[NLRS] Downconverters

[John P. Toscano]

john scherer wrote:

> I notice some people use radios capable of receiving 23cm and run
> hardline from the antenna to the radio, while others use a
> downconverter to bring the signal to a more "cable" friendly region
> like 2 meters.

Hi, John.

As others have already told you, the best reception will come from a
system that has at least the first stage of RF amplification at the
antenna, and doing the downconversion right there is the best way to
go.  The nice thing about this, at least for Oscar-40, is that you
can buy a fully weatherized MMDS downconverter quite inexpensively,
and either modify it for best reception, or do minimal mods and add
a weatherized preamp in front of it.  That's the route I'm going.  I
have both a Drake 2880 and a CalAmp xxxx to play with, and a Down
East Microwave preamp to go in front of either one.  I plan to run
RG-59 down to the shack for the 122 MHz IF, and will probably need
to insert even MORE attenuation once the signal reaches the shack.
I am probably also going to split the feed and send it to two radios,
so I can use my FT-847 for CW/SSB reception and my IC-706 to monitor
the Middle Beacon telemetry at the same time.  Each radio can be
tuned independently to a different part of the passband from the
single downconverted feed.

There is a wonderful Excel spreadsheet on the AMSAT site that lets
you play with the whole receive chain setup, and see just what
happens with various combinations.  It basically has cells that you
fill in for each step:
  antenna ......... gain, and linear vs. circular polarity
  feedline 1....... loss, antenna to preamp (0 if no feedline)
  preamp .......... gain, NF  (or insert 0 if no preamp here)
  feedline 2 ...... loss, preamp to converter (0 if none)
  converter ....... gain, NF (0 if using a 13cm receiver instead)
  feedline 3 ...... loss, converter to IF radio (0 if no feedline)
  receiver ........ NF
 
You can see the effect of a long run of coax from antenna to shack
(a serious problem if feeding 2400 MHz, and nearly of no consequence
if feeding 122 MHz or 144 MHz), how much improvement to expect from
adding a preamp, etc.  

The spreadsheet was prepared by Gene, W3PM, and can be found at:
  http://www.amsat.org/amsat/ftp/software/spreadsheet/w3pm-ao40-v2.1.zip

Good luck with preparing your station!
John (W0JT)