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 Angle of Arrival statistics 
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Post Angle of Arrival statistics
A number of HRD members asked for a piece on "Angle of Arrival", so here goes!

An HF signal which propagates via the ionosphere will arrive at the receiver site at some angle above the horizon - we call that elevation angle the "Angle of Arrival"(AoA). The AoA depends on many factors. Obviously simple geometry tells us that the closer the Receiver site is to the Transmitter site, the higher the AoA is likely to be; however there are many other factors at play including the effective height of the ionosphere and the number of "hops" the signal takes, which in turn depend on the frequency, the time of day, the season of the year, and the point in the sunspot cycle.

It helps to know the likely AoA for a particular path because if we compare it to the elevation pattern of an antenna it helps us judge how well that antenna might perform over that particular path.

The ARRL undertook a major project in 1993 to tabulate the arrival angles for signals between major areas of the globe using a computer prediction programme called IONCAP. Many thousands of predictions were run for all levels of solar activity, for all months of the year, and for all hours of the day, and the results were presented statistically as the probability that a particular AoA would be experienced. I've converted the ARRL data into a series of charts based on the UK(London) as the receiving site; to reduce the amount of data I've omitted the WARC bands. The charts show the arrival angle along the horizontal axis, and the probability of it occurring on the vertical axis.

The full ARRL data set is available on the CD which comes with the Antenna Book.

There's a few things to note:
* Modes such as Ground Wave, Sporadic E etc are not included in the analysis
* Most paths are "reciprocal", so the arrival angles on Rx are also the Take-off angles on Tx
* These are long-term average statistics, which means that over the course of a week, a month, or even a year, the full spread of angles shown in a chart may well not be experienced.

Here are the charts:

Image
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So that you can assess the performance of an antenna against the stats, I've also plotted the elevation response of a horizontal half-wave dipole at various heights over average ground. The vertical axis is the response in dBi; the same relative figures will apply to any horizontal antenna - even beams. I've also include a quarter-wave ground-mounted vertical.

The height is expressed in wavelengths so you can apply it to any band. For example 1/8 wavelength would be an 80m dipole at 34ft; 1/4 wavelength would be an 80m dipole at 68ft; 1/2 wavelength would be a 20m dipole at 34ft or a 10m dipole at 17ft etc etc.

Image

Let me know if anything is not clear. I'd also be interested to hear any conclusions you draw from the data.

73,
Steve G3TXQ


Mon Jan 10, 2011 3:42 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
M0TNX wrote:
What do the horizontal figures represent on the antenna figures please Steve? The ones depicting the actual antennas that is....

Sussed it.. Angle of Take off...

Yes - arrival angle or take-off angle. I restricted the range to 1-35 degrees so you can compare directly against the stat charts.

I hope that's clear: if your interest was mainly 20m USA say, ideally you'd want an antenna which covered 3deg - 20deg well. Looking at the antenna charts, a horizontal at 1 wavelength height would be about optimum. 2 wavelengths is too high because it has that nasty null right in the middle of the range.

73,
Steve G3TXQ


Mon Jan 10, 2011 4:09 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
M0TNX wrote:
Looking at it, and from what I read, a good DX antenna will have a strong lobe between
3 and 19 degrees take off?


That would be useful.

What's difficult is to achieve it on a multibander. What's 1 wavelength up on 20m is also 2 wavelengths up on 10m, where there's a horrible null at 15 degrees. See what that would do to the 10m Europe path stats !!!

When trying to choose a "good height" it sometimes pays to watch the nulls rather than the peaks - better to sacrifice a couple of dB of top-end performance than have a 20dB null right in the middle of the expected arrival range.

You'll find the serious contesters often have antennas at different heights on the tower - sometimes to stack them, but more often so that they can switch between them.

73,
Steve G3TXQ


Mon Jan 10, 2011 4:56 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
I should have added that what I do on my web site is explain how to map the antenna elevation pattern against a stats chart to arrive at a single "figure of merit" for any height, expressed in dB.

This is the resulting chart for three 20m paths and three 10m paths. It shows much more clearly how it's possible to be too high - particularly on 10m for the Europe path; that's the 5dB "dip" in the magenta curve as you go from 33ft to 74ft.

Image

Steve G3TXQ


Mon Jan 10, 2011 5:02 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
Ah crap.

Here's one conclusion Steve although I'd happily be corrected.

On the majority of bands on my inverted L, when modelling the antenna the max gain is at a takeoff angle of around 30 degrees on my antenna and when you get down to 10 degrees takeoff, its usually 0dB or a slight loss. However that is 0dBi.

Looking at where I want to work and what I can do here, a far better solution than my inverted L would be a Cobwebb which I can get up 26-30ft and a 40m monoband vertical or something like a Hustler 4/5BTV using the radial field the Inverted L is using or even just whacking up a vertical tuning it on multiple bands using the SGC.

Well, if nothing else Steve, that entire post has told me exactly why its hearing what it is and why I stand a cat in hells chance of working some of the stations I'd like on the current setup.

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Mon Jan 10, 2011 9:05 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
Just realised it's much more meaningful to overlay all the antenna responses on one chart; so I've edited the original posting - do take a look.

I've also added for reference the response of a quarter-wave, ground-mounted vertical, over a fairly good radial system. Notice that a horizontal dipole up at a half wavelength always beats it.

73,
Steve G3TXQ


Mon Jan 10, 2011 9:19 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
M0GVZ wrote:
Looking at where I want to work and what I can do here, a far better solution than my inverted L would be a Cobwebb which I can get up 26-30ft and a 40m monoband vertical or something like a Hustler 4/5BTV using the radial field the Inverted L is using or even just whacking up a vertical tuning it on multiple bands using the SGC.


Yes - a horizontal at around 30ft for 20m thru 10m, plus a vertical for the lower bands is a nice combination to have.

What may surprise some folk is just how low the arrival angles are, even for relatively close-in stuff. For example, at 750 miles range the arrival angle never goes above 50 degrees, and is often much lower.

73,
Steve G3TXQ


Mon Jan 10, 2011 9:30 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
Sorry to have to ask Steve but I'm struggling with the charts as despite reading it twice I'm not really sure what the charts are conveying. :oops:

I'll re-read as I'm sure the penny will drop eventually but the mention of the 1/4 wave vertical has me even more confused as a half wave dipole at 66 feet above ground might well beat my fence mount 1/4 wave but not as much as my neighbours would after installing the dipole ends 66 feet above their gardens.

Cheers,Dave.


Mon Jan 10, 2011 9:51 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
This is something that everyone should learn before they start building and putting up antennas - it'd stop much of the "why can't I hear X, Y or Z when matey up the road can?"

I know for me personally, it'd have saved a load of grief and time trying things that quite plainly were not going to work. Money wise, very little difference thankfully as I've not rushed out and spent money on commercial antennas and all materials are re-useable. At least I've got to practice antenna construction, lol.

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Mon Jan 10, 2011 9:51 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
G7VQE wrote:
Sorry to have to ask Steve but I'm struggling with the charts as despite reading it twice I'm not really sure what the charts are conveying. :oops:

I'll re-read as I'm sure the penny will drop eventually but the mention of the 1/4 wave vertical has me even more confused as a half wave dipole at 66 feet above ground might well beat my fence mount 1/4 wave but not as much as my neighbours would after installing the dipole ends 66 feet above their gardens.

Cheers,Dave.


Simple example. All the top ones show the arrival angles and the probability of getting a contact. The angle is along the bottom and the probability up the side.

Lets take Europe on 20m. The angle with the most probability, the longest bar, is 7 degrees. So with an antenna with the best take-off angle at 7 degrees has the highest probability of making a contact.

Next we have a look at the best antenna height of a dipole or whether a 1/4 wave vertical is better. If you look at the graph with the different coloured lines on, we're looking for the best antenna at an elevation height of 7 degrees - the one we've just worked out has the highest probability. Looking at the graph, a dipole 1 wavelength above ground has the highest gain of the dipoles but the 1/4 wave vertical has a higher gain so the 1/4 wave vertical would give a stronger signal.

Now doing the same for Europe on 10m where 14 degrees elevation has the highest chance, even a 10m dipole just 1.25 metres or about 4ft off the ground has 3dBi more gain than a 1/4 wave vertical.

Doing all of this, you can work out what gives the better performance depending on where you want to work on what band. A 1/4 wave vertical is worse than even a dipole 1/8th wave above the ground where the elevation with the highest chance of a contact falls within 14-16 degrees.

Hope that helps.

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Last edited by M0GVZ on Mon Jan 10, 2011 10:05 pm, edited 2 times in total.



Mon Jan 10, 2011 10:02 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
G7VQE wrote:
Sorry to have to ask Steve but I'm struggling with the charts as despite reading it twice I'm not really sure what the charts are conveying. :oops:


Let's use the Oceania-80m one as an example. The chart tells me that, averaged over a full sunspot cycle, signals on that path will arrive at the UK with a 1 degree elevation angle for 85% of the time, and at 2 degrees elevation angle for 15% of the time; the arrival angle will never get above 2 degrees. That tells me if I want to work Oceania on 80m from the UK I better have an antenna with a very low take-off angle.

Does that help? Do say if it doesn't - there may be others silently watching who don't like to ask ;)

Steve G3TXQ


Mon Jan 10, 2011 10:03 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
Its interesting that apart from Europe on 10m, the 1/4 wave has the better takeoff angle. A simple 1/4wave groundplane is hard to beat.

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Mon Jan 10, 2011 10:13 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
I "double-posted" with Conor.

What he said, except don't focus on just the single most likely angle. In his example, the arrival angle is only 7 degrees for 16% of the time - that means there's 84% of the time it's not 7 degrees.

For Conor: On my web site I show a mathematical method for taking each elevation angle, scaling the antenna Gain at that angle by the probability of occurrence, and averaging over all angles. That gives an accurate single figure of merit for the antenna on that path - that's how the chart in my 4th post is derived.

73,
Steve G3TXQ


Mon Jan 10, 2011 10:14 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
M0GVZ wrote:
Its interesting that apart from Europe on 10m, the 1/4 wave has the better takeoff angle. A simple 1/4wave groundplane is hard to beat.

Conor,

Not sure I follow that conclusion. A dipole at 1/2 WL is always superior to the vertical at any take-off angle - so it must beat it on every path.

73,
Steve G3TXQ


Mon Jan 10, 2011 10:17 pm
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 Angle of Arrival statistics 
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Post Re: Angle of Arrival statistics
Thank you gentlemen,I'm going to look at the charts again only this time with a slightly smug 1/4 wave vertical on 40 look on my face :lol: now if I could only figure who is transmitting all that hash I'm doing such a great job of receiving too!

Cheers,Dave.


Mon Jan 10, 2011 10:21 pm
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