433 MHz and 50 MHz range

[alphactif]

Hello I m new here, this is my first thread
I have question about RF link, what is the difference between VHF and UHF transmission range.

example : I have to transmit data from point A to point B, and i have the possibility to transmirt at 50MHz, or at 433MHz.

If the Tx output power is the same ( exemple 30db), the receiver sensibility is the same also (exemple -70db), and if I use a simple dipole antenna (+3dbi) in transmission and reception in the same cases (at 50 and 433MHz), what is the best range between this two transmissions ???

If I use RF link budget calculator, the range of 50Mhz transmitter is greater than 433MHz, but RF calculator (like this : **broken link removed**) calculate range in line of side conditions but, in my application, i have two conditions :

1/ No objects between Tx and Rx, but i need distance up to 5 miles (5 to 6 miles) !!! ( curvature of the Earth = - 23 ft !!!)
2/ the height of Tx and Rx antennas are only 3 to 5 feets

So in this conditions, what is the best frequency : 50MHz or 433 MHz ?

If i dont"'use directive antennas, how much Tx power is necessary to cover this distance ?

Thks

 

[JimB]

OK, if no one else want to have a go, I will start with a rather rambling discourse.


The Free Space Path Loss between two pont is given by the expression:

FSPL (dB) = 32.45 + 20Log(f) + 20Log(d)

where f is the frequency in Mhz and d is the distance in kilometers, the Logs are to base 10.

So for an 8km (5 miles) path, the loss at 50Mhz is 84.5dB and at 432Mhz is 103.2dB.


We also need to consider if the path is "line of sight".
The path length for grazing the horizon for two points of known height is given by the expression:

D(max) = √(2 x 1.17 x (h1 -h3)) + √(2 x 1.17 x (h2 - h3))

where h1 and h2 are the heights of the antennas in metres, h3 is the height of the midpoint obstruction (zero in our case as we are grazing the earth surface), and the 1.17 allows for some atmospheric refraction, ie a bit of "bending" in the atmosphere.

Setting the antenna heights at 1.5 metres, gives a maximum range of 12.5km.
So your 8km path may work with a bit of luck.

To see if the RF link budget is sensible, we previously calculated the path loss as 84.5 and 103.2dB at the frequencies of interest.

The transmitter has an output of 30db (???) I will assume 30dbm, ie 30db above 1mW, which is 1 watt.
The receiver has a sensitivity of -70db (???) again I will assume you mean dBm. (This is pretty poor sensitivity by any standard, but hey ho, on we go).
Antenna gain of +3dBi for a dipole is also rather wishfull thinking, +2dBi is nearer reality.

So to find the signal power at the receiver:

Rx signal = Tx power + Tx antenna gain - path loss + Rx antenna gain

= 30 + 3 - 84.5 + 3

= -48.5 dBm at 50Mhz

and at 432 Mhz
= 30 + 3 -103.2 +3
= -67.2 dbm at 432Mhz

So, the received signal level at 432Mhz is just above the recever sensitivity, the path may work, but there is only a 2.8dB fade margin.
At 50 Mhz we have a 21.5dB fade margin, this is more likely to provide a working solution.

There are several big assumptions in these calculations, but they show the general principles.

JimB

 

[alphactif]

The receiver has a sensitivity of -70db (???) again I will assume you mean dBm. (This is pretty poor sensitivity by any standard, but hey ho, on we go).

thanks Jim ...
yes -70 dbm (not db sorry) is verry poor, but this is the sensitivity of standard TV tuner, i'm wrong ? some TV tuner are more sensitive than other ?
Of corse, I have the possibility to add VHF antenna preamplifier +15 or 20 dbm, (I forgot to mention it in my first post)