[VHFcontesting] Circular polarization for terresterial modes

[Joshua M. Arritt]

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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