Jim Weir or other qualified persons: a tangent on the 2 radio 1 antennathread

In article ,
Brian Whatcott wrote:
On Mon, 14 Jun 2004 15:17:48 GMT, Dave S
wrote:
//
My question is, given the limited "resistance" of some of the radio
components (and the ability to tolerate less than a watt input if I
paraphrased it correctly) I am wondering just how much energy the radio
system is being exposed to flying by the transmitting elements a mile
away laterally, and how prudent that is for the longevity of the
components. Lets use 50,000 watts if that is appropriate for the example.

Dave

You are not the only one who has experienced breakthrough
near a big transmitter tower. Here's a rough, rough estimate of
intercepted power.
If 50 kw were distributed through a spherical surface of 1 mile in
radius, what would the power intercepted by one square yard?
(arbitrary cross-section value for a 1/4 wave whip...)

power times Antenna cross-section / Extended surface area
[4/3 pi r squared] = 4 milliwatts

Correction: surface area of a sphere is 4 pi r squared
(volume is 4/3 pi r cubed)

1.294 milliwatts per SQUARE YARD of surface area, at 1 statute mile
0.995 milliwatts per SQUARE YARD of surface area, at 1 nautical mile.

0.995 milliwatts/square yard is the same energy density that a _FIVE_WATT_
transmitter creates at a distance of 20 yards.

Does anybody worry about 5 watts @ 20 yards? Assuming you don't have a
pacemaker, that is. grin


The above is -not- 'fair' to the big transmitter sites, however. It's
true, they they are limited to 50kw 'out the back of the transmitter' ,
*BUT* 'gain' antennas are almost universally deployed by VHF (and above)
stations. An 'effective radiated power' in the several _megawatt_ range
is not uncommon. One of the stations in downtown Chicago announces
itself at at least 8 megawwatts (ERP) -- might be 9 megawatts, memory
isn't giving a firm answer on -that- point. grin

8 megawatt ERP is 160 times the effective energy of a 50kw output.
Or about 53Mwatt/sq.yd at 1 statute mile (40mw/sq.yd at 1 naut. mi.)
Roughly equivalent to a FIFTY WATT transmitter at 40 yards. (many taxicab
companies use 30-watt VHF radios in the vehicles, and it usually doesn't
affect the FM receiver in the cab itself -- with maybe _two_ yards between
the tx and rx antennas.)

A typical VHF aircraft antenna is, electrically, about 4/3 of a yard long.
if it is 1/4" in diameter, it presents a maximum cross-section of just
about 1/100 of 1 square yard. Which, at 100% capture/conversion efficiency
would pick up just under 0.5milliwatts of energy. v^2 would be 0.025 -- the
peak voltage would be about 0.158 V.

Capture/conversion efficiency is nowhere *near* 100%. If it was, there
would be a 'dead zone' behind _every_ receiver. 'gain' figures for a
3-element beam antenna suggest that capture efficiency for a single
element is on the order of _one_ percent.

Which would equate to 5 microwatts of power, and an induced voltage of about
15 millivolts. _Not_ threatening to the 'health' of the equipment, but
definitely strong enough to produce enough 'distortion' in a 1st RF amp
stage to create enough 'in-band' signal to pass through the rest of the
receiver.