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 keep 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 problems 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,
I'll 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, I’ll 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 multiply 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 filter 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 channel spacing of 50
kHz is required.
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