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



 
 
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  #1  
Old June 14th 04, 04:17 PM
Dave S
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Default 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

  #2  
Old June 14th 04, 06:32 PM
Brian Whatcott
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Default

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
  #3  
Old June 14th 04, 08:33 PM
jerry Wass
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Default



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 ! ??


  #4  
Old June 15th 04, 01:14 AM
Brian Whatcott
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Default

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

  #5  
Old June 15th 04, 01:40 AM
TaxSrv
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Default

"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.

  #6  
Old June 15th 04, 02:05 AM
Rich S.
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Default

"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.


  #7  
Old June 15th 04, 02:08 AM
John Ammeter
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Default

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
  #8  
Old June 16th 04, 10:27 PM
Corrie
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Posts: n/a
Default

"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.
  #9  
Old June 17th 04, 09:32 PM
Gene Seibel
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Posts: n/a
Default

Dave S wrote in message ink.net...

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



Actually, many UHF stations will have effective radiated powers up to
5 million watts, if you are directly horizontal from their antenna.
One on which I work is directional, and I have flown a circle around
it and been able to hear the bleed over/sync buzz in my VHF radios
increase and decrease according to the pattern of the antenna. I also
had a handheld GPS permanently quit working once when I flew by that
antenna. Fortunately, Magellan fixed it under warranty.
--
Gene Seibel
http://pad39a.com/gene/broadcast.html
Because I fly, I envy no one.
  #10  
Old June 18th 04, 04:37 AM
Bob Martin
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Posts: n/a
Default

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.
 




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