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Author Topic: Best Doppler Practices  (Read 176 times)

W9AC

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Best Doppler Practices
« on: March 24, 2021, 07:36:02 AM »

Yesterday, I made my first connection with the CAS-4A, CAS4B and AO-7 linear transponder satellites.  No QSO yet, but one step at a time.  As I began calling CQ in CW mode, I was not sure how to best compensate for Doppler.  It it best practice to only adjust the transmitter?  The receiver?   Seems like either way has the potential to unintentionally cause QRM. 

The CAS satellites are moving along so fast that it seems to require constant Doppler compensation.  With a near overhead pass, it's only about 13 minutes from horizon-to-horizon.  Anyway, I was happy just to have made it into the satellites after two months of gear prep. 

Paul, W9AC
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W9IQ

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Re: Best Doppler Practices
« Reply #1 on: March 24, 2021, 09:28:31 AM »

Congrats on getting the setup working. I still recall the thrill the first time I heard my call in the downlink passband.

You will need to adjust both transmit and receive for Doppler correction. The goal is to make your transmitted signal hit the bird at a fixed frequency. You then need to adjust your receive frequency to correct for the Doppler that occurs from the downlink Doppler shift. This works fairly well because your QSO partner is likely experiencing a different Doppler shift than you are.

The satellite antenna tracking software handles this transmit and receive Doppler shift quite nicely. If you aren't using it for tracking, you may wish to consider it just to manage your VFOs.

Have fun.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W9AC

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Re: Best Doppler Practices
« Reply #2 on: March 24, 2021, 09:49:53 AM »

The satellite antenna tracking software handles this transmit and receive Doppler shift quite nicely. If you aren't using it for tracking, you may wish to consider it just to manage your VFOs.

Have fun.

- Glenn W9IQ

This afternoon, I just made my first recent CW QSOs through RS-44.  Last time was back in high school when working AO-7 in the mid 1970s. Hard to believe it's still with us today -- even if it's on crutches.

Reminds me of flying a plane - always making an adjustment of some kind.  The lobes of the portable Arrow antenna have been broad enough that I don't feel like auto AZ/EL tracking is needed.  So far, more adjustments to polarity are needed for optimum signal strength.  The antenna boom rotates through an encased ball bearing.   

Hopefully, SATPC32 software can manage TWO Icom 705s.  I just sent a message to the author for confirmation.  Info is updated for the 9700, but I've not seen a reference to the new Icom 705 running as a pair.  I loathe the thought of adding a tablet PC to the sat kit, but it will probably be worth having.   

Paul, W9AC

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WD9EWK

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Re: Best Doppler Practices
« Reply #3 on: March 24, 2021, 10:04:08 AM »

Yesterday, I made my first connection with the CAS-4A, CAS4B and AO-7 linear transponder satellites.  No QSO yet, but one step at a time.  As I began calling CQ in CW mode, I was not sure how to best compensate for Doppler.  It it best practice to only adjust the transmitter?  The receiver?   Seems like either way has the potential to unintentionally cause QRM. 

Ideally, you would adjust both frequencies, so you would stay on the same frequency at the satellite. If you can't adjust both frequencies during a pass, the normal convention is to make your adjustments on the higher of the two frequencies - regardless of whether the higher frequency is the uplink or the downlink. The higher frequency will have a larger Doppler shift, so that is why the recommendation is to at least make adjustments on that frequency.

73!
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Patrick WD9EWK/VA7EWK
http://www.wd9ewk.net/ - Twitter: @WD9EWK or http://twitter.com/WD9EWK

WD9EWK

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Re: Best Doppler Practices
« Reply #4 on: March 24, 2021, 10:13:17 AM »

Hopefully, SATPC32 software can manage TWO Icom 705s.  I just sent a message to the author for confirmation.  Info is updated for the 9700, but I've not seen a reference to the new Icom 705 running as a pair.  I loathe the thought of adding a tablet PC to the sat kit, but it will probably be worth having.   

SatPC32 should be able to manage two IC-705s. SatPC32 has been able to manage other two-radio setups, like two Yaesu FT-817s or FT-818s. I sent Erich an e-mail years ago on hints for configuring SatPC32 to manage the two FT-817s I have, and he quickly responded.

Especially away from home, I haven't wanted to become reliant on computer control for my satellite station, so I have continued to use manual control for my radios when working satellites. The small Windows tablets from a few years ago had sufficient CPU and memory to run SatPC32, and the only limiting factor might be the number of USB ports on the tablets (although a USB hub might get around a shortage of USB ports on a tablet).

73!
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Patrick WD9EWK/VA7EWK
http://www.wd9ewk.net/ - Twitter: @WD9EWK or http://twitter.com/WD9EWK

WE4B

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Re: Best Doppler Practices
« Reply #5 on: March 24, 2021, 04:37:19 PM »

Since I'm totally manual, I fix the RX frequency and tune the TX frequency on U/v satellites. On V/u satellites, I fix the TX frequency and tune the RX frequency.

I'll be honest, to me it seems that people that manually tune often do a better job than relying on CAT control. I worked a station today on one of the CAS birds and he told me his station was computer controlled yet his station couldn't seem to hold a steady frequency.

I think that the brain sometimes is the best computer control and it's just so easy to listen and make the minute adjustments that must be made as a satellite traverses the sky.
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