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

Something about the thread, regarding transmitting power from one radio
being funnelled into the other radio made me take pause. In the Houston
area (and I'm sure most other major cities) there is an antenna farm
that has a collection of FM, AM and TV broadcast antennae. I would
estimate there are 10 in a several square mile area and have radiated
power in the Tens of Thousands of watts. This area lies just to the
southwest outside the surface area of Hobby's Class B and the tops of
the antennae reach up to the floor of the next ring of Class B.

When circumnavigating the Class B its not uncommon to be as close as a
mile to these towers and once or twice I've heard bleed-over on the VHF
radios of the aircraft.

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

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

Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Brian W

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

Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Brian W

And the sensitivity of most receivers is rated in MICRO volts ! ??

On Mon, 14 Jun 2004 14:33:22 -0500, jerry Wass
wrote:
/// 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

///
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

Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Brian W

And the sensitivity of most receivers is rated in MICRO volts ! ??


Compare with the ultimate sensitivity of the human eye: one quantum
in the visible (at low quantum efficiency), but it can stand a
fleeting exposure to 1400 watts/meter^2 i.e. direct sunlight
That is an extra ordinary range.

Brian W

"Brian Whatcott" wrote:
...
Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....


Maybe you didn't intend excess details here, but this .4V calc is not
the whole story if the receiver is not tuned for a band which includes
the offending freq. Even 107.9 FM bleeding over into 108.0 on a VOR
receiver will be some decibels down -- the VHF rcvr presumably feeding
the antenna input into at least one passive, tuned circuit before
meeting up with a semiconductor. And you'll have some loss in the
antenna itself at the extreme ends. So maybe .2V tops at 107.9? And
107.9 FM will be way, way down in a comm rcvr's front end, the type of
rcvr is at issue here.

Fred F.

"TaxSrv" wrote in message
...
"Brian Whatcott" wrote:
...
Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Anybody around here wanna perk up the old johnson, you kin fly across the
opening of the blind canyon where Jim Creek radio has it's antennas.
Wanttaja went up there once in Moonraker and it turned his hair red.

Rich "Light up your life" S.

On Mon, 14 Jun 2004 18:05:35 -0700, "Rich S."
wrote:

"TaxSrv" wrote in message
...
"Brian Whatcott" wrote:
...
Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Anybody around here wanna perk up the old johnson, you kin fly across the
opening of the blind canyon where Jim Creek radio has it's antennas.
Wanttaja went up there once in Moonraker and it turned his hair red.

Rich "Light up your life" S.



I thought that was a "long wave" antenna.... no effect on
short objects...

John

"Rich S." wrote in message ...
"TaxSrv" wrote in message
...
"Brian Whatcott" wrote:
...
Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Anybody around here wanna perk up the old johnson, you kin fly across the
opening of the blind canyon where Jim Creek radio has it's antennas.
Wanttaja went up there once in Moonraker and it turned his hair red.

Rich "Light up your life" S.


A few years back a buddy of mine was driving past the big USAF base in
North Dakota, when they apparently tested *something.* His dash lit
up like something out of a Spielberg movie, and the radio never worked
again.

Brian Whatcott wrote in message . ..
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

Into 50 ohms, that would amount to v^2/50 = 0.004
v^2 = 0.2 v^2 so V = 0.4 volts very roughly....

Brian W


Could something similar happen if I get painted by a NEXRAD weather
radar? There's one about a quarter mile away from our airport, and
every now and then (seemingly random but it only happens in the
pattern) a get a quick "fweem" over the intercom... I've noticed it in
Cessnas once or twice, and the RV. Some days I don't get it; other
days I'll hear it three or four times.

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.