[TowerTalk] Dipole length vs diameter -- a theory question

[David Gilbert]

As you point out, the resonance of a conductor is determined by length 
(inductance) and diameter (distributed capacitance to itself).  I don't 
know the formula for that either, but I'm pretty sure that whatever you 
get by reply to your question will be for a straight conductor.  A bent 
conductor like your halo will have somewhat more capacitance to itself 
than a straight one.

Also, proximity has more effect at high voltage positions than at low 
voltage positions ... which is how top hats work.

All that is why I usually just generate an EZNEC+ model, which at least 
tries to geometrically take into account distributed capacitance.  As a 
general rule, almost every model I've ever done says that as I increase 
the width (as long as it's an appreciable percent of a wavelength) the 
resonant frequency goes down and the bandwidth increases ... but 
configuration has a large effect.

73,
Dave   AB7E



On 1/28/2020 11:19 AM, Larry Banks via TowerTalk wrote:
> Hi TTers,
>
> A friend of mine asked me what first appeared to be a simple question.  Paraphrasing:
>
>            “How do I calculate the length of my HB 2M halo, based on
>              the diameter of the aluminum rod.  Is it like propagation
>              velocity with coax?”
>
> My quick answer was: “No, propagation velocity only relates to transmission lines.  Use the graph in the literature for your design to start.  Modeling will help.  But let me do some research.”
>
> I had realized that I really didn’t know the answer.  I have looked in my two usual places: the ARRL Antenna Book and Wikipedia and found lots of hand-waving and the usual references to the “K-factor” graph, which appears to be derived experimentally.  BUT NO THEORY, other than vague references to the capacitance and inductance of the rod changing with dimensional changes which, in fact, is similar to transmission lines.
>
> Do any of you have a reference to some real theory and an equation that allows me to calculate this based on length, diameter, and material characteristics?  (Ignoring environment effects of course.  This would be for free space.)
>
> 73 -- Larry -- W1DYJ
>
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