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Re: Ditting to locate



>From: Dave Guimont <dguimon1@san.rr.com>
>I really can't believe anyone one has difficulty locating themselves
>in the passband.
>
>Pick a frequency pair according to the published ones. Of course, way away 
>from the
>beacon.  Once you establish a pairing write that pair down or put it in
memory
>if you have that capability...
>
>I've got a pairing that I know works, albeit at one point in the doppler, and
>I've got it in memory.  When starting a new session, it may take me all of
five
>seconds to find myself with a very little  RECEIVE FREQUENCY HUNT!!
>I just cannot imagine anyone swishing his xmit frequency to find himself,
>much less thru the beacon...TUNE YOUR RECEIVE!!
>
>I also use an audio input that I made from a 556 timer. I inject it at the 
>mike,
>and I can find myself very easily on ssb and never have to switch to or 
>back from
>the cw position.  It makes it easier for beginners to use, once they hear 
>their
>tones they put the switch back in the mike position.  I have a choice of
three
>tones, and one of the sounds is exactly like LEILA!  Come on by AO40 and 
>I'll give
>you a demo!!

Subjects: "Ao-40, FT-847 AO-40 and Doppler Uplink Correction, and Ditting"
all pertain

Audio generator is a good idea for a tuning tool, Dave.

Best is to calculate the uplink offset using doppler observed from the
beacon {per Chris Hill}.  But you need to know your down convertor LO
frequency and uplink frequency accurately.  My Drake is 5 KHz high and the
FT-847 appears to be 13 KHz low on 435 MHz. 

Saving a good freq. pair is good {or just write it down} if you operate at
the same part of the orbit all the the time.  But if you operate when the
satellite is approaching you then the numbers are different than when it is
moving away {per Chris this would be +2 vs -2 KHz}.  Actually doppler can
be as much as 25 KHz, so uplink offset can be as much as +/-4.5 KHz.  A
simple rule is if receive doppler is below, Tx offset is above {or vice
versa}. 

I think a good rule would be to always find the beacon first and measure
the doppler offset {from 2401.323}.  Then move away from the beacon at
least 30 KHz, calculate your Tx offset, enter it, and finetune it in
{either tune Rx or Tx at this point as you should be within a couple KHz}.
If you use Bruce's technique with say an FT-847, you need to switch
"tracking" to none, else the Tx tracks the Rx when you tune.  Be sure to
switch back to TCK-REV after netting your frequency.

I encountered a strange circumstance with my FT-847.  Evidently it is -13
to -14 KHz off frequency.  So tonight when doppler was -12 KHz, alI needed
to do was subtract 12 KHz from the calculated 435 MHz freq.  Chris' method
would have me be +2 KHz!

In the final analysis, there is no need to go "ditting" across the band and
beacon to find yourself.

Ed
PS: finding the beacon also gives you a signal level reference to start
with for setting your uplink, and if you read the status and mailbox from
the tlm then you know what mode it is currently in and any schedules.
BTW: the above applies to mode-US.  You get to recalculate if on mode-LS
{hint the uplink is three times higher so doppler offset will be three
times larger}. 

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