On 6/14/2013 3:39 PM, Paul Decker wrote:
I thought it would be a 3db loss to linear polarization.
Correct -- you're dividing power between a V and H components
simultaneously. Therefore, the we de-rate the ERP / RX gain /per/
/plane/ (and thus in the overall system) by 3dB... or about 50%. Not a
deal killer, perhaps, but something to consider.
OTOH, for tropo/ms, who really knows what the polarization is at the two ends.
Correct -- components of both polarities are present, but for most of
the time in terrestrial work, the originally transmitted polarity is
what is /dominantly/ present at the receiving end, with few exceptions.
This fact shouldn't discredit CP for terrestrial operation, however.
It's a balanced field of pros and cons with merits highly dependent on
the system designer's/operator's goals.
There are advantages to being in two planes at once, certainly in space
comm, but even in terrestrial work. Though changes in incident wave
polarization in transit we witness in EME and sometimes in Es are
terrestrially (Ts, Gw) a rare phenomenon, the "multimode op" can realize
mechanical system simplification in the basic CP antenna system by
eliminating the need for two booms/feedlines to accommodate both FM and
SSB/CW/DIG.
In transmit (and receive), multipath effects are less pronounced in CP
than in linear pols. Hence one reason why broadcast FM transmission
systems -- mobile receivers with V-pol only whip antennae are multipath
prone. Rotating the pol helps reduce this multipath. Another major
reason for broadcast FM use of CP is the dual function of TV RX LPDAs
(horizontally polarized) for FM. Two distinct audiences in need of
service in different planes. The broadcaster says: "Gain is cheap.
Take the 3dB antenna gain hit, make it up with amplifier, and fill both
planes with power." Still more advantages exist for FM broadcasters
with CP than is probably appropriate for this discussion.
A beacon transmitter is an ideal candidate for an omnidirectional CP
antenna!!!
But we must also examine the disadvantages one may encounter with CP,
again qualified by system operation goals. In CP receive, your antenna
noise is the sum of the noise in both planes. Speaking about CP gain
antennas such as yagis, eliminating the vertical plane reduces noise
somewhat more than eliminating the horizontal plane (vertical yagis tend
toward slightly larger sidelobes than horiz -- vertical mono- or
di-poles are more omni and thus more noise prone than a horiz dipole.).
This argument starts to fall apart if both planes are identically
omnidirectional in response, such as in an "eggbeater", Lindenblad,
"Superturnstyle" or similar design.
Complex feed structures with many junctions/connectors, etc., are prone
to failure, moisture intrusion, etc. By this feedline coupling, a
change in the characteristic of the antenna in one plane affects the
opposing plane. Further, it can be tough to evaluate the antenna's
health in the normal manner: since the power is divided, in a
compromised feed assembly -- all phase errors being equal -- a simple
VSWR evaluation at the transmitter port of the feedline may only reveal
an acceptable return loss/reflection. Evaluate the system in terms of
"Return Loss": if you're incident is down 3dB at the feedpoint by way
of the divider, your overall returned power indication at the
transmitter port may be down 6dB when compared from the standpoint of
the transmitter directly exciting the feedpoint without the power
divider in the line.
So the brave operator, who is unafraid of optimization,
mechanical/electrical failures and system complication, can envision a
righteously superior system ready for multiple propagation modes and
modulation-pol conventions, in which polarization sense is selectable --
TX/RX in either RHCP, LHCP, Horiz, or Vert, by way of some fancy relay
contraption.
She/He is ready for any polarity........ ;)
The RX noise issue is a serious consideration in weak signal work,
however..... and so some of these operators elect to restrict the
antenna plane to a single linear polarity in light of this and system
simplification.
73,
-Josh / KF4YLM
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