the attenuation due to rainfall calculated by function "rainpl" is different from ITU-R.838

the help of "rainpl" says：To compute the total attenuation for narrowband signals along a path, the function multiplies the specific attenuation by the path length R. Then, total attenuation is Lr = Rγr. but when i test this by code:

 tau=0;
 elev = 30;
 freq = 11.0e9;
 rng = [1:1000]*1e3;
 rr = 10.0;
L = rainpl(rng,freq,rr,elev,tau)
plot([1:1000],L)
grid
xlabel('range(km)')
ylabel('Attenuation (dB)')

since the γr is fixed,it seems that the Attenuation isn't linear function of the the path length.is there anything i ignored?

i also test this with software p.838 on ITU software. And i get quite different result. so i am confused about to trust which one?

Answer 1

[Honglei Chen]

The plot you see is in dB scale, that's why it does not look like a linear model, but it is indeed linear.

I'm not familiar with the ITU software but as the documentation indicates, the function is based on ITU-R P.838-3, so it could be different to the original P.838 if that's what the software is based on. I did try compute values mentioned in P.838-3 using the function and was able to get the matching result.

HTH

[liang]

Thank you for your answer.

But you can see from my code above,the unit of total attenuation Lr is dB,and i do not use log scale plot. According to ITU-R P.838, the unit of the specific attenuation γr is dB per kilometer.Since γr is fixed because it's not a function of Signal path length, the function to compute the total attenuation for narrowband signals along with equation Lr = Rγr should be linear function of R.

There is only small difference between each version，and the method is not changed.The ITU software of P.838 is available on https://www.itu.int/en/ITU-R/study-groups/rsg3/Pages/iono-tropo-spheric.aspx, the name is "Specific attenuation model for rain".

you can try software "rainattnver2.exe" with parameters above,the total attenuation is 242dB with Signal path length equals to 1000km.

[Honglei Chen]

Ah, looks like the documentation is not accurate and needs to be updated. The ITU standards only gives the parameters. The computation of the final loss also includes some distance based scaling factor and that is covered in John Seybold's Introduction to RF Propagation book, section 10.3.2 on page 224. Essentially the distance is adjusted by a factor of r where r = 1/(1+d/d0) and d0=35*exp(-0.015*RR) where RR is the rain rate.

HTH

[liang]

Thank you again! I have known how it works from your answer.

I find another way to calculate the effective path length in Recommendation ITU-R P.618. Those two methods are totally different, that's why i get different result.

Thaks.

[Honglei Chen]

Thanks for pointing out the doc issue. I'll take a look at P.618.