At 07:56 AM 12/16/2006, David Robbins K1TTT wrote:
>I am trying to decide what to use to build 4 new 80m verticals for a
>4-square upgrade here. My criteria are:
>1. full size 1/4 wave verticals, no top or base loading.
>2. must be able to standup to new England winters at 2000' with ice and wind
>3. must be new material, I don't want to go scrounging for used tower or
>4. prefer only 1 guy level, but will do 2 if necessary.
>5. prefer to stand up complete, but will climb to add top section(s) if
>necessary, crane or truck access will be very hard to this spot.
>6. prefer something that can be delivered to here, truck delivery is not a
>problem, I don't want to have to haul long stuff myself.
A couple things to think about, and a few questions..
What are you going to use as a feed network? A Comtek box, A
Lewallen current forcing, Full on Gehrke networks?
Are you operating over the entire 3.5-4 MHz? Or just over a small slice?
In any phased array the mutual impedances make it so that "resonance"
is sort of poorly defined. When the array is driven, none of the
elements are going to be a nice 50+j0 ohms, so obssessing about the
length may not be worthwhile. The vertical radiation pattern of a
single element doesn't change much with, say, a 10% length change one
way or another, so you can choose a height that is convenient mechanically.
If you want a particular impedance at the feedpoint for individual
elements, some lumped components at the feed point would probably
meet the need quite nicely. A big inductor with low loss might be
easier to build than a low loss capacitor, so cutting the element
"short" might be a plan. On the other hand, vacuum variable caps
show up surplus all the time, and you're going to be investing
several thousand dollars in the rest of the array, so a few hundred
bucks for some caps might not be a problem. And, this gets back to
my question about how you intend to feed the array. Some feed
methods (e.g. "current forcing") aren't all that picky about
feedpoint impedances, although, if you get too far away, the losses
in the element feedline *might* become significant (you'd have to
calculate and see..)
On one hand, it might seem that making large diameter elements for
broadband is useful. On the other hand, the spacing isn't a fixed
fraction of a wavelength as frequency varies, either, so it's not
clear that a real broadband element is needed. Either you tune for a
particular frequency (including spacing and relative electrical
phasing) and accept less performance at other frequencies or you use
multiple networks and customize.
Given all the practical uncertainties in things like ground radials,
soil properties, local environment, etc, I think that one's best bet
is to design for mechanical considerations, and then fool with the
electrical issues second. You want something in the quarter of a
wavelength area (unless you're going to do a vertical dipole array),
so you need something that's reasonably structural for a 60-70 ft
length. I think that tubing, in some form or another is probably the
best deal overall. Thin wall irrigation tubing in the 3-4" diameter
range, with guys, can easily do those heights, and it's inexpensive
(especially if you buy it used) and easy to handle. You could step
the diameter down as you go up to make it lighter and easier. If you
want free standing, then maybe something like a flagpole is the way
to go. (or, since you won't have the wind load of the flag, you can
use dimensions from commercial flagpoles, and build it
lighter). 60-70 ft flagpoles are about a foot in diameter at the bottom.
While I know you don't want to go scrounging used tubing, if you call
the places that sell the new stuff, they'll usually also have a line
on used stuff. Or, if you call a place like "Rain for Rent" which
does irrigation stuff in general, they'll have tubing that can't be
rented out any more (too many dings and/or pin holes.
Do they have 4" irrigation tubing in New England? It's one thing
where you've got miles of flat ground to irrigate like in
California's central valley or around here in the Oxnard plain..
totally another in rolling hills with more natural rain where it
might not be the usual thing.
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