|
Circular Polarization
Circular Polarization is a relatively untalked about subject on 11 Meters, perhaps
this is the first time you have ever heard of such a thing. Most antennas are
oriented to produce linear polarization - either horizontal or
vertical polarization. Sometimes, a creative CBer will turn their beam
at a 45 degree angle, half-way in between horizontal and vertical polarization.
This results in a signal that is weaker than if the owner would have
oriented their antenna the same (polarization) as their neighbor's antenna. This
is still a "linear" polarization.
To make sure we have vertical and horizontal polarization down, lets look
at some animated figures again (these also are in the "Antenna Basics" section).
Do not think these animations are an actual representation of a signal,
they are to merely demonstrate how the electric field is parallel
to the radiating elements.
Horizontal Polarization
Vertical Polarization
It is known that if one station transmits a vertically polarized signal and the local receiving
station is using a horizontally polarized antenna, this will result in
a signal that is 20db weaker at the receiving station's receiver!
Matching the polarization's is a good idea for maximum signal strengths
between two stations. When talking "Skip" (Dxing), you might think you
need to make sure you are using the same polarization as the distant
station for maximum signal strength. In actuality, it might be impossible
to match the polarization of the Distant station - this is because when
the signal "hops" off the ionosphere - it's polarization usually gets rotated -
at a random amount.
(((If you are unsure how signals "skip" great distances,
check out the "Angle of Radiation" section for more insight.)))
To make matter worse, the polarization does not maintain
a set polarization during a period of time. When the incoming signals polarization
matches your antennas polarization, you get the strongest signal strength possible. And when
it is 90 degrees
different (meaning, say your listening on vertical polarization and the signal is coming in on
horizontal) you get the weakest signal possible. As the polarization swaps between
matching your antennas polarization and varying from it, you will hear the signal "fade"
(known as "QSB" to Amateur Radio
Operators). Sometimes it will fade in and out fast - sometimes is will fade out and
say down in strength,
This is just one condition that causes fade that you can help eliminate. Just so you know,
there are a few other phenomena that cause signals to fade. The two other major
things are:
The reflectiveness of the ionosphere - The ionosphere isn't a perfect reflector, and
perhaps is varying the signal quality. This is one type of fading you don't have any
control over!
"Multipath" fading - As the signal travels
from the distant (DX) station, the signal takes more than one path. This results in
the signal arriving at the antenna at slightly different times. Does this sound familiar?
If you read the section on "Yagi", you know that if signals arriving at the driven
element out-of-phase results in signal cancellation, which results in signal loss. As the effect
of multipath varies, it causes signal fade. There isn't too much you can do about
this either! A good beam with a high Front-to-back and Front-to-side ratio can
help combat this. Multipath can cause signals to fade out almost entirely!
Another situation that you might not be able to match your polarization with
the receiving station is that, you are talking to more than one local station, and
they have opposite polarizations (i.e. one station is using vertical and the other is
using horiztonal). When you are listening, you could flip
between polarizations to match the transmitting station for maximum signal,
but when you transmit -- well, I hope you see the problem. Perhaps you can
flip your antenna switch between horizontal and vertical about 10000000 times a second and neither station
will notice.
So, what we know now is, we would like to match our antenna polarization to the
station we are talking to. This is easy when we are talking to a local station, where
our signal travels along the ground. If we have a beam that is capable of both
horizontal and vertical polarization, then we just switch between the two. Now, how do we
deal with this random polarization change of "Skipping" signals? Simple, we
make our antenna receive and transmit horizontal, vertical and EVERY angle between
horizontal and vertical polarization. This is known as "Circular Polarization".
Once we have set our antenna up for circular polarization - it will be really
effective for dealing with talking to vertical and horizontally polarized stations
and for reducing signal fade when talking to distant stations!
I mentioned the two other conditions that cause signal fade to let you know, it (fade)
can't easily be eliminated, so don't think you did something wrong when you still hear signal
fade on "skip" signals with your new circular polarized antenna.
Now, for a crude visual representation of a circular polarized signal.
Circular Polarization
Compared to horizontal and vertical polarization, you can see, it encompassed
every angle. As the signal leaves the antenna, it is actually spinning, not maintaining
a set angle. This is good because no matter what polarization the receiving station
is using, your signal will still come in at the same intensity. On receive,
the same thing is true, no matter what polarization a signal is coming in at,
it will still have the same intensity. When facing the same direction as
the antenna is pointed, a signal that rotates clock-wise as it leaves the antenna
is known as "Right Handed Circular polarization". As signal that rotates counter-clock
wise is known as "Left Handed Circular polarization".
Suitable Antennas for C.P.
Now, lets talk about what antennas we can use to transmit and receive Circular Polarization.
(I am abbreviating Circular Polarization = "CP" from here out!)
Mainly, only dual-polarity beam antennas are good for this. Either yagi, quads or hybrids
that can produce two polarization's can be used. If your beam has two coax connections
on it, you should be good. The Moonraker 4, Moonraker 6, Shooting Star, Comet, PDL II, are some
that I can think of that will work good. It doesn't matter how many elements
your beam has, by the way. Sorry to all the people reading, wanting to make
their vertical radiate CP. You need two opposed polarization.
I can't guarantee results from Jo-Gunn
Star series. I am not entirely convinced those antennas produce two polarization's
that are 90 degrees opposed to each other (exact opposites). Man, I seem to pick
on that antenna! Sorry! However, I was just reading their site, the "V-Series" does
NOT radiate a CP signal.
The magic, how it's done. Its quite simple - you just need to feed both
polarization simultaneously - but most importantly - one polarization must be fed
90 degrees out-of-phase with the other one. What does this mean, you might be thinking,
This is similar to co-phasing, in that you have to make up a special coax harness
to get everything to work correctly. In the case of co-phased beams, you use equal
lengths from the "T" connector so each antenna is fed in-phase. In the case
of CP, you want to make one leg longer than the other, so there is a timing
delay in which one polarization receives the energy first.
Some operators already have switch boxes that can switch to using both horizontal
and vertical polarization at the same time. This does not produce
CP, as CP requires very precise lengths of 50,75 and 95 Ohm coax.
Whether your SWR meter indicates it or not, SWR will double when
paralleling horizontal and vertical polarization when using 50 Ohm coax.
To achieve the proper 90 degree phase difference, you must make one leg of the harness 1/4
Wavelength longer than the other. See figure 1 for details. For best results,
coax harness lengths should be verified electrically for length. This
can be done with a M.F.J. SWR meter or similiar device.
Figure 1 - Special harness to produce Circular Polarization.
Lead / Lag.
I suggest feeding
the vertical polarization feedpoint first. This will produce right hand
polarization. If all DX stations agree to use right hand CP, then losses
can be avoided when other stations running CP use opposite CP .
If stations are using opposite handedness CP, they will experience
the normal 20db reduction found in crossed linear polarization. So, it
will be helpful if everyone that chooses to do CP sticks to one
sense (hopefully clockwise - or right handed). Also, notice the 75 Ohm
matching sections of coax used, like when co-phasing antennas. Since paralleling
two antennas will produce a lower impedance at the "T" connector, we must
use 75 Ohm matching sections to correct the mismatch!
This is a single harness and is a setup for running CP full-time. Although CP has
its advantages, it also has its disadvantages. First off, you must know
that you are likely to receive more "noise" on your receiver when using
CP to receive. This is a side effect from using CP to receive. Secondly,
you must also know your signal will 3db lower when using CP to transmit.
This is because power (watts) is split exactly at the "T" connector to each antenna,
meaning if you are using 4 watts, 2 watts will go to one polarization and 2 watts will go to the
other polarization. From reading this website, you should already know that
doubling your output power results in a 3db gain in your signal - so it stands to reason
that cutting your power in half will result in a reduction of -3db. The splitting
of your power to each polarization results in a signal that is 3db lower to most
linear polarized receiving stations (but NOT to other CP receiving stations,
they pick up all your power).
Although your signal will be 3db lower, the effect of using CP to combat signal fade
will sometimes be a greater advantage than that losted 3db.
Since at any one time it is hard to say which is best (horizontal, vertical or CP) when
talking "Skip", it
is best to be diverse, and have the ability to switch between all three at any one time.
You will probably find that when receiving, its best to switch between vertical
and horizontal to find the best signal and when transmitting, the receiving station
will receive the least amount of fade on your signal if you transmit with CP.
When talking to local stations (groundwave) that have opposite polarization's, it is definitely
to your advantage to use CP.
So that wraps up the talk for now. I want to get feedback on this page and make
some updates. I was working on a simple switching system that allows you to
switch between vertical, horizontal and CP from the shack - but the solution
is not so simple. Hopefully, in the near future I will have a good solution
to this problem. I am open to suggestions...
|
