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Fw: Geosynchronous Satellites




----- Original Message -----
From: Jose Mihotel <jmihotek@gte.net>
To: Jon Ogden <na9d@mindspring.com>
Sent: Monday, December 25, 2000 11:57 AM
Subject: Re: [amsat-bb] Geosynchronous Satellites


> Jon
>
> Lets get a few things straight
>
> When you say FM, lets agree you mean class C.
> When you say cellular you mean class A
> When you say SSB or AM or FM  you mean same bandwidth.
>
> If you want to compare things , I can compare them with you and we can
talk
> about it, provided we are comparing apples and apples.
> I dont care what you sell or what your background is. I think you are very
> opinionated and very sly with your remarks and the result is that you
> confuse a lot of people that are not in the field of electronics and
> telecommunications.That is called alienation.
>
> Presently I design class A PAs for wideband  MMDS wireless-cable  around
the
> world. I know how innefficient class A PAs can be. ( Cellular uses class A
> power amplifiers) I'm not arguing that.
> But please, when you say FM lets agree we are talking about FM repeaters,
> and the like . (class C)
>
> Now you  tell me , how efficient is the PA in the local club FM repeater?
> Most likely a little better than  ~50%
> That is Power in = power out.
>
>  Say 100 Watts DC power in (12Vdc at 8.33Amps) and you get ~50 Watts of RF
> out into a Wattmeter.
>
> The point I'm making is that FM PAs are much better than 7% efficient.
>
> Another thing to set straight.
> Back in the days of  Mayor Armstrong, the inventor of FM radio, he proved
> mathematically that for narrow band FM,  if you make the occupied
bandwidth
> the same, FM will talk practically as far as AM. The noise power is the
same
> for both cases.
>
> Noise Power = 10 log (Bandwidth of your IF filter)
>
> And you can make your occupied bandwidth anything you want. In AM you
limit
> your frequency response (say 300 to 3000Hz like a telephone channel) and
you
> adjust your modulation depth. In FM, you adjust your deviation.
>  You can get an aproximation of the bandwith required for transmission (or
> reception) of a FM carrier modulated by a complex (voice, multitone)
signal
> by:
>
> Bandwith = 2(delta F + 2fm)
>
> where,
>
> delta F = peak deviation for the system
> fm = highest baseband frequency.
>
> So there, you can make your FM occupied bandwith 2 KHz if you like.
>
> The main point here I think is that for LEO sats. at VHF or UHF either
> system will talk just as far,  but for  geosycchronous satellites, AM or
SSB
> systems have an edge.
>
> As for the reason EME and other modes don't use FM is because by nature FM
> detectors (discriminators, ratio detectors, phase locked loops,etc...)
need
> better than 8dB carrier to noise ratios to provide the signal to noise
> improvement that you get from FM de-emphasis. With C/Ns lower than that
the
> FM detector falls appart very quickly. That is to say for 10 dB C / N you
> get 10 dB S / N but for 5 dB Carrier / Noise you  only get say 1 dB Signal
> / Noise. So in situations where your signal might be burried in the noise,
> say 1 dB carrier to noise, an FM detector does not work.
>
> AM / SSB detectors, (diode, synchronousdetector, product detector) on the
> other hand, don't have that problem. They hold their own all the way down
> into the mud.
>
> So the moral of the story is to be fair and compare Apples and Apples.
>
> Merry Chritmass !!!
>
> KF4VND
>
>
>
>
>
> ----- Original Message -----
> From: Jon Ogden <na9d@mindspring.com>
> To: Jose Mihotel <jmihotek@gte.net>
> Cc: <amsat-bb@amsat.org>
> Sent: Sunday, December 24, 2000 5:13 PM
> Subject: Re: [amsat-bb] Geosynchronous Satellites
>
>
> > on 12/24/00 10:12 AM, Jose Mihotel at jmihotek@gte.net wrote:
> >
> > > your statements about PAs that have 7% efficiency is plain not true.
> Another
> > > half truth you stated is the one about that FM occupies INHERENTLY
more
> > > bandwidth than SSB or AM. Just not true.
> >
> > Excuse me, Jose, but they are.  I used to design PAs for Motorola as
well
> > and now I sell them for another major amplifier company.
> >
> > Most of the amplifiers produced and used in the cellular industry today
> are
> > Feed Forward Multi-Carrier LINEAR amplifiers.  These amplifiers are VERY
> > linear, have a capability of handling peak to average ratios of 10 or
> higher
> > and as a result are VERY inefficient.  If you doubt me, look up the
specs
> of
> > amplifiers from companies like Powerwave or Spectrian and see how much
> > current they draw at 27 Volts and see how much power they rate their
> modules
> > at.  Then you'll see I AM correct!
> >
> > And FM DOES have more bandwidth than SSB or AM!  Why don't they then use
> FM
> > down in the MW or Shortwave broadcast bands?  After all, the fidelity
> would
> > be so much better!
> >
> > I can listen to an SSB conversation on HF with a 2 KHz filter.  Put two
> KHz
> > guard band on either side and you basically have 6 KHz total bandwidth
for
> > an SSB channel while well over 25 KHz for an FM channel (yes, there is
an
> > FCC move to make manufacturers develop FM radios with 6.25 KHz
channels).
> > You can have narrow bandwidth FM, sure.  But I am speaking in the
context
> of
> > a TYPICAL ham radio setup.
> >
> > > It's because AM (as well as SSB) allows you to hear everybody at the
> same
> > > time. And it has the same efficiency as FM.
> >
> > Not true.  AM has less efficiency than FM voice in a single channel.  FM
> > voice can utilize saturated power amplifiers.  AM requires linear
> > amplification.
> >
> >
> > > I gues you know all these facts anyway, and about PA's efficiencies
they
> > > reach tipically 50% and if you optimize them, they can reach close to
~
> 70%.
> > > I know, I used to design them at Motorola.
> >
> > No kidding!  Gee, I didn't know that you could get a 50% efficient PA.
I
> > guess the one I built for HF is 50% efficient just by accident!
> >
> > Oh stupid me.  I guess I don't know jack!  It doesn't matter that I used
> to
> > build, test and install linear amplifiers and it doesn't matter that I
now
> > sell them.  I guess I an everyone else in my industry are just plain
> wrong.
> >
> > Jon
> >
> > -------------------------------------
> > Jon Ogden
> > NA9D (ex: KE9NA)
> >
> > Member:  ARRL, AMSAT, DXCC, NRA
> >
> > http://www.qsl.net/ke9na
> >
> > "A life lived in fear is a life half lived."
> >
>

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