ERP derivation?  RF Cafe Forums

Mr.Whatever

Post subject: ERP derivation?
Posted: Tue Sep 05, 2006 4:08 pm



Captain 

Joined: Fri Apr 07, 2006
6:10 pm Posts: 18 
I was reading the Electronics Engineers' Handbook
and they have the following formula:
E =
[7 * sqrt(pt)] / d
where E is the field strength
in V/m, d is the distance in meters, and pt is the
effective radiated power (ERP), which includes the
transmitter gain with respect to a halfwave dipole.
How was this formula derived?
Thanks!





Mr.Whatever 
Post subject:
Posted: Wed Sep 20, 2006 11:59 pm



Captain 

Joined: Fri Apr 07, 2006
6:10 pm Posts: 18 
I found the answer to this question after reading
some more wave propagation stuff. I thought I'd
share it.
power: (V^2) / R = P
power
flux density: s = P / (4*pi*r^2), where pi = 3.14
and r = distance
field strength: e = sqrt
(120 * pi * s) = sqrt(30 * P) / r
A halfwave
dipole has a 1.64 gain in its equatorial plane,
therefore:
e = sqrt(1.64*30*P) /r = 7*sqrt(p)
/ r
Pretty simple, but I hate it when other
books try to make me memorize a formula. I prefer
finding out how its derive, so I can start from
"scratch" whenever I forget the formula.


Posted 11/12/2012


