[BC] Skirt feeds.....

[Cowboy]

On Saturday 20 February 2010 12:33 pm, Craig Healy wrote:

>  But if the wires up the side are connected top and bottom, doesn't it
>  effectively shield the support tower? 

 Truly, that depends on how it's tuned.
 Detuning skirts are tuned to "effectively" shield the tower.
 What they actually do, is tune the structure such that the radiation
 from the skirt is out of phase with the stick, "effectively" canceling
 the radiation, but the circulating currents can be substantial.

>  Some years ago I put a wire up the 
>  middle of a 50' tower to use as an EBS receive antenna.  The desired station
>  was a non-d 5kw probably two miles away.  The receiver couldn't even detect
>  it.  I moved the wire to the outside and it blasted in.

 Depends on how/where that wire connected to the tower.
 If not at all, you effectively constructed a piece of coax, and as you might
 expect, saw the result.

>  I would think the 
>  external wires would take the vast majority of the current.  Almost like a
>  great big skin effect...

 No. The currents are all in parallel, just like they are in each leg of a
 lattice tower.

>  Or, how about modeling a tower structure...  Make one out of a pipe with
>  three wires up the sides connected top and bottom.  Then make the same thing
>  out of a two by four piece of wood.  How different would the resistance and
>  reactance be?  I would expect it to be essentially the same, if the wires
>  were large enough copper gauge.

 It will be quite different, as you've in one case had 4 conductors in parallel,
 and only three in the latter.
 Whether the wires surround the tower, or are all adjacent makes no difference
 in this case for this discussion.

 The one case I'd like to model ( but don't have the software ) would be to run
 a single wire up the center of a tower, ground the tower and feed the wire !
 A "reverse bazooka" if you will....

 I'd expect it to work, but be very narrow banded.
 Again, bonded at 90 degrees, and insulated the rest of the way.

-- 
Cowboy