Re: [amsat-bb] 12/9600 baud Mobile Sat Msg tests (D700)

[Jon Ogden <jono@xxxxxxxxxxxx>]

on 7/24/00 7:33 PM, Bob Nielsen at nielsen@oz.net wrote:

> This doesn't sound too good.  What is the shortest TXD that can be used
> with the D700?  Most "9600-ready" radios have pretty miserable TXD
> performance, but I would expect that a radio designed with a built-in
> TNC would be optimized a bit.

This is a BIG problem with synthesized radios.  When talking to the packet
guys I know, they say that most "9600" baud radios aren't really good at it.
Most either use a rig that has two genuine synthesizers (like your FT-847
and IC-821) or crystal rigs.

The problem is that the synthesizers take too long to lock onto a frequency.
Yes, the internal synthesizer frequency of the radio changes from TX to RX.
You have an TX mixer and an RX mixer.  Both are fed with different LO
frequencies.  It's the LO frequency shift that's a problem.  By the time the
radio shifts from TX to RX, the *ACK* packet from the other station has
already been sent!  I think that one guy I know said that he measured some
TH-D7As that had turn around times of between 250 and 500 milliseconds!
That's fine for voice and even 1200 baud data, but not for 9600 data.

I have tried the TH-D7A on 9600 baud packet and it is miserable.  A local
9600 node is about 3 blocks away.  It was slower than 1200 baud.

Now, one way to overcome this is to use a digipeater.  By the time the
digipeater gets the packet and re-transmits and the other station has sent
an *ACK*, there is more than enough time for the synthesizer to settle down.

Now, I haven't tried 9600 baud on my TM-D700 yet.  I imagine it suffers from
the same problem.  However, there is a work around with it.  The TM-D700A
DOES have two bands.  Therefore it has 2 VFOS!  Say your 9600 baud frequency
is 430.35 MHz.  Set both bands to 430.35 MHz and in the proper menu (I
forget which) set one band to TX packet and the other for RX packet.  You
set it up like you would for satellite except both frequencies are the same.
This I think may work.  Now, you can do the same thing on the newly upgraded
TH-D7A(G), but ONLY on 2 meters as you can't do UHF/UHF on that radio.

Have fun, folks!

73,

Jon
NA9D

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."

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