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Re: Quadrifilar home brew

Hi Folks,

if you research a little bit in several articles published in
non-ham magazines and have a look at commercial use of LEO
satellites in the UHF band the circular polarised antenna of
choice in most commercial applications is in fact the quadrifilar
helix antenna (QFH). It offers excellent circularity over a very
wide hemispherical range and offers a good performance/size ratio
apart from being an omnidirectional antenna.

As mentioned by Art KC6UQH in his message the shape (specially
the lenght/width ratio, but also the number of turns and wavelength
of the four helix elements) of the antenna greatly influence the
shape of the vertical pattern.

The pattern for the antenna published by Dave WB6LLO on his website
can be found in the satellite experimenters handbook. The same antenna
was promoted also by Domenico I8CVS )with exactly the same dimensions
as in Daves construction details.

This antenna works quite well, but has one mayor drawback when it comes
to work the weak signal LEOs (such as AO-51, SO-50, AO-27) at low elevation
angles. This becomes obvious when you look to the antenna pattern, which
in vertical position has the maximum gain overhead (where you dont need it
due to the closer satellite distance and less path losses) and has already
-3dB at 30° elevation and even less gain at lower elevations. This is
confirmed by my observations in the real world, where AO-51 or SO-50
start to get very weak when below 25-30° elevation, but excellent above
that. (with a GAsFET preamp)

I Have built a couple of LEO antennas lately such as an eggbeater or the
Eggbeater II, (both very lousy performers at low elevations - in my opinion
due to their horizontal polarisation at low angles, and very "nully").
Both were easily outperformed by a simple 1/4 groundplane + preamp.

The QFH based on Dave/Domenico dimensions works well (much better than
most other omnidirectional antennas out there). But the one promoted by
KC6UQH is much better offering maximum gain at approx 25° Elevation and
showing a low gain overhead (compensated by less path loss) and thus
giving you a "true" almost hemispherical coverage.

Currently I am building the "tall" variant based on the design of Art

This antenna was mentioned here by Art a couple of times, but never got any
response. I think the people underestimate QFH antennas - but the many
commercial users of this antennas for LEO applications cant be wrong !

I am almost finished with building the antenna and I have taken a lot of
photos from the construction process. I am planning the write a small
article on how to construct this antenna for my website including the
photos for illustration. It will be made public here on the mailing
list as soon as it is ready. I will also post some reports on signals
received at different elevations with the new antenna when ready.

Most important - always use a good preamp with omnidirectional antennas !

Best 73 de Oscar DJ0MY
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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