An interesting discussion guys. I'm glad I was the one that provoked
it. Some interesting and very good points were made.
1. A dipole has a larger null off its ends.
2. The vertical component of LOCAL noise is probably most predominant
because the vertical component propagates farther. (That would be
assuming the noise source is something like a city, and not the power
line on your property.)
Did anyone add this one? The polarization of atmospherically propagated
noise is not constant and not predictable, so that source is just as
likely to be received just as strongly on either antenna.
That leaves any significant difference due to local noise. So you guys
in quiet rural areas already have the answer. For the city guys, lets
look at some simulation results and see if any significant differences
can be identified. (Experimental data may prove difficult because of the
problem of erecting two different antennas at the same location facing
the same direction without interaction.)
I'm going to write this up as I go, so maybe I'll be surprised too.
The first problem is what height to assume for these antennas. To have
the best chance of seeing any difference, a relative high height should
be assumed since at low heights both antennas are going to be very
omni-directional. Inverted vees and dipoles don't have the same gain if
both are erected at the same height, but as a practical matter that is
what we usually compare. Since we need to compare signal to noise ratio
maybe that doesn't matter a lot. For the most vertical polarization,
lets assume the inverted vee has a 90 degree included angle. At much
less than that it becomes pretty much useless anyway.
So we are comparing a signal arriving at either vertical, horizontal, or
tilted polarization to a noise source that is predominately vertically
polarized. Of course there is also circular or elliptical polarization,
but I think I will ignore that for now.
At what elevation angle do we make the comparison? We are assuming that
the predominant local noise source is at a very low angle, maybe 10
degrees or less, but the signal can be at any elevation. Obviously the
signal to noise ratio varies a lot with the signal arrival angle and
polarization, on both antennas. For starters, and for the best chance of
finding a difference, lets assume the signal is coming in at the same
angle as the noise, 10 degrees and it is horizontally polarized. (If
both the noise and signal were vertically polarized, the signal to noise
ratio would be the same for both antennas because both sources would see
the same gain.) Now some data.
80 meter inverted vee with apex at 100ft, 90 degree included angle,
azimuth response at 10 deg el. Numbers are: Az angle in degrees,
Horizontal component in dBi, Vertical component in dBi, signal to noise
ratio in dB. 90 degrees is perpendicular the plane of the antenna.
AZ Horz Vert S/N
90 -2.7 -100 97.3
45 -6.2 -9.3 3.1
25 -10.9 -7.4 -4.5
10 -18.7 -6.8 -11.9
0 -45 -6.4 -38.6
80 meter horizontal dipole at 100ft, azimuth response at 10 degrees
elevation:
AZ Horz Vert S/N
90 -1.1 -100 98.9
45 -4.9 -16.1 11.2
25 -10 -14.5 4.5
10 -18 -14 -4
0 -100 -13.9 -86.1
The huge numbers like 99 dB says there is no significant noise and the
very small numbers like -86 dB says there is no detectable signal, and
these are in the same place for both antennas, so the only difference is
going to be at an off-angle. Looking at the 45 degree angle, the dipole
has a better signal to noise ratio by 8.1 dB and at 10 degrees azimuth
the dipole is better by 7.9 dB, for the conditions assumed. I'm a little
surprised that it is that large.
For signals arriving at a tilted angle, the dipole will still win but
the margin will be smaller (since when the noise and signal are both
vertical, the S/N is the same for both antennas).
When I made the original statement about there being no difference
between a dipole and an inverted vee, I was thinking for ALL noise
sources. I think this is generally still true, since on the low bands
atmospheric noise usually predominates and the polarization of this
noise source is not predictable. There could be exceptions for city
dwellers. However a new statement is now applicable: The dipole has a
BETTER signal to noise ratio for ground wave noise sources if that
source is outside the near field of the antenna.
Kudos to W6WRT and N6RK for good points.
Jerry, K4SAV
Bill Turner wrote:
>ORIGINAL MESSAGE:
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>On Thu, 02 Nov 2006 16:51:32 -0600, K4SAV <RadioIR@charter.net> wrote:
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>>An inverted vee is not more susceptible to QRN than a dipole.
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>------------ REPLY FOLLOWS ------------
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>I have to disagree, for two reasons:
>
>1. An inverted vee has a vertical component of radiation, and man-made
>noise is predominantly vertically polarized, so an inverted vee will
>pick up more man-made noise than a dipole.
>
>2. An inverted vee with an apex angle of about 90 degrees is almost
>perfectly omnidirectional in the azimuth plane, whereas a dipole is
>quite directional, having sharp nulls off the ends. If you are lucky
>enough to have the dipole's end pointed toward a noise source, the
>dipole will be quieter than the vee.
>
>Bill, W6WRT
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