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Re: initial reflections on AO-40 24GHz




>X-Sender: charles.suckling@pop.ntlworld.com
++snip++
>I am using a semi-homebrew system here for 24GHz narrow-band based on the
>DB6NT Mk 2 transverter, which I obtained fronm DB6NT a while back as a kit
>consisting of a pcb and two diodes.  Added to this are a HB waveguide
>filter, a DB6NT ready-made WG in/out 30dB gain preamp and a HB horn feed.
>An ex 11GHz sat-tv dish completes the system.  
>
>We are really still in the early stages of evaluating the performance of
>the system.  We have upgraded from a 20cm offset dish which we used to
>receive the first signals from AO-40,  to a 60cm, which produces about
>5-6dB sun noise.  It was just detectable on the 20cm.  The first AO-40 test
>with the 60cm were spoiled to some extent by rain, the effect of which
>still needs evaluating.  At times signals were better than in the first
>test with ssb being copied and some partial blocks decoded.  We also
>managed to "transpond", hearing our (L2) uplink carrier coming back on
>24GHz.   Looking forward to Monday, our next good slot for tests!
>
>You raise the question of polarisation.  It was a surprise to me, but the
>polarisation of the K-Band transmitter is linear. So, in the long term with
>a 3-axis stabilised satellite, there is no need for CP and indeed it would
>cost you 3dB (which judging by signal levels now is 3dB that can be
>ill-afforded).  However, while the satellite remains in spin-stabilised
>mode, the use of a linearly polarised receiver of course results in a
>signal with two very deep fades per rotation.  Possible to make QSO's and
>hear the satellite, but no good for tlm reception!  I am currently building
>a scaled W2IMU horn which I plan to configure for CP with either a
>dielectric slab or screws, simply to get rid of the fading.  
>
>I wonder what the interest would be in a home-constructible design for a
>24GHz receiver?  These days it should be possible to do some or all of the
>more difficult "plumbing" functions on a pcb, provided that some design
>choices were made at the beginning.  If you want a 144MHz IF, for example,
>then the image filter will need to be waveguide.  The choice of a higher IF
>(1296, or even 2401?) would make printed filters a possibility.   If a spec
>could be developed which was interesting to folks, then perhaps some of the
>microwave technical fraternity could take up the challenge and design
>something that could be homebrewed.  This would not satisfy the "convert
>surplus" brigade, but would at least provide some alternative to the
>present supply situation.  SMT construction skills would or course be needed!
>
>As to a draft spec, my proposal would be something like:
>
>Conversion gain:	30dB min
>Noise figure:		2dB max (target)
>IF			commonly used ham band, high enough to allow printed image filters
>			could be 1296, 2401 or other
>RF input port:	waveguide or coax.  Design to include a "reproducible" SMA
>to WG transition.
>stability:		good enough for ssb reception and tlm collection
>LO:			included in the design (possibly a separate unit)
>			
>73
>
>Charlie G3WDG

Charlie,

Re: linear polarization...if, when momentum wheels orient the sat toward
the earth then the spin can be eliminated {or significantly slowed}.  One
would only need to rotate their linear feed for best polarization match,
probably requiring adjustment ocassionally thru the orbit.  Most circular
polarization method introduce loss ahead of the preamp which hurts the
noise figure.

Re: dish...45 to 60 cm sounds practical from the pointing difficulty
standpoint.

Re:  The spec...sounds reasonable to me.

Re: conversion frequency.  Assuming most would use Mode-U for up link,
since most are using mode-US, then the IF of either 144 or 2401 sounds like
the best:  

- 144 is currently used by most for mode-S downlink IF.  
- 432 is not good since this would be used on the uplink.  
- 1296 would require purchasing additional 1296/144 converter for most
stations.  If 1296 is chosen for mode-K first IF, then mode-LK would not be
available for use.  

So I see using 2401 as a good IF since one's existing mode-U downconverter
may used as the second IF [24048-->2401-->145].  The only negative aspect
in using 2401 is that one could not dual-receive both mode-S and mode-K.
Keeping the antenna on point while monitoring the S2 beacon would be would
be convenient for narrowing in dish pointing on 24G.

My harmonic down converter uses an LO of 10,368 MHz and a first IF of 3456
MHz.  My DEM 3456/144 MHz xvtr then will convert to 145 MHz for input to
the FT-847.  Here I run inot a math problem: originally I have F =
(2*10,368) + 3456 = 24,192.  For AO-40 I would need to shift down to 10,298
MHz for the LO.

Ed

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