Jim Brown wrote:
> On Sun, 27 Jul 2008 20:10:42 +0100, Dr. David Kirkby wrote:
>
>> I wish to know if the reactance of a dipole that is physically 0.5000
>> wavelengths in length depends on the diameter of the wire or not.
>
> So study a book on antennas! It is very well known (not far from the
> beginning of the discussion in almost any text) that the physical length of a
> resonant conductor depends on its cross section. There are many excellent
> texts on antennas, including the one by Kraus. The ARRL Antenna Book is also
> quite good. Both are widely available new and used. And yes, it is quite far
> off topic for this list.
Hi Jim,
I agree its a bit off-topic, but I think many might agree its a bit more
interesting than some of the junk on here.
Any votes for what is most interesting - this or parasitic oscillations?
> This sure sounds like a troll, but for a hot minute, let's pretend that it
> isn't. :)
It is not a troll. I think you are the only person to think so.
> The confusion here is between a wavelength in air (or free space) and a
> wavelength in the conductor that makes up the antenna. When we decide how
> long the antenna should be, we're concerned with the wavelength in that
> conductor, not the wavelength in free space. When we design for the spacing
> between elements of a parasitic array (like a Yagi), we are concerned with
> the wavelength in free space.
Equations can be found in Krass as you say. Here is a plot of one of
those (the one I gave earlier), based on the formula of Schelkunoff and
Friis. It shows reactance for 3 different diameters. If you look at the
graph you can see the reactance is independent of diameter at lambda/2,
3*lambda/2 5*lambda/2, but defendant on diameter everyone else. (I know
it's hard on a graph to see this too clearly, but it is so. If you feel
like it, plot if yourself)
http://www.g8wrb.org/schelkunoff-full.jpg
So please don't take me for an idiot and think I have not looked at this.
It appears there are problems with this approximation.
The other problem is summed up by Roger K8RW, when he says "I'd have to
add the math on the previous two pages before getting to the general
case, would scare off the average ham and I mean no disrespect to any
one. I have a math minor and I'd have to do more than a little review."
I've never rated my maths as very good, although it is probably better
than average. It is certainly not very good.
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