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Re: Ham radio?



Bob,

I have been thinking about your last response a lot.  I have a few
comments.  My proposal for a dual operation constellation of sats
containing a 2 meter digipeater for UI and APRS packets, as well as a
mode J bent pipe subsystem per my last post, would, in my opinion, be
easier for the portable, possibly handheld operator, that a mode B FM
system.  Allow me to explain.

A 3 db link budget is substantial for a 70 cm handheld device.  I use a
cat whisker antenna for AO-27 now, and the 70 cm downlink is marginal. 
If I were to use the same antenna as an uplink radiator, the 3 db link
budget would probably keep me out of the bird.  I would be forced to
carry a small yagi, or perhaps a turnstile or Lindenblad antenna with
me, or double my power (carry a small amplifier?) to make up that 3 db. 
If the bird had a 30 kHz wide IF, the signal to noise ratio would
probably require an even stronger signal.

Doppler correction on an FM bird is really not too difficult on a mode
JA FM bird.  For instance, on my FT50, which I use regularly for AO-27
QSOs, has 5 frequency pairs programmed in for Oscar 27 operation:

		436.805/145.845
		436.800/145.850
		436.795/145.850
		436.790/145.850
		436.785/145.855

I start off with the first frequency pair, and tune "down" as the bird
approaches, then recedes.  It works quite well, and a rubber ducky will
work.

For what it's worth, we will all get a chance to try a mode B FM bird
soon, when AMSAT LU launches VOXSAT.  I pulled this from their web site:

		C.- Operation modes & frequencies

		Downlink frequency one : 145.990 MHz FM, (Voice & Telemetry).
		Downlink frequency two : 145.910 MHz FM, (cross band repeater uhf/vhf)
		Uplink : 435.990 MHz FM 

Their website is: http://members.xoom.com/Amsat_LU/

I am very anxious to try out this bird, and this mode portable.  

Speaking of LUSats,  Richard, G3RWL, tells me that LU-19 is in digipeat
mode.  It seems this bird, with it's wide coverage might be a good
candidate for some APRS experiments.  Have you ever experimented with
this bird and UI packets?

73, and thanks for your kind words Bob!

Mike in Fort Myers, FL

Bob Bruninga wrote:
> At first, 70 cm as an uplink seems to require doppler tracking by the
> user, but if the satellite simply had a 30 KHz wide IF, then FM would get
> through just fine without doppler compensation.  Only penalty would be
> about 3 dB in link budget.  But the ground user can afford the added
> 3 dB uplink power.
> 
> The 2m downlink is required due to the 9 dB advantage.  This reduces the
> XMIT Power needed by the tiny satellite.  AND the small doppler on 2m,
> then does not require the users to have wide band receivers or to track
> doppler.

                                       __\/__ 
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