Topband: circular polarization on 160m

[Herb Schoenbohm]

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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