by Ray Soifer, W2RS
Since my article on AO-27 was published in the January 1998 issue of QST, I have received many inquiries from readers, some of whom were experiencing difficulty in hearing or working through them.
Most of those who wrote in are now hearing and working through AO-27 and SO-50 with no problems. From our shared experiences, here are the suggestions they found helpful most often:
AO-27 transmits with a power output of 0.5 W into a quarter-wavelength whip antenna. The power output of SO-50 is approximately 3 dB lower, into a quarter-wavelength whip canted inward at a 45-degree angle.
Both satellites are approximately 500 miles (800 km) distant when directly overhead and over 2,000 miles (3,200 km) distant when near the horizon. This means that you will need a good receiver and antenna in order to hear them.
The "rubber duck" antennas with which most handheld radios are supplied are not large enough to hear AO-27 or SO-50, except under ideal conditions.
As mentioned in my article, a half-wavelength whip antenna has worked very well for me on AO-27. Dual-band whips which operate as a half-wavelength at 70 cm include the MFJ-1717, the Comet SMA-24 (or BNC-24) and the Diamond RH-77CA, among others.
Fixed and mobile vertical antennas are not recommended, because they cannot be tilted to follow the polarization of the incoming signal. With these antennas, results on AO-27 are very marginal.
Quarter-wavelength and 5/8-wavelength whips also are not recommended, because unlike a half-wavelength antenna, they require a ground plane to achieve their designed patterns and a hand-held transceiver does not provide one.
For use on AO-27 with a half-wavelength whip, your receiver sensitivity at 436 MHz should be at least 0.18 uV for 12 dB SINAD, which corresponds to the approximate signal strength of AO-27 at 10 degrees elevation when your whip antenna is correctly positioned for the polarization of the incoming signal. At the horizon, AO-27's signal strength, under similar conditions, is approximately 0.13 uV. Most modern, high-quality amateur radio transceivers will meet these specifications if designed to operate at this frequency (i.e., without modifications). Most scanners, and most radios which have had to be modified to cover 436 MHz, will not.
As mentioned earlier, SO-50’s power output is significantly lower than that of AO-27. My experience has been that to hear and work SO-50 with a half-wavelength whip, its elevation angle should be at least 30 degrees.
If you want a larger, better antenna than a whip, the Arrow 146/437-10 dual-band handheld beam provides forward gain of approximately 10.3 dBd at 70 cm and 4.6 dBd at 2 meters (measured at the CSVHFS test range in 2001), and is highly recommended. I have one, and it works very well. With the Arrow, I can work SO-50 down to about 10 degrees elevation, and AO-27 nearly all the way out to the horizon.
For further information, write to Al Lowe, N0IMW, the owner of Arrow Antenna, at (new address) 911 East Fox Farm Road #2, Cheyenne WY 82007, by e-mail at firstname.lastname@example.org, or visit his web page at http://www.arrowantennas.com.
Reflections from the ground or other conductive surfaces (such as a car body) can improve your received and transmitted signals by up to 6 dB. That's why you'll sometimes see hams who are working AO-27 with whip antennas moving their whips around as if they were dowsers looking for the next water well. Give it a try! In order to do this, however, you'll want to use a speaker-mike (or better yet, a headset with a boom mike attached) so you don't have to keep your handheld radio near your mouth and ears.
Check your frequency -- AO-27 and SO-50 transmit FM on about 436.795 MHz, plus/minus Doppler shift of up to 10 kHz on either side. Their uplink frequency is 145.850 MHz, plus/minus Doppler corrections of up to approximately 3.4 kHz.
SO-50 employs tone squelch (CTCSS) on its uplink, so set your transmitter for a tone of 67 Hz (also known as PL XZ). When it’s turned on, no CTCSS tone is required to access AO-27.
AO-27 and SO-50 signals are unlikely to be strong enough to trip the squelch on most FM receivers. So, be sure to open your receiver’s squelch all the way before you begin listening.
Try listening on a pass for which the maximum elevation angle is at least 30 degrees above the horizon, until you're sure that your receiver and antenna are capable of hearing the weaker, farther-out passes.
If you still aren't hearing AO-27, you could be listening when the satellite isn't turned on. AO-27’s command stations maintain a Web page, http://www.ao27.org, providing current information on the satellite’s status and operating schedule. Be sure to check it regularly.
Also, check your satellite tracking program to be sure that you have the proper Keplerian elements loaded and that your QTH file is correct. The exact times when the satellites are in range will vary from day to day -- that's why you need software to track the satellite for you. For best results, Keplerian elements should be updated at least once a month. If you don't have a tracking program, check out the ones available through AMSAT for a donation or for free download.
Tracking predictions for the current day (only) may also be obtained by visiting http://www.heavens-above.com and following the instructions you'll find there. That is not an AMSAT web site, so no promises, but it's working as I write this.
AO-27 and SO-50 are very popular. Unless you happen to be operating from the middle of the Pacific Ocean, central Siberia or some other extremely isolated place, you are likely to find it fully occupied whenever it is in range. Please wait until you can hear the other hams who are using it, then give your callsign during an "over", much as you might when entering a conversation in progress on your local repeater. If you detect a carrier but cannot hear voices, do not call CQ. The satellite is probably in use but its downlink signal is simply too weak at your station at that particular time. If you transmit under such conditions, you may cause interference to others who are using the bird.
If you're fortunate enough to be operating from a place within, say, a few hundred miles of the coastline, you'll probably find it easier to get into AO-27 and SO-50 with low power when the satellites are out over the ocean than when they're overhead or passing over central North America or Europe. That's because there will be fewer stations within the footprint to compete with you, and most of your competitors will be farther from the satellite than you are so the inverse-square law of signal strengths will work to your advantage.
Many readers have written to ask how I programmed the Yaesu FT-50R/RD to transmit and receive on different bands. Follow the directions under "Custom Tx Offset" in the instruction manual. In the edition I have, they appear on page 26. Sorry, I haven't tried it with other radios so please ask someone who owns a radio like the one you have, or the manufacturer.
Another frequently-asked question is "What other satellites can I hear and work with my AO-27/SO-50 equipment?" The answer changes over time, so check the latest AMSAT News Service bulletins (particularly the weekly satellite report), and the AMSAT-NA web site http://www.amsat.org/, for the latest information.
You might want to try listening for the research and beacon satellite UoSAT-OSCAR 11, which transmits on 145.826 MHz. UO-11, however, does not have two-way communications capabilities. Check http://www.amsat.org/ for more information.
At this writing, Amateur Radio on the International Space Station (ARISS) is operating FM voice and packet with a downlink at 145.800 MHz. Voice uplinks are 145.200 over ITU Region 1 (Europe, Middle East and Africa) and 144.490 elsewhere. I have worked them several times with a handheld transceiver and whip antenna. Although packet operation is outside the scope of this discussion, the worldwide packet uplink is 145.990. See the AMSAT and ARISS web sites for more information.
There are plans afoot for more LEO FM satellites. AMSAT expects to have AMSAT-OSCAR E (also known as ECHO) in orbit later in 2004. Again, AMSAT News Service bulletins and http://www.amsat.org/ are your best information sources.
I hope these suggestions prove helpful. CU on AO-27 and SO-50!
73, Ray, W2RS
Updated January 9, 2004