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