ARLA/CLUSTER: Cover very short distances on HF.(NVIS reports byTom(W6QJI)

[Mariano Gonçalves]

Carlos,

Nalguns países isso é um dado. Seria importante, ao menos para treino e situações de emergência apenas.

Esperemos a ver ...

73, Mariano
  
  ----- Original Message ----- 
  From: Carlos Mourato 
  To: Resumo Noticioso Electrónico ARLA 
  Sent: Monday, July 09, 2007 6:48 PM
  Subject: Re: ARLA/CLUSTER: Cover very short distances on HF.(NVIS reports byTom(W6QJI)


  O ano passado apanhei NVIS a chegar aos 14Mhz!,  mas foi menos de uma semana. Depois estações a 100km em 20m "fugiram". Enquanto deu, cheguei a escutar estações de Lisboa com 59+30!
  ...Precisamos da banda dos 5Mhz como os ingleses tem.

  73 de CT4RK

   
  2007/7/9, Mariano Gonçalves <ct1xi  sapo.pt>: 
    João,

    O que o Tom diz: «rarely exceeds 6 MHz ...» isso depende da latitude do
    lugar e do campo magnético terrestre (incluindo a cintura de Van Allen). 

    Em África, recentemente, quando estive na EUFOR Kongo e Gabão, por exemplo
    cheguei aos 12 MHz, e o típicos é 8 ou 9 MHz. Já aqui na Península Ibérica,
    raramente excede os 5 ou 6 MHz, por acaso, este ano chegou aos 8 MHz, muito 
    excepcionalmente.

    Se quiseres, tenho aqui instalada uma antena NVIS da HARRIS, estou a fazer
    estudos nessa área, para desenvolver novas antenas (de âmbito industrial e
    militar).

    73, Mariano



    ----- Original Message -----
    From: "João Gonçalves Costa" <joao.a.costa  ctt.pt>
    To: <cluster  radio-amador.net >
    Sent: Monday, July 09, 2007 1:52 PM
    Subject: ARLA/CLUSTER: Cover very short distances on HF.(NVIS reports by
    Tom(W6QJI)


    Hi,

    I've read discussion in this session about the inability to cover very short 
    distances on HF. There just has to be something wrong or overlooked. The
    two things that come to mind are a sick rig, or bad coax, at one end or
    excessively high NVIS Critical Frequency (f0F2).

    F0F2 can be seen at Near-real-time F2 Layer Critical Frequency on the 
    Internet. F0F2 --- the
    highest frequency at which NVIS propagation is supported by the ionosphere -
    rarely exceeds 6 MHz, in the northern hemisphere, at this point in the solar
    cycle, this time of year and the time of day. And, it does vary considerably 
    from day to day. The global curve set is updated every five minutes While
    the curves shown on the site are not brick walls, 40 meter NVIS, with f0F2
    at 6 MHz, is unlikely.

    Absent rig troubles, 75 Meter NVIS should work very well over the ranges 
    indicated.

    A suitable NVIS antenna is a dipole with the feed point at 15 feet AG and
    the dipole ends at 2 feet AG. The length will be a little longer than
    468/f, so suggest you cut it a little longer then tweak it on. The 
    bandwidth will be about 100 Khz. The feed point Impedance will vary with
    ground conditions - but usually very close to 50 ohms - no tuner required.
    The impedance will go up, if the ends are raised.

    W.Wa. Section Mgr., N7NVP and I have developed a series of NVIS designs 
    which we presented at the NW Comm. Academy. Ed and I have used these
    antennas extensively. I my case, and I'm sure Ed's experience is the same,
    coverage is solid from the antenna out in a circle with a -3db radius of 600 
    miles - a little farther at night. A CD, containing Power Point
    Presentations of some of our NVIS antenna work, is available via the ARRL.

    Vertical antennas are hard to achieve at 75, Some loading has to be used, 
    to keep them reasonably short, and an efficient ground is very difficult.

    Ground Wave --- with the stations only two miles apart, ground wave with
    just about anything made of wire for an antenna and 100 watts, should 
    produce a readable signal on 75 or 160.

    In the 1980's we ran a social net on ten meters with members checking in at
    distances 30 or so miles. We had antenna parties where we built 10 meter J
    poles. When we all got the J Poles up, we had solid copy every night. So, 
    maybe 10 meters and a simple J Pole antenna would get it done.

    Finally, Mark Yordy, W7BBO, and I played with a full wave loop on 160 lying
    flat on the grd. It was 20 or 30 feet too long from 468/f. But, after we 
    got it on 1.965, we got excellent reports all over the NW with a 100 watt
    mobile rig.

    I hope these thought are helpful.

    73 de Tom (W6QJI)

    tjsand  wavecable.com 

    [Non-text portions of this message have been removed]



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  -- 

  Best 73 from:
  regards from:
  CT4RK
  Carlos Mourato
  Sines - Deserto do Jamais Ocidental
  (Deserto livre de dromedários!...foram todos para a Praça do Comércio)

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