Re: ARLA/CLUSTER: Calculador/Indicador de Propagação, para VHF e UHF.

Carlos Mourato radiofarol
Quarta-Feira, 12 de Dezembro de 2007 - 17:38:11 WET

É isso mesmo, sem tirar nem por!

73 de CT4RK

2007/12/12, antonio matias <ct1ffu>:
> Pois Pois Colega Mourato-
> há aqui apenas um ligeiro problema de lingua.
> Quando o colega se refere a Disperção e Duto eu referia-me a Scatter e
> Ducting
> Voce em Portugues e eu em Ingles mas estamos a falar da mesma coisa hi hi
> Estas termos normalmente são mais conhecidas em Ingles do que na lingua de
> Camoes.
> Mas EStamos de acordo :-)
> Melhores 73's
> ------------------------------
> From: ct1ffu
> To: cluster
> Subject: RE: ARLA/CLUSTER: Calculador/Indicador de Propagação, para VHF e
> UHF.
> Date: Wed, 12 Dec 2007 17:28:18 +0000
> Então se nunca leu, leia lá agora hi hi :
> ------------------------------
> ------------------------------
>   *Tropospheric V/UHF DX Modes..* *Line-of-Sight (GW)* normal
> continuous reception where the receiving and transmitting antennas can see
> each other..taking into account the 4/3 Earth curvature of radio waves.
> *Tropospheric Scatter (TrS)* ever-present under normal conditions.
> That's the mode that produces the distant fluttery signals that randomly
> fade in and out. These are your most distant regular stations that barely
> make it in. Depending on your location and equipment..tropo scatter can
> extend to 300..500..or even 700 km. The theoretical maximum limit for most
> TV/radio DXers is 800 km (500 mi) (Some semi-professional setups can extend
> furthur). Scatter is caused by small particles/droplets in the air such as
> haze, dust, volcanic ash, clouds, etc.
> *Tropospheric Enhancement (TrE)*..(aka*Tropospheric Refraction*) is common
> under normal conditions. On most clear nights with calm or light winds..the
> ground radiates and the air near the ground cools. Eventually an inversion
> is formed and signals begin to refract off the inversion. Stations that
> normally fade in and out via tropo scatter come in continuously..with
> increasing strength. Also..weaker tropo scatter stations that are normally
> not heard (because their signal strengths never cross the background noise
> threshold signal level) also begin to appear. When the sun comes up..the
> ground & air heats up..the inversion breaks down..and the enhancement
> disappears. The enhancement is subtle on some nights..and very obvious on
> other nights. Distances are no different than your tropo scatter
>'s just that the signals are stronger. Tropo enhancement is
> greatly influenced by terrain..with valley and coastal paths favoured.
> ("Fog-prone" areas are also "DX-prone" areas!!). From a DXers point of view,
> multiple directions usually are enhanced at the same time.
> *Tropospheric Ducting (TrD)* an abnormal condition. An inversion has
> formed at a much higher level above the ground...the vast majority of
> duct-producing inversions lie between 450 and 1500 m (1500 to 5000 ft)..with
> a few between 1500 and 3000 m (5000 to 10,000 ft). These inversions are not
> formed due to nighttime radiation/cooling..but rather because of some other
> weather phenomenon (high pressure subsidence aloft, warm frontal boundary,
> cold frontal boundary, oceanic or lake inversion, Chinooks, etc.). Because
> of this..ducting can occur day or night (though it strengthens at night)
> not usually influenced by terrain (the exception being large mountain chains
> like the Rockies)..and from a DXers point of view is usually either uni- or
> bi-directional. In fact..typical ducts are sharply directional. Signals
> refract off of and also travel along the inversion..thus the analogy of a
> duct. Distances are theoretically unlimited. One large area can have
> multiple ducts going on simutaneously..but they are usually parallel paths.
> It is possible in a very strong high pressure system to have large areas of
> ducting creating multi-directional openings. These are the rare
> "blockbuster" openings that make DXers' mouths water. They are most common
> over the oceanic areas in the tropics and sub-tropics.
> *Additional Characteristics of Ducting..*
>   Ducting may or may not occur simutaneously with enhancement (caused by
> nighttime cooling). Often there is both a low-level radiational inversion
> caused by nighttime cooling (producing enhancement)...and a mid-level
> "system-produced" inversion above that (producing ducting). However..just as
> often there is only the higher duct-producing inversion..especially if the
> skies are cloudy or if it is windy. not use your regular
> scatter/enhancement stations as propagation beacons for longer-distance DX
> acheived via ducting! Sometimes ducting can even display a "skip-like"
> character where distant stations on the same frequency *and bearing* can
> be received while closer-by stations are nowhere to be seen.
>   Ducting is also very height selective..with maximum signal transmissions
> at and just below the altitude of the inversion. DXers normally receive
> ducting DX via ground-based inversions. Occasionally inversions can be based
> above ground, in which case a DXer beneath the duct could completely miss
> out on the DX opening. Meanwhile, DXers at a higher elevation could, at the
> same time, be in the midst of a strong opening. It is also possible that a
> DXer at too high an elevation (above the top of the duct) could also miss
> out.
>   Ducting conditions usually vary over short time periods as opposed to
> enhancement which is more stable. Ducts located behind cold fronts
> ("post-frontal ducts") are notoriously unstable as paths can even be
> interrupted by things such as heavy rain showers associated with the cold
> front itself. Expect the unexpected from these types of ducts with sudden
> and rapid changes in signal strengths quite common (some post-frontal ducts
> last only 15 to 30 minutes). High-pressure and oceanic ducts are a bit more
> stable and can last for days..but again expect the unexpected as changes can
> occur quickly.
>   Frequencies affected by ducting are determined by the vertical thickness
> of an inversion. Individual ducts will have a LUF (Lowest Usable Frequency)
> associated with them. Thin inversions (i.e.-thin ducts) will only
> propagate Microwaves. Thicker inversions will propagate UHF signals as
> well..while the thickest inversions will also propagate VHF signals.
> Unfortunately there is no reliable method known for forecasting inversion
> thicknesses. Also, in real-life tests, it has been found that reception at
> frequencies below the theoretical LUF is usually possible, although there is
> usually a sharp drop-off in signal strength at the LUF. See *LUF*<>
>   page.
> ------------------------------
> *Special Cases (Exotic DX Modes)..* *Rain Scatter (RS)* a rare mode
> that sometimes occurs on the higher UHF-TV channels. A band of very heavy
> rain (or rain and hail) at a distance can scatter or even reflect signals.
> The effect is the one used for microwave Weather Radars. Distances are
> typically around 160 km..though up to 650 km (400 mi) is theoretically
> possible. (Note that heavy snow is not an useful reflector).
> *Ice Pellet Scatter (SS)*..(called *Sleet Scatter* in the US) similar
> to Rain Scatter but is caused by bands of Ice Pellets in the wintertime.
> *Aircraft Scatter (AS)*..(aka *Tropospheric Reflection*) is simply
> reflection off of aircraft..although reflections off of flocks of birds are
> also possible. A rare form of reflection is "Chaf Scatter". Chaf is strips
> of metal foil sent out by the military during training exercises. Chaf helps
> to confuse enemy radars..but also helps to produce DX. Maximum distances for
> all reflection modes are again up to 800 km (500 mi).
> *Lightning Scatter (LS)* a mode that is sometimes discussed..but there
> is little documentation on it. The theory is that lightning strikes produce
> ionized trails. Reception is similar to other forms of scatter except that
> the DX is more burst-like similar to MS. LS is a mode that is very hard to
> distinguish and rarely reported.
> ------------------------------
> Reflections off of hills and mountains..and Knife-Edge Diffraction are not
> considered true DX modes since they are omni-present..though they can help
> to extend DX via the other modes. So these are the conditions in the
> troposphere that allow reception of VHF and UHF signals beyond their normal
> range. Basically..these are DX modes that are affected by the weather.
> In the forecasts.....the reason that I stick with just Ducting is because
> it is a large-scale phenomena..can be forecasted in a reasonable amount of
> time..and produces the best tropo. Enhancement is forecastable..but it is so
> dependant on regional and local terrain and conditions that it would be a
> labour-intensive effort (not to mention a very lengthy one) to forecast for
> all of North America. The process that one would have to use to properly
> forecast Enhancement for a particular area is the same that a meteorologist
> would use to forecast overnight low tempertatures, chance of fog patches,
> etc.
> ------------------------------
> ------------------------------
> ------------------------------
> ------------------------------
> Melhores 73's CT1FFU
> > From: joao.a.costa
> > To: cluster
> > Date: Wed, 12 Dec 2007 16:51:49 +0000
> > Subject: RE: ARLA/CLUSTER: Calculador/Indicador de Propagação, para VHF
> e UHF.
> >
> > Caro Matias,
> >
> > Esta é totalmente nova para mim:
> > "O Modos de propagação via Troposfera em VHF é, por exemplo mais
> influenciado pela poluição e poeiras no ar do que pela temperatura ou
> humidade."
> >
> > Tudo o que lie até hoje, a "tropo" acenta em 3 factores importantes:
> > - Pressão Atmosférica.
> > - Temperatura.
> > - Humidade.
> >
> > agora " pela poluição e poeiras no ar " é que nunca tinha lido.
> >
> > João Costa
> > CT1FBF
> >
> >
> >
> > ________________________________
> >
> > From: cluster-bounces [mailto:
> cluster-bounces] On Behalf Of antonio matias
> > Sent: quarta-feira, 12 de Dezembro de 2007 14:51
> > To: Resumo Noticioso Electrónico ARLA
> > Subject: RE: ARLA/CLUSTER: Calculador/Indicador de Propagação, para VHF
> e UHF.
> >
> >
> > Ora vamos lá então ver...
> >
> > Existem 13 tipos distintos de propagação em VHF !
> >
> >
> > Aurora
> > Aurora E
> > Back Scatter
> > EME
> > Esporádica E
> > FAI -field alinement irregularities
> > F2
> > Iono Scatter
> > Meteor Scatter
> > Reflexão -Satélite/avião
> > SSSP/WSSP -summer solistice short path propagation/ winter
> > TEP - trans-equatorial Path
> > Tropo
> >
> >
> >
> > naturalmente embora o possa fazer, não vou explicar agora as diferenças
> entre elas.
> > e entre estas 13 apenas o TROPO é influenciado pelo estado do tempo.
> > Mas por norma os dados que esses programas fornecem não indicam
> absulutamente nada em rigor para a previsão eficaz de propagação.
> >
> > Isto porque há outros factores a ter em conta e muito mais importantes.
> > O Modos de propagação via Troposfera em VHF é , por exemplo mais
> influenciado pela poluição e poeiras no ar do que pela temperatura ou
> humidade.
> >
> > e estes dados são dificeis de obter.
> >
> > no entanto o Tropo Ducting já depende sim das correntes do ar, camadas
> de ar com temperaturas e direcções diferentes em sobreposição.
> >
> > mas tambem aqui os dados não são facilmente equacionados
> >
> > Bom isto é materia para escrever um livro hi hi
> > Mas uma coisa é certa!
> > NÂO HÀ formulas para previsão de propagação rigorosas. apenas a
> experiencia de anos anteriores que pode garantir alguma ciclicidade para
> quem estiver atento...
> >
> >
> >
> >
> >
> > _______________________________________________
> > CLUSTER mailing list
> > /cluster/listinfo/cluster
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Best 73 from:
regards from:
Carlos Mourato

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