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Re: initial reflections on AO-40 24GHz


Thanks for your posting - it puts the record straight re equipment needs.
In response to some previous comments on the BB, I was in the middle of
preparing a posting along the lines of:

 "The kind of components found in free-running oscillator type ("wideband")
equipment such as police radars, intruder alarms etc will not be of much
use, unfortunately, as they are not stable enough (by a long way).  Don't
waste your money."

You have saved me the trouble!

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)

Charlie G3WDG

At 19:21 14/09/01 EDT, you wrote:
>Hi Folks, 
>I visited again Michael's OH2AUE amazing web-site (tnx Michael) and heard
>K band signals from AO-40, wow!!!!
>Next and as part of my 24GHz endeavour I went trough my microwave books 
>(basically those from ARRL and RSGB) and start digesting info on the topic. 
>Also visited some other repected sites in the net (e.g.. W1GHZ and others). 
>Based on this preliminary review of information below are my initial 
>reflections on AO-40 24GHz. I am writing this because I would like to share 
>them with you, so you can challenge, agree or disagree and most important 
>contribute with info and your knowledge and suggestions on how to walk the 
>path from 2.4 to 24.
>1. 24GHz is going to be a real challenge for those of us that are microwave 
>illiterate (I guess a bunch of us here at the bb). BTW, I am refering to 
>24GHz narrowband (SSB, CW) rquired for AO-40 vs wide band FM (WBFM). Yes, I 
>know we have been doing 2.4 GHz Rx for a while and that technically speaking 
>this is microwaves, but from X band up things are very different (that's why 
>I consider myself illiterate).
>2. At this point I really don't know how though the learning curve is from 
>2.4 to 24 without stopping at X and WBFM. Theory is not that though, but
>the practical point of view it does not seem easy to me (even going the way 
>as per #4 below).  I was expecting to cut my teeth on the high freq
>bands with X band on AO-40 and move from there to K, but it seems it may not 
>be the case. It also seems to me that X band would be easier, more fun (and 
>cheaper) than K mainly because of the availability of a variety of equipment 
>from different sources (surplus included). The later takes me to point #3
>3. There is not much 24GHz gear out there (consumer's worst enemy, no 
>choice). I have not been able to identify sources of equipment other than 
>Khune (DB6NT), and wish we had some other options, do we?
>4. Yes, you could do almost one stop shop (willing to spend some real money 
>1K3 DM in EU, $xxxx in US) and get a beautiful state-of-the-art DB6NT
>mixer, LO, etc... another stop (and toll) to get a 24GHz band pass filter, 
>horn and antenna and assemble all together. However, I wonder how much fun 
>and learning is in there (am I wrong?). Please do not misunderstand me, 
>probably this would be my ultimate goal but would like to walk the path if 
>possible (I really love junk ;-)). As an example of this I had much more fun 
>modifying and improving my TSI AIDC 3733 D/C (now a great performer, tnx to 
>Bob) than installing my SSB UEK2000 in the antenna mast.
>5. Even having and willing to spend the money, some items may be difficult
>very difficult to get (e.g. RHCP, a WG band pass filter, etc)
>6. All the previous makes 24GHz a very attractive proposal
>7. We could discuss this also in Atlanta, would be very nice to have 
>somethign like a working breakfast (don't see much space in the program
>now) in which some 24GHz experts would enlight the illiterate crowd.
>Best 73 de antonio, kc2hax/ea4le
><A HREF="http://www.qsl.net/kc2hax";>http://www.qsl.net/kc2hax</A>
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