[amsat-bb] Re: EggBeater
bruninga at usna.edu
Thu Apr 16 10:36:45 PDT 2009
> I have both a commercial and a homebrew Eggbeater
> and am disappointed with both,
Yes, because the best omni in the world is no better (generally)
than a dipole made out of a piece of wire. By definition, these
antennas are OMNI's so they can hear in all directions. This
means they have zero gain. That is the basic law of physics.
> ... performance is far less than with an HT and
> an Arrow.
This is because the arrow is a "gain" antenna. Gain means that
the structure sacrifices gain in all directions to concentrate
it in only one direction. Which then you have to point. It is
impossible to have both gain and omni at the same time.
But what about Gain Vertical OMNI antennas? Well, they have
GAIN in all directions but only on the horizon. To get that
gain on the horizon, they MUST sacrifice gain somewhere else.
They sacrifice all gain that is UP. But that is where
satellites are, so it makes little sense to try to use one of
these, because you still wont hear any satelites, because the
gain you get "on the horizon" (maybe 3 to 6 dB or so) is just
barely enough, but then as the satellite gets higher, it gets
into the null of the antenna.
> Guess an omni can't cut it, at least not from my QTH.
Yes, by definition. An omni cannot hear most of the amateur
satellites on the horizon, because there just isnt enough signal
(except for ISS). You need some gain to hear them on the
horizon. BUT when the satellite gets above 20 to 30 degrees, it
can be TEN times stronger and then ANY omni antenna will work
quite well (and you don't have to point it).
BUT... plot the amount of time a satellite is in view above 25
degrees and it is less than 1/4th of the time.
So there is no free lunch. Either put up an omni and only be
able to hear the satellites for on the order of 25% or less of
their available time in view (but have no moving parts). Or use
a beam to get the gain on the horizon where you need it most,
and plan on keeping it pointed at the satellite.
> Maybe if I could get it up higher, clear of all
> roofs, it would do better.
Not really. Because getting "high" only gets you visiblity to
the horizon where, by definition, you already do not have enough
gain to hear any of the low powered satellites.
> I can make contacts at relatively high sat
> elevations, but can do just as well with a
> $10 dual band ground plane.
Yes! That is exactly what we are talking about here. You
cannot have both. If you want an omni, then a simple ground
plane antenna made out of a few pieces of wire will do just
about as good most of the time as the most expensive "omni"...
Some will argue that you need circular polarization, to
eliminate some fading, but again you can also say that many
times the polarization is opposite and so even the right hand
circular antnena hears fades too when the circularity gets
Go with a simple 1/4 wave ground plane antenna with a preamp,
and hear well, less than 1/3rd of the time. Or us a beam...
> My friend has an Eggbeater and the same preamp
> at a clearer QTH, and he hears substantially better,
> down to 10 degrees elevation in some directions.
Yes, some satellites are stronger than others. But most are not
designed with the 10dB link margin they need to hit omni's
horizon to horizon. Most cannot afford to waste that much power
for the benefit of people with omni's.
Plenty of people corrrectly swear by their omni that it can hear
very well. And tthis is true, but not at low elevations -and-
with the low power satellites. So always make sure you are
comparing apples and apples.
I was shocked during a class today when I went out to listen to
VO-52 and the signals sounded like 20 over S9... But then 30
seconds later, they were down where I expected. Looking at the
track, it was almost directly overhead (and 10 dB) stronger than
when it is a few minutes later.
More information about the AMSAT-BB