[450] Goshen ham fest

[Rich]

Workin on the roof :-(

Have fun if you go!

TMA

-- 
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The Truth is realized in an instant; the Act is practiced step by step. - Zen saying
  ----- Original Message ----- 
  From: Vinnie Grosso 
  To: 144.450 Mailing List 
  Sent: Friday, May 21, 2010 8:43 PM
  Subject: [450] Goshen ham fest


  Is anyone thinking of going to Goshen tomorrow for the Ham Fest?

  I'm seriously thinking

  Vinnie

  On 5/17/2010 6:19 PM, John Foege wrote: 
Well, after searching Craigslist this morning, I found a Uniden
Bearcat 560xlt for $10. I didn't have any specs and I had no clue what
the block diagram of the receiver looked like. The price was right,
however, so I picked it up.

Turns out this Bearcat is almost identical in how it operates as, I
would assume, almost all others. It's a standard 2 stage conversion
receiver. The first IF is 10.7mhz and the second IF 455khz. The first
IF stage filter is just a run-of-the-mill 120kc or something wide
crystal filter. The FM IF demodulator section utilizes an MC3359 IC,
which incorporates a built-in mixer and FM demodulator and various
other assorted goodies. The MC3359 has a section for the external
455khz IF filter, which in this scanner is a Murata SFR-450D 5-pin
ceramic lattice filter. It has a 6db bandwidth of +/- 10khz. That's
pretty standard for a scanner reciever.

The weather satellites are FM modulated and deviate generally +/- 15kc
around the 137mhz carrier. So a 2nd-stage IF filter BW of 30kc would
be appropriate, however, one must also take into account the doppler
shifft of approximately +/- 4.5kc during a pass. Therefore, the
optimal 2nd IF filter BW is +/-20kc or 40kc total.

Unforunately, it is extremely hard to find a ceramic filter that has
this much BW. Best I could find was +/-30kc. According to the NOAA's
manual on constructing a station for APT (Automatic Picture
Transmission) reception, modified scanners with a 30-40kc 2nd IF
passband perform well. So, albeit 10kc low and sub-optimal, the 30kc
bandwidth on my modified bearcat will produce much better pictures
than the original 20kc. It is, after all, a 50% increase in bandwidth.

The only thing to do now is homebrew a quadrafiliar helix antenna for
the 137mhz band and scrap together a simple mast mount preamp for said
QFH antenna to make up for the less-than-spectacular sensitivity of
your typical scanner front-end.

Any comments? Suggestions? Anyone interested in the results or want me
to take some pics and document it? It's a pretty cool little project
for $15.

Scanner $10
New Ceramic filter $5

$15 is way better than $250++ for a new Weather Sat receiver from
various vendors brand new, and it should work just as well!

John
KB1FSX
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-- 
Vinnie Grosso
Vinnie at Vinnievision.com
Office 917-546-6661
Cell 917-697-6229
Fax 212-501-7955
Skype/Twitter: Vinnievision

Let's do work worth doing !!
Megan Blewett

The Best Way to predict the Future, is to invent it !!
Peter Drucker 


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