[amsat-bb] Re: Quadrifilar Helices

[Robert Bruninga]

>> Does anyone know of a website or of construction 
>> plans for an 70 cm Quadrifilar Helix antenna?
>> I have heard that they work  quite well for an
>> omni-directional LEO antenna.  I am looking for 
>> an improvement over my Eggbeaters...

Not to belabor the point, and just to clarify... but the "ideal"
omni antenna can never be better than - 2 dB over a dipole.  In
other words, "worse" than a dipole by 2 dB.  So be careful about
trying to get "omni" improvement over what might already be all
there is.

A cross polarized antenna like the quadrifilar can be as much as
5 dB worse than a dipole, while still correctly making the claim
as being the BEST hemispherical omni antenna.  Which it is...
If...  It all depends on what "best" means.  No antenna can give
gain anywhere, without giving up gain somewhere else.  The ideal
omni is equally poor in all directions.

The "best" antenna with equal gain in all directions and
horizon-to-horizon cannot possibly have enough gain at the
horizon to pull in some of our weak AMSATS, while it will have
excess gain overhead where it is not needed.  

A possible shaped gain antenna might try to add more of the gain
at the horizon at the expense of less gain overhead will result
in more or less weak signals everywhere.  But on the other hand,
satellites spend 70% of their time below 22 degrees.  If you can
see down that far, then go for it.  But the satellites are 10 dB
farther away down there and most simply will not be decodable on
any OMNI that low.

A simple antenna that concentrates its gain above about 15
degrees is simply a 1/4 wave whip over a ground plane.  Yes it
has a null directly overhead, but the satellite is in that cone
above 60 degrees less than 2% of all times in view.  So it is
better to give up that gain where it is not used, and let more
of that gain contribute to the 5 dBi or so gain that the 1/4
wave whip gives between 15 and 60 where the satellite will come
in nice and strong.

If you cannot see below 25 degrees from your location, then you
can get 3 dB more gain and get great signals, but only above 25
degrees.  Use a 3/4 wave whip which will give almost 8 dBi gain
between the 25 to 70 degree range, and since the satellite is 6
to 10 dB closer in this area, then you get almost 14 to 18 dB
better performance above about 25 deg than the BEST theoretical
omni (isotropic) antenna that is trying to pull in the weak ones
at the horizon and everywhere else.

There is no holy grail.  Quadrifilar's and Eggbeaters are ideal
and the BEST hemispherical antennas in all directions and
polarizations... They are perfect for good strong satellite
downlinks.  But only if there is enough signal there in the
first place to be above the noise floor of your preamp.  In most
AMSAT's, on the horizon, that signal simply is not strong
enough.  SO in my opinion, it is a waste to have the gain on the
horizon at the expense of gain higher up where you can use it
for better signals...

Just a data point.  See the discussion at the bottom 1/3rd of my
ASTARS page:

http://www.ew.usna.edu/~bruninga/astars.html


Bob, WB4APR