[TowerTalk] New Tower/Antenna Installation
Pat Thurman
thurman@primenet.com
Wed, 10 May 2000 20:59:58 -0700
Paul,
Yes, I know what you mean. I did look at reversing them and I still
haven't
made up my mind for sure, but I think for me the best strategy is to
have my
40m at 70 feet. Here's my reasoning...
First and foremost is that when I looked at the sample data from N6BV's
program
YT, I discovered that here in the Phoenix area, statistically, the vast
majority
of arrival angles for my 40m european path are at 10 degrees and
higher. Then I
looked at the penalty for using a 70 foot height on my terrain vs. a 100
foot height
over flat ground and discovered that at worst it shows a 2 dB penalty
for using the lower
height for my 340N. At 4 degrees they are actually identical (probably
due to
diffraction effects seen at my 45 degree azimuth).
Now, given that background, I looked at what was needed for my C31XR
stack.
First need of course is mechanical safety. Then I began to look at
electrical performance.
I'm limited to a 70 foot tower plus mast. The only real place to put my
lower
stack antenna is at the top of the middle section of the Skyneedle
(45'). So the only adjustment
I really have is to vary the upper stack elevation. As I began to model
the stack using NEC2, I
noticed a pretty good gain bandwidth at most reasonable choices of upper
placement, but I saw a fairly
big difference in F/B effects. IOW, if I choose the smallest stack
spacing (70/45) I appear to
lose as much as 6 to 7 dB of front to back. When I look at at 80/45 or
88/45 I see similar (better) gains,
but better F/B.
Assuming I'm not making a big mistake here, I think I'd be better off
using the the larger stack
spacing (approx 40') and just living with the 40m antenna at 70 feet or
so and eating the 2 dB penalty. What
I haven't done is model the interraction effects of having the 340N 'in
the middle'... Natan did say that
the effects should be minimal with this kind of spacing so I suspect
it'll be fine...
All of this of course depends on how the analysis goes for wind loading
etc vs tower capacity. <g>
73,
Pat WA9NGP
Paul Christensen wrote:
> Pat,
>
> Have you considered reversing the strategy at the top of the Sky
Needle?
> The stack matching may be better if the C-31XR is at the bottom of the
mast,
> with the 340N at the top of the mast. I would think that in terms of
w.l.,
> it would be better to have the low band antenna on the top of the heap
too.
>
> I did get lots of answers to my question. Although higher-angle
lobes are
> produced above a height of 1/2 w.l., the main lobe becomes lower with
> height. Someone ran a NEC program for me using three different
heights and
> at angles below 10 degrees, it's an absolute must to have the 40-meter
> antenna height above 90 feet. Also, this all brought me back to what
I'm
> using now: a 1/2 w.l. elevated vertical on 40-meters. I need to get
the
> 340N up to about 150-feet before it comes close to offering the low
angle
> afforded by my vertical. Even with the gain factor of the 340N, I
wonder if
> this is a rational approach. I guess I can always leave the vertical
in
> place for the really low angle paths. It's just discouraging to see
how
> much power never gets below 10 degrees when using a Yagi at heights
below
> 100-feet.
>
> -Paul
>
> -----Original Message-----
> From: Pat Thurman <thurman@primenet.com>
> To: Paul Christensen <paulc@mediaone.net>
> Cc: towertalk@contesting.com <towertalk@contesting.com>
> Date: Wednesday, May 10, 2000 2:32 PM
> Subject: Re: [TowerTalk] New Tower/Antenna Installation
>
> >This almost exactly what I'm planning to put up... Except that I'm
going
> to
> >stack
> >C31XRs on the Skyneedle using a Tic Ring at the 45' level. As a
result, in
> >order to get
> >the best stack performance and minimize the tower loading, I was
going to
> keep
> >the
> >MAG340N close to the top of the tower with the other C31XR up near
the top
> of
> >the mast.
> >I played around with YT and I don't think 10 or so feet (80' vs 70')
makes
> a
> >huge difference
> >in performance with the 40m. I'd love to hear what the gurus have to
say
> >though.
> >
> >Pat WA9NGP
> >
> >
> >
> >Paul Christensen wrote:
> >
> >> I am considering the purchase of a Force12 40-meter monoband MAG
340N and
> >> multiband C-31XR antennas to be mounted on either a Tri-Ex Sky
Needle or
> >> LM-series 70-foot tower. Apparently, the new MAG 340N is resistant
to
> >> mutual coupling effects and can be mounted within a reasonably
close
> >> proximity to multiband Yagi antennas on a shared mast. With this
in
> mind, I
> >> would like to mount the MAG 340N on top of the C-31XR by a distance
of 15
> >> feet, both pointing in the same azimuth. My question concerns the
height
> of
> >> the antennas and specifically, given the constraints of a single
tower
> with
> >> these two antennas, is there an optimum "overall" height for the
40-meter
> >> antenna? I want to place as much emphasis as possible on the
40-meter
> >> operation. It would appear that the "optimum" height for 40-meters
is a
> >> half-wave above ground in order to minimize the effect of
high-angle
> lobes.
> >> However, I see many single 40-meter monoband antennas mounted up to
> 100-feet
> >> above ground. Why, if more high-angle lobes are produced? I
realize the
> >> ideal situation is to incorporate more than one level of stacking,
but I
> >> want to stay with one antenna at a "best overall" height. Thanks!
> >>
> >> -Paul, W4XR
> >>
> >> --
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