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Re: Thoughts on future "FM birds" (was: proposal formaximizing fm le o contacts)



Concerning that a 10m rig and downconverter cannot operate full duplex..

> > Thats another good point.  But its still cheaper to buy 2 for full duplex
> > at $139 than to buy one and have to add a $700 all mode 2m rig to it...
> 
> Good point.

AND I just thought of another great advantage of "MODE-V", and that is
that the $149 UPLINK rig combined with the $49 downconverter allows
AUTOMATIC Transmit/Receive TRACKING!  (At least within 600 Hz) which is
darn close when you are tuning across a 30 KHz wide transponder)... Even
some of the the $1500 satellite rigs dont have this!  Here's how:

1) In MODE-V, the uplinks remain fixed (+/- 600 Hz worth of Doppler) and
users compensate for the total +/- 3000 Hz doppler on their RECEIVERS.

2) All cheap 10m rigs have 1 KHz stepped tuning.  THis leads to
more-or-less non varying channelized operations in the uplink passband.

3) By using a downconverter, the OFFSET between transmit and receive is
fixed.  Thus, as you QSY the uplink transmitter, your downlink (with the 
downconverter) moves with you.

4) By adding a 3 KHz "tuning" pot to the downconverter XTAL oscillator,
you can rubber the fixed converter's offset to account for the slowly
changing +/- 3 KHz you will experience during your pass.  But this offset
is short-term stable throughout your pass.

5) Thus, at any time in the pass, you can instantly QSY up and down the
passband through ALL 10 other QSO's using the radio's tuning knob  and the
downconverter MOVES WITH YOU and with the SAME Doppler adjustment that
applies to your station geometry at that time.  

Yes, the QSO's will have some uplink doppler, but it is never more than
600 Hz relative to your combined tuning.  THus, you can hop about the
passband and never be more than 600 Hz from zero beat on a QSO.  As it is
with Mode-A, you always have to find the QSO first with your RECEIVER,
then tune all over the 30 KHz until you hear yourself.

Of course, the NEATNESS of the above low-cost auto T/R tracking is diluted
somewhat by the fact that you are operating HALF-DUPLEX and will usually 
be several hundred Hertz off, since you cannot hear yourself.  In a
roundtable, this is a mess, but in a one-on-one dialog, where you each
simply adjust to the other, this will work beautifully for such a low cost
10m rig/downconverter $200 off-the-shelf station.

(I hope I got that right...)

de WB4APR, Bob



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