Optimizing Link Budget for 1.1km Transmission with 875MHz Carrier Frequency

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Discussion Overview

The discussion revolves around optimizing the link budget for a signal transmission over a distance of 1.1 km using an 875 MHz carrier frequency. Participants explore the calculations involved in determining received power, considering factors such as bandwidth, antenna characteristics, and losses due to free space path loss and additional antenna attenuation.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • Participants discuss the relevant equation for received power, which incorporates transmitted power, gains, and losses.
  • Some participants propose that losses may include free space path losses, with calculations provided for these losses.
  • There is a question regarding whether the 0.1 dB attenuation applies to each antenna or the system as a whole, indicating a need for clarification.
  • One participant calculates the free space path loss (FSPL) and derives a received power value, but there is uncertainty about the assumptions made in the calculations.
  • Participants express confusion about the interpretation of certain variables and calculations, particularly regarding the gain associated with isotropic radiation.

Areas of Agreement / Disagreement

Participants generally agree on the use of the free space path loss formula and the need to account for antenna losses, but there is no consensus on the specifics of the losses and gains, particularly regarding the interpretation of the 0.1 dB attenuation and its application.

Contextual Notes

There are unresolved assumptions regarding the definition of losses and gains, particularly the interpretation of antenna characteristics and whether the 0.1 dB loss applies to each antenna or the system as a whole. Additionally, the calculations involve approximations that may not be fully clarified.

Mickydawg25
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Homework Statement


A signal needs to be transmitted over 1.1km, with 12Hkz bandwidth on FM carrier.
Carrier freq=875MHz.
Allowed up to 1W carrier power transmitted and received thru omnidirectional antennas.
antennas low loss, max attenuation = 0.1dB


Homework Equations



Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

The Attempt at a Solution



I'm beat
 
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Mickydawg25 said:

Homework Statement


A signal needs to be transmitted over 1.1km, with 12Hkz bandwidth on FM carrier.
Carrier freq=875MHz.
Allowed up to 1W carrier power transmitted and received thru omnidirectional antennas.
antennas low loss, max attenuation = 0.1dB


Homework Equations



Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

The Attempt at a Solution



I'm beat

Well, at least you listed a Relevant Equation. That's good.

Now, what can you fill into that equation? What are the losses for example?
 
are the losses the free space path losses?
((4*3.14*1100*875000000)/300000000)^2

is the gain the attenuation ie G -0.1dB?

Power=1watt=30dB?
 
Mickydawg25 said:
are the losses the free space path losses?
((4*3.14*1100*875000000)/300000000)^2

is the gain the attenuation ie G -0.1dB?

Power=1watt=30dB?

What is "875000000)/300000000"?

The gain would be any antenna gain, but you are told the radiation is isotropic, so there is no gain.

It looks like you are trying to calculate the loss by using the surface area of a sphere of 1100m radius, which is correct. The 0.1dB would be an additional antenna loss to add in.

1Watt is 1000 times 1mW, so yes, that would be 10*log(1W/1mW) = 30dBm.


EDIT -- BTW, the question does not make it clear if it is 0.1dB max loss per antenna or for the two antennas as a system. You should see if you can clarify that (it oly changes your answer by 0.1dB, but it would be good to get it right the first time).
 
f=875000000
c=300000000

using ((4*pi*d*f)/c)^2=FSPL
((4*3.14*1100*875000000)/300000000)^2=FSPL

10log(FSPL)= losses in dB = 92.1dB

assuming 0.1dB attenuation per each antenna

Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)
Pr=30dbm + 0 - 92.1dB - 0.2dB

Pr=-62.3dBm?
 

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