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Re: New S Band Converter

Isn't there a full duplex problem using a 70cm RX converter IF if you ever
decide to TX uplink on 70cm, assuming we get that option working.  It
really will desense the RX side unless you have a very good isolation of
the RX IF.  I too would like to use a 70cm IF due to the greater tuning
range it will give me at the IF on my IC-475 than the 2M IF does on my
IC-275.  However the desense problem makes me want to go to the 2M IF.  Any
comments?  I'm still deciding.

Roy -- W0SL

paul.willmott@omsl.bm wrote:
> Well I went for a 70cm IF for RX Converter, as most TX converters are only
> available for 2m IF, ... I know there are some that use 70cm but not a lot.
> Given the above, 70cm down IF was the only choice.
> I think that L band up will be available a lot, as this RX is on anyway for
> commanding, ... both 2m and 70cm RX work on the bird, ... so maybe on at
> same time.
> -----Original Message-----
> From: Edward Hall [mailto:wc0y@hotmail.com]
> Sent: Monday, January 15, 2001 3:25 PM
> To: na9d@mindspring.com; paul.willmott@omsl.bm
> Cc: amsat-bb@AMSAT.Org
> Subject: Re: [amsat-bb] New S Band Converter
> I have been debating whether to choose VHF or 70cm as the IF for a converter
> that I plan to build.  One consideration is noise from the image frequency.
> This thread makes me think my concern is valid.
> The higher noise level when using a VHF IF may be caused by higher noise
> from its image frequency.  The image frequency is either 290 (2*145) MHz
> removed from the desired 2401 signal for a VHF IF, or 870 (2*435) MHz
> removed for a 70cm IF.  A non-filtered image passband can add 3dB to the
> receive noise figure; worse if signals other that thermal noise are present
> at the image frequency.
> The amount of image suppression is mostly determined the design of the
> front-end filters in the converter.  However, it is easier to suppress
> signals 870 MHz away (36% of desired frequency) than 290 MHz away (12% of
> desired frequency) from 2401 MHz.  Could this be why listening with a 70cm
> receiver as an IF is observed to be queiter than when using a VHF receiver?
> A second consideration for IF frequency is how the satellite transponders
> will be used.  Will VHF or 70cm be the preferred uplink frequency?  If I use
> a transmit upconverter for the L-Band transponder, will it be fed by the VHF
> or 70cm transmitter?  I would prefer to have the output of the receive
> converter on the band not used for transmit so that full-duplex will be
> possible.  It looks like it might be a while before we know what up/down
> combinations are possible.
> Ward - WC0Y
> >From: Jon Ogden <na9d@mindspring.com>
> >To: <paul.willmott@omsl.bm>
> >CC: <amsat-bb@AMSAT.Org>
> >Subject: Re: [amsat-bb] New S Band Converter
> >Date: Mon, 15 Jan 2001 10:10:50 -0600
> >
> >on 1/15/01 9:45 AM, paul.willmott@omsl.bm at paul.willmott@omsl.bm wrote:
> >
> > > <--- the 70cm IF is a lot nicer to listen to, .... I have both 2m and
> >70cm
> > > units, ... 1 days test is not conclusive, but today's pass was very
> >similar
> > > to a lot of other recent ones, I've never got above 50 CRCC A Blocks on
> >an S
> > > band pass before, ... a few more passes will get a better picture.
> >
> >Why is a 70CM IF better than a 2M IF?  I don't understand how one is
> >quieter
> >over another.  You would think that it wouldn't matter.  Sure 2m is
> >"noisier" but you aren't listening to the 2m on the antenna, but its being
> >pumped directly from the downconverter.  You'd think 70cm might be worse
> >because you would have more insertion loss in your coax.
> >
> >Perhaps I am missing something, but I am curious as to why the 70cm IF is
> >better.
> >
> >73,
> >
> >Jon
> >NA9D
> >
> >-------------------------------------
> >Jon Ogden
> >NA9D (ex: KE9NA)
> >
> >Member:  ARRL, AMSAT, DXCC, NRA
> >
> >http://www.qsl.net/ke9na
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