In producing a good satellite AMSAT antenna KLM uses the method of
quarter wave stagger of two yagis. One is about a quarter wave ahead of
the other and fed with a 1/4 wave delay line.
Polar plots of this antenna suggest that they are not really producing a
screw sense CP antenna but rather an Axial mode antenna that receives
both vertical and horizontal components of the arriving space signal as
they occur. As Tom points out this may be possible to make for 160
meters but the construction would require significant elevation.
Actually having a high dipole some distance away from a 1/4 vertical
might produce some signal diversity by an axial mode combination. Would
this not be easier to achieve with separate feeds to the receivers and
audio split to each ear?
Herb Schoenbohm, KV4FZ
On 2/5/2014 12:10 PM, Tom W8JI wrote:
Hi Carl,
This has to be the big picture of the system and the goals, and not a
narrow picture of what a wave is doing.
I think in the big picture we all agree it is useless.
First, when I said "advantage", all I meant was there is less fading
on HF
when receiving on a circularly polarized antenna. That's the common
conclusion of those "studies" that I referenced. Remember these
studies are
HF (80-10m), not MF (160m).
While G2HCG likely had circular polarization on ten meters, there
isn't much in the WA3's article that actually convines me he was
observing circular polarization. If he did have circular
polarization, which he probably did have some, it was only basically
"straight up".
This is entirely different than circular polarization at modest or low
angles, which is terribly difficult on any lower band. To be circular
polarized at modest to low angles, the horizontal antenna would have
to somewhat high above ground and broadside to the DX, and the
vertically polarized antenna would have to cross the center line of
that antenna, or have some planned offset.
In other words, it would have to actually be a circular polarized
antenna.
EZNEC actually provides a way to look at this. At the bottom of the
arrow tabs is Desc Options. Click on that, a choice of fields appear
that includes "circular". The bottom choice, "Linear, Maj, Min", gives
a relative comparison of circular to linear. Do a Far Field plot and
look in the FF Tab on the left for a level comparison between linear
and circular fields.
I've seen several enthusiastic studies where a lot of time was spent
with an antenna that really could not measure what the experimenter
concluded he was measuring.
Second, polarization is not purely random. There is more order to
polarization that we generally think due to the ionosphere being
immersed
in a magnetic field. What's important is where the wave enters and
exits the ionosphere - and how well the polarization of the ordinary and
extraordinary waves that propagate thru the ionosphere couples to the
polarization of your antenna. In my mind that theory translates
nicely to
the real-world. One of G2HCG's conclusions from his 10m study
unwittingly
confirmed magneto-ionic theory. I don't think he was even aware of the
effect of a magnetic field on a plasma, so that makes his conclusion all
the better. Yes, the ionosphere is dynamic and varies over the
short-term -
so there is some randomness imparted on the what the ionosphere
dictates.
For the record, G2HCG's conclusion referenced above stated that "It was
immediately apparent that the number of hops to the ionosphere and
back was
totally irrelevant. The polarization of signals must therefore be
controlled by the last hop."
The first issue is actually creating a circularly polarized antenna at
a useful angle that does not deteriorate signal-to-noise. I think that
is a very difficult thing to do unless the target is nearly straight
up. Most people think grabbing any horizontal antenna and delaying or
advancing phase 90 degrees aganst something vertical produces a
circular polarized antenna in any direction at any angle. Nothing is
further from the truth.
A poorly planned antenna might do that in some directions or at some
angles in some directions (with or without the 90 shift), but it will
also result in pattern tilt and pattern change. Adding signals and
noise unpredictably is not a good thing to do. Even if we somehow
manage to improve absolute signal level, we can also easily "improve"
noise level just as much or more.
I haven't looked at higher bands, but on 160 through 40 adjusting for
some optimum mix largely appears to be either a random thing or
useless. On most of HF, at least where I have looked, the same.
Having said all the above, I still say circular polarization on 160m
would
not be beneficial due to just the ordinary wave being useful.
The bottom line is we are S/N driven on HF, not absolute signal level.
What ratio of V to H signal levels do you expect, Carl? What direction
is the rotation? I'm assuming this is actually a circular signal, and
not something rotating very slowly that is causing fades?
73 Tom
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