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FM Sat Channel Spaceing 15 vrs 25k



Satellite FM Channel Spacing.

By Miles, WF1F

There has been some talk on the AMSAT list regarding
FM channel spacing of 15k for multiple down links from
the same satellite.  There may be some frequency
overlap issues that need to be investigated before we
jump in and start designing multiple FM down links
from one bird.

In the near future, the ISS will be operating multiple
Amateur Radio projects simultaneously on a regular
basis.  There have already been some experiments with
Mir and ISS on using multiple simultaneous projects. 
With proper planning we should be able to support
multiple simultaneous projects and at the same time
kept the projects affordable to amateur radio
operators.  We want to avoid designing projects that
force everyone to upgrade their existing radios
stations.  And we need to be aware that not every
country follows the same radio band plans.

ISS Example:
There have been times when both the D700 packet and
the PCSat-2 packet systems have been running at the
same time.
D700 Packet on 145.800 TX, 145.990 RX
PCSat-2 Packet on 145.825 TX/RX and 435.275 TX/RX

In the near term most projects will be separated by
Bands to prevent interference with each other.  Each
project will be primarily used as a mono band project
(Uplink and Down link channels on the same band). 
Occasionally some cross band repeater type activities
may be activated.

Example:  (Just a suggest plan theory being considered
by ARISS)
10 Meter Mono-Band (29.300 – 29.500 MHz)  Slow Scan TV
in FM
2 Meter Mono-Band:  FM Voice  
70 cm Mono-Band (435.000 – 438.000 MHz): FM Data
(packet)

Benefits of Mono Band operations:
With Mono-Band operations you will only need 1 Antenna
for each project you choose to use.

Transmissions will be typically Half-Duplex mode,
which is the most common mode we use today.

On the ISS we always need to be concerned with the
Weight, Size and complexity of all projects.  With
Mono-Band projects we will not need Cavity filters for
most projects and thus will save in all three
categories.

Band Access:  In many countries, Amateur Radio
operators do not have access to the 70cm satellite
band.  Or the band is too full of pirates or
commercial traffic to make it useful. Running three
(3) Mono-Band projects will provide the greatest
access to Amateur Radio stations around the World. 
And at the same time, help keep the costs to the
Amateur Radio operators as low as possible.
 

Common Down link band:
There have been some suggestions to use multiple down
links simultaneously from ISS and or other low
orbiting satellites on 2-meter FM by using 15 kHz
channels spacing.  There are a few problem with this
suggestion.  The first problem is that FM (5k
deviation) 15 kHz channel spacing does not work on
Earth and will be even worse in space.

First some basics about FM bandwidth requirements and
receiver specifications, ill try to keep it simple.

FM Transmitters:
Carlson Rule for calculating FM Bandwidth.
FM Bandwidth = 2 x (Peak Deviation + Highest
Modulating Frequency)
Most Ham FM mobile rigs are setup for a Peak Deviation
between 4.1 – 5.0 kHz.
The Highest Modulating Freq is around 3 kHz.
To keep the Math simple, Ill use a Deviation of 5k and
Modulation of 3.  Your actual bandwidth will typically
be a bit bigger.
FM Bandwidth = 16 kHz = 2x (Deviation 5 + Modulation
3)

For a typical mobile amateur radio FM Transmitter, you
will need a minimum of 16 kHz of radio spectrum when
you transmit (plus more for harmonics and transmitter
noise).

Let’s assume you are transmitting on 145.800 FM
Your carrier plus voice or data will extend from
145.800 to 145.808 (all values are approximate).
That is ~8 kHz of RF spectrum is used on either side
of your center frequency.
There will also be some additional transmitter noise
and junk extending beyond 145.808.  For now lets just
focus on the main part of the spectrum that has the
important data.

 
Receivers:
All receivers have filters to pass the channel you
want, and block the channel you do not want.  However,
these filters do not work very well, when the
Signal/channel you are tying to block is TOO close.  A
channel separation of 15 kHz is too close for just
about all radios made today for the 2-meter band.

Here are the specifications for two Icom radios.

Mono-Band 2-meter only 2200H
Selectivity (wide/narrow)	
More than 6/3 kHz at -6dB
Less than 14/9 kHz at -60 dB

To make these values match the normal way they should
be published you have to multiple the frequencies by
two (2).  If you are just looking at the filter on one
side of the Primary transmit frequency use the numbers
above.   If you need to know the total size of the
filter look at the numbers below.
Converted More than 12/6 kHz at -6dB
Converted Less than 28/18 kHz at -60 dB

Icom 706MKIIG (Multi band)
Selectivity:
More than 12 kHz/-6 dB (published)
Less than 30 kHz/-60 dB (published)

These values need to be translated a bit.  Even from
the same company, the format values are completely
different and a bit deceiving.  As you can see, the
Mono-band radio has slightly better specifications
than the multiband radio.  These values are typical
for most Amateur Radio transceivers in the FM mode for
2-meters.  Note, as you go higher is Band frequency
the selectivity of the receiver typically gets worse.

So what do these numbers mean.
Lets assume you have two consecutive channels spaced
15 kHz apart, 145.800 and 145.815. You are listening
to 145.815 at the same time the satellite is
transmitting on 145.800. Assume the signal strength
coming from both channels is the same (7 S-Units).
 

Let’s looks at the first set of the receiver filter
values
More than 12 kHz/-6 dB 
(This is the full size of the filter, we will look at
just one side for this example)

This means that if our center frequency is on 145.815,
that any signal more than 6 kHz from you center
receive frequency will be attenuated (reduced) by 6 dB
or 1 S-unit. Any signal below 145.809 will start being
attenuated by at 6dB or 1 S-unit (or less, notice how
they fool you with the wording). 

The next value (Less than 30 kHz/-60 dB) means, that
any signal more than 15 kHz from your center receive
frequency will be attenuated (reduced) by 60 dB or 10
S-unit. In this example, any signal at or below
145.800 will be attenuated by at 60 dB or 10 S-units.
(Notice how they change the wording from “More than”
to “Less than”, confusing ain’t it.)
 

At first this filter may look great, but let’s dig a
little deeper. We will assume the attenuation curve is
almost linear from 145.809 through 145.800.

>From the center frequency of the receiver on 145.815
Frequency	Attenuation	S-Units
145.815	<1		N/A 
145.809	6		1
145.808	12		2
145.807	18		3
145.806	24		4
145.805	30		5
145.804	36		6
145.803	42		7
145.802	48		8
145.801	54		9
145.800	60		10		

If our adjacent transmitter is S7 on 145.800 and we
are listening on 145.815, then the part of the primary
transmit signal at 145.800 will be reduced by our
receiver by 60 dB or 10 S-units.  However, the signal
on the adjacent channel on 145.800 extends out to
approximately 145.808 and 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.  If we assume
there is a voice conversation going on at 145.800, we
will hear voice peak distortion bypassing the receiver
filter on 145.815.

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

This is why on Earth we should be using at least 20
kHz channel spacing FM transmitters.  In space the
problem would be even worse because we have to contend
with Doppler and Antenna Preamps.
 

Doppler and FM 15 kHz channels.

On the 2-meter band at the 250 mile altitude of ISS,
the Doppler shift is approximately Plus 3.3 kHz and
Minus 3.3 kHz.  With two or three FM transmitters on
ISS spaced at 15 kHz intervals the Doppler will be the
same for all three radios.  The Doppler signal drift
will cause even more problems for your Earth bound 5k
channel locked receiver.

Lets assume you are three (3) consecutive channels
spaced 15 kHz apart, 145.800, 145.815 and 145.830.
You are listening to 145.815 at the same time the
satellite is transmitting on 145.800.
Assume the signal strength coming from both channels
is the same (7 S-Units).
As the Satellite or ISS comes over the horizon and is
approaching your QTH, the Doppler for the first 3
minutes of the pass the Doppler will be approximately
Plus 3 kHz.

The transmit signal from ISS will be 145.800, plus
Doppler = 145.803 Center frequency.  The top edge of
the primary signal (Carrier and voice, etc.) will now
extend to approximately 145.811.  This puts 3 kHz of
the adjacent channel signal right inside our receiver
filer that will not be attenuated.  And a large
portion of the adjacent signal is so close that our
stock filter will be ineffective.

Your receiver on 145.815 will only start attenuating
adjacent channel interference from signals below
145.809.  After you add in Doppler, you can see that
the two consecutive channels (145.800 and 145.815)
will over lap.  The receiver on 145.815 will not be
able to block the signals from the adjacent channel.

After ISS passes your QTH the Doppler will shift in
the other direction.  The interference from 145.800
will be reduced and the interference from 145.830 will
increase (assuming 3 projects running simultaneously
using the down links at 15 kHz).

Note:  Most FM 2-meter radios under $1000.00 do not
have the ability to adjust for Doppler in less than 5
kHz channel steps.
 
Receiver Preamps:

When used properly, Receiver preamps improve your
radios ability to hear weak signal.  Preamps boost the
good signals and noise.  The closer you mount your
preamp to the antenna, the better it will perform. 
Mounting a receiver preamp in your shack will have the
least benefits.

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.

 
2-Meter 25 kHz FM Channel Spacing for Satellites:

As you have read from my previous analysis, 15 kHz
channel space will not work on earth or in space.  The
FM 20 kHz channels will have good results between
earth stations assuming some reason separation between
the two transmitters.  The best bang for the buck for
satellite operations of the FM mode is 25 kHz channel
spacing.

Assume 3 channels, 145.800, 145.825 and 145.850 on the
same orbiting satellite.
You are listening to 145.825 at the same time the
satellite is transmitting on 145.800.
Assume the signal strength coming from both channels
is the same (7 S-Units).
The ISS is approaching your QTH and for the first 3
minutes of the pass the Doppler will be approximately
Plus 3 kHz.

The transmit signal from ISS will be 145.800, plus
Doppler = 145.803 Center frequency.  The top edge of
the primary signal will extend to 145.811.

Your receiver on 145.825 will be able to attenuate the
145.811 signal by over 54 dB or 9 S-Units.  The
receiver’s stock filters will also be able to do a
good job at blocking the harmonics generated by the
adjacent channel transmitter.  Your ability to listen
to a specific channel will improve greatly when
satellites that use multiple down links use the
correct channel separation.

Receiver frequency listening on 145.825, trying to
block signals from 25 kHz away.
Frequency	Attenuation	S-Units
145.825	<1		N/A
145.819	6		1
145.818	12		2
145.817	18		3
145.816	24		4
145.815	30		5
145.814	36		6
145.813	42		7
145.812	48		8
145.811	54		9
145.810	60		10
145.808	>60		>10
145.800	>60		>10



In closing:
If you are planning on designing projects for FM
satellites or ISS, you will need to be aware of proper
channel separation and Doppler issues.   For multiple
FM down links on the same satellite on 2-meter, a
minimum spacing of 25 kHz is required.
For the 70cm band for multiple FM down links a
channels spacing of 50 kHz is required.
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