take off angle

[wavrider]

It appears you have done the research.

You are working on a design, experimental and interesting.

You most likely know the radiation pattern of a simple dipole at given height.

If you do not then GOOGLE is a great tool.

Arrival of signals varies from day to day, hour to hour, minute to minute.

Some say the fading effect is the signal changing polarization, I tend to believe it is most likely mother nature putting her twist on things.

You want to find the best height on any given day for arriving signals?

Mount that experimental antenna on a hazer and raise and lower it until you find the signal strength you want.

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?

 

[Lil'Yeshua]

He's been doing his homework.

 

[ghz24]

Take off angle proven real

He's been doing his homework.

Actually I was hoping to copy off someone else and not have to do the home work.
But I've done it now.
You guys can copy mine now instead. <gotproof>
Critical angle is directly related to critical frequency.
Critical frequency can be found in "near real time" here

Then you can use Snells or Secant law F = Fc / ( sec θ )
Where :
Fc is critical frequency
F is a higher frequency (~27.1 for CB)
θ "is the angle of incidence, measured from normal to the plane of the layer".
that is the angle that a radio wave would make to a vertical line.(angle to the horizontal is 90- this angle)

This site gives a better description that I can at the end of chapter 2 see figure 2-3 and previous paragraph.

So the angle above which CB frequency waves are lost into space and your antenna doesn't need to put sensitivity/gain can be calculated from the critical frequency using the formula (solved for θ ).

θ =90-(Inv cos Fc/F)

Feel free to check my math.

Watching the real time map for a few days the highest critical frequency I saw was 12 MHz in the tropics.( 5 was the best in temperate North America at my 20)


From that optimistic value of 12 MHz I get a max angle of arrival of just over 26 degrees.
(only 3 degrees off my earlier estimate)
@ Fc = 5 the max angle was 10 degrees, much lower than even I expected.

So does that prove reality of take off and arrival angles?
Did I miss something?

 

[Shockwave]

Changing from 36 feet to 72 feet ruined my ability to talk DX in the areas I normally did. What a disappointment that was. It would seem that 14 degrees is an angle that works much DX on this band. It is the nulls that will get you because at 72 feet the deepest null is placed right where the primary lobe is at 36 feet!

Another thing I'll mention is last week I heard Motor Mouth Maul talking about raising a huge beam up to over 80 feet and that he was going to add an azimuth rotor to make the TOA adjustable. At HF frequencies you CANNOT employ mechanical beam tilt because the ground reflection will ALWAYS be the dominant factor in determining TOA.

On this band you can move the front of the antenna up or down 20 degrees but you'll never move the TOA even a single degree. What you will do is lower the gain by not keeping all of the elements inline with each other with respect to the ground reflection.

 

[Lil'Yeshua]

I thought about making my CP antenna tiltable for that very reason

 

[Shockwave]

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?

I see you're tired of the 20 db cross polarity null rolling through your signal every few seconds too? While you may not be able to control Mother Nature, you sure can use technology to limit her negative propagation effects.

The vast majority of large signal fluctuations here are the direct result of polarization shifts due to the signal refracting off the ionosphere. They are not the result of conditions becoming weaker or stronger.

With true circular polarization, your signal is immune to all fluctuations caused by polarization shifts! Hardly something to be overlooked if a stable DX signal is your goal.

 

[Captain Kilowatt]

I see you're tired of the 20 db cross polarity null rolling through your signal every few seconds too? While you may not be able to control Mother Nature, you sure can use technology to limit her negative propagation effects.

The vast majority of large signal fluctuations here are the direct result of polarization shifts due to the signal refracting off the ionosphere. They are not the result of conditions becoming weaker or stronger.

With true circular polarization, your signal is immune to all fluctuations caused by polarization shifts! Hardly something to be overlooked if a stable DX signal is your goal.

Except when the incoming wavefront is left hand circular and you are right hand circular or vice versa.

 

[Lil'Yeshua]

Except when the incoming wavefront is left hand circular and you are right hand circular or vice versa.

I'd put a gear shift knob on mine to change from right hand to left hand

 

[Shockwave]

Except when the incoming wavefront is left hand circular and you are right hand circular or vice versa.

That would only happen if the station you were working was also using CP since a linear polarized signal has no direction of rotation. On the other hand you do have me wondering about something now. If both DX stations are running CP in the right hand mode, I suspect a refraction on the ionosphere could reverse the direction of rotation. Am I right on this?

 

[Shockwave]

I'd put a gear shift knob on mine to change from right hand to left hand

Changing from left to right hand just involves switching the location of the 1/4 wave delay line from one antenna feedline to the other. If you do your phasing inside the shack polarity changes are easy to do.

 

[ghz24]

I suspect a refraction on the ionosphere could reverse the direction of rotation. Am I right on this?

Yes reflected CP signals reverse with each reflection.
so local would require both left or both right handed ends of the link.
Dx would require one left and one right.

I saw somewhere that linear signals are converted by the ionosphere to a more elliptical. Which direction the spin was imparted had to do with direction on the globe (and maybe time of day).
I'll see if I can find it.
That seems the most likely reason that CP is not mainstream at CB frequency.
And potentially the hardest to solve but worst case more can use the same Channel.

Changing from left to right hand just involves switching the location of the 1/4 wave delay line from one antenna feedline to the other

Unfortunately the design I'm working on doesn't work that way.

But to solve both problems I was thinking two arrays one left one right, two radios one feeding each ear .

 

[Shockwave]

Last fall I experimented with a PDL-II in the CP mode. I can say it definitely stabilized DX signals. Being that it was only a 5 foot boom the gain was not impressive but this fall I'm working on 8 crossed elements on a 18 foot boom that should give more gain.

If DX causes significant rotation of linear polarized signals it may be possible that mixed polarity of vertical and horizontal would be more beneficial than CP since rotation would be reduced and you could only be 45 degrees away at most from any polarization. Not enough to cause a large drop in signal.

 

[Lil'Yeshua]

CP antennas might be the deal with the Sun going towards solar minimum. I've never experienced solar minimum.

 

[Shockwave]

When we actually hit the solar minimum, you won't hear a peep of DX for years on this band. No antenna will change that either.

 

[Lil'Yeshua]

When we actually hit the solar minimum, you won't hear a peep of DX for years on this band. No antenna will change that either.

Ok. Last time I was on the radio was in the eighties.