[ARC5] Lopsided modulation

[Richard Knoppow]

    If you can tune in the oscillator on a receiver see if you 
hear any modulation when you modulate the transmitter. A 
variation in AM sidebands comes from phase modulation so I am not 
sure you would see it on a waterfall display. I know this is 
covered in some older textbooks but I don't have a ready 
citation. My idea is that the modulation is causing a change in 
the B+ voltage which is, in turn, modulating the oscillator. 
However, there are other causes such as some effects of tuned 
circuits.
     I think its unlikely an antenna can cause the problem unless 
its a VLF antenna with extremely high Q. Even when AT&T used 
single sideband transmission to England in the late 1920s and 
early 1930s a filter was used to suppress the unwanted sidband. 
Fairly complete descriptions of this system exist in the Bell 
System Technical Journal, which is available free on the web.

    There is a little on incidental FM in the handbook at:
http://www.darc.de/fileadmin/_migrated/content_uploads/Guide_toSpectrum-and_SignalAnalysis__engl..pdf

     Also see
http://www2.ee.ufpe.br/codec/HP_AM_FM.pdf

Also
 
https://books.google.com/books?id=BgLyCAAAQBAJ&pg=PA177&lpg=PA177&dq=incidental+fm&source=bl&ots=U5pLyPPC9v&sig=p7yTUPEGxoCi2pRp3s_FOCi-oyY&hl=en&sa=X&ved=0ahUKEwiUrf7h2L_ZAhUr0YMKHbB8DN44ChDoAQgzMAI#v=onepage&q=incidental%20fm&f=false

    Lots of other stuff via Google.



On 2/24/2018 2:07 PM, AKLDGUY . wrote:
> FM does not seem to be present in either his signal or mine. The 
> carriers are straight lines on the waterfall display. Actually, 
> there is one operator on the net whose carrier is a fuzzy line 
> when he speaks, but his sidebands are symmetrical. I'm stumped.
> 
> Neil ZL1ANM
> 
> On 25/02/2018 10:30 AM, "Richard Knoppow" 
> <1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>> wrote:
> 
>          That affects the upward vs downward modulation but not
>     the strength of the sidebands. A frequent cause of
>     asymetrical sidebands is incidental FM. If the oscillator is
>     being affected by the modulation it can be the cause.
>          The scope display will not show assymetry of the upper
>     vs lower sidebands although it will certainly show assymetry
>     of upward vs downward modulation.
> 
>     On 2/24/2018 1:18 PM, Jim Wiley wrote:
> 
>         One reason could be that your voice in itself is
>         non-symmetrical.  If you can do so easily, try reversing
>         the "polarity" if the modulation transformer, and see if
>         that helps.  If it does, your can leave it set the way
>         that works best for you. Monitoring the modulation
>         envelope with an oscilloscope is always  the best way.
> 
>         - Jim, KL7CC
> 
> 
> 
>         On 2/24/2018 11:38 AM, AKLDGUY . wrote:
> 
>             A friend and I are running Command transmitters on
>             the local 75 meter AM net, with full plate and screen
>             modulation. We monitor our transmissions on the local
>             SDR receiver site which has a waterfall display. In
>             both cases, the lower sideband shows as considerably
>             stronger than the upper, and this has us perplexed.
>             Our stations are the only ones affected.
> 
>             I have never seen anything in the literature to
>             explain this. Downward modulation is a problem with
>             AM, but articles do not indicate whether lopsided
>             modulation results.
> 
>             My transmitter is the AN/ARC-5, absolutely unmodified
>             and running 40W input due to dynamotor constraint.
>             His is the SCR-274 modified for plate & screen
>             modulation and running twice that.
> 
>             Anyone suggest a reason for the unequal sidebands?
> 
>             Neil ZL1ANM
> 
> 
> 
>     -- 
>     Richard Knoppow
>     1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>
>     WB6KBL
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL