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Re: FM Sat Channel Spaceing 15 vrs 25k



I'd like to offer some counter arguments to opinions in the
original email on this topic:

>>> MM <ka1rrw@yahoo.com> 06/01/06 1:40 PM >>>
>There has been some talk on the AMSAT list regarding
>FM channel spacing of 15k for multiple down links from
>the same satellite.  ...
>In the near future, the ISS will be operating multiple
>Amateur Radio projects simultaneously on a regular
>basis....   Each project will be primarily used as a mono 
>band project (Uplink and Down link channels on the same 
>band).

There are severe shortcomings to that approach, since each
such operation  then excludes all other uses of the same
band.  To activate a second system on VHF for example, 
then requires the crew to re-configure to shut the first one 
down.  And not only do we not have routine access to the
crew, anytime one has to "reconfigure" one system to 
allow another one to operate has all kinds of potential
for errors.  ALso, monoband operation completely excludes any
kind of repeater.  If you activate a repeater, then TWO
systems/bands have to be shut down, not just one.

It is far better to put ALL downlinks on one band so that many
multiple systems can each/all be on/off independently without 
any impact on each other and also so that the other bands can 
be used  exclusively for uplinks without any impact from any other 
system.

>Example:  
>10 Meter Mono-Band: (29.300...)  Slow Scan TVin FM
> 2 Meter Mono-Band:  FM Voice  
>70 cm Mono-Band: (435.000...: FM Data (packet)

What about school contacts?  FM repeater?
PSK-31=>FM transponder?  Linear Transponder?

The monoband approach further  guarantees that *only 1 system*
at a time can be downlinking on 2 meters which is the only band
that can be reliably worked horizon to horizon on an omni.

Further it requires the casual operator to have  both a 2m,
70CM and HF rig to simply monitor what is going on, and
even more receivers if more modes are on more bands...
again, just to monitor!  This is a very poor approach for
an "outreach" system like on ISS.

Further it implies there can only be 3 simultaneous systems 
without having to always turn something off in order to turn 
something else ON.  One thing that is really disadvantageous to
amateur radio outreach is not being able to have consistent 
modes that people can count on.  Plan on giving a PACKET demo, 
and it comes over in voice.  Plan on voice, and it is on packet.  
Plan on the PSK-31 transponder, it is off.   Plan on ANY demo
and it is off because during mode-shifting, a mistake was made!
This is furstrating...

>Benefits of Mono Band operations:
>we will not need Cavity filters.

Neither does operating cross band with lots of simultaneous
donwlinks all on 2m for the casual listener.

>Band Access:  In many countries, Amateur Radio
>operators do not have access to the 70cm satellite
>band.  Or [some bands are] too full of pirates or
>commercial traffic to make it useful.

This is an impact to both monoband and crossband.  But with 
crossband and all downlinks on  2m then OUR downlinks become 
the interference to the pirates, not the reverse...  Let them
move.  Not us.

>Common Down link band:
>15 kHz channel spacing does not work on
>Earth and will be even worse in space.

Not true.  It is much worse on Earth because one
repeater may be 5 miles from a receiver and cause
splatter when that station is trying to listen to a different
far distant repeater only 15KHz away.    BUT, when
both 15 KHz split repeaters are far-away and one is
not 60 dB stronger than the other, then 15 KHz works
just fine.  And from space, they are both from 500
to 1500 miles away all the time. But please separate
any objections to 15 KHz operation from the concept
of cross-band operation.  They are independent 
issues.

Receivers:
>A channel separation of 15 kHz is too close for just
>about all radios made today for the 2-meter band.

Only if there is an adjacent channel transmitter that
is 60 dB closer and stronger, but not if both signals
are orders of magnitude weaker as from space.

>From the center frequency of the receiver on 145.815
>Freq      Attn    S-Units
>145.815  <1    N/A 
>145.809     6      1
>145.808   12      2
>145.807   18      3
>145.800   60    10		

>[at] 145.808 ... this portion of the signal will only be reduced 
>by 12 dB or 1 S-Units.  Which leaves ~5 S-units in this example 
>still sitting in the pass band of our receiver on 145.815.  

But you assume all the TX power from the 145.800 signal was on 
145.808 which it isnt.  In fact the modulation sidebands from the 
145.800 signal are quite a bit down from the carrier.  Add this
rolloff (say 10 dB) and the actual energy in the 145.815 receiver is 
then down over 22 dB and since FM has the "capture" effect, then
 this trivial amount of energy is completely swamped by the desired 
signal on 145.815.

FURTHER:  Any builder of a 2m transmitter for space downlink
will not use 4 KHz deviation as you did in your examples.  He will 
reduce it to 3 KHz deviation so that all of the trnsmitted energy 
DOES remain in the passband of a typical 15 KHz receiver used
on the ground without tuning for Doppler.

>In short, the Transmitter of one radio will overlap the receiver 
>of the second radio when using FM 15 kHz channel spacing.

That conclusion is incorrect because it assumes all of the
adjacent channel energy is on 145.808 instead of only 
the tapered sidebands.  And it assumes 4 KHz deviation
and not 3 KHz as is used for space downlinks.

>This is why on Earth we should be using at least 20
>kHz channel spacing FM transmitters. 

No, on Earth the reason is because *one* of the two 15 KHz 
signals may be a lot closer to the receiver and hence maybe 20 
to 60dB stronger.  Yes, then splatter is a problem.

>In space the problem would be even worse because we 
>have to contend with Doppler and Antenna Preamps.
 
Neither apply. 1)  THe Doppler does not apply as much as you
imply becaues both signals have the same Doppler (being from 
the same spacecraft), so the two signal remain the same 
separation and same characteristics as without Doppler.  and
still being equal in magnitude (both coming from 1000 miles
away) the intended one with FM capture will be the only
one heard in a receiver (even though it is 3 KHz off).  And
again, space downlinks reduce their deviation just for this
purpose.

2) ANtenna preamps mostly cause intermod and increase
"splatter" on the Earth, because of *overload* of closer
stronger signals.  SInce both signals are coming from 500-
1000 miles away and are both weak, there is no front-end 
overload to contend with.

>Doppler and FM 15 kHz channels.

I do agree that we should investigate the typical receiver
response when listening to two EQUAL and weak signals
and being off-tuned by 3 KHz, but this is -totally- different
from the terrestrial experience of 15 KHz cross channel
interference on which the above arguments were based.

And if 15 KHz will not work then so be it.  But this is independent 
of any arguments about having all ISS downlinks on 2m so
that the maximum number of HAMs can *hear* them without
any gain antennas.  Having all ISS outputs on 2m has a 9 dB
signal advantage to all stations listening with mobiles and 
HT's and omni antennas.  We should not overlook that single
BIG advantage for this outreach mission.

>Receiver Preamps:
>The down side to receiver preamps is that they reduce
>the Selectivity of your receiver.  Your receiver will be affected 
>even more by adjacent channel  interference when you use 
>an external preamp.

That is true, but only if the preamp is driven into overload
and nonlinear operaton where it generates splatter.  This only
occurs on *strong* signals and there is no such thing as that
*strong* a signal coming from space.  Again, we need to
be careful about translating terrestrial issues to space issues.

>As you have read from my previous analysis, 15 kHz
>channel space will not work on earth or in space. 

Interesting, considering that there are many, many dozens 
of FM voice repeaters in the Baltimore Washington DC
area that are ALL operated on 15 KHz splits.  They all
work very well.   As noted before, when listening to
a distant repeater, a closer repeater only 15 KHz away
that is 30 to 60 dB stronger will cause splattter, but
again, two 5W signals coming from ISS will both be weak, 
will be using 3 KHz deviation, and will not splatter, and will 
not overload the receiver preamp.

> The best bang for the buck for satellite operations of the 
>FM mode is 25 kHz channel spacing.

Not true.  AMSAT has been operating on 20 KHz channel
spacing for dozens of years without any problems.
If there has been any adjacent channel "splatter" observed
it is from two *separate* satellites, one coming and the
other going so that one is 3 KHz high and the other 3 KHz
low.  Again, this is exceedingly rare and is simply not the 
case for the two 15 KHz signals coming from the same 
space platform.

>In closing:
> For multiple FM down links on the same satellite on 2-meters, 
>a minimum spacing of 25 kHz is required.

With only 200 KHz in the 2m band, we can not afford 25 KHz 
spacing.  20 KHz has worked fine for all existing satellites and
there is potential for investigating 15 KHz spacing if we want
to see ISS come over with ALL of our favorite modes hearable
on a 2m mobile whip antennas at the same time.

HOWEVER, I will grant you that there is probably a good 
not-yet-mentioned reason why 15 KHz channels might not be 
a good choice even though technically possible, and that is for the
muilti-satellite-constellation concept.

That is, I would like to see a standard channel system for
these primary modes to be supported by multiple
satellites.  So the APRS channel is always 145.825, the
FM repeater downlink is always XXXX, the SSTV downlink
is always YYYYY and the PSK-31 FM downlik is always
ZZZZZ no matter what satellite it is on.  Since 2 meters
can easily provide strong signals from horizon to horizon
to any mobile with just its own mobile whip antenna, we
dont need "tracking" to know where to point antennas,
and so we just monitor our favorite mode channel and
use it if we hear it.

In this situation of multi-satellites supporting multiple
standard channels, then 15 KHz channels would not be
a good choice because of the combined efffect of opposite 
Dopplers when two are in view at the same time..

So my conclusion is that 15 KHz channels from one 
platform could work, but should probably not be persued
if our goal is multiple satellite constellations supporting
common dedicated channels on 2m downlinks.  And this
only works on 2m because it is the only band that 
everyone can hear and that everyone can hear with
a 9 dB advantage on their mobile whip.

I will be happy to revise my proposal for all-2m-downlinks from
the 15 KHz special spacing for one multi-channel satellite to
to 20 KHz so that it can operate in conjunction with lots
of multiple-satellite-common-mode operations.

thanks
de WB4APR, Bob
----
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