Repeater Antenna

[LeoG]

Pretty sure I won't be able to help myself about reporting what ends up happening with my system.  I'm hoping that getting above the trees is one of my main issues.  If I had patience I would wait for winter to come in and all the leaves on the tree to be on the ground.  If my signal at the house and elsewhere improved then I would know for a fact that one of the more prominent issues I'm dealing with.  I know in the direction without trees I have a good signal 6 miles away.  Which of course isn't that far, but the hilly areas I live around are somewhat insurmountable issues that can't be worked around other than height.

[LeoG]

On 9/19/2024 at 12:22 AM, SteveShannon said:

Max is 9.78 dB (assuming dBi) at theta = 90°, horizontal. That’s the blue horizontal line to the right, and it’s 9.77 dB along the green horizontal line to the left (theta = -90°).

The red lines are about angles above and below horizontal. Theta is the angle along the circle.

So at theta = 84° (which is the upper red line in the diagram) the gain is 5.74 dB. That’s 6° above horizontal.

At theta = 95° (the lower red line) the gain is 6.77 dB.

Everywhere within the 11° angle between theta = 84° to theta = 95° has gain greater than 5.74 up to 9.78 and back down to 6.77. 

Steve, I wonder if we could discuss the diagram further.  I assume this is a view of the horizontal.  the 90º being the ground or parallel to the ground.  The circle around it in degrees are 0º being up towards the sky and 180º being down to the ground.  I see 90 and -90º being front and rear, but in an "omnidirectional" antenna it shouldn't really matter.  Obviously there was a .01dB difference at 90º but that's moot in real life and only exists in a lab.

What I want to understand are the shapes of the lobes.  I also see the concentric circles going inward there are also numbers which I assume are decibel ratings going from 0 from the outside circle to -40 on the inside circle.  The look like they are spaced with an algorithmic strength.

So how are the lobes equated to distance from the central point which is the antenna?  And why are the 0 through -40 numbers in that arrangement and not reversed? As the distance from the center goes out further I would assume the signal gets weaker.

I assume I'm getting confused because this is a gain diagram and not a power diagram.  I would really like to understand how to read this as well as you do.

[SteveShannon]

17 minutes ago, LeoG said:

Steve, I wonder if we could discuss the diagram further.  I assume this is a view of the horizontal.  the 90º being the ground or parallel to the ground.  The circle around it in degrees are 0º being up towards the sky and 180º being down to the ground.  I see 90 and -90º being front and rear, but in an "omnidirectional" antenna it shouldn't really matter.  Obviously there was a .01dB difference at 90º but that's moot in real life and only exists in a lab.

What I want to understand are the shapes of the lobes.  I also see the concentric circles going inward there are also numbers which I assume are decibel ratings going from 0 from the outside circle to -40 on the inside circle.  The look like they are spaced with an algorithmic strength.

So how are the lobes equated to distance from the central point which is the antenna?  And why are the 0 through -40 numbers in that arrangement and not reversed? As the distance from the center goes out further I would assume the signal gets weaker.

I assume I'm getting confused because this is a gain diagram and not a power diagram.  I would really like to understand how to read this as well as you do.

Sure! And trust me on this, I’m no expert on reading this either!

You’re exactly right, this shows gain, (no relationship to the distance the signal might carry!) with the outermost ring being the reference based on the maximum signal strength.  That’s all it means. We all look at these and wonder what they have to do with distance, but that the way to torture yourself.

Just so I can look at it easily while talking about it:

Those lobes are actually what has either been simulated or measured.  Whoever put this together found the direction of max gain and measured the strength of the signal in that direction.  Then, in other directions they measure the signal strength and compare the max against the strength in that direction to determine how many dB down the signal is in any other direction. The blue lines showing the lobes simply indicates the loss of signal strength with the smallest lobes showing greater loss.
So, for instance, there’s a lobe nearly centered on the 60° angle.  The intersection of the outermost extreme of the lobe falls between the -10 dB and -20 dB circles, but I would estimate between -16 and -17 dB.  So that simply tells us that at that angle, the measurement of signal strength was about 16 or 17 dB weaker than the maximum signal.

Note that directly above and below (0° and 180°) there is no blue. That means there’s a point above and below where there was no signal, a null reading on the field strength meter. That null actually appears to subtend an angle about 10° right and left of vertical.  That’s why placing a repeater on your roof and trying to hit it with another radio that happens to be within that “cone of silence” could be really frustrating.  You’re within eyesight of the antenna, looking straight up at it, but you’re in a dead spot.

Jist as a comment, the lobes are not symmetrical with respect to the horizontal axis.  That’s probably because of the proximity to the ground.  As you move the antenna up, the lobes below the horizontal axis will become more and more like the upper ones.  Sometimes, whe people want to have their signals go skyward, they actually lower their antennas.  
I’ve got to go to bed, but I’ll check in the morning to see if I made things worse.

[piggin]

34 minutes ago, LeoG said:

Pretty sure I won't be able to help myself about reporting what ends up happening with my system.  I'm hoping that getting above the trees is one of my main issues.  If I had patience I would wait for winter to come in and all the leaves on the tree to be on the ground.  If my signal at the house and elsewhere improved then I would know for a fact that one of the more prominent issues I'm dealing with.  I know in the direction without trees I have a good signal 6 miles away.  Which of course isn't that far, but the hilly areas I live around are somewhat insurmountable issues that can't be worked around other than height.

I am looking forward to the leaves being down here too. I am very happy with my system working better as the leaves fall. Winter is wonderful here on UHF. The only downside is ducting is much poorer in the winter VS fall and the spring. As it is, I am pretty happy with my current antennas.

[LeoG]

7 hours ago, SteveShannon said:

Sure! And trust me on this, I’m no expert on reading this either!

You’re exactly right, this shows gain, (no relationship to the distance the signal might carry!) with the outermost ring being the reference based on the maximum signal strength.  That’s all it means. We all look at these and wonder what they have to do with distance, but that the way to torture yourself.

Just so I can look at it easily while talking about it:

Those lobes are actually what has either been simulated or measured.  Whoever put this together found the direction of max gain and measured the strength of the signal in that direction.  Then, in other directions they measure the signal strength and compare the max against the strength in that direction to determine how many dB down the signal is in any other direction. The blue lines showing the lobes simply indicates the loss of signal strength with the smallest lobes showing greater loss.
So, for instance, there’s a lobe nearly centered on the 60° angle.  The intersection of the outermost extreme of the lobe falls between the -10 dB and -20 dB circles, but I would estimate between -16 and -17 dB.  So that simply tells us that at that angle, the measurement of signal strength was about 16 or 17 dB weaker than the maximum signal.

Note that directly above and below (0° and 180°) there is no blue. That means there’s a point above and below where there was no signal, a null reading on the field strength meter. That null actually appears to subtend an angle about 10° right and left of vertical.  That’s why placing a repeater on your roof and trying to hit it with another radio that happens to be within that “cone of silence” could be really frustrating.  You’re within eyesight of the antenna, looking straight up at it, but you’re in a dead spot.

Jist as a comment, the lobes are not symmetrical with respect to the horizontal axis.  That’s probably because of the proximity to the ground.  As you move the antenna up, the lobes below the horizontal axis will become more and more like the upper ones.  Sometimes, whe people want to have their signals go skyward, they actually lower their antennas.  
I’ve got to go to bed, but I’ll check in the morning to see if I made things worse.

I think a lot of my problem is this is a graph of gain and not power.  And your explanation of the concentric circles makes more sense to me because the outer circle is of max gain and as you go inward you are subtracting dB gain as compared to the outer circle of max gain.

With my current antenna, I have no diagram of gain and I don't have an issue with trying to hit the repeater whilst under the antenna.  I have my HT on low power and I'm using a stubby antenna.  Although I have heard crackling in my signal which I though of as strange because of my proximity closeness to the antenna but it is likely I'm in the dead spot and my signal is superficially getting to the antenna only because of the brute strength of the signal by being so nearby.

So if someone was making this graph they would need to have a constant power radiating signal and they would move it around the antenna in a 360º spherical pattern while taking measurements at each azimuth point and recording the strength of the signal and then find the strongest and weakest signals and determine the logarithmic strength between the high and low and then plot it on the graph.

Not sure if they need to do a spherical measurement or just a simplistic 2D simple circle going around the antenna in a vertical pattern.

[SteveShannon]

57 minutes ago, LeoG said:

I think a lot of my problem is this is a graph of gain and not power.  And your explanation of the concentric circles makes more sense to me because the outer circle is of max gain and as you go inward you are subtracting dB gain as compared to the outer circle of max gain.

With my current antenna, I have no diagram of gain and I don't have an issue with trying to hit the repeater whilst under the antenna.  I have my HT on low power and I'm using a stubby antenna.  Although I have heard crackling in my signal which I though of as strange because of my proximity closeness to the antenna but it is likely I'm in the dead spot and my signal is superficially getting to the antenna only because of the brute strength of the signal by being so nearby.

So if someone was making this graph they would need to have a constant power radiating signal and they would move it around the antenna in a 360º spherical pattern while taking measurements at each azimuth point and recording the strength of the signal and then find the strongest and weakest signals and determine the logarithmic strength between the high and low and then plot it on the graph.

Not sure if they need to do a spherical measurement or just a simplistic 2D simple circle going around the antenna in a vertical pattern.

Yes, or simply transmit a constant signal from the antenna and move a field strength detector around it.

Most antennas don’t have a complete null above and below.  That’s characteristic of a high gain antenna.  In fact, multiple lobes like you see in that chart are characteristic of high gain antennas. If you look at the charts for a low gain antenna, such as a vertical dipole you’ll see two graphs, one from the side and one from above.  The graph from above (called azimuth in this image) looks like a fairly smooth circle, showing that the signal strength is approximately even in all directions.  Taken from the side (elevation) the signal strength of a dipole looks like a squashed figure eight. The three dimensional colored image helps understand that the squashed figure eight is swept or rotated around the z axis. The vertical dipole forms the z axis.

By the way, while looking for these images I found this description on how to understand antenna graphs.  I haven’t read it yet, but it appears to explain a lot of different aspects.

https://gristleking.com/all-about-antennas-radiation-patterns/comment-page-1/

 

[LeoG]

Well it came in.  The packaging freaked me out a bit.

It said 70cm which I know is 440MHz, but it also had a "C" sticker put on the end of the original printed label.  I wrote Jim and he assured me it was the correct antenna and said it came that way from the manufacturer.  After I got a response from Jim from Chatt Radio I noticed the antenna had a sticker on it saying it was indeed the CA-712EFC antenna

On top of that the stated 9.0dBi gain listed on all the websites I've seen but the gain is listed on the package and in the literature as 9.8dBi gain.  Not much of a boost.  But going from 7.2 to 9.0 vs 7.2 to 9.8 is 1.8 change vs 2.6 additional gain.  Nice.