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

[Carlos Mourato]

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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