[amsat-bb] Re: give the newbie the full story on ELK and ARROW please
glasbrenner at mindspring.com
Sun Dec 28 09:15:48 PST 2008
> Arrow and Elk are LINEARLY polarized antennas . . . . the 17,000 mph
> moving satellite is throwing off CIRCULAR polarized RF energy . . .
> please SUBTRACT 3 db or more of gain from your Elk or Arrow when
> receiving the satellite's signal . . . some still do not understand this
> concept and never will . . . ignorance is bliss . . . .
This is not always correct. AO-27 and SO-50 both use linear antennas for
their downlink transmitters. Even with AO-51, the downlink signal is likely
to be elliptical most of the time depending on squint angles. The point to
take away from this is when using a handheld yagi, always try to peak the
signals on both uplink and downlink by rotating the antenna. The up to ~20db
of potential mismatch is more important than the gain of the antenna. All
the current FM satellites use linear whips for the uplink receivers; peaking
the polarity from worst case to best case is like adding a 500 watt
amplifier to your 5 watt HT. To do this on transmit requires full duplex.
Here's another way to look at it. After the launch of AO-51, we heard
several people say AO-27 seemed to have a stronger signal. AO-27 typically
ran a 500mw downlink, and AO-51 ran similar power. Users with handheld yagis
or whips who matched polarity gained 3 db on AO-27 reception as compared
with AO-51 (assuming a close to circular signal from AO-51). That's the same
as effectively doubling the downlink power on AO-27.
Sorry to impede anyone's bliss.
73, Drew KO4MA
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