<h3 class="post-title entry-title">More on HF – SSB - Antennas </h3>
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<div class="post-body entry-content" align="justify">Carrying on from my previous blog <a href="http://www.expeditioncoppelia.com/2010/10/engineers-antennas-hf-ssb.html" target="_blank"><font color="#888888">on engineers antennas HF and SSB</font></a>, in this second part I will more fully describe our emergency antenna setup. </div>
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<div class="post-body entry-content" align="justify">I draw your attention to a very useful set of resources available free on the internet:</div>
<div class="post-body entry-content" align="justify"><a href="http://frrl.files.wordpress.com/2009/02/usmc-antenna-hb.pdf" target="_blank"><font color="#888888">Field Antenna Handbook - US Marine Corps</font></a></div>
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<div class="post-body entry-content" align="justify"><a href="http://hilltoparmyradios.com/usmc-radio-ops-hdbk.pdf" target="_blank"><font color="#888888">Radio Operators Handbook - US Marine Corps</font></a></div>
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<div class="post-body entry-content" align="justify">In the previous post I worked out the length of antenna for a given frequency that would provide the best chance of effective communication. This certainly does not mean for a given frequency if the length is different you can not communicate, it just means that there is more chance at the lengths specified. </div>
<div class="post-body entry-content" align="justify">So how is it all going to work on the boat. </div>
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<div class="post-body entry-content" align="justify"><u>Situation one – the antenna is broken, but the tuner is intact. </u></div>
<div class="post-body entry-content" align="justify">Cut a new antenna from a piece of wire of a length greater than 7m as specified for our ICOM tuner. </div>
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<div class="post-body entry-content" align="justify">One end of the antenna would need to be lifted towards the sky, if the mast or part of it is still standing the wire could be hoisted on it, otherwise some other pole, like the prod, kite pole or boom could be used. The resulting antenna would have the characteristics of this sloping wire antenna show below:</div>
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<div class="post-body entry-content" align="justify"><a href="http://lh6.ggpht.com/_rK1oaS0yOa8/TNNujQUNbNI/AAAAAAAAFTc/kuMS4ZHLEUg/s1600-h/sloping%20wire%5B2%5D.png"><font color="#29303b"></font><img style="BORDER-BOTTOM: 0px; BORDER-LEFT: 0px; DISPLAY: block; FLOAT: none; MARGIN-LEFT: auto; BORDER-TOP: 0px; MARGIN-RIGHT: auto; BORDER-RIGHT: 0px" title="sloping wire" border="0" alt="sloping wire" src="http://lh4.ggpht.com/_rK1oaS0yOa8/TNNukTB_XHI/AAAAAAAAFTg/vAJPy-LFRb8/sloping%20wire_thumb%5B2%5D.png?imgmax=800" width="343" height="259"></a></div>
<div class="post-body entry-content" align="justify">Sloping Wire – source: US Marine Corps (1991) “<em>Field Antenna Handbook</em>” pg 4-34</div>
<div class="post-body entry-content" align="justify"><u>Situation two – the antenna is broken, the tuner is broken</u></div>
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<div class="post-body entry-content" align="justify">This is the situation where having the antenna length appropriate for the frequency you want to transmit of is important. Normally the tuner, tunes between length suitable for the frequency and the actual antenna length using a series of capacitors and inductors. Without a tuner each time you change frequency, to optimise the chance of being able to communicate the length of the antenna must be changed. </div>
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<div class="post-body entry-content" align="justify">The length of antennas required for international emergency frequency range between 32.6m for 2182 Hz to 11.6m for 6125Hz. </div>
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<div class="post-body entry-content" align="justify">32.6m is greater than distance between the back of Coppelia and the mast head. Therefore the sloping wire arrangement used above can not be used. Moreover, it is likely that if we have broken the tuner, we might have suffered damage to the mast, and therefore we would not even be able to make a 11.6m sloping antenna. </div>
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<div class="post-body entry-content" align="justify">The answer is a different sort of antenna, a vertical half rhombic antenna. </div>
<div class="post-body entry-content" align="justify"><a href="http://lh6.ggpht.com/_rK1oaS0yOa8/TNNumH0uAaI/AAAAAAAAFTk/r08mwsGQ8lI/s1600-h/Slide2%5B1%5D.png"><img style="BORDER-BOTTOM: 0px; BORDER-LEFT: 0px; DISPLAY: block; FLOAT: none; MARGIN-LEFT: auto; BORDER-TOP: 0px; MARGIN-RIGHT: auto; BORDER-RIGHT: 0px" title="Slide2" border="0" alt="Slide2" src="http://lh6.ggpht.com/_rK1oaS0yOa8/TNNunQ55UoI/AAAAAAAAFTo/T_8O3PI31yg/Slide2_thumb%5B1%5D.png?imgmax=800" width="337" height="254"></a> </div>
<div class="post-body entry-content" align="justify">Vertical half rhombic antenna – source: US Marine Corps (1991) “<em>Field Antenna Handbook</em>” pg 4-37</div>
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<div class="post-body entry-content" align="justify">The plan onboard would be to make some sort of pole in the middle of the boat. A shielded cable would then be run from the radio to the back of the boat, and a second wire from the ground plate to the same location. A piece of wire equal to the length of antenna required for the frequency to be used would then be attached to these wires, and hoisted aloft in the middle, then fastened at the bow. The resulting antenna setup would form a vertical half rhombic antenna. </div>
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<div><b>Emergency Antennas for VHF and UHF Use</b></div></td></tr>
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<div> <br> Get a piece of RG-58 coax and using the chart below, strip the outer jacket of the coax to that appropriate length, depending on the band you choose to work. Once the outer covering has been stripped, carefully open the copper braid near the bottom of where you stripped the outer jacket. Once the braid is open, carefully bend the center conductor over and pull it out of the hole you opened in the copper braid. Once you have the center conductor pulled free, straighten it and either form a loop at the top end for hanging, or solder on a terminal lug. Remember, the antenna length is measured from the very end of the look or terminal lug, and they do add length to your antenna. Trim as necessary.</div>
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<div><b>MHz</b></div></td>
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<div><b>Length in Inches</b></div></td>
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<div><b>Length in Metric</b></div></td></tr>
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<div>52</div></td>
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<div>54"</div></td>
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<div>137.2 cm</div></td></tr>
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<div>146</div></td>
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<div>19.2</div></td>
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<div>48.8 cm</div></td></tr>
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<div>223</div></td>
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<div>12.5"</div></td>
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<div>31.8 cm</div></td></tr>
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<div>445</div></td>
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<div>6.3"</div></td>
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<div>16cm</div></td></tr></tbody></table>
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<div><img border="0" src="http://www.wcerc.org/Projects/images/emergency_antennas_1.gif"></div></td></tr></tbody></table></p>
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<div>Fontes: <font size="1">Expedition Coppelia</font> e <font color="#1122cc">WCERC <strong>VHF UHF Emergency Antennas</strong> </font></div>