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RE: Satellite Inquiry



An in-band S-band setup need not be that complicated or expensive.  A
dual-polarity CP patch feed can be easily built which provides roughly
37dB of isolation between ports.  Filters can easily and relatively
inexpensively extend the isolation to over 90dB.  Used to feed a dish of
reasonable gain (1.2m or larger), this would be a great ground station
for LEOs and HEOs and portable enough for Field Day or other portable
applications while making doppler tuning a breeze.

73 de Rick, KG6IAL
www.teksharp.com


-----Original Message-----
From: owner-AMSAT-BB@amsat.org [mailto:owner-AMSAT-BB@amsat.org] On
Behalf Of i8cvs
Sent: Tuesday, February 22, 2005 11:43 AM
To: KC6UQH; AMSAT-BB; William Erhardt
Subject: R: [amsat-bb] 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
linear
> 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
choice.

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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