Emily: Go look at the article on EaglePedia discussing the SD meeting.
In the appendix you will see spectrum analyzer plots.
Thanks & 73, Jim wb4gcs@amsat.org
Emily Clarke wrote:
I rarely post to this list but I hope you will accept my apologies for doing so on this topic.
At 05:31 PM 9/7/2006, Matt Ettus wrote:
I have a few problems with this.
1 - It has bad selection bias. Amateurs who are already on satellites and who use S-band downlinks are the ones who don't have problems with interference. If they had interference, they wouldn't be on those bands.
This is IMHO a very bad assumption. If this was a problem with AO-40 or AO-51 (since the assumption is the problem grew expotentially since 2003) then I think we would have heard volumes about it. We haven't. I would be the first to defend that AO-51 is not the best example to use, but I swept my yagi and downconverter around 360 degrees with my spectrum analyzer attached. The spikes were s1 or below. It's not a calibrated measurement, but I'd have to side with the "this isn't a problem" folks. So I feel strongly more tests are needed.
Additionally, people currently on satellites tend to be those with lots of space for big dishes.
I live in a townhouse - this is again a bad assumption. I work AO-51 with a Yagi for 2.4G. If the wind conditions here in San Mateo are light (which they generally aren't) I might put put up a 60CM dish, but generally I won't. Even with the Yagi - WiFi is not a problem.
The point here is to open this up to people who won't, and would have more interference problems, since they live in denser areas and can't put up big dishes with lots of directional gain.
San Mateo has almost a million people in a 12 mile radius. I get no little inteference in the lower S band, and only a very small amount of popping (that my DSP removes completely) in the upper S band. So I personally need more convincing that this isn't a problem. At worse we may have to convince our members to buy better downconverters or add on notch filters so that their wideband downconverters don't suck up noise from outside the band. But telling people to get a filter isn't a big inconvenience. It's been done before.
2 - Anecdotal "evidence" is pointless, especially since it is 5 years old. WiFi is a lot more popular now than it was 5 years ago.
Matt, I don't want to seem confrontational, but so far all I've seen is anecdotal eveidence from the Eagle team. I will admit I don't read everything, but I do read things more closely when I see things like camera shots from spectrum analyzers that are real experiments. I haven't seen it.
It is important to provide empirical data. I have a very open mind, but so far the only thing I've seen are predictions of impending doom and statistics, not hard data.
3 - Could you justify putting up an X million dollar satellite that uses a band which is questionable at best, just because some complainers who don't actually volunteer to do anything say that it works for them?
I think this is a very good question - so if that is the argument, why would you put up a mode U/V transponder? It's as usable as mud in a gas tank.
4 - Why should we have to justify why we're not using a band?
There are several reasons. The first is because we have spectrum allocated there, and if we don't use it we simply lose it. In the future we may need to rely upon it.
The other reason is because the people who bought into the technology 5-10 years ago have an investment that hasn't yet been fully realized. I don't want to go down the road questioning the judgement of people who told our customer base to go down the road, but if it was flawed we haven't told people we are sorry. It's also why none of the commercial broadcasters cut off analog TV transmissions when the FCC set the 2006 deadline. People hang on to technology.
As we agreed at the SD meeting, we are looking at providing _services_ and the best way to do that, not how to best make use of old hardware, which wouldn't be usable anyway.
I wasn't invited to that meeting, but can you tell me why you don't think it wouldn't be usable? I think that is a very bad assumption.
X-band is usable too. Why isn't anyone asking us to justify not using it?
I'm not an expert at that level of microwave technology, but I think that the biggest hurdle (and I could be slammed here) is that if we were to try to launch an X-Band satellite I'm not sure the baseband oscillator technology would be stable in space at a price we can afford. Rick can comment on that - he's the GPS expert and I'm commenting with only terrestrial experience. But putting a GPS corrected clock for X-Band stabilization is something we haven't tried before. However, perhaps we should, and I think the membership would get behind that experiment.
73,
Emily
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