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Written by HighGain
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Wednesday, 24 May 2006 |
After seeing the omni vs. sector comparison come up again, I have decided to share my putty explanation
My putty analogy
My Putty Analogy
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An omni and sector antenna comparison is easy to explain, but you must understand a few characteristics of antennas.
1. Antennas do not create RF, they only shape it
2. A properly manufactured and installed antenna should not consume much RF or reflect it back to the radio if the VSWR (Voltage Standing Wave Ratio) is kept to 1.2:1 to 1.5:1 by the manufacturer
An omni antenna
Imagine or ”just do it”… A ball of soft putty about the size of a baseball is a good size.
Form the putty into a nicely shaped ball, lay it down and poke your finger into the center until you touch whatever’s at the bottom. What you have done is created a somewhat doughnut shape. Taller then a doughnut and rounder then a doughnut but starts to take on the shape. Let’s call this a 1dBi radiation pattern.
Now start to flatten the ball until it is half as high but twice the diameter. You now have a 4dBi radiation pattern.
Flatten the ball again until it is half as high as the previous and again twice the diameter. You have now created a 7dBi radiation pattern.
Every time you flatten the ball to half the height and double the diameter you increase the gain by 3dBi
If you continue to do this 3 more times you create a 16dBi radiation pattern and what you’ll have in front of you is a very flat but very large diameter putty disc.
As you noticed, the height decreased as the diameter increased which is exactly what your ability to reach customers will do. You can now reach out further but may start to overlook close customers due to the thinner pattern.
A sector antenna
Right where you left off, you can now make a sector antenna.
Fold the flat putty disc over like a perfect omelet, and now what you have created is an 8dBi 180 degree sector pattern. You have not changed the distance it will travel in one direction but you have doubled your vertical height.
Now flatten the omelet shaped putty until it reaches out twice as far and is half as high but still retains the 180 degree / omelet shape. You have just doubled your gain.
Fold the putty now into a quarter round shape by folding it sideways. This creates a 90 degree sector pattern that has the same capable distance but the vertical coverage has doubled again.
Disclaimer
This should help explain RF shaping differences between omni antennas and sector antennas. I use it when talking to my customers and hope it doesn’t insult anyone’s intelligence
To keep the explanation simple I have not brought into the mix what side lobes, front to back ratios, down tilt etc are. If this makes some sense, you can go on to study what that’s all about some other time.
Rich
»www.highgainantennas.com
PersComp:
***I didn't see this thread until after I had posted in the other one, so I copied it here as it fits better.***
Excellent method of illustrating!!! I thought I had the antenna concept ironed out (flattened doughnut, etc), but this makes me think a little deeper.....and tell me if I am thinking correctly.
So as you created the 8 dBi 180 sector and then flattened it , the gain returned to 16dBi. However since the coverage distance did not reduce when the putty was folded in half, the 16 dBi sector has the potential to cover MUCH more area than the 16dBi omni (not a true doubling, but probably 20-30% further).
I guess I just thought that 16dBi was 16dBi and was only concerned with additional interference when using an omni vs a sector. But, your illustration tells me that not all antenna gain is created equal!!!
rfnut:
Very nice analogy.
BTW.
VSWR is Voltage Standing Wave Ratio.
When a transmission line is terminated with an impedance, that is not equal to the characteristic impedance of the transmission line, not all of the incident power is absorbed by the termination. Part of the power is reflected back so that phase addition and subtraction of the incident and reflected waves creates a voltage standing wave pattern on the transmission line.
anon:
I debated mentioning this but your analogy is SO good that I think I should.
A 3 dB increase is worth 1.414 times as much distance. A 6 dB increase is worth 2.0 times distance. Clearly you know this, and I have forgotten it many times; but as I said, your example is just SO good that this deserves mention.
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