[amsat-bb] Re: eggbeater II antenna performance oscar 50-52
aa2tx at comcast.net
Sat Aug 18 13:20:52 PDT 2007
At 10:20 PM 8/17/2007, Bruce Robertson wrote:
>Do I remember correctly that designs such as this are less circularly
>polarized at the low elevations than they are overhead? If so, have you
>considered the Lindenblad by Tony, AA2TX?
>If you're building either antenna specifically for VO-52, you might
>consider making it LHCP, which I believe is the polarization of the 2m
>signal transmitted by Hamsat. (Tony reports that his RHCP design
>nonetheless receives VO-52 well.)
>A low noise preamp at the antenna will make a huge difference also at 145 MHz.
I have designed, built and used both "Eggbeater" and
Lindenblad (omni-directional) antennas for LEO satellites.
(You can find my "Eggbeater" design in the 1998 AMSAT-NA
Space Symposium Proceedings.)
Both antennas are perfectly reasonable for LEO satellites.
The performance differences between them are subtle and
may not easily be noticed in a simple A/B test.
The "Eggbeater" (i.e. full-wave loop turnstile) antenna
is horizontally polarized at the horizon and at low
elevation angles. At elevation angles above around 50-60
degrees, the vertical component starts to become significant
and the antenna becomes elliptically to circularly polarized.
Directly overhead, the antenna is circularly polarized.
The Lindenblad is circularly polarized at the horizon and at
low elevation angles. As the elevation angle increases, the
vertical component decreases and the axial ratio becomes
bigger (i.e. the circularity gets worse.) A Lindenblad
has a null directly overhead although in actual use,
it is pretty hard to detect this and of course the satellites are
almost never directly overhead anyway.
When the satellite antenna is circularly polarized (i.e. like AO-51,)
there is virtually no overall performance difference between the antennas
although the instantaneous signals levels may differ.
But, when the satellite has a whip antenna, the Lindenblad will
provide less fading at low elevation angles due to its ability
yo accept any linear angle of polarization whereas the "eggbeater"
will show a deep fade if the incoming polarization is nearly vertical.
Many of the current analog satellites use circularly
polarized antennas, so there would not be a great deal of difference
between the two antennas on these satellites.
The Lindenblad has one other advantage and that is that work reasonably
well for terestrial FM repeaters so you can get away with having only one
omni antenna at your station for each band.
73 to all,
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