Just Launched AO-91 Will be a Great Bird


AO-91 is an FM repeater [1] satellite with a strong 2 meter downlink (receivable with a rubber duck) and an incredibly sensitive receive system on the 70 cm uplink. There are reports of people getting into the bird using a handheld inside a house. That is incredible. If there are no issues, this will likely become another “Easy Sat” like AO-51 was. It’s currently being tested by AMSAT-NA but it will be open for general use very soon.

For years, the only FM bird was SO-50. It has a great receiver but the downlink is not strong (only 250 mW), so it isn’t easy for beginners to use. AO-85 is another FM satellite that was launched a couple years ago. It has a powerful transmitter, but due to an antenna system issue, it doesn’t receive well. Getting into it with a handheld at only 5 watts can be difficult for beginners and experienced ops alike. Many people use 50 watt mobile FM rigs to compensate for this issue. Its transmit audio can also be low and/or “muffled,” making it hard to complete contacts with some low audio portable stations (on sats it’s better to talk close to the microphone and loud than far away and low).

AO-91 is of the same design as AO-85 but the AMSAT engineers seem to have addressed the issues AO-85 had. It has a great receive system and great transmit audio. Compared to SO-50, the things that may be difficult for beginners to deal with are tuning the uplink to compensate for the doppler effect, and to a lesser degree, dealing with signal fades.

SO-50 is 2 meters up and 70 cms  down. The doppler effect is more pronounced the higher in frequency you go. On 2 meters with FM, you don’t need to compensate much, if at all. On SO-50 you never have to tune the 2 meter uplink. You do tune the downlink, which is easy to do because while other people are talking on it, you can hear when it’s off frequency, and can easily adjust until it’s right.

AO-91 is the opposite. It’s 70 cms up and 2 meters down. If you are off frequency on the uplink you may not be able to get in at all. If you don’t have full-duplex capability so you can hear the downlink while you transmit (highly recommended), you won’t be able to tell.  I believe they chose a 70 cm uplink instead of a 2 meter uplink because there is considerable unlicensed 2 meter usage in certain parts of the world that can interfere with the operation of satellites on that band. For example, I have heard reports of hearing cordless phones on sats like AO-51 while it was in range of  South America, and taxi cab drivers while it was in range of Mexico.

The other thing beginners may be affected by are signal fades. AO-85 and AO-91 both sometimes seem to have transmit signal fades as they tumble. It probably depends on where you are in relation to the satellite and its antennas, and how it’s tumbling, because at least with AO-85, it isn’t a persistent issue. But SO-50 doesn’t have strong fades at all in my experience.

I personally am very excited about how much excitement and how many new hams AO-91 could bring into the ham radio space communications community. During launch on Saturday the amsat.org website was hard to reach because there were so many people trying to load the liveblog about the launch. Here’s to AMSAT-NA for putting this bird up there and for its good life and long service to the community.

NOTE: Even though you can hear the downlink with a rubber duck and maybe even get into the bird with an HT inside a house, you should ideally use a directional antenna like the Arrow II or the WA5VJB cheap LEO yagi. Full duplex operation is also highly recommended.

NOTE: Do not try to use the repeater until AMSAT-NA has announced that it is open for general use to the public. Currently it is in beacon mode and occasionally they are turning on the repeater for use by designated AMSAT engineers and support. Check the News and Events section at the bottom of the AMSAT.org website for updates.

AO-91 frequencies:

Downlink: 145.960 MHz.

Uplink: 435.250 (67.0 Hz tone)

  1. Technically it’s a “transponder” not a repeater but to make it easy to understand you can think of it like a standard analog FM land based repeater, except it’s cross band.



11 thoughts on “Just Launched AO-91 Will be a Great Bird

  1. Awesome post John. I’m excited for this bird as well. It’s great to finally have a powerful satellite in the sky. I remember my early days of Satellite operation (I’m not terribly active these days) and operation AO-51 which was just loud as heck.

    If you’re interested I blogged about one of the boards inside AO-91 that I helped build. So far it appears to be working fine! It’s a neat story about how the Fox-1 Maximum Power Point Tracker came to be and is now on it’s first flight. Hopefully one of many reasons it’s a loud bird as we’re hoping to give it as much power as possible ;). Power budgets be less constrained.


    Liked by 1 person

    1. Right before it was packed up to launch one of the antennas on 1A broke. The regular guy who would fix it was unavailable in the time period allowed, so they used a conductive epoxy to re-attach it. This had the side effect that the bird is a little ( a lot) deaf. It also has some thermal issues. They took the lessons learned from 1A and fixed it with 1B.

      Liked by 1 person

  2. Excellent post.
    Caught a couple of passes yesterday afternoon (in New Zealand) with my 3 element yagi, newly constructed from old TV aerial bits. Strongest signals I’ve heard from a satellite with lots of traffic, mostly from Australia.

    Liked by 1 person

    1. Thanks Terry. It is indeed a strong signal. I’m not sure if AO-85 is stronger, but they are similar. The only ham sat signal stronger than either for sure is the ISS’s Kenwood TM-D710E in the Russian Service module used for SSTV and some school contacts, at 50 watts.


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