How is it possible that you can hear two voices simultaneously on AO-27?
AO-27 is an interesting bird. Never before on an FM bird or any terrestrial FM have I heard two signals at the same time. I thought the capture effect basically made that impossible, and yet I often easily hear two voices at the same time on AO-27 when people are transmitting at the same time.
I believe AO-27's transponder was made for data, so perhaps the filtering is different? Or it has a wide receive bandwidth and differently doppler shifted input signals can both be demodulated at the same time?
Examples from tonight's pass can be heard at 01:28 and 01:39. There are probably other examples in the recording but those are the ones I found first.
https://www.dropbox.com/s/r8bkhlf24aod2t6/2021-05-14_0041_UTC_AO-27_FM05_KG4...
73, John Brier KG4AKV
My guess is that the wider filter bandwidth on AO-27 makes FM capture less effective.
This thesis I found says "The simulation results establish that the lowpass filtering portion of frequency demodulation accounts for the capture effect of FM receivers."
https://apps.dtic.mil/dtic/tr/fulltext/u2/a208479.pdf
-Stephen N8URE
On this discussion of hearing two simultaneous signals from aO-27. I have not heard any discussions of the fact that SO-50 and AO-27 passes have been happening for many in northern latitudes, happening, at close to the same times. SO 50 does get earlier each day by a few minutes while AO-27 is fairly sun synchronous. Both satellites use the same uplink and downlink frequencies. 27 does have the 4 minute timer, which will limit the window of signal collision, but still that window can occur. As there is doppler going on between each signal, both up and down, so I would expect that different receivers would handle the competing signals differently. If both satellites were active at the same time, with signals on the opposite sides of your recievers passband, I bet you could hear two discriminated signals at the same time. There could also be an instance of both satellites getting the same uplink and that would only add to the already raucous conditions on these satellites.
-----Original Message----- From: sjdevience@gmail.com sjdevience@gmail.com Sent: Saturday, May 15, 2021 8:57 AM To: amsat-bb@amsat.org Subject: [AMSAT-BB] Re: How is it possible that you can hear two voices simultaneously on AO-27?
My guess is that the wider filter bandwidth on AO-27 makes FM capture less effective.
This thesis I found says "The simulation results establish that the lowpass filtering portion of frequency demodulation accounts for the capture effect of FM receivers."
https://apps.dtic.mil/dtic/tr/fulltext/u2/a208479.pdf
-Stephen N8URE
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Very interesting Stephen. I apparently don't understand FM well enough because I always thought the capture effect had to do with whichever signal was stronger, the reciever would "lock" on that one, unlike AM which is simple enough for me to understand why you can hear two signals at once.
Thanks for looking for that and sharing it.
73, John Brier KG4AKV
On Sat, May 15, 2021, 09:58 sjdevience@gmail.com wrote:
My guess is that the wider filter bandwidth on AO-27 makes FM capture less effective.
This thesis I found says "The simulation results establish that the lowpass filtering portion of frequency demodulation accounts for the capture effect of FM receivers."
https://apps.dtic.mil/dtic/tr/fulltext/u2/a208479.pdf
-Stephen N8URE
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Acceptable Use and Privacy Policies available at https://www.amsat.org/about-amsat/
View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/amsat-bb@amsat.org To unsubscribe send an email to amsat-bb-leave(a)amsat.org Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org
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