COMMENTS ABOUT CHOICE OF VHF and UHF UPLINKS AND DONWLINKS de WB4APR ---------------------------------------------------------- --------- At the AIAA/USU Small Satellite conference I observed a many student satellites (cubesats) that were going to use 2m uplinks and 70cm downlinks (mode J) in their micro-powered designs. When I asked one of them why, the response was because that is the way the others did it (pacsats) etc... This caused me to make this posting to the AMSAT Bulletin Board and to collect responses to some of the issues I have looked at when choosing VHF or UHF for your uplink or downlinks: + UP/DNLINKS on VHF are 9 dB better to OMNI antennas than on UHF - In some areas there is a higher receive noise level on VHF in urban areas. - In some areas there is lots of illegal interferrence on VHF uplinks - In some areas there is radar interference to digital uplinks on UHF + Gain tracking antennas (if needed) are 3 times smaller on UHF than VHF + VHF in most cases does not require Doppler tuning - UHF in most cases always requires Doppler tuning + UHF is uplink band for big AO-40 so most experienced hams will have UHF uplinks + More hams have VHF capability than UHF - On the Satelite, UHF uplinks are vulnerable to 3rd harmonic of VHF downlinks - On the ground, UHF downlinks are vulnerable to 3rd harmonic of VHF uplinks So your best choice of uplink and downlink band depends heavily on whether you will use VOICE or Digital, Half or Full duplex, and whether your intended users will be using omnidirectional receiver or transmitting antennas or beams. The remainder of this file is several comments from people on the AMSAT Bulletin board in response to my postings on the subject. de WB4APR, Bob -------------------------------------------------------------------------------- EMAIL #1 From: Bob Bruninga WB4APR Subj: Mode J vs Mode B Mode J was a legacy of the pacsats where the lack of uplink Doppler was an advantage for the early digital operations and where full-duplex downlinks avoided any problems with 3rd harmonic desense. But other than those issues, Mode J might be a poor idea as it results in almost a 23 dB difference between the uplink and downlink power budgets using omni antennas on the ground. This number comes from the fact that the 2m link is 9 dB better (to an omni) and the ground user running 50W has 14 dB more power than a typical satellite 1W transmitter. In the case of trying to close the link to handhelds and mobiles, mode J is not a good idea. This is not to say, however, that mode J may be a good idea if your satellite transmitter power is so low in the first place as to require beam antennas on the ground anyway. Then it will be easier to track with a UHF beam instead of a 2m beam... But with the popularity of handheld Satellite operations and the built-in packet TNC's of the Kenwood HT and Mobile, it might be much better to design with a 2m downlink if you want everyone to hear your bird... The downlink will be about 9dB stronger to an HT for the same satellite power. Although this puts a bigger burden on the uplink, the 5W HT usually has more power than the bird and can afford it. An additional advantage of the 2m downlink is that it may in some small way, alert the non-amateur interlopers that the 145.8 to 145.99 MHz frequencies ARE IN USE. With enough 2m downlinks maybe they will go elsewhere... Now that the 2m uplink on UO-40 may be never used, this does give an opportunity to take a second look at the 2m satellite band,... Of course, a mitigating factor is the higher observed noise floor in urban areas in the 2m band making it harder to hear such a 2m downlink... Just thinking out-loud... de WB4APR@amsat.org, Bob ------------------------------------------------------------------------ Email #2: From: Bob Bruninga Subject: [amsat-bb] 2m Satellite Uplink QRM! VHF UPLINK Lessons Relearned Wednesday at the AIAA/USU Small Satellite Conference, we relearned a very important lesson in Amateur Satellite Operations. Mark, N4TPI demonstrated an FM Satellite transponder which used an unpublished and unannounced uplink in the 2m Amateur Satellite band. There were only 2 other stations involved in the test, and so the transponder was 100% devoid of any QRM from other radio amateurs. But, it took an Arrow antenna to get in much of the time. A 5W HT with rubber duck was never heard. What was heard was constant spanish? speaking signals and/or typical FM doubling QRM from those same stations whenever one of the high power BASE stations was not talking. The uplink channel was full of non-amateur QRM. This was devistating news to me. The link budget for a 2m FM uplink shows that a 5W HT should have no problem getting into an amateur LEO satellite.. This test proved what most operators have always known, that sometimes you can, and mosttimes you can't. And the reason is because of INTERLOPERS in the western Hemisphere on the 145.8 to 145.99 satellite uplink band! This is terrible prrof that we must somehow take on the political resolve to preserve our most valuable satellite band. Only on 2m can we easily close the link to amateur satellites with handhelds and whip antennas. Since it is technically feasable, it is frustrating to be denied such access due to the lack of political wherewithall. If we let this band get ruined by foreign interesets, then we have lost. de WB4APR@amsat.org, Bob ------------------------------------------------------------------------- EMAIL #3: Date: Fri, 17 Aug 2001 11:02:58 -0400 (EDT) From: Bob Bruninga Subject: Re: [amsat-bb] Handheld Radios for Satellites: > > I would like to know which of the many dual band handhelds > > has the best feature set for FM satellite operation.... Actually, after returning from the Small Sat Conference in Utah yesterday, I should note that there are 18 CUBESATS to be launched next May. When I asked which of them will have Amateur Radio, 18 hands shot up Plus or minus a few. Most will have AX.25 for telemetry and command and control. Jan King was also at the conference and we made a good case for these teams to get in contact with AMSAT mentors to help them resolve any frequency and operational issues. Here is my list of AX.25 Satellites that are usable for Packet-Digipeating: When Satellite Downlink Uplink VIA BAUD notes ----- --------- -------- ------- ------ ---- ------------------------ NOW ISS 145.800 145.975 NOCALL 1200 See ww,ariss.net NOW OPAL 437.100 437.100 KF6??? 9600 NOW UO-22 435.120 145.900 UOSAT5 9600 See my ASTARS page below Sept PCsat 145.825 145.825 APRSAT 1200 for HT's only Sept PCsat 145.825 435.250 APRSAT 9600 for mobiles only Sept Sapphire 437.100 437.100 TBD 1200 May a dozen more..... Sunsat was a GREAT AX.25 bird, but died earlier this year... de WB4APR@amsat.org, Bob ----------------------------------------------------------------------- EMAIL #4: Date: Fri, 17 Aug 2001 18:55:10 -0600 From: Jim White Subject: Re: [amsat-bb] 2m Satellite Uplink QRM! Bob, One small clarification and some thoughts from one of the 'high power' ground stations Mark was talking to during the AIAA demonstration. I heard him just fine until the footprint included the area starting at about the middle of Mexico. This is consistent with previous tests we have done. Also, I believe I was hearing him fine on just the rubber duck until the QRM was in the footprint, then he was covered unless he used more EIRP. There is no significant QRM from non-satellite users in the US or Canada that I've been able to detect. It is perhaps more likely that high power uplinks to the proliferation of cubesats would be heard by AO-40 than these land mobile comms. Of course this will be mitigated by the small amount of time cubesat ground stations will be pointed in the direction of AO-40. (This assumes future cubesats spread across the band. With dozens planned this seems possible if not likely). All off which reenforces my view that 2M is over subscribed and other bands need to be considered. If you chart the 2M frequencies used by all birds present and planned every single 'channel' on 2M is used, many are shared by several birds. jim@coloradosatellite.com --------------------------------------------------------------------------- EMAIL #6 Date: Fri, 17 Aug 2001 14:34:27 -0400 (EDT) From: csuprin@lynx.neu.edu Subject: Re: [amsat-bb] Mode J and B on Small Satellites Bob, My understanding is that mode J came about after mode B. In some places in the world (Japan?), the interference environment was such that mode B was impossible. If this scenario is till around, we should either have cubes that are scheduled according to where they are in the world. or go with what works everywhere. Charles ---------------------------------------------------------------------------- EMAIL #7: From: Graham Shirville Subject: Re: [amsat-bb] 2m Satellite Uplink QRM! Hi Bob, Don't worry - the Western Hemisphere is not alone! A couple of years back I checked 2 metres in the South China Sea and surrounding area and 145.8 -146 is full wall to wall every 5, 10,12.5,15,20,25 kHz with FM signals - all non amateur of course. There are tens of thousands of rigs being used out there.... At least one of the Surreysats has done some monitoring world wide around 145Mhz and I have a strong memory of large areas of very red colour on the globe that its data produced! Graham G3VZV (in The UK) ------------------------------------------------------------------------- EMAIL #8: Date: Mon, 20 Aug 2001 09:39:21 -0700 (PDT) From: Jon Neher Subject: Re: [amsat-bb] 2m Satellite Uplink QRM! You were hearing the Mexico City Taxi company. Really! There are quite a few "utilities" in Mexico you will hear on the uplink (and hence downlink) on UO-14 and possibly others. Jon ----------------------------------------------------------------------- EMAIL#9: Date: Sun, 19 Aug 2001 20:22:56 +0100 From: Ib Christoffersen Subject: Re: [amsat-bb] Mode J and B on Small Satellites Hi Bob, According to DB2OS, Peter, UHF radar interference is a problem for digital communications - not for SSB and FM. The Pave Paws radar systems on UHF obviously are in operation still. 73 OZ1MY/Ib ------------------------------------------------------------------------ EMAIL #10 Date: Sun, 19 Aug 2001 14:51:59 -0400 (EDT) From: Bob Bruninga Subject: Re: [amsat-bb] TH-D7A(G) & UO-22 Full Duplex Operation On Sun, 19 Aug 2001, Ken Patterson wrote about using the D7 HT on UO-22: > My TNC configuration string is pretty simple. > > MYCALL: N5EQT-3 > HBAUD: 9600 > UNPROTO: APRSAT via UOSAT5 > > I used an uplink frequency of 145.900 & a downlink frequency of 435.120 And although everyone on AMSAT will recognise the need to tune the downlink for DOppler, I will just remind new users to start the pass on 435.130, then tune down to 435.125 and on down to end at 435.110 at the end of the pass... It is very easy to do, and you can HEAR when you are tuned because the UO22 has a constant 9600 baud donwlink all the time. I might point out that this becomes nearly impossible with a Mode-J downlink that is "bursty" like most of the CUBESATS and other low power satelites. When you hear a burst of 9600 baud noise that lasts only 0.2 seoncds once a minute, I dont think ANY ear can tell if it is centered. But with UO22 or any of the 100% full duplex downlinks, it is easy.. Humhh... this is another reason why CUBESAT low duty cycle designs should consider using mode B instead of mode J. Of course, the blind doppler problem then shifts to the uplink, but still, "you cant work em if you cant hear em".. just a thought... de WB4APR, Bob --------------------------------------------------------------------------- EMAIL #11 From: Gary "Joe" Mayfield Subject: Re: [amsat-bb] Mode J and B on Small Satellites I for one would vote for mode B FM (yes, I have changed my position). As I think everyone will agree mode B provides a superior downlink over anything else we have used. I think we are much better off when users (new or otherwise) can hear the downlink easily. If you can hear the downlink well, but can't hear yourself you will realize you need to improve your uplink. On the other hand we all know what happens when people uplink to a bird they can not hear! ... I have been asking some questions of my ham friends about satellites:... * Over half of them are NOT on the internet. * Those who were at all interested in satellite operation all agree that what should be built.was "another AO-13". * Over 85% have no interest in "digital modes" [and most arent on the Internet] My conclusions? The AO-40 CONCEPT was sound. Build a bird with mode B hooks to get them started, and let it have some higher configuration to strive for. Joe ka0yos@amsat.org ------------------------------------------------------------------------------- EMAIL #12 Date: Sat, 18 Aug 2001 14:07:46 -0600 From: "Vince Fiscus, KB7ADL" Subject: Re: [amsat-bb] 2m Satellite Uplink QRM! At 06:55 PM 8/17/01 -0600, Jim White wrote: >I might also note that I suspect interference to the AO-40 uplink on 2M >from this type of operation is unlikely to be an issue. If you listen to >the FO birds over the US and south you will hear little through the >transponder from this activity even though the FM birds hear it and repeat >it well. Nah, I hear a lot of terrestrial FM activity in the fo-20, fo-29 uplink passband. I can live with the ISS, & UO-14 activity since it appears at the lower edge of the downlink band on these birds. But just last week I heard a couple US ops having their qso around 145.950 on FO-29. I almost caught their callsigns but didn't quite make it. >>>>>>AT YOUR CLUB MEETINGS EVERYONE AND ON YOUR VHF/UHF RADIO NETS: Please inform(educate) amateurs in your area not to use satellite passbands for terrestrial communications. {145.8 ~ 146.0 mHz}<<<<<<<< KB7ADL ----------------------------------------------------------------------------- EMAIL #13 From: Trevor M5AKA Subject: Re: [amsat-bb] 2m Satellite Uplink QRM! The reality is that countries in Central and South America have a tiny number of hams in relation to their populations and the 2 metre and 70cm bands have little ham activity. The problem is more acute on 2 metres since these freqs offer greater range which is what those interests want and a good case could be made for having more strong downlinks on 2m (intruders only use clear frequencies). Ultimately the only way to protect the frequencies is by getting more hams in these countries. Strong ham organisations are needed for effective political lobbying. Trevor M5AKA -------------------------------------------------------------------------- EMAIL #14 From: Al Lawler Subject: Re: [amsat-bb] Mode J and B on Small Satellites >Mode B is also in my opinion vastly superior to Mode J for LEO, One other consideration in favor of Mode-B is that AO-40 also has a 70cm uplink. IMHO, the amount of blown Mode-S converters will be directly proportional to the number of mode-J satellites that are launched... :^) Keeping 70cm as the general purpose uplink band for both types of communications would not only cut back on the number of "accidents", but also carries the secondary benifit of allowing folks to invest ONCE in a high-power general purpose uplink capability, freeing up more funds for improving their recieve capabilities on the various bands. It will be far easier to sell "microwave" satellites (and 2m downlink satellites) in the future if the only investment is another dish feed and down-converter, (and a small yagi for a 2m leo downlink) while preserving the 70cm uplink investment. WB1BQE --------------------------------------------------------------------------- EMAIL #15: From: "John Stephensen, KD6OZH" Subject: Re: [amsat-bb] Mode J and B on Small Satellites I remember reading something about the choice of a 2 meter uplinks many years ago. There was a problem with 70 cm military radars desensing uplink receivers on LEOs. John, KD6OZH -------------------------------------------------------------------------- ========================================================================== DISCLAIMER! All of these are opinions and may or may not have any real basis for consideration in your design process.