[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: circular polarized Arrow antenna

> I entered the numbers for the Arrow antenna into the Gene Marcus AO40
link calculator and wasn't thrilled with the uplink I would get with
50W, 1 dB cable loss, and (optimistically) 11 dB gain with linear
polarization.  But the uplink would be more respectable if I convert the
antenna to circular polarization.

That is only true when both antennas are pointed toward each other.
In most cases, the signal will be somewhat elliptical relative to each
other in practical applications. You will only realize 3dB under ideal
conditions using a CP antenna. In practical terms, the ideal condition
does not exist. When the circular polarization becomes highly
elliptical, it starts to resemble a linear polarized signal.

The greatest thing about the handheld Arrow is the ability to rotate the
boom to change the polarization at any instant. This is also a great
learning exercise about the changing polarity of the signal from a
moving satellite. As it moves through space, you can rotate the axis
of the boom to change polarity to peak it for the strongest signal

When satellites reach the horizon you may hear folks speak about the
polarization changing from RHCP to LHCP and vice versa. The happens
as the signal becomes elliptical. When the signal is elliptical it is
also attenuated under CP conditions since the antennas are no longer
pointing directly at each other to provide a truly circular pattern.
Thus, the end result implies that the 3dB advantage has narrowed to a
smaller number or less of the polarity has changed. Without the ability
to change between RH and LH sense with the portable antenna, the
linear antenna may tend to outperform the CP antenna since the Arrow
can be rotated on it's axis. The point I am trying to make is when the
CP signal becomes elliptical it starts to resemble a linear polarized
signal in some respects. A linear antenna may be a good practical
compromise in keeping the antenna smaller and compact to tuck away
in a suitcase. It also takes less time to assemble and disassemble.

> The objective is to have a circular-polarized AO40 uplink antenna that
can be easily disassembled to fit in a suitcase.  Is there an easier way
to get a "traveler's" CP antenna than converting 2 Arrow antennas into a
single CP 70cm antenna?  Is it worth the trouble to make a CP antenna,
compared to just making do with a linear-polarized uplink?  Of course I
should TRY the linear Arrow antenna on the AO40 uplink to see how good
or bad it is.

In my own personal opinion, a portable CP antenna may not be worth the
effort unless the satellite antennas are pointed toward earth to create
an ideal situation to realize the extra gain. A linear Arrow antenna
requires no polarity switching hardware since you can rotate the axis of
the boom to compensate for the relative change in polarity. Hopefully
this does not start a big debate on CP versus linear as I am only
talking in terms of practical real world applications for portable


Tim - N8DEU
Huntsville, Alabama

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org