WA8LMF Mirror of WB4APR Website - 21 July 2008
MODE-H/V
EASY-SATS FOR EVERYONE
Bob Bruninga, WB4APR
115 Old Farm Ct
Glen Burnie, MD 21060
EASY-SATS: The great popularity of the FM satellites, AO-27, UO-14, SO50 and
ECHO shows how much interest and excitement can be generated amongst the
Amateur community if satellites are made easier to access using simple
equipment. Of course, single-channel FM
while popular and simple, is very difficult to use reliably due to the heavy
congestion and FM capture effect that only allows one user at a time to gain
access to the bird and always at the expense of others.
Although
we have had linear Amateur Satellite Transponders for more than 30 years that
can support numerous simultaneous conversations, their popularity has fallen
recently probably because of the significantly higher cost of all-mode radios,
antennas and tracking required compared to the FM “easy-sats”. What we need is an “easy-sat” that is
designed from the ground up to optimize the functionality of the satellite to
many users while also addressing the needs of low cost user equipment. Also, it should allow operation from
stations with only omni-directional whip antennas for ease of use.
BACKGROUND: Frequently, experienced satellite operators remember the first
satellites operating in MODE-A and offer this mode as the best solution to get
more low tech operators on the satellites using the linear modes. But the problem with MODE-A is the
requirement for the user to transmit an SSB signal on the 2 meter uplink and
use a separate HF receiver for reception on 10 meter downlink. This typically takes a $700 all mode
transceiver or a do-it-yourself transverter as well as an HF rig. In my mind, these are high cost and not
readily available items and not suitable for an entry level easy-sat
satellite mission. Also, the current
satellites have insufficient link budget to be worked reliably from the mobile
or omni antenna environment.
MODE
H/V: But if we analyze the overall requirements
and availability of low cost off-the-shelf equipment, and simply invert the
frequency bands of MODE-A to what I will call MODE-H/V there accrues a lot of
advantages:
10 meters UPLINK can use low cost 25 W Radio
Shack $149 transceivers
10 meters UPLINK has very low Doppler
compared to all other bands
10 meters has plenty of bandwidth
2 meters DOWN has a 9 dB link budget
advantage over 70cm for mobiles
2 meters DOWN has significantly less
Doppler than 70 cm
2 meters DOWN can be received by the same
$149 radio and a $49 converter
Of
these advantages, the LOW COST user station is the main factor involved from
the user requirements standpoint while the 9dB lower power budget is the most
significant factor involved in the satellite design. This 9 dB link advantage is what makes this satellite design
possible in the currently popular small microsat design package. Conserving power in the link budget by
wavelength selection allows for adding enough power in the transmitter to close
the link to a user with only an omnidirectional receive capability. No need for beams, tracking, or tracking
computers. Thus newcomers would find it
trivial to get onto the bird.
Another
way to conserve satellite power and the precious 2 meter satellite spectrum is
to keep the total width of the satellite transponder relatively narrow compared
to previous linear transponders. By
limiting bandwidth to only 30 KHz, it takes up not much more bandwidth than a
single FM channel yet it can support over 10 simultaneous 3 Khz voice
QSO's. This is actually a lot,
considering the amount of existing linear satellite usage which has fallen to
only one or two QSO’s per satellite per pass during non-prime hours.
COMMON
USER GROUNDSTATION: By designing for a low cost 10 meter
transceiver as the common uplink investment and for a link budget that will
work to an omni directional antenna, this design will strongly encourage a
relatively consistent baseline for all user ground stations. By having a common baseline, then the
performance of a linear transponder is much more consistent across all users
and is far more predictable in performance.
There are several factors that would tend to standardize on this common
consistent baseline.
·
The availability
of the $149 SSB transceivers all with 25 watt power
·
The
lack of availability of high power amplifiers for 10 meters
·
The
higher link budget on 10m can allow a shortened 10m RX antenna on the satellite
·
The
ease and success of using an OMNI vertical will make little sense for
users to develop large
high gain tracking antennas on the uplink.
By
having this common consistent baseline ground station that only uses 25W into
an omni directional antenna as a standard uplink configuration would
drastically level the playing field.
Not only are the transmitted powers quite consistent, but the
combination of the consistent transmit power and the antenna elevation pattern
of an omni-directional vertical combine to give an almost constant level at the
satellite receiver from all users. This
solves one of the main disadvantages of linear transponders when some
individuals greatly overpower the uplink at everyone else’s expense.
Although
there is a 10 dB variance between the satellite on the horizon compared to the
satellite overhead due to range between each user and the bird, this “range
gain” is almost perfectly offset by the vertical elevation pattern of a
vertical whip antenna which is 10 dB or more down in the same area where the
satellite is 10 dB closer. This self
regulating configuration is a great advantage as long as everyone uses their 25
watt transmitters and vertical whip antennas.
SATELLITE
DESIGN: One of the most critical factors in
satellite design is the transmitter power budget. Especially when you have a broadband transponder which must
support up to 10 simultaneous QSO’s.
This need to conserve satellite transmit power is what drives the
downlink to use 2 meters. This downlink
has a 9 dB advantage over 70cm, thus requiring only 11% of the power (to users
with omni receive antennas). One other
advantage of the linear transponder over an FM satellite is that the
transmitter uses little power when it has no users in the footprint. This can give a savings of almost 90% since
only 10% of the world’s surface has any appreciable HAM population.
Fortunately,
we think this MODE-H/V transponder can be built into an 5 inch or smaller type
satellite. Such a satellite can have an
average DC power budget of about 2 watts in low earth orbit and this is more
than adequate to provide usable downlinks across a 30 Khz bandwidth containing
as many as 10 simultaneous QSO’s during the 10% of the earths area where it
would see use.
Finally,
the need for a 10 meter dipole antenna on the small satellite suggests that the
satellite will inherently align itself to a gravity gradient attitude (if small
masses are placed at the ends) and thus will provide a worldwide vertical
polarization that is ideally matched with all the users who are also using
vertical whip antennas. There will be a
null beneath the satellite, but as noted before, this will help balance all
uplinks to a consistent level that is ideal for linear transponders. Further the satellite will never be in the
null of any one station for more than a minute or so per day.
It is
these subtle but significant differences between the existing MODE-A and this
MODE-H/V relative to satellite power and getting more people and newcomers
involved in satellites that makes this such a viable design opportunity.
1) SSB
uplink on 10m needs only a Radio Shack 10m transceiver ($149).
Mode A needs an all mode 2m rig ($700?).
2)
Using 2m as the SSB downlink requires only a 2m to 10m converter ($49).
Mode A requires both the 2m SSB transmitter
and 10m HF receiver ($139).
3) 2m
down is quieter and requires less satellite power. Mode A's weak
downlink must compete with higher atmospheric and man made noise.
DOPPLER:
Another argument can be made for MODE-V that minimizes the problems
encountered by newcomers in tuning for Doppler. Unless the current convention (which for mode-A is to tune the 2m
UPLINK) is used, then QSO’s tend to wander up and down the band by as much as 3
Khz during a pass and this wandering can be in either direction for each
different QSO geometry. The result is
that some QSO’s slide into each other and cause unnecessary QRM.
In
contrast, MODE-H/V places the uplink on 10 meters where the Doppler is 5 times
less. This establishes QSO’s to remain
relatively fixed in the passband relative to each other and allows a much
closer spacing between QSO’s without encouraging QRM. Thus we can get more users in the same bandwidth on MODE-H/V
compared to MODE-A in the situation where not everyone is properly following
the MODE-A convention.
Although
the MODE-H/V user still has to tune an overall Doppler that is the same as the
MODE-A user, he is tuning it on his receiver where it affects only his
reception, and does not affect the spacing of QSO’s in the passband. This has clear advantages on a satellite
designed for newcomers and minimizing bandwidth.
HALF-DUPLEX
OPERATIONS: Usually for the MODE-A linear birds, users
operate full duplex so that they can find themselves on the downlink and so
that they can tune their uplink on MODE-A to balance out the Doppler effect on
their signal. This Full-Duplex
significantly adds to the cost of the new-user station requirement. But with the minimum Doppler of MODE-V on
the uplink, half-duplex operations are quite possible. Thus the minimal newuser only needs a single
cheap 10m transceiver and a down converter instead of two radios.
The
minimum uplink Doppler, and the adjustment for Doppler on the downlink and the tendency to tune into a station by
tuning one’s own receiver leads to almost chanelized uplink operations. Users simply set their transmitters to one
of the 10 uplink channels and then all they need to do is tune their receiver
to the QSO in progress. There can still
be up to 600 Hz between different users in a roundtable discussion, but at
least half-duplex single-radio operation with a 2-station QSO is quite
possible.
AUTOMATIC
TRANSMIT/RECEIVE DOPPLER CORRECTION AND TUNING:
Because of the relatively low UPLINK Doppler and therefore operation in
sort of a fixed channelization on the uplink, it is very easy to implement
automatic Transmit/Receive tracking when tuning among the 10 or so simultaneous
QSO’s on the downlink. By adding a pot
to the oscillator crystal in the downconverter, the tracking relationship
between your transmit and receive signal can be adjusted in real time. Since the major component of the Doppler by
a factor of 5 to one is your own receiver, at any instant, your adjustment of
this pot for zero-beating a QSO will be the same for all QSO’s in the passband
(+/- their minimal 10m uplink Doppler).
So, you
tune your 10m rig from fixed channel to channel on the uplink, and the
downconverter offset tracks beautifully with you through the channels. You only need to adjust the donwconverter
offset to correct your receive signal for Doppler as it varies through the
pass. But still you can return to any
of the other uplink channels and your tuning offsets will still be correct
within the 600 Hz variability of the other QSO’s uplinks.
Thus
you never have to search for your donwlink, it is always the same fixed offset
from your transmit signal as the XTAL in your donwconverter. For any instantaneous Doppler observed at
your station, ALL channels are affected the same way, so you can QSY among all
the QSO’s on the bird with the same relative setting, only making minor
adjustments to the pot during the pass, no matter what “channel” you are on.
With
the 1 KHz step size of most cheap 10 meter radios, this “channelized” operation
with automatic transmit/receive tracking of the fixed offset downconverter is
actually a significant advantage. Using
this method, you will never be more than 600 Hz away from any QSO you tune
in. This is far simpler than trying to
use two separate rigs full duplex which would have to be separately retuned by
as much as 20 KHz every time you jumped from channel to channel.
DISATVANTAGES:
Although this new MODE-H/V has many advantages and benefits as a
newcomer’s satellite that is easy to work with simple equipment requiring no
antenna tracking and useable while mobile, it does have two minor
disadvantages:
·
First
is the requirement for a General Class license to transmit on 10m
·
Second
is the unknown worldwide nature of interlopers and unintended stations
operating on 10 meters in the satellite segment of the band.
Actually,
the first problem is really no problem at all, but can be considered as an
incentive to upgrade. The second
problem is also not considered significant, since we are only talking about a
single 30 KHz portion of the 10 m band and these days, the problem is just as bad
on 2 meters as on 10 meters.
COMPARISON
TO FM EASY-SATS: Although most of the excitement about the FM
Easy-sats is their direct accessibility from a Handheld HT transceiver, under
favorable conditions, most operators prefer a relatively large handheld beam
antenna to pick up many dB on both the up and downlinks. Thus, the weak downlink signals are not
considered that much of a disadvantage since the user’s radio easily fits into
one hand and the antenna in the other.
Thus, using a handheld ARROW antenna with an FM HT is a very easy form
of operating FM.
But,
this ease of operation and workability with a weak downlink using handheld beam
antennas with significant gain, does not carry over well to linear operation using
SSB with new users on 10m for several reasons.
·
Doppler
demands constant hands-on tuning of the receiver
·
Radios
are not handheld, but large and separate for transmit and receive
·
Uplink
power requires bulky power supplies.
But
conversely, the MODE-H/V permits mobile operation with only a vertical whip
antenna, which in many cases is far more comfortable, just sitting in the car,
out of the weather.
MODE-I:
By taking the search for the cheapest off-the-shelf USER Satellite
station to the extreme at the expense of satellite complexity, a 10 meter INBAND
transponder would provide the ultimate in simplicity for operation with the
$149 low-cost SSB transceiver. The
advantages are:
1) One radio and it is cheap.
2) With an inverting transponder, you get virtually
NO DOPPLER
3) With no Doppler you get instant QSY all
over the passband
4) On 10m you get maximum link budget for
minimum satellite power
BUT,
there are two problems to consider:
1) It would need to be a split-tethered
dual satellite to get a few
hundred feet separation between the XMTR
and the RCVR. But this
can be done and it provides gravity
gradient stabilization
for an easy vertical polarity to
mobiles.
2) You go up LSB and come down USB. Lets assume that someone will
figure out a single-wire mod to hook the
PTT to the SIDEBAND
switch to make it change between USB and
LSB on transmit.
But
imagine the popularity of this Satellite.
Anyone with $149 and a CB whip antenna can work it a few times a
day. Now that is an EASY-SAT. And a good satellite design challenge. Although operating in-band on 10m will have
to use shared spectrum, it is possible within the ITU regulations and this
mission would be an ideal use of the 10m band during the several year long
solar minimums.
CONCLUSION:
Searching for an optimum new-user easy-sat design has been a fun
exercise. There are so many variables
to balance both on the satellite, on the ground in the choice of user
equipment, modes, antennas and operating options, that even if no one builds
one, it is still a good design project for students looking for a satellite
mission. Although my usual interests
are in looking for low-cost access to satellites for digital applications (see
other paper in these proceedings), a strong case can be made that the ANALOG
modes of future amateur satellites might be more popular than the digital
ones. With today’s worldwide internet
connectivity and the ability to send bits and megabytes anywhere on the planet
almost, the appeal for digital Amateur Satellites might actually decline. In competition with the Internet, maybe the
only niche for HAM radio will remain the thrill of a live voice QSO using just
your simple radio wherever you may be…
Doing
it with a $149 radio instead of a $2000 computer is still fun to me…
De WB4APR,
Bob