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Since the Alpine Rally I have been investigating why a mobile whip
may not work properly when mounted on a pole, and how to fix this.
This has involved variants of the flower pot design. More on this
anon. <br>
<p>Along the way I was reminded that Graeme VK3BXG drew my attention
to a website which supposedly calculates the dimensions for a
flower pot antenna (here:
<a class="moz-txt-link-freetext" href="https://nomonsuhendar.blogspot.com/2020/12/flower-pot-antenna-calculator.html">https://nomonsuhendar.blogspot.com/2020/12/flower-pot-antenna-calculator.html</a>
)</p>
<p>This website uses the cable velocity factor as an input to
calculate the length of the cable for the coil (which is actually
a self-resonant choke). <br>
</p>
<p>Unfortunately, <i><b>there is no relationship between the cable
velocity factor and any dimension of the coil </b></i>(or the
rest of the antenna, for that matter). So the website cannot be
correct. <i> </i><br>
</p>
<p>Also, while the length of wire is the main effect which
determines the coil self-resonant frequency, <b><i>there are
other factors involved which must be included</i></b> to get a
sufficiently accurate result. <br>
</p>
<p>John Bishop VK2ZOI (sk) who developed the flower pot, specifies
that the self-resonant frequency of the coil should be about 3% to
4% below the operating frequency for best results.<br>
</p>
<p>So how to get the coil correct? <br>
</p>
<p>You could use the attached designs, which have been successfully
constructed by many WICEN members. <br>
</p>
<p>If you really need to use a coil with different dimensions (such
as different size pipe), use the coil calculator here:
<a class="moz-txt-link-freetext" href="https://hamwaves.com/inductance/en/index.html#input">https://hamwaves.com/inductance/en/index.html#input</a></p>
<p>It is a bit complex, but is well worth coming to grips with.
Once you have entered the required dimensions in the 'Input'
section, scroll down to the bottom of the Results section to Table
8, the Self-Resonant Frequency. That's the result you need. Vary
the coil dimensions to get the self-resonant frequency in the
target range. Incidentally, I use a wire diameter of 4mm (the
diameter of RG58 shield) and overall diameter of 5mm (needed to
obtain the length of the coil from the number of turns
close-wound). <br>
</p>
<p>For those more adventurous, if you keep scrolling past Results,
there is a FAQ section with a summary of the research behind this
calculator. And towards the bottom are the actual formulae used.
<br>
</p>
<p>And finally, if you are curious about the phenomenon of coil
self-resonance, Dr David Knight G3YNH has written a research paper
about it, here:
<a class="moz-txt-link-freetext" href="https://g3ynh.info/zdocs/magnetics/appendix/self-res.html">https://g3ynh.info/zdocs/magnetics/appendix/self-res.html</a> . (Dr
Knight's work is also included in the calculator). <br>
</p>
<p><br>
</p>
<p>Cheers all, and Happy New Year!</p>
<p><br>
</p>
<p>John VK3ZRX <br>
</p>
<p></p>
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