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R: Satellite Inquiry

----- Original Message -----
From: KC6UQH <kc6uqh@cox.net>
To: i8cvs <domenico.i8cvs@tin.it>; AMSAT-BB <amsat-bb@amsat.org>; William
Erhardt <k7mt@mt.net>
Sent: Tuesday, February 22, 2005 7:57 AM
Subject: Re: [amsat-bb] Satellite Inquiry

> Hi All,

> If we were to use 2304 and 2448 the difference would be 144 MHz. Make TX
> LHCP and RX RHCP some filtering on the satellite and you could have a
> in band transponder with a low Doppler shift. AO-51 has used this approach
> to separate two transmitters. At 2.4 GHz filters can be small and light
> weight.
> Here is some fresh meat for the nay sayers!
> Art, KC6UQH

Hi Art, KC6UQH

If the in band transponder is an inverting transponder than the total
doppler shift is the difference between the doppler at 2304 MHz and the
doppler at 2448 MHz so that it can be manually compensated on the groung
with easy on CW and SSB particularly if the satellite is a HEO type like
AO40 or OSCAR-10-13 wich in turn do not requires automatic antenna
tracking and so apparently things seams to be easy.

On the other side the ground station become more complicated because two
antennas in the same band are required one RHCP to receive and the other
one LHCP to transmit for example or just the reverse depending on our

In addition the ground RX must be saparated almost 90 dB from the in band
ground TX if you want to prevent RX desensing because even if the antennas
have opposite polarizations the IP3 and the -1dB compression points at 2.4
GHz RX input stage are very low and so you need a large use of ground RX
filtering and since the TX power is high than the ground TX side requires
also the use of good low loss interdigital filters.

Remember that with AO40 using a dual band feed or just two separate
ground antennas for the L and S bands a passband filter for 2400 MHz was
required in the S receiving input to prevent desensing from a very far away
1269 MHz uplink.

The effort to built a ground station like the above one using a in band S
transponder can be afforded only by a few numbar of very well skilled
microwave experimenters and it could be justified only for experimentation
if the satellite would be a HEO one offering a high worldwide communication
performance for many years because in this case you have the time to
experiment with your antennas and RX/TX while the HEO satellite is operating
for hours for the experimenter who is investing time and money but also
operates in much lower frequencies in V-U and S bands for the DX user who
buy everyting and like mostly QSLs and DXexpeditions.

You probably remember that a in band transponder operating in 2 meters was
used for OSCAR III just early in 1965 where the satellite was placed in to a
LEO orbit and it was receiving around 145.100 MHz and it was transmitting
around 145.900 MHz in to a 50 KHz BW with a power of 1 watt.

At that time it was easy to sufficiently separate the ground RX from the
ground TX using big cavity filters originally designed for terrestrial
repeaters having shift of 600 KHz but unfortunately the satellite lasted
only for 18 days of operation but about 1000 amateurs in 22 countries
where heard through it.

Why a big success in only 18 days ?

Because the 2 meter band and CW - SSB but perticularly CW where widely used
worldwide in 2 meters and so it was exciting to find a duplexer for a
terrestrial repeater retune everyting in a hurry and try to contact by
satellite having already in hand the right equipments.

If one year from now a new satellite will be placed in a HEO orbit how many
amateurs have actually in hand equipments and antennas ready to work for
2304 MHz and 2448 MHz in addition considering that the Amateur  Satellite
Service is alloved to operate only from 2400 MHz to 2450 MHz and the FCC
never permitted to "officially" switch ON the OSCAR-7   2304.100 MHz
beacon due to regulatory constraints ?

In conclusion a 13 cm in band transponder could be made only between
2400-2450 MHz with a maximum of  less than 50 MHz difference between the
uplink and downlink and not 144 MHz difference wich made things more and
more hard to made both in the satellite and the ground station.

Best 73" de

i8CVS Domenico
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