Getting Started on the Amateur Radio Satellites (Part V) for Beginners
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For Beginners Getting Started on the Amateur Radio Satellites (Part V) by Keith Baker, KB1SF/VA3KSF, [email protected] (The bulk of this article was previously published as “How To Use Linear Amateur Satellite Transponders” in the November 2010 issue of Monitoring Times, Brasstown, NC 28902) n previous columns, I’ve been discussing operator’s uplink signal frequency is on satellites currently in orbit (including ways to find, track, listen for and then the high end of the uplink passband, their FO-29, VO-52 and HO-68) use inverting communicate through our expanding downlink signal will also be in the high end transponders. The one exception is our old Ifleet of Amateur Radio satellites. In this of the downlink passband. AO-7 satellite that was launched back in installment, I’ll be introducing you to another Conversely, in an inverting transponder, 1974 and which is still “sort of” operational (non-FM) type of satellite transponder called when an operator’s uplink frequency is on (that is, it operates only when it is in a “linear” transponder that is carried aboard the high end of the uplink passband, it will sunlight). It uses a non-inverting, linear many of our current amateur satellites. I’ll become “inverted” (hence the name) and transponder. also discuss how our OSCAR satellites get come out on the lower end of the downlink As with the FM birds, common operating their names on orbit. passband. Put another way, inverting practice on amateur satellites with linear You’ll remember from previous columns transponders make mirror images of the transponders is to first listen for your own that a transponder is the circuit in a satellite signals they pass. signal on the downlink. You’ll remember that receives an uplink signal and then This holds true for the sideband sense as from my previous columns that working retransmits what it hears via its downlink well. In a non-inverting transponder, the through a satellite transponder is a full duplex transmitter … much like your local FM signals an operator sends up to the satellite operation, much like talking on a telephone. repeater does. However, unlike your local (USB or LSB) will come out the same This means that others can usually hear you FM repeater (and most of our FM satellites way on the downlink. However, in an as well as you are hearing yourself. which have specific input and output inverting transponder, a USB uplink will Finding your own signal in the downlink frequencies) most of our so-called “linear” be “inverted” and come out as LSB on the passband of a satellite with a linear satellite transponders (sometimes also downlink. Conversely, a LSB uplink will transponder the first few times can be called “analog” transponders … the terms be “inverted” and come out as USB on tricky. However, I’ve found that placing your are often used interchangeably) receive and the downlink. The latter approach (USB transmit frequency somewhere in the middle then retransmit a whole band of frequencies signals on the downlink) is also what’s of transponder’s passband and then sending a commonly called a passband. most preferred by operators using our linear few “dits” of CW while tuning your receiver What’s more, linear amateur satellite satellite transponders today. Fortunately, CW to find your own downlink signal works best. transponders come in one of two “flavors”. will be CW regardless of the transponder’s Once you’ve located your own signal, you’re These transponders are usually classed as variety! ready to try making a contact. inverting or non-inverting. If the satellite Note that most linear-transponder-equipped However, unlike the FM birds, calling has a non-inverting transponder, when an CQ on these satellites is acceptable and you’ll usually find the convention of CW operations in the low end of the passband with phone operation in the upper part of the passband (an arrangement common to High Frequency (HF) Amateur Radio operation) generally also holds true for satellite work as well. As I have also noted in previous columns, since a satellite is a moving target, its downlink signals will exhibit a pronounced Doppler shift, just like the changing pitch of a train whistle as it approaches and then passes. During a satellite QSO, the “old” (that is, prior to computer frequency control) so-called “One True Rule” of thumb for linear satellite operation is that if the uplink band is higher in frequency than the downlink, you should slowly shift your transmit frequency on the uplink as the Doppler effect changes the frequency of your downlink signal. Conversely, you should shift your receive frequency if the Photo1: Dubbed Phase 3-B prior to launch, the satellite that would eventually become AMSAT- OSCAR-10 on orbit hangs in the payload integration hall prior to its launch from Koruou, French uplink band is lower in frequency than the Guyana. (Courtesy: AMSAT-DL) downlink. 4 The AMSAT Journal March/April 2011 www.amsat.org our satellite contacts, commodity, most linear satellite transponders this age-old problem are built to use the most efficient operating should eventually modes possible. Normally, this equates to take care of itself. A Single Sideband (SSB) voice and/or Morse much more detailed (CW). discussion on this Therefore, it should also come as no surprise (often controversial!) that another big “no-no” is running FM subject is contained on through linear satellite voice transponders. the AMSAT Web site FM signals occupy a much larger bandwidth at: http://www.amsat. and take a significantly greater portion of org/amsat/features/ a transponder’s precious output power to one_true_rule.html handle than do CW and SSB signals. And Keep The Power while some people have met with moderate Down! success operating through linear satellite Because it is generated transponders by “simulating CW” ... using Photo 2: Phase 3-B (AO-10) sits atop its carrying structure prior to by relatively small solar the push-to-talk circuit on a 2 meter FM launch. Although its onboard computer was eventually hobbled by panels illuminated radio for example ... this approach often radiation damage, AO-10 provided many hams with their first taste of by the Sun, satellite produces a wide (and very “chirpy”) CW a truly high altitude, linear-transponder-equipped satellite. (Courtesy: AMSAT-DL) power is a finite (and, downlink signal. therefore, scarce!) Practically speaking, this means that you Either way, your FM signals will gobble up resource. That’s why it is very important should shift your transmit frequency on the lots of downlink power and stick out like a to use only enough power on your uplink Mode U/V (the old Mode B) satellites, and sore thumb. Just imagine how obnoxious transmissions to produce a readable signal your receive frequency when operating on you’d sound running SSB signals through on the downlink. the Mode V/U (the old Mode J) satellites. your local FM repeater and I think you’ll This approach will usually help prevent As I’ve said, when operating on the satellites, an inadvertent shift of your conversation you need to get used to the idea that satellite into someone else’s conversation on the work is weak signal work. It’s not like transponder. However, unfortunately, in operating on HF (or terrestrial VHF or UHF) the “heat of the moment” with everyone where the use of more power is usually frantically searching through the passband for “better”. Besides being potentially harmful to their own downlink signals, my experience a satellite’s battery life by using more uplink has shown that these conventions are often power than is necessary, overpowering your ignored. uplink signal beyond the point of creating a discernable signal through the satellite’s Hopefully, as more and more of us employ linear transponder on the downlink will not computer-aided frequency control for appreciably improve the overall strength of Photo 3: By far, AMSAT’s most successful, high your downlink signal. altitude linear transponder equipped satellite was Phase 3-C. Later named AMSAT OSCAR On the contrary, such activity will do little 13 (AO-13) on orbit, it provided many years more than “pump” the satellite’s automatic of long-duration, satellite DX QSOs to users gain control as it tries to compensate worldwide. (Courtesy: AMSAT-DL) for the onslaught you are creating with agree that all use of FM should be avoided your overpowered uplink signal. Such when operating through a linear satellite activity will only gobble up yet more of the transponder. satellite’s precious available power, not to Linear vs. FM mention limiting the overall downlink power Of course, all of this begs the obvious available for others using the transponder. question as to which mode is “better”. Unfortunately, all it takes is one overpowered Having operated in both modes on a number uplink signal in the linear passband to of different satellites over the years, suffice drastically cut the strength of everyone else’s it to say that neither mode is “better” than downlink signal. As you might expect, such the other; they are just “different”. activity will not make you a popular camper Clearly, our FM birds offer an easy path for on the satellites for crocodiles ... those who newcomers to get started as they usually only operate with “all mouth and no ears” ... are require an FM handheld (and something like about as welcome on the birds as lids are Photo 4: AMSAT’s satellites are usually at the an Arrow or Elk handheld antenna) to use. “bottom of the launch stack”. Here, the faring used elsewhere in Amateur Radio.