This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF
When you're on the ground listening to SO-50 as it approaches you, the RF waves are compressed, and thus the frequency you receive at is higher than the frequency the satellite is transmitting at.
When you're transmitting to AO-91 as it approaches you, you need to compensate for the fact that your RF waves are going to be compressed and appear at the satellite at a higher frequency than you are transmitting at. So you have to transmit lower than the nominal frequency.
73,
Paul, N8HM
On Wed, Jun 5, 2019 at 1:45 PM Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Look at it from the satellite’s point of view. The satellite is receiving your Doppler-shifted signal. If you continue transmitting on a fixed frequency, the satellite sees the frequency dropping. To keep the frequency seen by the satellite steady, you need to offset the drop by increasing your transmit frequency at the same rate.
Dave, W8AAS
On Jun 5, 2019, at 1:41 PM, Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
SO-50 is a VHF uplink and UHF downlink (V/u) satellite. AO-91 and AO-92 are UHF uplink and VHF downlink (U/v) satellites. While there is some doppler effect on VHF, the amount is minimal (2-3 KHz). We can pretty much ignore that, with the exception of AO-92, which can require you to make an adjustment near the end of the pass. (You will notice your received audio getting scratch. So, let's focus on UHF doppler effect, which can change as much as +/- 10 KHz during a pass - Remember the train example. The sound of a train at a distance and approaching will sound higher pitched, continually decreasing in pitch until it arrives (on frequency) at your location (closest), and continues decreasing in pitch as it passes and pulls a way from you. - On SO-50, you have to adjust your UHF rx frequency to the higher pitched frequency at the start of the pass (AOS), on frequency at mid-pass (TCA), and lower pitched frequency at end of the pass (LOS). Thus, you will need to be 10 KHz higher at AOS, 5 Khz half way between AOS and TCA, on frequency at TCA, -5 KHz halfway between TCA and LOS, and -10 Khz at LOS. Just remember you are adjusting your RX frequency to compensate for the effects of Doppler - On AO-91 and AO-92, it is the opposite. You adjust your UHF uplink frequency so the satellite hears your signal at the designed uplink frequency. At AOS, doppler effect will cause your signal to sound higher to the satellite, so you need to start at a lower pitched frequency (-10 KHz), Halfway between AOS and TCA, you adjust to -5 KHz, and so on, and so on.
I hope this helps. If not, don't hesitate to ask. 73, Robert Bankston, KE4ALAMSAT-NA VP of User Services Twitter: @KE4ALabamaWebsite: KE4AL.wordpress.com
On Wednesday, June 5, 2019, 12:46:07 PM CDT, Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Philip you might have a typo here:
You say, for SO-50, "from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS." You do lower the frequency through the pass, but you actually start by raising the frequency, then you lower. So with AO-91 you start by lowering the frequency and then raise it through the pass.
73, John Brier KG4AKV
On Wed, Jun 5, 2019, 13:56 Paul Stoetzer via AMSAT-BB amsat-bb@amsat.org wrote:
When you're on the ground listening to SO-50 as it approaches you, the RF waves are compressed, and thus the frequency you receive at is higher than the frequency the satellite is transmitting at.
When you're transmitting to AO-91 as it approaches you, you need to compensate for the fact that your RF waves are going to be compressed and appear at the satellite at a higher frequency than you are transmitting at. So you have to transmit lower than the nominal frequency.
73,
Paul, N8HM
On Wed, Jun 5, 2019 at 1:45 PM Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my
head
around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird
approaches
LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from
AOS, I
go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when
transmitting
to a satelllite?
So, my question boils down to - why should transmit doppler shift go in
the
opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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.
Not an AMSAT-NA member? Join now to support the amateur satellite
program!
Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
And to add to this, when the satellite passes by you, what was a compression of the downlink as it approached turns into an expansion as it passes away. So the downward tuning of the received signal continues. It started above "actual" at the start of the pass, was dead-on right as the satellite got to its closest point, and then continued below actual as it trails off to the setting horizon.
And everything is swapped for your uplink. You start out below center at the start of the pass, even with it in the middle, and above towards the end of the pass.
FM satellites (such as SO-50) benefit from the "FM Capture Effect", where a strong enough FM signal can be a little off frequency and still get received correctly. This can let you not have to mess with the 2 meter band (whether up or down link), and just focus on the 70cm part. Tuning both is best, if you can, as it gets you a bit more margin.
Good luck on the birds,
Greg KO6TH
Dave Taylor via AMSAT-BB wrote:
Look at it from the satellite’s point of view. The satellite is receiving your Doppler-shifted signal. If you continue transmitting on a fixed frequency, the satellite sees the frequency dropping. To keep the frequency seen by the satellite steady, you need to offset the drop by increasing your transmit frequency at the same rate.
Dave, W8AAS
On Jun 5, 2019, at 1:41 PM, Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Let me take a stab at this... The obvious thing about doppler is that, as the sat comes at you the frequency sounds higher than its normal frequency and lower when it goes away from you. Hence we set our receivers at a slightly "higher" frequency at AOS and step lower until LOS. The not-so-obvious thing is that a similar effect happens "at the satellite", so if we transmit at the normal frequency, it will appear slightly higher at the satellite, as it comes at you. Since the satellite receiver cannot change, we compensate by transmitting at a slightly lower frequency so it is "normal at the satellite" and we keep stepping it up until LOS. The amazing/confusing thing about space is that everything is relative, i.e., the sat and the earth station are just moving relative to each other. Hope that helps ! 73! Umesh k6vug
On Wednesday, June 5, 2019, 10:45:46 AM PDT, Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
It appears, the second part of the query is about adjusting for doppler at VHF vs UHF frequencies.
Doppler shift is like a percentage, so the amount of, say 5%, is a bigger number for UHF compared to VHF. Here is the kicker, due to the way FM receiver works, it is tolerant even if the signal is a little off-frequency. Since the doppler shift at VHF is quite small, we rarely have to adjust the FM on VHF. However the doppler shift at UHF is beyond the tolerance of a FM receiver, so we have to frequently adjust UHF for doppler. 73! Umesh k6vug
On Wednesday, June 5, 2019, 11:31:18 AM PDT, k6vug@sbcglobal.net k6vug@sbcglobal.net wrote:
Let me take a stab at this... The obvious thing about doppler is that, as the sat comes at you the frequency sounds higher than its normal frequency and lower when it goes away from you. Hence we set our receivers at a slightly "higher" frequency at AOS and step lower until LOS. The not-so-obvious thing is that a similar effect happens "at the satellite", so if we transmit at the normal frequency, it will appear slightly higher at the satellite, as it comes at you. Since the satellite receiver cannot change, we compensate by transmitting at a slightly lower frequency so it is "normal at the satellite" and we keep stepping it up until LOS. The amazing/confusing thing about space is that everything is relative, i.e., the sat and the earth station are just moving relative to each other. Hope that helps ! 73! Umesh k6vug
On Wednesday, June 5, 2019, 10:45:46 AM PDT, Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
A band on a train explains Doppler effect the best. On board the train, the sounds are normal, I.e. the frequency is constant.
KC9SGV
Sent from my iPad
On Jun 5, 2019, at 12:54 PM, Paul Stoetzer via AMSAT-BB amsat-bb@amsat.org wrote:
When you're on the ground listening to SO-50 as it approaches you, the RF waves are compressed, and thus the frequency you receive at is higher than the frequency the satellite is transmitting at.
When you're transmitting to AO-91 as it approaches you, you need to compensate for the fact that your RF waves are going to be compressed and appear at the satellite at a higher frequency than you are transmitting at. So you have to transmit lower than the nominal frequency.
73,
Paul, N8HM
On Wed, Jun 5, 2019 at 1:45 PM Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Thanks for all the replies, both on and off list.
I should have given my satellite background in my first email; it would have saved some of you quite a bit of typing! :-)
I'm an AMSAT Ambassador, AMSAT-NA Life Member, and my very first satellite QSO was on AO-13 in 1993 (but then I was inactive on the birds for 20 years) - and did learn a few things I didn't know at Hamvention's AMSAT Academy. :-) I have given "Satellite 101" talks to about 10 clubs in the past 2 years, have manned info tables at a few hamfests, and may tackle a couple of hamfest forums solo during the next year.
I had hoped my question about receive versus transmit was specific enough (since I know how to operate the birds and explain doppler when I give presentations at clubs), but as I said, it would have been helpful to you respondents if you had already been familiar with my background. Perhaps I could have worded it better, and been more succinct. I'm glad that I got so many answers - and I did get a comprehensible, clear, answer right off the bat - and hopefully your responses will help those who just "lurk" on the BB to understand doppler better and how to work the sats more effectively.
So thanks again to all who replied!
Philip N4HF EM85
On Wed, Jun 5, 2019 at 1:41 PM Philip Jenkins n4hf.philip@gmail.com wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF
More train Doppler effect. These are sound waves, but the RF FM principle is the same. Not sure why they don't use AM in space, like in aviation...I.e. NO Doppler effect.
KC9SGV
Sent from my iPad
On Jun 5, 2019, at 9:39 PM, KC9SGV kc9sgv@gmail.com wrote:
A band on a train explains Doppler effect the best. On board the train, the sounds are normal, I.e. the frequency is constant.
KC9SGV
Sent from my iPad
On Jun 5, 2019, at 12:54 PM, Paul Stoetzer via AMSAT-BB amsat-bb@amsat.org wrote:
When you're on the ground listening to SO-50 as it approaches you, the RF waves are compressed, and thus the frequency you receive at is higher than the frequency the satellite is transmitting at.
When you're transmitting to AO-91 as it approaches you, you need to compensate for the fact that your RF waves are going to be compressed and appear at the satellite at a higher frequency than you are transmitting at. So you have to transmit lower than the nominal frequency.
73,
Paul, N8HM
On Wed, Jun 5, 2019 at 1:45 PM Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF _______________________________________________ Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
But AM is just amplitude modulation imposed on an electromagnetic wave. If you've got waves, I'm pretty sure you're going to have a Doppler effect.
Maybe aviation doesn't show much Doppler because they are going relatively slowly (compared to spacecraft) and they operate in the VHF realm, where Doppler is less obvious (like 2m on the satellites).
Steve AI9IN
On 2019-06-06 8:42 am, KC9SGV via AMSAT-BB wrote:
More train Doppler effect. These are sound waves, but the RF FM principle is the same. Not sure why they don't use AM in space, like in aviation...I.e. NO Doppler effect.
KC9SGV
Sent from my iPad
I'm pretty sure the reasons for AM in aviation was because equipment was cheap and easy to build when the aviation band was formed and if 2 stations doubled they would both be heard.
My guess for not not having perceived doppler in aviation is because aircraft aren't flying at 17,500mph and they are using VHF frequencies. Feel free to contact me if I'm wrong.
Dave-KB1PVH
Sent from my Galaxy S9
On Thu, Jun 6, 2019, 8:44 AM KC9SGV via AMSAT-BB amsat-bb@amsat.org wrote:
More train Doppler effect. These are sound waves, but the RF FM principle is the same. Not sure why they don't use AM in space, like in aviation...I.e. NO Doppler effect.
KC9SGV
Sent from my iPad
the other reason to stick with AM is that you can hear if other stations are trying to break in. FM has capture and blocks all but the strongest station. AM can superimpose multiple signals and hear if emergency calls may be made. Additionally, in the '30's FM was nearly impossible to generate reliably. AM doesn't depend on the transmitter's stability (to an extent).
Noel Petit - WB0VGI
On 6/6/2019 8:51 AM, Dave Webb KB1PVH via AMSAT-BB wrote:
I'm pretty sure the reasons for AM in aviation was because equipment was cheap and easy to build when the aviation band was formed and if 2 stations doubled they would both be heard.
My guess for not not having perceived doppler in aviation is because aircraft aren't flying at 17,500mph and they are using VHF frequencies. Feel free to contact me if I'm wrong.
Dave-KB1PVH
Sent from my Galaxy S9
On Thu, Jun 6, 2019, 8:44 AM KC9SGV via AMSAT-BB amsat-bb@amsat.org wrote:
More train Doppler effect. These are sound waves, but the RF FM principle is the same. Not sure why they don't use AM in space, like in aviation...I.e. NO Doppler effect.
KC9SGV
Sent from my iPad
Sent via AMSAT-BB@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. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
About AM vs SSB - AM signals sound the same even if the carrier is a bit off frequency, whereas SSB signals can become difficult to copy when off-frequency. Being able to copy without having to fiddle with the controls would be important in aviation. Umesh, k6vug On Thursday, June 6, 2019, 7:38:34 AM PDT, Noel Petit via AMSAT-BB amsat-bb@amsat.org wrote:
the other reason to stick with AM is that you can hear if other stations are trying to break in. FM has capture and blocks all but the strongest station. AM can superimpose multiple signals and hear if emergency calls may be made. Additionally, in the '30's FM was nearly impossible to generate reliably. AM doesn't depend on the transmitter's stability (to an extent).
Noel Petit - WB0VGI
On 6/6/2019 8:51 AM, Dave Webb KB1PVH via AMSAT-BB wrote:
I'm pretty sure the reasons for AM in aviation was because equipment was cheap and easy to build when the aviation band was formed and if 2 stations doubled they would both be heard.
My guess for not not having perceived doppler in aviation is because aircraft aren't flying at 17,500mph and they are using VHF frequencies. Feel free to contact me if I'm wrong.
Dave-KB1PVH
Sent from my Galaxy S9
On Thu, Jun 6, 2019, 8:44 AM KC9SGV via AMSAT-BB amsat-bb@amsat.org wrote:
More train Doppler effect. These are sound waves, but the RF FM principle is the same. Not sure why they don't use AM in space, like in aviation...I.e. NO Doppler effect.
KC9SGV
Sent from my iPad
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"A perceptive question deserves a proper answer..." - Umesh, k6vug
On Thursday, June 6, 2019, 7:34:51 AM PDT, Philip Jenkins via AMSAT-BB amsat-bb@amsat.org wrote:
Thanks for all the replies, both on and off list.
I should have given my satellite background in my first email; it would have saved some of you quite a bit of typing! :-)
I'm an AMSAT Ambassador, AMSAT-NA Life Member, and my very first satellite QSO was on AO-13 in 1993 (but then I was inactive on the birds for 20 years) - and did learn a few things I didn't know at Hamvention's AMSAT Academy. :-) I have given "Satellite 101" talks to about 10 clubs in the past 2 years, have manned info tables at a few hamfests, and may tackle a couple of hamfest forums solo during the next year.
I had hoped my question about receive versus transmit was specific enough (since I know how to operate the birds and explain doppler when I give presentations at clubs), but as I said, it would have been helpful to you respondents if you had already been familiar with my background. Perhaps I could have worded it better, and been more succinct. I'm glad that I got so many answers - and I did get a comprehensible, clear, answer right off the bat - and hopefully your responses will help those who just "lurk" on the BB to understand doppler better and how to work the sats more effectively.
So thanks again to all who replied!
Philip N4HF EM85
On Wed, Jun 5, 2019 at 1:41 PM Philip Jenkins n4hf.philip@gmail.com wrote:
This came up at AMSAT Academy at Hamvention, and I still can't wrap my head around it (something simple I'm not getting, I'm sure). I know the xmit/receive frequencies aren't shifted, stay the same at the satellite.
SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I lower my receive frequency (and continue lowering it as the bird approaches LOS). So far so good.
AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I go up in my transmit frequency.
Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when receiving a satellite, but *raise* the 435 Mhz frequency when transmitting to a satelllite?
So, my question boils down to - why should transmit doppler shift go in the opposite direction from receive on the same band? In both cases, the satellites are approaching me (from AOS).
Basically, why the difference when I'm transmitting and when I'm receiving?
73
Philip N4HF
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On 2019-06-06 09:37, Noel Petit via AMSAT-BB wrote:
the other reason to stick with AM is that you can hear if other stations are trying to break in. FM has capture and blocks all but the strongest station. AM can superimpose multiple signals and hear if emergency calls may be made. Additionally, in the '30's FM was nearly impossible to generate reliably. AM doesn't depend on the transmitter's stability (to an extent).
On linear transponder satellites (https://www.amsat.org/two-way-satellites/), you _can_ hear other stations doubling.
AM is wasteful of power, as others have commented, and has no place on a power-limited satellite. The carrier, containing no information, consumes 2/3rds of the transmitted power. The rest of the power is split between two sidebands carrying the exact same information. (https://www.tutorialspoint.com/analog_communication/analog_communication_amp...)
I disagree strongly with your assertion that FM was "nearly impossible" to generate. (https://en.wikipedia.org/wiki/Armstrong_phase_modulator)
--- Zach N0ZGO
participants (12)
-
Dave Taylor -
Dave Webb KB1PVH -
Greg D -
John Brier -
k6vug＠sbcglobal.net -
KC9SGV -
Noel Petit -
Paul Stoetzer -
Philip Jenkins -
Robert Bankston -
skristof＠etczone.com -
Zach Metzinger