Dan, Gary, and everybody who took the time to read and respond...thank you very
much for your time and effort to provide some very thoughtful and fact based
feedback.
This has been a great learning experience for me. Obviously I did some
selective reading of ON4UN's book and falsely concluded that I can get away
with 1 to 4 raised radials. Well, the models and practical experience shared
here, as well as re-reading the book, made me rethink this array.
I will lower the verticals and add ground radials. Definitely not looking
forward to laying the radials, but I can't argue with the facts.
Lowering the verticals has the benefit of the wires being below the 68' KT36XA.
The wires and the yagi director are perpendicular to each other and unlikely to
interact, but moving them further apart can only help.
The other no-regrets work will be adding more common mode choke resistance to
the DXE phasing lines. DXE admitted that the current number of beads is too low
and will be increased in the very near future, which of course didn't help me
with the current project.
Finally, I am contemplating replacing some of the wire with aluminum to
hopefully gain some more bandwidth. The question is how tall they can go
without guy wires.
Again, sincerely appreciate everybody's input.
P.S. If anyone has built a really long boom (2 or 3 wave lengths) wired inv V
yagi for 20m and has the EZNec models, please let me know. I am considering a
wire yagi fixed on 355 degrees to help me with polar region contacts. My
station is in VE3 and this path is tough.
Rudy N2WQ
Sent using a tiny keyboard. Please excuse brevity, typos, or inappropriate
autocorrect.
> On Sep 4, 2017, at 2:19 AM, Dan Maguire via TowerTalk
> <towertalk@contesting.com> wrote:
>
> Gary, NA6O wrote:
>>>> Why the manuals say to tune the verticals 150 or 200 kHz low makes no
>>>> sense to me.
>
> I spent some time with that question. From the DXE Transmit Four Square
> System (DXE-TFS4-80B) manual:
>
> "Set the resonance to the low edge of your desired operating range. This is
> because array performance peaks approximately four percent higher in
> frequency than the low resonant point of each individual element. Example: To
> obtain minimum dump power for 3.65 MHz, each vertical is tuned for resonance
> at 3.51 MHz."
>
> The Comtek (COM-ACB-80-A) manual has similar wording except it recommends
> element self-resonance of 3.55 kHz for an array design frequency of 3.65 MHz.
>
> So I started with a model having these characteristics:
>
> a) Conventional 4-sq (ground mounted qtr-wave verticals) with an assumed
> ground loss in the radial field of 5 ohms per element.
> b) Diameter of the vertical elements set to simulate the DXE-7580FS-VA-1
> Qtr-Wave Vertical (stepped diameters ranging from 2.125" to 0.375" over 16
> taper sections).
> c) Array design frequency at 3.650 MHz.
> d) No modifications to the hybrid coupler.
>
> Then I ran comparisons for the gain at 20° elevation, the F/B at 20°
> elevation, and the dumped power, with the elements set for resonance at
> either 3.51 MHz or 3.65 MHz (140 kHz difference).
>
> https://s26.postimg.org/hmszkjhs9/NA6_O-1.gif
>
> https://s26.postimg.org/6yp8lp7t5/NA6_O-2.gif
>
> https://s26.postimg.org/4jxd19rk9/NA6_O-3.gif
>
> If one considers either maximum gain or minimum dumped power (per DXE) as the
> metric for "array performance at 3.65 MHz" it looks like the elements should
> be tuned about 90 kHz lower, not 140 kHz. Of course, that's just for this
> particular modeled scenario. And although that F/B peak of ~52 dB looks
> impressive it's important to keep in mind that's for exactly 180° azimuth.
> The Front/Rear response (gain at 0° azimuth compared to the range 90-270°,
> not just 180°) is almost identical for both cases.
>
> The above study was done using the "Vertical 4-sq Hybrid sweep.weq" AutoEZ
> format model, available for download at the end of the first section on this
> page:
>
> http://ac6la.com/aecollection8.html
>
> ======
>
> Gary also wrote, regarding the N2WQ "single radial, 'L' shaped wire elements"
> array:
>>>> Single radials are not good for pattern, as previously explained. ... I
>>>> cannot see what adding 1/8-wave radials would accomplish, intuitively nor
>>>> in simulation.
>
> I ran some models for that as well. N2WQ had proposed adding three 1/8-wave
> radials per element, in addition to the single 1/4-wave radial. A top-down
> view of the model looks like this, with the central tower and its grounding
> system temporarily removed to avoid clutter and confusion. (Those wires were
> put back in for the actual modeling runs.)
>
> https://s26.postimg.org/7cv5z1hah/N2_WQ-5.png
>
> With the extra 1/8-wave radials added, the vertical portion of each element
> was changed from 68.5 to 68.7 ft to maintain element resonance at 3.413 MHz,
> same as the previous single radial model. Then I ran sweeps to compare the
> gain at 20° elevation, the F/B at 20° elevation, and the dumped power. For
> all sweeps and with both model configurations, the elements are being fed via
> a commercial hybrid coupler (DXE or Comtek) with no adjustments or
> modifications.
>
> https://s26.postimg.org/q6gyw1fih/N2_WQ-6.png
>
> https://s26.postimg.org/5wjngwedl/N2_WQ-7.png
>
> https://s26.postimg.org/f31y46jm1/N2_WQ-8.png
>
> I'd have to agree with Gary, it's hard to see any advantage with the extra
> 1/8-wave radials. The gain is worse, the F/B is better, and the dumped power
> is about the same.
>
> If anyone would like to experiment with Rudy's setup I have AutoEZ format
> models available for both the single 90° radial and the "1x90° + 3x45°"
> configurations that I'll be happy to share. Drop me a line off-list.
>
> Dan, AC6LA
> http://ac6la.com
> _______________________________________________
>
>
>
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