|[AMRadio] Re:Reactor Wanted|
ars.w5omr at gmail.com
Mon Nov 12 07:36:54 EST 2007
Thomas Adams wrote:
> Howdy, Geoff!
> >>> Thomas Adams wrote:
>> and maybe even make the legal limit output for AM (375 watts of
> Hi, Tom.
> This is an incorrect statement. There is no -legal limit carrier
> level- for AM.
> The legal limit on -any- mode is 1,500 watts -PEP -output-. Period.
> If you have a transmitter that has it's audio at the base-line on
> negative peaks and positive peaks that reach beyond a 2:1 ratio, then
> your 375w carrier is producing more than 1,500 watts PEP output. <<<
> In the (unlikely) event of an FCC inspection, I doubt that the
> inspecting engineer is going to go with
> that sort of reasoning and observe such niceties, even tho it IS correct.
Man, I dunno... the -federal law- is pretty clear...
> The "right" answer in this case is the one that they tested me on for
> my (now defunct) 1st Class Radiotelephone
> license; the peak power of a carrier at 100 percent modulation is four
> times the carrier power. Anything else
> leaves too much wiggle room, and opens the door for lawsuits...
> especially if your rig doesn't use any sort of
> processing tricks like negative peak clippers.
Which mine doesn't. I, too, have a modulation reactor (thanks, to
Brett/N2DTS) that's 50Hy @ 400mA that I'm using to shunt the DC off of
the secondary of the modulation transformer.
My audio chain starts with a Shure SM-7 Mic, a Yamaha 31-band graphic EQ
(balanced line in/out) into a Bogen 120w PA system, used as a
speech-amp, and the audio comes straight off of the plates of the (4)
8417's in push-pull, parallel to a 1:1, 6k ohm isolation transformer.
The secondary of the isolation transformer acts as a High Z choke, and
then the audio goes to the bases of a pair of horizontal output
transistors in emitter-follower configuration, which then directly
drives the grids of the 250TH's in Class B.
Those 250TH's, with nearly 3kV on their plates, idle at 120mA, resting
current, when B+ is on. They are modulating another pair of 250TH's in
the final, with a max of 1500v (when the variac is cranked up) at around
250mA for max smoke (which I hardly ever run). The -normal- operation
of the final is 800~1000v @ 150mA for around 150w DC input, and 100w of
carrier output. With that light of a load on the final, there's -more-
audio available. Glad to say that I don't get an SR of 4, when running
250w carrier output, but it ain't 2, so I don't go higher.
> This is a matter of absolutes... and absolutes ONLY exist in
> bureaucracy and statutes. In those realms,
> reality means little or nothing. And the bureaucracy says that without
> processing, PEP equals four times
> the power of the unmodulated carrier.
I read it differently, Tom...
1500w PEP output (regardless of mode) IS the absolute maximum legal
output. Period. Punto. Absolutely. End of discussion. It's written
She Retracted her Ball Point (that's all she wrote)
If I want to stay -legal-, I need to keep my carrier power just under
100w, because my rig -will- produce an SR of 4. Re-read that page
again, and then read it again, and let it soak in.
Here's a scope video of my rig. Just slightly over 100% at times (and
don't mind that 'hum' look on the carrier - that's just a noisy
near-field transformer) but it describes my point dramatically. this is
a 100w carrier, with an SR of -4-.
The carrier consumes 2 divisions, and the peaks (from top to bottom)
consume 8. 8:2 is reduced to 4:1. 100w at 2:1 = 400w PEP output. at
3:1, that's an increase of 3db, or 800w. You can see where this is going.
No audio, but it's me, saying "hellooooo, one, two threeee, four five.
Driving your AM Rig without a scope,
is like driving your car at night, without headlights. (K4KYV)
73 = Best Regards,
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