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

[Bob Snyder <kd1vv@xxxxxxxxxxxx>]

I did some quick helix antenna runs using NEC.  These runs use an
infinite backplane, so they are only approximations, but may be useful
for comparisons.  The 7-turn helix you describe (with 44 mm dia. i.e.
1.106 wavelength circumference; and 37 mm spacing, i.e. 15 degree pitch)
has 11.9 dBi gain.  The 16-turn helix (I assume it is built to nominal 1
wavelength circumference and 12 deg pitch) has 14.8 dBi gain.  The
parabolic reflector on the 7-turner may be enhancing it's gain somewhat.

73,
Bob KD1VV


K5OE@aol.com wrote:
> 
> 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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