# directivity of parabolic antenna

Discussion in 'Wireless & RF Design' started by muni, Sep 18, 2008.

1. ### muni Thread Starter Active Member

Jul 29, 2008
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a parabolic dish antenna has a conical beam 2° wide, the directivity of the antenna is approximately
a) 20 dB b) 30 dB c) 40 dB d)50dB

i've read from some net files that directivity =(41253)/(BW theta χ BW phi) but even how does it shall it be in dBs ?
if i'm not right what is the right formula?

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Apr 5, 2008
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3. ### AlexR Well-Known Member

Jan 16, 2008
735
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NOT true!

The gain of any antenna is solely dependant on beam-width however it is true that beam-width is dependant on frequency but since beam-width is given in this problem frequency plays no part in the calculations.

Take a look at the attached .pdf for a good explanation of antenna gain.

• ###### TN_WJgain.pdf
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Apr 5, 2008
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Hello,

In theory it is o.k. dut the practice shows somthing else.
There is a graph in the VHF-UHF manual from the RSGB that shows the relation between frequency and gain and beam width.

Greetings,
Bertus

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5. ### nanovate Distinguished Member

May 7, 2007
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It is not solely dependant. The Directivity depends on the antenna pattern (beamwidth is the angle between the -3dB points). If you assume no losses (100% radiation efficiency) then gain and directivity are interchangeable. But in real life nothing is 100% efficient and so gain < directivity. For a given antenna the directivity is related to the wavelength.

The formulas the OP mentioned are based on antennas that have a major lobe and very small minor lobes where the bulk of the energy is contained within the beamwidths in two perpendicular planes.

6. ### AlexR Well-Known Member

Jan 16, 2008
735
55
This is perfectly true. It is also utterly irrelevant in relation to the OP's original question.

The OP is obviously quoting from a test/exam question about an idealised antenna. As no information about sidelobes is given none can be inferred so any discussion about sidelobes and frequency is pointless in relation to the question.

I suppose that one could always say that the answer is the lowest gain (20db) and that any discrepancy between your answer and what the examiner expects is due to power in sidelobes, but I somehow doubt that the examiners would accept your answer.

7. ### muni Thread Starter Active Member

Jul 29, 2008
45
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sir i've checked this site already. the attachment which i posted is from that site only. it says that directivity = 41253/(beamwidth in theta χ beamwidth in phi).
then i felt somethinf wrong because in the Q it is provided with only beamwidth as 2° and is not mentioned whether it is phi or theta.

8. ### muni Thread Starter Active Member

Jul 29, 2008
45
0
thank you sir it is really helpfull for me. this notes cleared many of my doubts. it really really good. i got the concept of directivity gain and directivity, efficiency etc.

thank you sir
muni