[AMRadio] AM Transmitter Advice??


Donald Chester k4kyv at hotmail.com
Sat Feb 18 20:45:58 EST 2006


John, WA5BXO wrote:

>As I understand it the trick, that the FCC was to prevent, and someone was
>trying to get away with, was to run a 304TL with about 100 Volts on the
>plate in a GG configuration.  Forward bias it to a high plate current like 
>1
>Amp, so that it acts like a switch that is on.  Then drive it with 5 KW 
>PEP.
>It may have been some other some scenario as this but I think you get the
>picture.

I had never heard of that, but it makes sense that someone might try it.


>     Then there was the trick that a gentleman up in 3 land, I think, was
>going to run the high level double sideband reduced carrier generator type
>rig but he was not reducing the carrier just increasing the SBs via an 
>extra
>upside down tube, as it was commonly called.  The sideband power would
>continue to go up without distortion (if copied on a proper synchronized
>product detector) after the first tube was over modulated in the negative
>direction. The voltage and power would be diverted to the upside down tube
>where sideband power would continue.  There was trouble with the specific
>rule interpretation at the time in the FCC. Of course any of us today, 
>would
>be able to see that the upside down tube's audio plate current and audio
>voltage must also be counted as part of the input power.  But the FCC was
>having trouble deciding, at least as I understand it.  At any rate, I think
>they got him for being outside the 40 meter band limits.  You may remember
>more of the specifics on this Don.

I knew the gentleman personally.  It was Fred, W3PHL, near Phila, PA.  I met 
him at many hamfests, and visited him one weekend back in about 1971.  I saw 
his rig, but by that time he had converted it to a big SSB linear.  He liked 
to ragchew with VK's and ZL's in the pre-dawn hours on 40m, using a 120 ft. 
high beam.  He not only fought the FCC, but had to deal with a tower case as 
well (which he won).

The loophole in the regulations was that the definition of power was DC 
input to the final.  With the upside down tube circuit, he ran about 600 
watts DC input, and then applied several kw of audio.  The rig was basically 
a high-level balanced modulator, but with DC applied to one tube, which 
effectively unbalanced the modulation, he claimed it was a plate-modulated 
AM rig, and that the legal power measurement was limited to the DC input to 
the final.

The signal was double-sideband reduced carrier, with several kilowatts in 
the sidebands and less than 500 watts carrier power.  Althhough a 
synchronous dectector would have have taken full advantage of both 
sidebands, most of the people he worked actually used SSB receivers, and 
simply copied either USB or LSB, and used the carrier only as a pilot 
carrier for setting the frequency on their receiver.

The FCC couldn't make up its mind on how to  deal with the issue, even 
though they could have cited a rule on the books that limited modulation to 
100%, and they could have said he was modulating over 100% in the positive 
direction, regardless of the fact that the signal was clean.  Instead, they 
ended up citing him for splattering outside the 40m band.  He liked to 
operate at 7290, and even though he had engineering data to prove that his 
signal met all FCC specifications regarding spurious sideband products, they 
said that the rules allow no detectable signal whatever outside the limits 
of the amateur band, and he had detectable sideband products above 7300, 
even though they might have been 50-60 dB down.

I understand this whole thing was part of an ongoing feud between Fred and a 
SSB group that was  competing for the frequency, and the issue was brought 
up when the SSB group complained to the FCC.  They suspended Fred's  licence 
for six months based on the citation for out-of-band distortion products.

The FCC referenced that case when they railroaded through their p.e.p. power 
rule.

Don K4KYV

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