[BC] Skirts

[Phil Alexander]

>From: Steve Lewis <steve at theengineeringbureau.com>
>
>Another (may be stupid) point to clarify.

     NOT stupid! The only stupid points are those not
     understood because a question was not asked.  

>The top end of 
>the drape wires are connected to the tower through the 
>girdle and spider.  Would insulators be used at the top 
>to eliminate connection to the tower via the support
>structure?

     No need for spider at the top. Provided the tower and
     attachment/support arms are galvanized with sound
     electrical connection the arms act as the conductors. 

>The bottom of the wires *are not* electrically connected 
>to the tower in any way (by the use of insulators), correct?

     Provided wires are taut and clearance is as previously
     indicated, this is correct.

>And the drape is not electrically connected to the tower 
>at any point except the top, as I understand it, right?

     That is the correct way. There is a school of thought
     that believes the tops should be insulated and a "50
     ohm" jumper point should be found to eliminate the
     ATU. I would suggest that anyone considering this
     idea might have a brief conversation with Ron Rackley
     before creating yet another problematic skirt 
     installation.

>I'm asking so many questions because I have one of these >things in Mississippi that I've received varied opinions 
>on its performance.  The tower is listed in the CDBS as 
>205 degrees at operating frequency but no mention of a 
>skirt.  

     This is considered too tall for skirt/shunt excitation 
     by most. It should be accurately modeled to determine 
     the best skirt configuration. The steel is inherently 
     in the capacitive region of height and this can 
     complicate the issue.

     Normally the excitation method is shown on the license, 
     but not on CDBS. Complete information about the skirt
     should be in the station's files. (See 73.54 and also
     73.45 and 74.51)

>The theoretical field strength at 1 KW is 
>419 mV/m at a kilometer.

     I think you will find that is based on tower height
     alone.

Phil Alexander, CSRE, AMD