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Re: Mode-S Antennas

Mark and all:

Here is what I'm building.  

I have a couple RCA 18 inch [46 cm] offset feed dishes* designed for the
ku-band DSS satellite TV service.  I plan to feed the dish using a LHCP
5-turn helix [which will result in receiving RHCP].  I will mount a preamp
at the feed and split the output to two Drake 2880 convertors.  One conv.
is modified for the 122 MHz IF [no xtal change], and the second conv. is
modified for 433.9 MHz IF.  This way I'll be ready for modes US and LS.  No
RF stage modification has been done on either; the preamp should overcome
the NF shortcomings, I hope.  W1GHZ is the expert in setting up these
dishes and has been [previously mentioned on this list].

The 5-turn helix feed was chosen as easy to make.  The helix size is
selected to match the apparent dish f/d of .70.  The dishes were obtained
from MCM Electronics:  http://www.mcmelectronics.com

I purchased the SMC dish [p/n 221198] which is listed for $ 24.25.  They
also list 18 inch steel dishes for $17.66.
This price is for the dish, only.  You also need the feed mounting bracket
[221200] $16.38, and dish mounting hardware:  nuts [212056] $1.84 and
washers [212057] $1.08.  The total comes to $43.55 plus shipping from MCM
in Ohio [1998 prices!].  You definitely need the feed mount bracket to
properly place the feed.  You will need to make an additional bracket that
will allow moving your 2.4G feed in and out to find the optimum focal
point.  Point the feed at the dish center.

There is a wall-mount base tube which you may want [221202] $13.50.  I will
mount my dish on a cross boom so I don't need this last item. Note that all
the above p/n's refer to the SMC dish; different p/n's apply for the metal

A question that may occur to you is whether the 18 inch dish is large
enough?  Based on the pre-launch predicted link analysis this dish will
result in a SNR of 20 dB [with a 1.5 dBNF preamp].  It may be a bit better,
say 21 dB, due to the better efficiency of the offset dish vs a centerfed
dish.  So I believe this 18 inch dish will do fine for AO-40 after it is
commissioned in a stable orbit and earth-oriented attitude.

For receiving telemetry now?
In the current attitude the 18 inch dish may not suffice for other than
optimum MA and using a low NF preamp ahead of the Drake.  Larger dishes are
made for ku-band sat-TV.  I have a 33 inch dish for TV on my roof.  There
are 39 and even 48 inch dishes available [at a price].  Finding a source
for just the antennas is the problem.  You probably can get them through a
local satellite dealer, but at full retail price!  It might be interesting
to see if MCM can get the 33 or 39 inch dish.  I would guess a price range
of $70-100.

Downeast Microwave is selling spun aluminum dishes.  The DH6 is 60 cm and
DH9 is 90 cm.  The DH6 dish starts at $40-75.  No price listed for the 90
cm dish.  http://www.downeastmicrowave.com/Antennas.htm

Regarding receiving current tlm, I have an old 4 foot fiberglass microwave
dish in the barn which could be pressed into use [need a mount].  I also
have an 8 foot [2.5m] dish which isn't trackable at present [pointed at
az=180, el=36 and looking for SETI on 1420 MHz right now], and a 16 foot
[5m] dish in the barn...but nothing ready for the near future other than
the little 18 incher.

PS: *These were bought with 10 GHz roving in mind, not satellite specifically.
PSS:  If you use the conifer bbq grill dish you will need to line it with
mesh or foil to support circular polarization!

>From: "Mark D. Fossum" <n0nsv@AMSAT.Org>
>Hi Everyone,
>Well now that so many of us have our Drake 2880 downconverters and our
>crystals are on the way, I'm interested to know how everyone plans to
>get a signal to it. What is the consensus for a good 2.4ghz antenna?
>Helix, dish, yagi? 
>Personally, I'd like to find another Conifer Dish like Myers used to
>sell. I had one but it got mangled due to the effects of sudden
>deceleration. Prices became way out-of-line (my opinion) when Andrews
>bought the company so I'm looking for an alternative. Homebrewing isn't
>an option for me, I'm all thumbs and my last project made the mangled
>Conifer look like a masterpiece. 
>Are there any good commercial antennas that I should consider? It would
>be kinda fun to see if my surgical desoldering efforts in this 2880 are
>going to leave it operable. 
>Best wishes from the cold north of Minnesota! (-12F this morning) 
>Mark Fossum - n0nsv
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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