Maybe I'm missing something, but I don't see anything all that strange
with your results. The WARC dipole, for receiving purposes, might as
well be a normal 10m dipole. Your stack of C31's at 121, 81, and 41
feet should work great on 20m, but they are are pretty high and spaced
pretty wide for 10m. I realize HFTA (the terrain modeling program from
N6BV that comes with the ARRL Antenna Book) may not be perfectly
accurate, but I loaded your situation into it and got some interesting
results. I don't think this reflector allows jpeg enclosures so I'll
send them to you separately (and I'll send them to anyone else who asks).
I ran the model for your stack at 121, 81, and 41 feet with the
comparison dipole at 91 feet. At certain elevation angles the stack has
a deep notch in the pattern and the dipole will even beat it.
I also ran a plot for a stack at 105, 90, and 75 feet to show the
comparison for the stack with closer spacing at the same height as the
dipole. The peaks in the pattern match those of the dipole but there is
still a pretty deep notch just above 10 degrees takeoff. That might be
an issue at some times for some paths.
Finally, I ran a plot for a stack at 70, 55, and 40 feet (arbitrarily
chosen) to show how a lower height for 10m actually improves the pattern
... the lowest notch goes away.
As an aside, the C31 specs show a gain of roughly 7.5 db over a dipole.
The maximum gain you can expect from stacking two identical antennas
(even with optimum spacing) is about 3 db over that of a single
antenna. Adding a third antenna to the stack will give you maybe 1.5 db
additional gain. When I add those numbers (7.5 + 3.0 + 1.5) I get 12 db
gain for your stack over the dipole if the spacings were optimized, and
in your case they are not. That doesn't sound too much different than
some of your observations. The primary benefit of an optimized stack is
more it's ability to remove notches from the takeoff pattern than it is
simply to add some gain. Removing a notch, as you can see from the HFTA
plots, might result in several S-units benefit for certain conditions.
Hope any of this helps. 73,
Dave AB7E
StellarCAT wrote:
> I have a stack of C31's up - top at 121 then 81'/41'. They work
> great on 20 and 15... and I had believed on 10 as well... but
> recently I installed a simple D3W warc dipole on the side of the
> tower about 10' above the middle C31. The dipole is inline with
> the elements of all the antennas (not turned 90 degrees to
> them). The C31's drivers are on the opposite side of the tower
> (this is a rotating tower so all antennas are mounted to the
> tower) which places them not directly under the dipole - and the
> dipole is about 11' total distance away... and of course the
> dipole is not resonant on 10... however...
>
> I noted today that the difference in signal is much more often
> than not just a couple 2 - 3 S units (Orion II). I have seen,
> once, with one station and then briefly, a difference of about 7
> S units... but I have also seen almost no difference at all (1/2
> S unit)... this seems incredibly strange to me. Since I had
> nothing to compare it to before putting up the WARC dipole I
> don't know if it has changed (the array) due to
> mechanical/connection issues, or there is interaction of the
> dipole, or it has always been this way due to interaction with
> the metal guys (with insulators). But these findings (closeness
> of
> the dipole to the array in signal strength) just do not seem
> right.
>
> I am using an Ameritron switch to switch between them. The
> switch is at 60'. There is also a StackMaster box for selecting
> the array antennas separately or together.
>
> The SWR has not changed since installing the WARC dipole
> that I can see.
>
> Any "modeling experts" have any educated guesses and/or
> experience similar to the above?
>
>
> thanks.
>
> g.
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
--
save the cheerleader ... save the world
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|