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Re> Quadrifilar home brew



Hi Domenico,

I have a comment on the "calculation" made by John G3BVU:

>I believe about 3.5 dBi is a more honest value for the gain of a good
>quality, uniform distributed radiation, Quadrifilar Helix Antenna.
>

This theory is quite correct, BUT, since many signals on the UHF downlink
are circularily polarized you may add the famous 3dB to that gain.....

So you are back in the 6.x dBi range when receiving a circular polarized
downlink signal with the QFH. (as compared to receive the satellite
with a isotropic "linear polarized" radiator)

This is why some of the commercial users do not measure the gain of
the QFH in dBi, but in dBic (dB over isotropic "circular").
In case of dBic it would really be arround 3.5 dBic... :-)

Furthermore a "good quality" QFH for earth station LEO use will NOT
be "uniform distributed". The good quality ones are usually unevenly
distributed with more gain towards the horizon and less gain overhead.
(therefore compensating "as close as possible" the variation in path loss
due to different satellite/groundstation distance throughout the orbit)

This "uneven distributed ones" (=good quality) QFHs usually tend to start to
pick up the satellite with a few deg. of elevation already in rather good
quality and deliver a very constant good quality signal with almost no
fading
or nulls in the antenna pattern throughout the rest of the orbit regardless
of maximum elevations and directions of the overflight - theoretically.
(of course only if the antenna has a free line of sight)

73 de Oscar DJ0MY
----
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