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take off angle
- Thread starter ghz24
- Start date
Very simply, there isn't one. A desired signal doesn't always arrive at any particular angle, low, medium or high. What may arrive at one angle today may arrive at a different angle tomorrow. Propagation plays a big part in that along with how/where the antenna is mounted/situated.
So what do you do? You pick a particular 'type' of antenna that generally has a good radiation pattern to fit your particular need, omni-directional/directional, then polarity, horizontal/vertical, and then it's height above ground in some cases. Optimize all of that for the best signal from someplace. The 'catch' is that tomorrow it may change because of propagation and your 'perfect' antenna system ain't what it was yesterday (or an hour ago). So there's no -one- characteristic of any antenna that's always going to be the best.
In general, meaning that it's always going to change, a moderate to low angle of arrival/TOA is nice to have. But don't obsess about it.
- 'Doc
So what do you do? You pick a particular 'type' of antenna that generally has a good radiation pattern to fit your particular need, omni-directional/directional, then polarity, horizontal/vertical, and then it's height above ground in some cases. Optimize all of that for the best signal from someplace. The 'catch' is that tomorrow it may change because of propagation and your 'perfect' antenna system ain't what it was yesterday (or an hour ago). So there's no -one- characteristic of any antenna that's always going to be the best.
In general, meaning that it's always going to change, a moderate to low angle of arrival/TOA is nice to have. But don't obsess about it.
- 'Doc
http://www.worldwidedx.com/amateur-...myth-surrounding-antenna-take-off-angles.html
Take note of what post #21 in that thread says.
Take note of what post #21 in that thread says.
Not always a myth?
I was aware of the thread (in Amateur Radio Antennas)
That's why I opened a new thread here in CB antennas instead of posting to that thread.
Is the Myth of take off angle article relevant to the narrow band of CB ?
Does MUF ever approach 27 MHz?
Don't get me wrong I'm in agreement with a lot of the article.
However, the author is a ham who uses frequencies that are close to the MUF and therefor have high critical angles (up to 90 deg.)
While trying to find an answer through google I stumbled on this blog.
http://www.qrz.com/db/EI8GQB
It's a large page so I'll quote a small bit so at least you can find where I'm referring to.
So there is at least statistical evidence for a maximum angle beyond which signals escape into space.
I did apply some trig to the numbers 781 Km and the (height of ) the highest and lowest possible reflective layers of the atmosphere and got 18 and 23 degrees as the two answers. Suggesting any signal above 23 degrees is not reflected and escapes to space.
But I used a flat earth instead of curved (to simplify the math) and 28 MHz is higher than 27 for CB.
Overall I considered it a rough approximate.
I guess I'm arguing that at least at CB frequency there is a statistically optimal angle where it would be nice to concentrate the center of the sensitivity of your antenna around.
I was aware of the thread (in Amateur Radio Antennas)
That's why I opened a new thread here in CB antennas instead of posting to that thread.
Is the Myth of take off angle article relevant to the narrow band of CB ?
Does MUF ever approach 27 MHz?
Don't get me wrong I'm in agreement with a lot of the article.
However, the author is a ham who uses frequencies that are close to the MUF and therefor have high critical angles (up to 90 deg.)
While trying to find an answer through google I stumbled on this blog.
http://www.qrz.com/db/EI8GQB
It's a large page so I'll quote a small bit so at least you can find where I'm referring to.
So there is at least statistical evidence for a maximum angle beyond which signals escape into space.
I did apply some trig to the numbers 781 Km and the (height of ) the highest and lowest possible reflective layers of the atmosphere and got 18 and 23 degrees as the two answers. Suggesting any signal above 23 degrees is not reflected and escapes to space.
But I used a flat earth instead of curved (to simplify the math) and 28 MHz is higher than 27 for CB.
Overall I considered it a rough approximate.
I guess I'm arguing that at least at CB frequency there is a statistically optimal angle where it would be nice to concentrate the center of the sensitivity of your antenna around.
Yes
Yes there are times, depending on conditions, when the MUF is high enough to make skywave contacts on the CB band with other parts of the world.
The DB
Antennas are antennas it doesn't matter if they are CB or ham antennas or CB or ham bands. TOA is TOA.
There are times when the MUF exceeds 50 MHz ie. F2 propagation on 6m. No band whether 40m or 11m or 6m has an optimum TOA. Any TOA is determined by distance from the transmitter and height of the reflecting layer.
Try not to take this the wrong way but it seems like the only ones really wound up about achieving the optimum critical TOA are the newbies that have read a couple of articles or listened to a couple conversations. The old timers never seem to get too wound up about it. They have the experience to be able to understand what really matters and that TOA only matters if you have the ability to change it. Since distances vary and the reflection layer height changes it is necessary to be able to vary your TOA in order to achieve the optimum TOA. Unless you have multiple stacked antennas to choose from you can't change your TOA. As I have repeated many times, placement of nulls is many times more important than maximizing your main lobe since the difference in maximizing the main lobe may make a difference of 2 or 3 dB while placing a null in your target area can drop your signal by 20 or 30 dB.
There are times when the MUF exceeds 50 MHz ie. F2 propagation on 6m. No band whether 40m or 11m or 6m has an optimum TOA. Any TOA is determined by distance from the transmitter and height of the reflecting layer.
Try not to take this the wrong way but it seems like the only ones really wound up about achieving the optimum critical TOA are the newbies that have read a couple of articles or listened to a couple conversations. The old timers never seem to get too wound up about it. They have the experience to be able to understand what really matters and that TOA only matters if you have the ability to change it. Since distances vary and the reflection layer height changes it is necessary to be able to vary your TOA in order to achieve the optimum TOA. Unless you have multiple stacked antennas to choose from you can't change your TOA. As I have repeated many times, placement of nulls is many times more important than maximizing your main lobe since the difference in maximizing the main lobe may make a difference of 2 or 3 dB while placing a null in your target area can drop your signal by 20 or 30 dB.
Thanks to all who responded .
I can tell by the replies that I misused the term MUF and should have used "critical frequency" instead of max useable frequency.
So yes I know it's possible to make skywave contacts on the CB band with other parts of the world.
So my question should have been does critical frequency ever approach 27 Mhz
I was under the impression that NVIS did not work at 27 MHz. and critical frequency never rose much above 7 MHz.
Doesn't that mean that there is some angle above which 27 MHz signals are virtually never going to arrive at?
Doesn't this quote support that supposition?
What I really want is that threshold angle and the angle that a reflected signal is most likely to arrive at, sort of a statistically median value.
I'm not overly infatuated with TOA to me it is just another number used to describe an antennas performance as in the angle at which an antenna shows highest gain.
I can tell by the replies that I misused the term MUF and should have used "critical frequency" instead of max useable frequency.
So yes I know it's possible to make skywave contacts on the CB band with other parts of the world.
So my question should have been does critical frequency ever approach 27 Mhz
I was under the impression that NVIS did not work at 27 MHz. and critical frequency never rose much above 7 MHz.
Doesn't that mean that there is some angle above which 27 MHz signals are virtually never going to arrive at?
Doesn't this quote support that supposition?
Doesn't the skipped zone between the circles prove that above some threshold angle reflection/refraction is not occurring?
OK very poor choice of the term "optimum" .I can see your objection.
What I really want is that threshold angle and the angle that a reflected signal is most likely to arrive at, sort of a statistically median value.
I'm not overly infatuated with TOA to me it is just another number used to describe an antennas performance as in the angle at which an antenna shows highest gain.
Not looking for a dynamic adjustable system just a "target area" to shot at in the design phase.
Rather than cover what others have said, I'll ask a question, what is it you are hoping to achieve?
Maybe no 'craps', but I'll bet you've taken chances on something at one time or another. It amounts to the same thing, you can only wait to see what happens. And hope it happens in your favor.
- 'Doc
I don't shoot craps either! I loose most of the time...
- 'Doc
I don't shoot craps either! I loose most of the time...
OK you may all consider this a fools quest. And it may well be but it's mine.
I'm working on the design (model) of a 4 element circularly polarized electrically steered phased array antenna.
I'm aware of the article denouncing circular polarization by Tom the author of the take off angle myth article. If you desire to discuss CP start another thread and I'll submit the reasons I still want to investigate this.
Basic stats:
max gain 7.98 dbic @ 14 degrees
F/b 21 Db to 31 Db depending on angle
-3 Db horizontal beamwidth 122 degrees
-3 Db vertical beamwidth (actually height)16 degrees ( from 8 to 24 degrees)
but axial ratio is real good ( > .95 ) from about 10 through 22 degrees.
Below 10 degrees the ratio is not bad but from 24 up the ratio degrades due to a null in the horizontal gain, at 32 degrees almost all the gain is vertical.
I could eliminate the null (by mounting lower) at the cost of a Db or so. and adjust the axial ratio but of course every thing is a compromise.
I got about 12 -16 degrees of performance( from this model) where I am happy with the predicted performance may get twice that from a less aggressive gain lower mounted version but ultimately I have to decide where to aim the point (angle) where the axial ratio will be the best I can get.
And if high angles of arrival are worth listening/designing for..
The model was created before I did any mathematical attempts to find the limits of the angle of arrival so any correlation in the two are coincidence.
If you subscribe to the height is might school of thought and you believe the
Myth of TOA is applicable to all frequencies you should see what a horizontal dipoles pattern looks like at any height above about 1/2 wavelength.
Maybe the fellow in Ireland got the results he did due to nulls in his front pattern, but it looked like the propagation software and the actual contacts correlated well.
Could that fellow ever talk to the coast of France or is that in a perpetual skip zone?