how do you read the polar curve of antenna radiation pattern? - Canadian TV, Computing and Home Theatre Forums

 
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post #1 of 9 (permalink) Old 2009-03-04, 10:05 AM Thread Starter
 
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how do you read the polar curve of antenna radiation pattern?

Sorry guys for the very basic question but I don't know how to read a polar plot. I has always been afraid to ask.

In particular, I don't undestand what the dB values mean at each concentric circle and why the values start at 0 on the outer circle and go inward with negative numbers

Also, does the radiation curve represent a physical distance from the antenna? in other words, how can one know what is the distance from the antenna does the curve represent?

can someone familiar with the polar curve please explain

thanks

here is an CM4221 curve
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post #2 of 9 (permalink) Old 2009-03-04, 02:16 PM
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Youre going to kick yourself when I tell you those are compass points on the right and left. The center numbers are db gain figures, which there should be also positive numbers listed, but only negative numbers are shown. The positive numbers are implied.

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post #3 of 9 (permalink) Old 2009-03-04, 02:32 PM
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I will explain since I am just beginning my journey into UHF antennas as well. My explanation will be rather simple. Hopefully I get is right.

The plot indicates it is an overhead view and the top of the graph looks to-wards the TV station tower. So imagine you are standing over the antenna looking toward the tower. Signals coming straight into the antenna from the front will have the strongest signal. Those off to the sides and back-end will see the signal drop off. The amount of drop-of depends on the direction the signal is coming from and what frequency (channel). This is what the plot tells you.

For example,

Channel 20 (dark blue line) will have a drop of 2.5dB if the signal is coming into the antenna from the front. Now if it is coming from the front left side (-45░) it will be -9.5dB. Look at the concentric dots. Each one represent 1 dB drop. Also, as you can see channel 50 (light blue line) is stronger from the front (-1dB) and weaker from the side (-10.5dB).

I believe halving of db values represents a 10X loss of power. So every little bit of dB helps with weak signals.
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post #4 of 9 (permalink) Old 2009-03-04, 03:04 PM
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Quote:
Also, does the radiation curve represent a physical distance from the antenna?
No, not a physical distance, but a relative distance of gain in that direction.

My builds/plans (not the latest models) are located here.
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post #5 of 9 (permalink) Old 2009-03-04, 03:18 PM
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Quote:
believe halving of db values represents a 10X loss of power. So every little bit of dB helps with weak signals.
Halving of a dB value has no particular significance.
100X loss of power = -20 dB
10X loss of power = -10 dB
1/2 loss of power = approx -3 dB
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post #6 of 9 (permalink) Old 2009-03-04, 03:31 PM
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Thanks for clarifying that.
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post #7 of 9 (permalink) Old 2009-03-04, 03:36 PM Thread Starter
 
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So if I understand well, looking at the plot above for the CM4221:

is my following assessment correct:

1- if the antenna is directly pointed toward the tv station, channel 20 will experience about 2.5 dB less gain than channel 60

2- if the antenna is off axis by 30 degrees left or right, the CM4221 performs about equally well for all channels, however the penalty is about 5 dB because it is off axis at 30 degrees

3- for any signal coming from the back, there is about at least 20 dB penalty, although channel 60 seems to be less penalized than lower channels if the signal is coming from the rear left or the rear right.

now my question is: looking at the plot, how can you tell the gain of the CM4221 vs 4228 etc. when the manufacturer advertise that a given antenna has a 10 dB gain for example?
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post #8 of 9 (permalink) Old 2009-03-04, 05:55 PM
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Quote:
is my following assessment correct:
Yep, you pretty well got it. For gain, you would really want to look at a gain vs mhz chart.

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post #9 of 9 (permalink) Old 2009-03-04, 06:08 PM
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The hdtprimer azimuthal Gain is plotted RELATIVE to the MAX Gain on the best tested channel.

The hdtvprimer "comparing" chart shows Gain versus Channel (i.e. frequency).
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