Linbudget for a 433 MHZ system

[weedogt]

Hi,

I'm designing a simple 433 MHz radio link to transmit some low rate measurement data from a bridge/quay to a building nearby. I need some help estimating if I will get a reliable communication.

Please have a look at this page (a draft of the surroundings):

http://www.trym.se/images/link.jpg

The end of the quay is just visible from the building. There's also about 10 big trees in the line of sight.

The link is one way only, that is I can send each data package several times but I can't get any acknowledge from the receiver.

I have tried to calculate the link budget like so:

Path loss (free space) = 20*log(433)+20*log(180)+28 = 69 dB

The transmitter power is +10 dB.
The receiver sensitivity is -100 dB.
Losses will probably be approx -5 dB.
Both antennas have +2 dB gain.
Link budget: +10-(-100)-5+2+2 = 109 dB

That would give me a margin of 109-69 = 40 dB.

My concerns are what attenuation I can expect from the trees and if the fact that the LOS is close to the ground will be troublesome.

Will I be able to get a reliable communication?

Any input is appreciated!

Thanks. /weedogt

 

[VEBill]

I see some errors:
Isn't the calculated pathloss more like 125 dB?
Tx power and Rx sensitivity should be in dBm (not dB)?

Might need to use higher gain antennas.

 

[weedogt]

VE1BLL, the path loss is correct, there's just a typo in the calculation. It should look like this:

Path loss (free space) = 20*log(433)+20*log(180)-28 = 69 dB

Yes, the Tx power and Rx sensitivity should be in dBm (not dB).

/weedo

 

[VEBill]

Why would it be MINUS 28?

If -28 were true, then the Universe would explode in a huge flaming ball of infinite energy as soon as the pathloss equation crosses zero.

"Bit closer, bit closer" <poof! we all die...>

 

[VEBill]

Apologies, the pathloss works out to about 70dB with either formula.

Regarding your question, I've not noticed much attenuation in UHF range by a scattering of trees. If you've got 40dB margin, then it should work and work well.

 

[Higgler]

I'd suspect two antenna related source of your problems;

1) ground bounce
2) side and backlobes from antenna degrading antenna pattern.

Regarding antennas
+2 dBi gain antennas? what type are they.
If they are patches require a ground plane behind them and the manufacturer usually doesn't give specifics on that. Some patches change frequency if you don't have a ground plane behind it.
Higher antenna gain; if you make a simple helical antenna, (18" long x 6" diam.), you easily get 10 dB of gain which could help (I recently make a 6" long helical antenna(2" diam.) for 1.5 ghz which had 10-12 dB gain. Look up helical antennas, there's alot of good data on this easy to make antenna. Each of your antennas may actually be -5 to -15 dBi due to stuff around it.


Regarding bounce off the ground. Changing antenna orintations, rotate both of them helps. Changing to circularly polarized also helps since the bounce off the ground changed polarization inherently and won't get into your receive antenna easily.

If you set the system up in a lab and put the antennas a few feet away from each other, that will give you a starting point on s/n for the system.

kch