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Re: Helical array (was ... well never mind)



Terry,

You ask some good questions.  I don't have all the answers, but perhaps
I can point you towards the web sites/people who might have the
answers.  Your plan to get school kids involved in satellites gets my
attention, and I am sure it will get the attention of other operators on
this reflector.

Are you a current operator?  What type of station do you have at home?

Terry Bendell wrote:
>  Actually was thinking of building one for 70 cm and 2 meters, I am
>  interested as I have a readily available supply of 1/8 and 1/4 inch
>  hard drawn copper line stock available. Also I agree that the setup
>  appears to be rather awkward. 
Have you ever seen a 2 meter helix?  Bigger than a bus was my
proclamation, the first time I saw one.  Since switchable polarity
sense, in my opinion, is crucial to satisfying satellite operation, you
would need 4 helixes total.  This would be, again in my opinion, a
monstrosity.  The 70 cm helix pair is manageable however.  My field day
2 meter link is made from a pair of $20.00 Radio Shack FM broadcast
antennas. Switchable, with a coax switch, a pair of tees, and a delay
line, the entire array can be balanced on one finger.  Chris Fargis,
WB2VVV has plans for modifying this antenna on his web site: 
http://www.wb2vvv.com   Let me know if you need further help using this
strategy.

>  Have you ever experimented with the mast
>  mount T.V. Antenna Pre-Amps? I am looking at one here now and it has
>  a noise factor of about 3 db, not too bad but I am going to play with
>  it to see if its feasable to improve the s/n ratio by narrowing the
>  bandwidth a bit. Seems to me with all the garage sales going on here
>  these days you can get some great junk to convert for sat use. 
Both Ramsey, and Hamtronics sell preamp kits that are adequate for mode
J work.  A 3 db noise figure, although marginally usable on 2 meters,
will fail miserably on 70 cm.  Most modern, mast mounted GasFet preamps
have noise figures around .5 db.  On 2 meters, a noise figure of 1.5 db
will work nicely.  Go for as low a figure as you can on 70 cm. 

>  How about feeding that Helical antenna with 75 ohm T.V. Coaxial cable?
>  Looks like it could handle maybe 25-50 watts? 
Hard line is 75 ohm TV coax.  Is that what you mean?  The garden variety
75 ohm line you use at home would certainly handle the power, but is so
lossy, a run of any length would introduce so much thermal noise to your
system, it could render it useless.  Ladder line could be used for
longer runs with proper baluns at both ends, and the loss/cost ratio is
good.  At a minimum, I would recommend 9913 coax.  How much do you need
for the school setup?  I might be willing to finance that part of your
school install.  I have a partial spool of 9913 in my garage.

>  Trying to get a cheap 
>  station setup for the local School who are interested in Sats and
>  Amateur Radio. Found a couple of Alliance U-101 TV rotator's last
>  Fall so may try to come up with a cheap interface circuit so it
>  can be computer controlled. My wife says I tend to be "Stingy", if
>  she means "thrifty" and "forward thinking" I would have to agree with
>  her! But I suspect she ment I am just too cheap to BUY the REAL stuff.
Thrifty is cool.  It is a ham hallmark. :-)  There are some operators
who have made interfaces for the Alliance rotator.  Message Bill Briles,
W0OQC, he made one a few years ago.  Jeff Griffin, KB2WQM, is currently
working on one.  I like the idea of using a basic stamp type controller
for the Alliance rotator.  Mark Hammond, KC4EBR helped develop one.  Try
contacting him.  All 3 of these operators have AMSAT aliases.

I hope to work you soon on one of the birds!

73, Mike
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