Mal,
"What is the difference between an antenna wire strung between two high rise
flats and two radio masts at 100 feet...?"
My best guess, in priority order ("+" in favor of two 100 foot masts; "-" in
favor of two high-rise flats):
(+): The (a) ratio of the impedance of your mast ground system to the impedance
(to ground) of your signal path (i.e. very low), vs. (b) the ratio of the
impedance of the building infrastructure (to ground) to the impedance (to
ground) of the connection between buildings (not necessarily low). Strike
current divides by this ratio, and this ratio can be made very small by a very
low mast-ground-system impedance (reducing the likelihood or extent of damage,
not substantially reducing the likelihood of a strike)
(+): The 100 foot height of your masts compared to the height of the
leader-source building in the video, which may have been > 100 feet (but
possibly not much greater, based on guesstimates from the slow-motion video).
See MIL-HDBK-419A Vol 1 of 2 Section 3.5.1 (link below), and attached plots. If
taller structures are absent from the vicinity of the masts, the probability of
strike is proportional to the attractive area shown in the attached plots.
Multiply attractive area in attached plots by the flash density in your region
(approximately 1 flash per square kilometer per year in your grid square, see
link immediately below).
http://earthobservatory.nasa.gov/IOTD/view.php?id=6679
Based on the above, your MTBS could be in the range of 5 to 10 years.
(+): The length and width of the leader-source building in the video; not a
large factor (see attached plots), mentioned here for completeness
(+): Specification compliant lightning rods on the building. Based on the video
image, the building may not have specification compliant lightning rods.
References:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=2&ved=0CDMQFjAB&url=http%3A%2F%2Fwww.commscope.com%2Fdocs%2Flightning_protection_for_cellular_tower_mounted_electronics_tp-101613.pdf&ei=NMfiUo76DcS7oQTL0YDwCQ&usg=AFQjCNGD9Bv8xnKTKd9OOwCJrshM8vwHFA&bvm=bv.59930103,d.cGU
("Lightning Protection for Cellular Tower Mounted Electronics", Andrew)
http://www.uscg.mil/Petaluma/TPF/ET_SMS/Mil-STDs/MILHDBK419.pdf
(MIL-HDBK-419A Vol 1 of 2)
http://www.wbdg.org/ccb/FEDMIL/hdbk419a_vol2.pdf
(MIL-HDBK-419A Vol 2 of 2)
Jim AA5BW
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of mal hamilton
Sent: Friday, January 24, 2014 9:52 AM
To: [email protected]
Subject: Re: LF: lightning strike
What is the difference between an antenna wire strung between two high rise
flats and two radio masts at 100 feet as I do ??
G3KEV
----- Original Message -----
From: "Julian Hardstone" <[email protected]>
To: <[email protected]>
Sent: Friday, January 24, 2014 2:42 PM
Subject: Re: LF: lightning strike
> (Lurking here :-) Very interesting videos, but why would anyone hang a
> cable between two blocks of flats?
> Maybe a boy in one wanted his own telephone line to a girl in the other,
> or something like that. Reminds me of the time, some decades ago, when I
> was working in a London hospital; one of the electricians could get away
> with anything, but when he strung a cable between the tower of labs and
> his workshop, he could see no problem so I had to refer the matter to a
> senior power to get it removed
> - Julian, G3TFR
>
> ----- Original Message -----
> From: "Eike Katzera" <[email protected]>
> To: <[email protected]>
> Sent: Thursday, January 23, 2014 10:51 PM
> Subject: Aw: LF: lightning strike
>
>
>> Hi,
>> that happened in russia.
>> http://www.liveleak.com/view?i=25f_1344496418
>> the same in slow motion http://www.liveleak.com/view?i=9cc_1344612168
>>
>> 73s de Eike DL3IKE
>>
>> Gesendet: Donnerstag, 23. Januar 2014 um 14:13 Uhr
>> Von: "Stefan Schäfer" <[email protected]>
>> An: [email protected]
>> Betreff: LF: lightning strike
>> Hi LF,
>>
>> Did someone see something similar in his own garden? :-)
>>
>> https://dl.dropboxusercontent.com/u/19882028/strike.jpg
>>
>> 73, Stefan/DK7FC
>>
>> PS: It is not my antenna
>>
>
>
Lightning Attractive Area ab.png
Description: PNG image
|
