier.  BTW, I had a daily sked with the original WX2S, Tom, before he passed away of a heart attack.
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QRP4U2
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« Reply #3 on: July 15, 2012, 02:00:56 PM »

There are at least three discussions on this very topic at AMFONE:

Here's the latest discussion as of April 22nd, 2012:

http://amfone.net/Amforum/index.php?topic=31177.0

Phil - AC0OB
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WX2S
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« Reply #4 on: July 15, 2012, 02:21:49 PM »

BTW, I had a daily sked with the original WX2S, Tom, before he passed away of a heart attack.
Thanks for the BTW. Although I never met the man, I've included a small memorial for him on my QSL card and added him to the National SK Archive: http://www.silentkeyhq.com/main.php4?p=bin/NSKALookup.php4&call=WX2S . Perhaps you have some additional info to share.

- Steve (WX2S.)
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
N2EY
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« Reply #5 on: July 15, 2012, 03:21:35 PM »

Something I've been curious about... Back in the day, the power limits were 1000 watts PEP for CW, 2000 watts PEP for SSB.

No, they weren't.

I'm not sure of the exact date of the change, but it was in the early 1980s.

From 1912 until the change, the US amateur radio power limit was "1000 watts input".

In spark days this meant 1000 watts AC input to the transformer.

In tube days it meant 1000 watts DC input to the plate(s) of the RF stages delivering RF power to the antenna" (usually the final stage".
For transistors, "collector" or "drain" was substituted.

On modes like CW, AM and FM, the measurement was simple: Transmit a carrier and measure DC volts and amps.

For SSB, things got a bit more complicated. FCC interpreted 1000 watts DC input to mean 2000 watts peak DC INPUT.

This was done because amateurs usually found it much easier to measure DC input than RF output. Particularly peak output.

It also meant that amateurs were quite concerned about transmitter efficiency. An amateur running the legal limit with a transmitter of 70% efficiency was 3 dB ahead of one running the limit and 35% efficiency.


Somewhere in the last 30 years, it appears to have changed to 1500 watts PEP RF OUTPUT for either. What happened?


FCC decided to bring ham radio into the same measurement system as other US rradio services. So the power limit was changed to 1500 watts PEP for all modes.

73 de jim, N2EY
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K4FMX
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« Reply #6 on: July 15, 2012, 06:40:34 PM »

Something I've been curious about... Back in the day, the power limits were 1000 watts PEP for CW, 2000 watts PEP for SSB.

No, they weren't.

I'm not sure of the exact date of the change, but it was in the early 1980s.

From 1912 until the change, the US amateur radio power limit was "1000 watts input".

In spark days this meant 1000 watts AC input to the transformer.

In tube days it meant 1000 watts DC input to the plate(s) of the RF stages delivering RF power to the antenna" (usually the final stage".
For transistors, "collector" or "drain" was substituted.

On modes like CW, AM and FM, the measurement was simple: Transmit a carrier and measure DC volts and amps.

For SSB, things got a bit more complicated. FCC interpreted 1000 watts DC input to mean 2000 watts peak DC INPUT.

This was done because amateurs usually found it much easier to measure DC input than RF output. Particularly peak output.

It also meant that amateurs were quite concerned about transmitter efficiency. An amateur running the legal limit with a transmitter of 70% efficiency was 3 dB ahead of one running the limit and 35% efficiency.


Somewhere in the last 30 years, it appears to have changed to 1500 watts PEP RF OUTPUT for either. What happened?


FCC decided to bring ham radio into the same measurement system as other US rradio services. So the power limit was changed to 1500 watts PEP for all modes.

73 de jim, N2EY

Still not correct. 
The old input power on SSB had nothing to do with PEP. It was average power input by plate current times plate volts. Plate current on voice peaks as read on the DC plate ammeter. The plate current meter could not have a time constant longer than .025 seconds (if I remember correct). That power input was limited to 1000 watts.

There was no limit on PEP. PEP could be as high as you wanted. In fact, average power as read on plate current meter peaks is as low as 20 to 30% of actual PEP. So in those days if you had an amplifier that was capable you could have run the 1000 watts average power input with PEP of 5 or 6 Kw input and have been perfectly legal.

The AM transmitters fell into the same boat. They were limited to 1Kw DC input but there was no limit on PEP. Some operators had super modulators that could modulate in excess of 200% on positive peaks while limiting negative peaks to 100%. This gave very high PEP power.

Now of course everything is limited to 1500 watts PEP output.
So while the AM guys lost some power privileges, so did the  SSB guys but few ever realized it as not many understood what PEP really is. Even a lot of the amplifier manufacturers had ads saying that their amplifiers would do 2000 watts PEP input while the maximum it would do on CW was 1000 watts. No way that amp would run 2000 watts PEP on SSB.
The general notion in those days was that whatever the DC  input rating was, just double that and that would give you PEP. Not correct of course.

73
Gary  K4FMX
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K7KBN
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« Reply #7 on: July 16, 2012, 09:37:08 AM »

The CB gang was all up in arms about that time, because the "5 watt rule" was also changed.  Before the "input vs. output" shift, CB transmitters couldn't produce more than five watts DC input.  And then the number dropped to FOUR! (4 watts RF output).  "The dang FCC stole a whole watt!".

It didn't do any good to explain to most of them that 5 watts input, assuming 60% efficiency in the final amplifier, would produce no more than THREE watts RF output.  The FCC actually GAVE them a whole watt.
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73
Pat K7KBN
CWO4 USNR Ret.
WX2S
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Posts: 673




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« Reply #8 on: July 17, 2012, 06:08:14 AM »

So, to make a long story short, the FCC changed from furlongs per fortnight to Smoots per second.  Grin

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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
K4DPK
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« Reply #9 on: July 17, 2012, 01:52:18 PM »

K4FMX has it pretty well right, although I don't remember the part about the .25s time constant on the plate meter.

The change actually was a very long time in developing, and as I remember, we started hearing about the use of PEP as early as the mid fifties.  I'm not sure when the law finally came to it's present form, but there were two or three variations presented between the early sixties and the late seventies.

One of the problems was the lack of any available (and affordable)measuring equipment.  There weren't any good PEP wattmeters in those days, in fact, we only had a couple of choices of RMS wattmeters, other than homebrew.

Most of us didn't have much in the way of an SWR bridge, even.   

My first SWR indicator was two flashight bulbs on a piece of twinlead.

Phil C. Sr.
k4dpk
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G3RZP
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« Reply #10 on: July 17, 2012, 02:20:53 PM »

From at least the early 1960's, the UK went for PEP, although not calling it that. You loaded the TX to 150 watts DC input (the maximum allowed), noted the deflection of the RF output on an oscilloscope, and then kept the peaks to less than twice that deflection. Assuming 66% efficiency, that gave 400 watts PEP.

With the benefit of hindsight, I suspect that the RSGB told the authorities how to measure it and they followed along....Not that I am really cynical, but have had too much experience professionally negotiating with administrations.
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K9FV
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Posts: 479




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« Reply #11 on: July 21, 2012, 03:25:28 PM »

So, to make a long story short, the FCC changed from furlongs per fortnight to Smoots per second.  Grin

Today was not wasted - I learned something new today - not sure how useful Grin - a new unit of measurement..... the Smoot.  Neat!

73 de Ken H>
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K7KBN
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Posts: 2756




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« Reply #12 on: July 21, 2012, 06:11:23 PM »

So, to make a long story short, the FCC changed from furlongs per fortnight to Smoots per second.  Grin

Today was not wasted - I learned something new today - not sure how useful Grin - a new unit of measurement..... the Smoot.  Neat!

73 de Ken H>

Not very many of us ever heard of the Smoot until Google Earth included it in their distance measuring feature.  I'm not sure, though: One Smoot, Two Smeet?
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73
Pat K7KBN
CWO4 USNR Ret.
KB1LKR
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Posts: 1899




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« Reply #13 on: July 24, 2012, 06:19:11 AM »

Plural of Smoot (named after Oliver Smoot, MIT '62) is Smoots: e.g. The Mass Ave Bridge is 364.4 Smoots plus one ear.
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K1ZJH
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Posts: 898




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« Reply #14 on: July 24, 2012, 06:53:37 AM »

Quote
The general notion in those days was that whatever the DC  input rating was, just double that and that would give you PEP. Not correct of course.
73 Gary  K4FMX

Best explanation yet. There is a lot confusion regarding PEP. Even today, companies like RM Italy obviously
double the PEP average power to claim an exhorbantly high power rating.
PEP--regarding ham amps--is the most misunderstood and misused rating that I can think of.

Pete
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