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RE: New S Band Converter

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 
> > units, ... 1 days test is not conclusive, but today's pass was very 
> > 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 
>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
>Jon Ogden
>NA9D (ex: KE9NA)
>"A life lived in fear is a life half lived."
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