[amsat-bb] S-Band Antenna Comparisons, etc.

[K5OE@xxxxxxx]

I did some antenna experiments this AM when AO-40 came over my horizon (1200 
utc, MA=4.5).  Those window-shopping the various options may be interested in 
these observations.  What can be determined from current observations with 
respect to the final satellite configuration is subject to considerable 
speculation:  these observations were taken with a squint of greater than 140 
degrees on AO-40's low gain antenna at low power.  What one can extrapolate 
to a favorable orbit, orientation, and squint, plus the change to the 
high-gain antenna and the high power amplifier, is clearly just speculative 
at this time. 

In a quasi-designed-experiment, I used several antenna and downconverter 
combinations using a Drake 2880 and my Yaesu FT-100 as a receiver, and 
recorded the results enumerated below.  One unknown in my measurement system 
is the accuracy of the meter on the Yaesu.  I have previously "calibrated" it 
using a variable power 2 m FM signal, but only in the crudest sense (after 
all, I am an "amateur").

1.  The 24 dBi dish antenna (bar-b-que style, borrowed from KK5DO), does 
indeed offer a 12 dBi gain (2 full S-units) over my 16-turn helix (estimated 
at 12 dBi).  This was consistent with-and-without a preamp.  Note to those 
without automatic antenna tracking:  this antenna's beamwidth was so narrow, 
that 12 dB of signal was lost in +/- 5 degrees of movement.

2.  The Conifer preamp does NOT provide the rated 10 dBi gain, but appears 
much closer to 6 dBi (when close-coupled to the Drake).  It was difficult to 
judge the impact on S/N ratio with this setup, but I believe it was 
substantially improved, even though the noise floor is up 6 dB as well.  

3.  At MA=18, I could detect the beacon with a coffee can and preamp/Drake 
combination, but it did not move the S-meter.  I could not hear the beacon 
without the preamp.

4.  Using an unmodified Drake 2880, I could not detect the beacon at 121.3xx 
MHz with both the dish and the preamp in line.  This surprised me.  If I have 
the frequency conversion for the stock crystal wrong, please let me know.

5.  In comparing the 16-turn helix I have been using for a month to a much 
smaller one built for me by my friend Oleg, RV3TH, the two were fairly 
comparable.  The interesting thing is that helix is only 7-turns!   Oleg's 
helix employs some non-conventional metrics:  the diameter is about 44 mm and 
the turns spacing is about 37 mm, plus it uses a 100 mm semi-parabolic 
reflector.  I will report more on this after I completely reverse-engineer it 
:-)

For those interested in seeing the hardware, I have put a few pictures of 
this equipment at:
http://home.swbell.net/k5oe

73,
Jerry, K5OE


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