Hi Bill and all, this is a difficult compromise, and I feel may differ from
site to site.
I see it like this.....
If you put top load on a vertical you can increase its effective height
and Rrad) from h/2 to close to h.
If you have to droop them then you will loose some of this gain ....Laurie
suggested to me it was of the order of 1/3rd of the overlap with the
vertical.
Yes the commercial boys use drooped loading wires on their Umbrella
aerials..... see the pics of CFH on Dave's site, and DCF39, and the new
Skelton 19kHz mast (cant remember the URL but it may be on Daves site as
well) so it must be economic in their terms.
BUT the thing is, Rrad is NOT everything that matters......increased
capacity means a smaller loading coil, easier to make and less loss there,
it also means a considerably reduced ground loss, particularly with amateur
sized aerials. Since ground loss is our main "power thief" this part can
well over-ride any small loss in Rrad because of the increase in aerial
current from a given available PA power. You still need to keep it clear of
the trees though.
So by my reckoning, flat (level) is good..... but longer is better even
drooped, and because the Rrad term is proportional to the square of the
height ..even an extra 50% in vertical section height is well worth having,
even if it then means drooping the top load a bit.
An "ounce" of experimental measurement is worth several weeks playing with
NEC !! Remote FS measurement as Bill says (and Jim has done extensively) is
the only real test.
Happy (aerial) farming
Cheers de Alan G3NYK
[email protected]
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