I have not personally seen work to validate signal strength and prove the
mechanics of arrival at various altitudes at 50 km. Tons of speculation,
extrapolation. NEC4 says that it doesn't continue. But NEC4 also nicely
predicts the 2.8 km helicopter measurements.
There apparently is completely zilch commercial interest or need for the
answers to our speculations. The question is in a niche within 160m
operation, itself a niche within ham radio, which is yet a niche within
general radio. PLUS our propensity to want things for free, is it really
any amazement that the issue is not researched?
In my college days at Berea, Ky, I would listen at night to clear channel
650 WSM in Nashville (Grand Ole Opry, etc), using a 195 degree tower,
supposedly the Cadillac for low angle transmission. In the daytime at a
distance of 165 miles, the 50 kW signal was there but very raspy to the
point of irritation at Berea. I learned not to bother turning on that
station during day. Complaints. Some folks in the dorm had the same
response to scratchy radio signals as drawing fingernails across a
chalkboard. At night the combination of a strong, clear WSM, live
broadcasts, and a wonderfully mellow 5 tube Zenith in a bakelite cabinet
was a near constant sound in that end of the hall.
I'm sure that WSM was happy to cover a radius most of the way to Knoxville
in the daytime.
At night they had a killer signal without any background noise. I would
hear fading as the signal started to come up at dusk, but nighttime fading
would have been from extremely loud to very loud, because I never heard
any.
So I would have to say that whatever passed for ground wave on that signal
was pure useless for DX in ham terms. "Ground enhanced" or "ground ducted"
, or whatever you would want to call that, should be the same day or night,
right? It was HUGELY attenuated from the night time sky wave.
So far, regarding this issue, I have seen/heard neither technical
measurement nor anecdote to contradict the NEC4 estimations. I have other
issues with NEC4, but none related to this. Yet.
Anybody know someone who owns a helicopter just looking for any excuse to
fly on a nice day? It is possible to stack up graphs, equations, and
quotes from old textbooks until the BS buffer completely overflows all over
the rug. But until we have sitting-man-with-meter-in-helicopter out at 50
km drop from operational ceiling to the ground, measuring all the while,
all we have is anecdotes that follow NEC4, and no contradictions at all
from technically proper measurements.
73, Guy.
On Wed, Oct 24, 2012 at 9:56 AM, Paul Christensen <w9ac@arrl.net> wrote:
> I have linked to an image of a NEC 4 vertical profile analysis of the
> WLS-AM radiator. As most everyone here knows, WLS is a legendary 50KW AM
> station in Chicago. WLS uses a 190 degree radiator and is the preferred
> antenna height for most AMBC purposes since it's the height that produces
> the most gain at zero elevation while simultaneously minimizing skywave
> lobes.
>
> http://tinyurl.com/9zuqpre
>
> Notice how the pattern is crushed to the horizon with very little of a
> high angle lobe produced. I ran a far-field analysis of three
> hypothetical models over: (1) average soil conductivity [green]; (2) poor
> soil conductivity [red]; and (3) highly conductive salt water [blue].
> Notice a few things from the colored overlay:
>
> 1) The WLS radiator over salt water produces a significantly improved
> field strength at a distance of 1km over the other two models. No surprise
> here;
> 2) The shape of the far-field profiles *appear* to change as a function of
> ground conductivity.
>
> However, if we were able to conduct a surface wave analysis and overlay
> the three curves (4Nec2 does not appear to allow that in the surface mode,
> but I'll check with the author), we would find that the shape of the curves
> going up in Z axis vertical distance is the same. I've run the model for
> many vertical radiators and that's what NEC 4 reports. This too should be
> no surprise as ground wave effects to 3 MHz have been extensively
> documented in academia since the 1930s.
>
> Here again, if we were to believe that WLS produces no far field strength
> at low elevation over anything but salt water, they would have very limited
> daytime listeners - even with 50KW of output power. So, to Rich's past
> points, we need to consider both the far field and surface wave components
> when computing far field strength over a range of elevations. Don't just
> believe that the far field plots we've seen time and again are always
> representative of the actual low-angle field strength. According to Jerry
> Burke, author of NEC 4, the surface wave component can be a major part of
> achieving ionospheric DX reach through at least 3 MHz.
>
> Paul, W9AC
>
> ----- Original Message ----- From: "Richard Fry" <rfry@adams.net>
> To: <topband@contesting.com>
> Sent: Tuesday, October 23, 2012 9:28 AM
> Subject: Re: Topband: Monopole Elev Pattern w.r.t. Earth Conductivity
>
>
>
> Cristi YO3FFF wrote:
>>
>>> That means, the radiation pattern will be affected too because the
>>> electromagnetic wave will be much curved to the ground, so the
>>> groundwave intensity will be direct proportional with the conductivity
>>> Is it right?
>>>
>>
>> Other things equal, the field values in the vertical plane radiation
>> pattern
>> first "launched" by a monopole are a function of earth conductivity. But
>> the shape of the radiation pattern close to the monopole remains
>> essentially the same for all conductivities, as shown in the link below.
>>
>> The fields in the NEC4 analysis below were calculated along a vertical
>> distance of 0-50 meters above the surface of the earth, at a horizontal
>> distance of 100 meters from the monopole. A vertical distance of 50 meters
>> for this chart is an elevation angle of 26.6 degrees from the monopole.
>>
>> Note in the chart that the radiated fields at elevation angles below 26.6
>> degrees are greater than the field at 26.6 degrees. For the lowest
>> elevation angles, those fields are FAR greater than those shown in a NEC
>> "far-field" analysis, which for real earth go to zero field in the
>> horizontal plane.
>>
>> At an elevation distance in the chart of of 15 meters (8.5 degrees), the
>> difference in the field shown for 1 mS/m conductivity and that for sea
>> water
>> conductivity is 2.6 dB -- probably less than intuition would expect.
>>
>> It is the low-angle radiation directed toward the ionosphere from the
>> fields
>> at ~this electrical distance from the monopole that can produce useful
>> skywaves having the greatest single-hop range.
>>
>> http://i62.photobucket.com/**albums/h85/rfry-100/Monopole_**
>> Surf_Wv_Compare.jpg<http://i62.photobucket.com/albums/h85/rfry-100/Monopole_Surf_Wv_Compare.jpg>
>>
>> ______________________________**_________________
>> Topband reflector - topband@contesting.com
>>
>
> ______________________________**_________________
> Topband reflector - topband@contesting.com
>
_______________________________________________
Topband reflector - topband@contesting.com
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