70 lines
3.4 KiB
Plaintext
70 lines
3.4 KiB
Plaintext
For those of you interested in listening to the 242 to 270Mhz
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Milsat band, here are some tips and equipment ideas to get you on
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the air. With our current activities in the Gulf, there's plenty to
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hear !
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The antenna I'm using is a 12 turn axial mode helix...the preamp is
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a 20Db gasfet from Advanced Receiver Research. They advertise in QST.
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They dont stock a model for the Milsat band, but they'll gladly build
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you one for around $100. Mine has a center freq. of 262Mhz.
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The receiver is a Yaesu FRG-9600. It's continuous coverage from 60 to
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905Mhz, all mode receive. Great little radio.
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The helix antenna is VERY easy to build. For a center freq. of 262Mhz
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the following dimensions will give you about 16Db gain over the entire
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242 to 270Mhz Milsat band: Diameter of the helix - 14.5 inches
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Circumference of helix - 45.4 inches
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pitch or spacing of turns - 11.35 inches
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The dimensions are not critical. If you're within 10% of these
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numbers it'll work fine. The helix element can be constructed from
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refrigeration type copper tubing, 1/4 inch diameter. The reflector
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can be anything from chicken wire to hardware cloth. If you make it
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square, it should be .8 to 1.0 wavelengths on a side...so 45 inches
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will work well.
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The polarization is Right Hand Circular...in other words,if you're
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standing behind the antenna, at the feedpoint end, the turns of the
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coil should spiral away from you in a clockwise direction. The nominal
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impedance of the above antenna is around 140 ohms. To get a good match
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to 52 ohm coax, try making the first quarter turn at the feedpoint end
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run parallel to the reflector at about 1/8 to 1/4 inch spacing from it.
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This will act as an impedance matching transformer and give you a
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pretty good match.
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DO use good low-loss coax. Belden 9913 will give you the best results.
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This antenna has a beamwidth of 30 degrees and is VERY broadbanded.
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Be sure to use a non-conductive boom...hardwood works, fiberglass works
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much better. PVC tubing has too much sag...don't even bother !
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What will you hear ?? ALL kinds of things !!! There are many
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different satellites and modes on this band. To really grasp what's
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up there, Larry Van Horn's book "Communications Satellites" is REQUIRED
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reading.( Grove Enterprises-Box 98-Brasstown,N.C.-28902)
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As an example, I've heard everything from the Secretary of
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State, to McMurdo Station Antarctica, to Soviet telephone intercepts to
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A-10 and F-16 drivers involved in the current conflict. And that's
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just the stuff that is "in the clear"! The Tactical traffic from Iraq
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is in the clear in am mode, and really should'nt be there at all. Due
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to poor frequency management, the UHF com frequency they've chosen
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falls within the uplink passband of one of the satellite transponders.
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So, they're getting re-broadcast on the satellite just like your local
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2 meter repeater ! (Very Weak...though copyable)
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80% of the signals in this band ARE digitally encrypted...but the
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other 20% are some of the most interesting I've ever monitored.
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The strongest signals are from Fleetsatcom 1(Pacific relay) in orbit
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at 100 degrees West, and Fleetsatcom 3 (Atlantic relay), at 23 degrees
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West. Try tuning 260 to 262.4 Mhz. Some of the signals are strong
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enough to copy with only a ground plane or discone antenna.
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Have FUN !!! 73 de Carl WN8U
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/EXIT
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