Some time ago someone (either Radio8z or Rich) posted the math necessary to calculate the actual voltage applied to a radio by a given field strength. I can't find it anywhere using the built-in search function, so I was wondering if that someone could repost it.
I've found a bunch of equations on the web, but I'm hoping that those in the know could simply them for a standard monopole such as a car antenna.
I generally use a factor of 2.5 (I remember it was around that for the examples used in the original post, i.e., a field strength of 10uv/m will induce a voltage of 4uv into the radio).
... but regarding the math necessary to calculate the actual voltage applied to a radio by a given field strength -- the correct answer depends on the gain of that receive antenna with respect to the net field intensity and polarization of that received signal, and the losses inherent in coupling that received r-f voltage from the receive antenna to the input terminals of the receiver.
We can make some assumptions. Same polarity, typical car vertical antenna (old style) maybe a meter in length and a typical 50 ohm run of rg-58 cable maybe 2 meters in length. I'm just interested in the equations and what everything means in the equations - I can plug in the values.
Below is a link showing the equation for the power flowing on a receive antenna based on the power radiated by a transmit antenna.
BUT NOTE:
- this equation is valid only for free space paths (no reflections or obstructions)
- accurate values must be known for the gains of the transmit and receive antennas
Those requirements are not likely to exist or be known for Part 15 FM systems.
http://www.antenna-theory.com/basics/friis.php
If a receiver complies with 15.239 we know the field strength does not exceed 250 uv/m@3m. That lets us use the equation in oet63rev.pdf. To keep the math simple, a numerical gain of 1 would be a 0dB gain.
So a part 15.239 transmitter would do no better than 1.875E-08 watts into a unity gain antenna.
So a part 15.239 transmitter would do no better than 1.875E-08 watts into a unity gain antenna.
...there is path loss, even for an isotropic transmit antenna.
So what E-field (or power) do you calculate as present across the output terminals of an FM-band receive antenna, for a 3m horizontal distance near the surface of the earth, when an EIRP of 1.875 E-8 watts is radiated ?
Enter the Site Attenuation Test that I got thrown under the bus about.
That bus was contributing to site attenuation.