On 23 jun, 22:34, (Richard Harrison) wrote:
George, K6GW wrote:
"What is the esiest/fastest way to calculate received power out of a
very short non-resonant antenna?"
Field strengths are given in volts per meter. This is related to power
by e squared / r.
The dissipationless resistance of free-space (a ratio between voltage
and current) is 377 ohms. Attenuation due only to reduction of power per
square meter as the radio wave envelope expands in space is 6 dB every
time distance from the transmitter doubles. That means getting only 1/4
the previous power every time distance from the transmitter doubles.
Volts per meter decline linearly with distance from the transmitter. At
10x the distance, the signal strength or volts per meter is 1/10 the
previous value. 1/10 the voltage is 1/100 of the power.
We have a participant in this newsgroup, Art Unwin, who disdains book
contents, but the subject is not nearly exhausted and I don`t intend to
exhaust it. Instead, I`ll recommend one of the best books:
"Radio-Electronic Transmission Fundamentals" by B. Whitfield Griffith,
Jr. Its 2nd edition has just been released by Scitech Publishing, Inc.
Whatever it costs it is well worth the price.
Best regards, Richard Harrison, KB5WZI
Hi Richard,
That E is proportional to 1/r is valid for free space propagation.
George's question relates to the frequency range where there is no
free space (obstacle free) propagation in most cases. For his case,
the most important obstacle is mother earth.
As in practical cases (distance about several miles, height of
receiving antenna about 5 ft), the earth reflected ray cancels most
part of the direct ray. Fast methods to get a result are the two-ray
formula and/or propagation curves for frequency planning. Add about
10dB loss for heavily populated areas (buildings).
Using one of the above methods, will result in E is proportional to 1/
r^3...to 1/r^4. This matches measurements I did myself. Therefore 1/r
gives a far from realistic result (too optimistic).
Best regards,
Wim
PA3DJS
www.tetech.nl