Wow.. a lot of interesting questions.. Some answers and directions for
further analysis are interspersed below...
----- Original Message -----
From: "WA4MWM" <wa4mwm@triad.rr.com>
> CALCULATING FEEDLINE LENGTH:
>
> The physical distance from the center of the tower to the C-51 (top)
driven
> element is 68" in front of the tower (i.e. the driven element is on the
> director element side of the tower). The physical distance fron the
center
> of the tower to the driven element of the C-31 driven element (20 mtr.)
is
> 56" behind the tower (i.e. the driven element is on the reflector side of
> the tower). Therefore the driven element of the C-51 is physically 124"
> (10.33') in front of the driven elements of the C-31s. Therefore I will
> have to make the feedline to the C-51 longer to keep signals in-phase.
>
> Calculating the additional length required for the C-51 feedline, the
10.33
> physical distance is reduced by the Velocity Factor of the RG-213 of .66
to:
> 10.33 * .66 = 6.82'. (Electrical length)
>
> The distance from the C-51 (top) to the switching network is 132.0'.
>
> The length of the feedline required for the middle C-31 (at 92') is
132.0' -
> 6.82' = 125.18'.
>
> The length of the feedline required for the bottom (52') C-31 is the
same;
> however, to reduce unneeded (I think) feedline, I will reduce it by one
> wavelength or: (983.6/14.2) * .66 = 45.72'. This will be 79.46'.
>
>
> QUESTIONS:
>
> 1. Is logic and math correct.
I assume you're basically just using the coax to "time align" all the
antennas, rather than to steer the beam up and down (no inphase/outof phase
stuff?). If you're actually trying to phase things, the coax would need to
be different lengths for different bands.
>
> 2. I am using the 20 mtr driven elements as the reference. Is this
> correct? Why? (I know stacked tri-banders are generally compromises and
I
> have read varying opinions on spacing distances (anywhere from 30' to
50+')
> and feeding, but nobody tells why for example, a 30' spacing is better
than
> 40' -- I chose 40' because that is what most opiners on the subject
> (including Force 12) recommend.)
Whether the 20m DE is the phase center of the antenna is tough to know.
(You'd have to model the antenna or look at it on an antenna range to find
out). With any of the modeling programs out there, it's pretty easy to
determine what the phase shift of the antenna is, relative to some
mechanical reference point. If you're looking at raw NEC output, look at
the phase and magnitude E(theta) and E(phi) columns on the RP card output.
I would think that you don't need an exact model to get a feel for it, but
be aware of this:
Changes in the position of an element (parasitic or not) of 1% or the
relative angle of 1 degree will change the phase of the radiated signal by
about 1/4-1/2 degree (depending on the level of coupling, etc.)
So, you DO need to model the tower (in some way) to understand if it's going
to have an effect (don't obssess about accuracy... just model the antenna
w/o tower, then stick in a single big wire to represent the tower, and see
if the pattern/phase changes a LOT or a little)
On a multiband antenna, the odds are that the phase center will vary with
the band. Probably less so on a trapped antenna (where all bands share the
same elements) than on a interlaced one like the Force 12.
Does it matter if you're off by a foot? A decision you need to make. Even
at 10m, a foot is only 12 degrees, and there are other factors that are
probably larger. (for instance, I wouldn't think trust 0.66 for the
velocity factor, if you're worried about inches here and inches there)
Jim, W6RMK
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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