<div>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!</div>
<div>...Precisamos da banda dos 5Mhz como os ingleses tem.</div>
<div> </div>
<div>73 de CT4RK<br><br> </div>
<div><span class="gmail_quote">2007/7/9, Mariano Gonçalves <<a href="mailto:ct1xi@sapo.pt">ct1xi@sapo.pt</a>>:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">João,<br><br>O que o Tom diz: «rarely exceeds 6 MHz ...» isso depende da latitude do<br>lugar e do campo magnético terrestre (incluindo a cintura de Van Allen).
<br><br>Em África, recentemente, quando estive na EUFOR Kongo e Gabão, por exemplo<br>cheguei aos 12 MHz, e o típicos é 8 ou 9 MHz. Já aqui na Península Ibérica,<br>raramente excede os 5 ou 6 MHz, por acaso, este ano chegou aos 8 MHz, muito
<br>excepcionalmente.<br><br>Se quiseres, tenho aqui instalada uma antena NVIS da HARRIS, estou a fazer<br>estudos nessa área, para desenvolver novas antenas (de âmbito industrial e<br>militar).<br><br>73, Mariano<br><br>
<br><br>----- Original Message -----<br>From: "João Gonçalves Costa" <<a href="mailto:joao.a.costa@ctt.pt">joao.a.costa@ctt.pt</a>><br>To: <<a href="mailto:cluster@radio-amador.net">cluster@radio-amador.net
</a>><br>Sent: Monday, July 09, 2007 1:52 PM<br>Subject: ARLA/CLUSTER: Cover very short distances on HF.(NVIS reports by<br>Tom(W6QJI)<br><br><br>Hi,<br><br>I've read discussion in this session about the inability to cover very short
<br>distances on HF. There just has to be something wrong or overlooked. The<br>two things that come to mind are a sick rig, or bad coax, at one end or<br>excessively high NVIS Critical Frequency (f0F2).<br><br>F0F2 can be seen at Near-real-time F2 Layer Critical Frequency on the
<br>Internet. F0F2 --- the<br>highest frequency at which NVIS propagation is supported by the ionosphere -<br>rarely exceeds 6 MHz, in the northern hemisphere, at this point in the solar<br>cycle, this time of year and the time of day. And, it does vary considerably
<br>from day to day. The global curve set is updated every five minutes While<br>the curves shown on the site are not brick walls, 40 meter NVIS, with f0F2<br>at 6 MHz, is unlikely.<br><br>Absent rig troubles, 75 Meter NVIS should work very well over the ranges
<br>indicated.<br><br>A suitable NVIS antenna is a dipole with the feed point at 15 feet AG and<br>the dipole ends at 2 feet AG. The length will be a little longer than<br>468/f, so suggest you cut it a little longer then tweak it on. The
<br>bandwidth will be about 100 Khz. The feed point Impedance will vary with<br>ground conditions - but usually very close to 50 ohms - no tuner required.<br>The impedance will go up, if the ends are raised.<br><br>W.Wa. Section Mgr., N7NVP and I have developed a series of NVIS designs
<br>which we presented at the NW Comm. Academy. Ed and I have used these<br>antennas extensively. I my case, and I'm sure Ed's experience is the same,<br>coverage is solid from the antenna out in a circle with a -3db radius of 600
<br>miles - a little farther at night. A CD, containing Power Point<br>Presentations of some of our NVIS antenna work, is available via the ARRL.<br><br>Vertical antennas are hard to achieve at 75, Some loading has to be used,
<br>to keep them reasonably short, and an efficient ground is very difficult.<br><br>Ground Wave --- with the stations only two miles apart, ground wave with<br>just about anything made of wire for an antenna and 100 watts, should
<br>produce a readable signal on 75 or 160.<br><br>In the 1980's we ran a social net on ten meters with members checking in at<br>distances 30 or so miles. We had antenna parties where we built 10 meter J<br>poles. When we all got the J Poles up, we had solid copy every night. So,
<br>maybe 10 meters and a simple J Pole antenna would get it done.<br><br>Finally, Mark Yordy, W7BBO, and I played with a full wave loop on 160 lying<br>flat on the grd. It was 20 or 30 feet too long from 468/f. But, after we
<br>got it on 1.965, we got excellent reports all over the NW with a 100 watt<br>mobile rig.<br><br>I hope these thought are helpful.<br><br>73 de Tom (W6QJI)<br><br><a href="mailto:tjsand@wavecable.com">tjsand@wavecable.com
</a><br><br>[Non-text portions of this message have been removed]<br><br><br><br>_______________________________________________<br>CLUSTER mailing list<br><a href="mailto:CLUSTER@radio-amador.net">CLUSTER@radio-amador.net
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<br><br clear="all"><br>-- <br><br>Best 73 from:<br>regards from:<br>CT4RK<br>Carlos Mourato<br>Sines - Deserto do Jamais Ocidental<br>(Deserto livre de dromedários!...foram todos para a Praça do Comércio)<br><br>-----------------------------------------------------------
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