Int Guide to Pricipals of Comm- Please help 23 Jan

[andrea]

I'm looking at the free space loss calc in the most excellent guide:
If range and wavelength are pulled out from under the original parenthesis, and we're calculating the dB conversion of (4*pi)^2,
I get 21.98, not 32.4. Looking at the example on the next page of the tutorial, it's clear that conversion terms are necessary to convert kilometers to meters, and wavelength to frequency. Including them in the first term, I get -87.55, not -92.4. What am I missing?

[aya2002]

hi freind

We combine the gain of the transmitting antenna with the effective area of the receiving
antenna to determine delivered power and path loss.

the path loss:
Pd/Pt= A₂G₁(ƒÆ, ƒÓ)/(4ƒÎR²)

Antenna 1 transmits, and antenna 2 receives. If the materials in the antennas are
linear and isotropic, the transmitting and receiving patterns are identical (reciprocal) [2,
p. 116]. When we consider antenna 2 as the transmitting antenna and antenna 1 as the
receiving antenna, the path loss is

Pd/Pt= A₁G₂(ƒÆ, ƒÓ)/(4ƒÎR²)

Since the responses are reciprocal, the path losses are equal and we can gather and
eliminate terms:

G1/A1= G2/A2= constant

Because the antennas were arbitrary, this quotient must equal a constant. This constant
was found by considering the radiation between two large apertures
G/A= 4ƒÎ/ƒÉ²

We substitute this equation into path loss to express it in terms of the gains or effective
areas:

Pd/Pt= G₁G₂(ƒÉ/(4ƒÎR))²= A₁A₂/(ƒÉ²R²)

We make quick evaluations of path loss for various units of distance R and for frequency
f in megahertz using the formula

path loss(dB) = K_U + 20 log(fR) − G₁(dB) − G₂(dB)

where K_U depends on the length units:

Unit K_U
---------------
km 32.45
nm 37.80
miles 36.58
m −27.55
ft −37.87

Example Compute the gain of a 3-m-diameter parabolic reflector at 4 GHz assuming
55% aperture efficiency.
Gain is related to effective area by

G = 4ƒÎA/ƒÉ²

We calculate the area of a circular aperture by A = ƒÎ(D/2)². By combining these equations, we have


G =(ƒÎD/ƒÉ)²ƒÅ_a =(ƒÎDf/c)²ƒÅ_a

where D is the diameter and ā_a is the aperture efficiency. On substituting the values
above, we obtain the gain:

G =[3ƒÎ(4 ~ 10⁹)/(0.3 ~ 10⁹)]²(0.55) = 8685 (39.4dB)




enjoy

[aya2002]

if it is not clear please see this link for my account on www.4shared.com

www.4shared.com/file/89580473/a67b69c0/ModernAntennaDesignJun2005eBook-DDU.html