From: royle@tekig6.PEN.TEK.COM (Roy W Lewallen)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Radio Wave Polarization
Date: 29 Sep 92 01:45:12 GMT



>What is the polarization of the signal from a 1/2-wave dipole antenna
>configured in the shape of the letter L? Assume one leg of the L is
>vertical, the other horizontal, and it is fed with balanced line.

>The obvious (?) answer is that the signal is part horizontal and part
>vertical, or is polarized at a 45 degree angle.

>If the obvious answer is correct (and I do not know if it is or not),
>then what is the signal polarization from a 1/4-wave ground plane or
>vertical antenna fed with balanced line?

>de Miles Abernathy, N5KOB
>miles@emx.cc.utexas.edu

In considering this kind of problem it's helpful to see what the antenna 
looks like from various positions. For example, if you look at a dipole 
from the side, you see a horizontal wire. The polarization from the antenna
is horizontal. But now look at the dipole from the end, above its level:
                                            
                                            you



           ---------- ---------
                  dipole

Now what you see is a vertical line. (That is, facing the antenna, the
apparent width is just the wire diameter, while the height is greater.)
The polarization from the dipole is VERTICAL off the ends. (This doesn't 
seem to be widely known.) The other thing you have to consider is the
nature of the currents on the antenna. For example, look at an inverted
vee from the side:
        .
      .   .
    .       .
  .           .

If the current in the left leg is going up, an equal current at the
corresponding point in the right leg is going down. A vector diagram
adding the currents (a vector up and to the right added to an equal
vector down and to the right) results in a purely horizontal result.
The polarization from this antenna is horizontal from the side. (From
the end it's vertical. Although the vectors are equal and opposite, the
fields arrive at the viewer at different times, so aren't completely out of 
phase.)

Ok, now let's look at your L. It's just a rotated version of the inverted
vee. From the side, the polarization is midway between vertical and
horizontal. (I'm assuming free space for simplicity. The presence of ground
adds a reflected field from each leg as well as altering the relative
currents in the legs, making the situation more complex. The principle is
still the same, though.) In the plane of the L, the polarization is
purely vertical. At other angles from the antenna, it's in between.

In a ground plane antenna, the fields from opposite pairs of radials
cancel directly broadside to the radials because the currents flowing
in them are equal and in opposite physical directions. However, for the
same reason the inverted vee radiates in the plane containing it, the
radials do generate a field in directions other than broadside. (It's much
weaker than a dipole field, though.) Like a dipole, the polarization of
the field from a pair of radials is horizontal when broadside to the pair,
vertical in the plane of the pair, and between at other angles. The
field from the radials is much weaker than the field from the vertical
section, however, resulting in a nearly vertically polarized total field
in all directions. If the radials are bent downward, they generate a
vertically polarized field broadside to each pair. (This works just like
the inverted vee except that the relative phase of the currents is
reversed, resulting in the horizontal, rather than vertical, component
cancelling.) A vertical dipole, of course, radiates a vertically polarized
field in all directions.

This is difficult to explain without pictures, but hope this answers your
questions.

     73,
     Roy Lewallen
     W7EL