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LF: Re: Top loading - to short or not to short?

To: [email protected]
Subject: LF: Re: Top loading - to short or not to short?
From: "Dave Brown" <[email protected]>
Date: Tue, 27 May 2003 21:16:46 +1200
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
I believe you will find the multiple vertical wire arrangements are
generally for mechanical stability and don't have a lot to offer
electrically. As amateur top loaded antennas are generally a lot smaller
than the commercial ones (There are some exceptions!) they don't need these
considerations. A large two wire flat top for example, typically has
stabilising halyards from each end of the spacers down to a comon anchor
point at both ends of the flat top and in the middle-. The ones in the
middle do double duty as the vertical element as well but will usually be
commoned some distance above the top of the loading coil structure so a
single wire forms the lower portion of the vertical element. Conversely, the
end halyards are usually tied down to an anchor point on the support masts
individually, so they can be adjusted individually as required. ( to keep
the flat top flat!)
73
Dave, ZL3FJ

----- Original Message ----- From: "Laurence J Howell" <[email protected]>
To: <[email protected]>
Sent: Tuesday, May 27, 2003 6:55 AM
Subject: LF: Top loading - to short or not to short?


Im  still struggling with a 110 foot dying spruce tree and wonder if Ill
every get the wire up it before it snaps in these high winds. Spruce trees
are dying/have died in large numbers in S central alaska. Beetle carrying
virus.

  In looking at my LF Inverted L, and specifically the 3 wire top loading
wires - I've a couple of questions...

1. I see a recommendation that the vertical part of the Inv L aerial
wire(s) should be a single wire (ZL recommendation), or if not then three
or how many wires should converge to a single point at the bottom. Why is
a
single wire recommended over  a multiple? Em thinking losses into nearby
objects? Wouldn't a multiple vertical wire feed increase  "good non lossy"
C and increase Ra?

2. Ive seen a large amount of schematics both for commercial and ham top
loading.of T and L's ...Say for a three wire top loading spaced 1m apart -
should the wires at the far/near end all be shorted together or left open?
Does this affect the total cap either way, or is it just for electrical
"circuit" stability?? Does it make any difference if they are shorted at
any point along the top?? Is there a recommendation which proves open
versus short is better ?

Mine are open all the way and Im using a converging 3 wire vertical wire
feed... I see shorted at the ends, shorted half way, open ended solutions
from my peers here...

Im trying to rationalize that the end of the loading wires are at Zero
Current (no different circulating/local currents between the wires?).. and
it may not be an impedance thing but may help reduce high voltages as the
surface area at the end is now a lot larger...and may reduce corona.

Most commercial T's for NDB short out at the far ends..and I do see single
and multiple wire converging vertical wires, so I guess they are all right
But a lot of the drawings ref materials show them open! Arghh.

Any experiences welcomed...

Laurence KL1X -






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