On 1/7/20 10:36 AM, Jim Brown wrote:
On 1/6/2020 5:47 AM, jimlux wrote:
On 1/5/20 7:11 PM, Jim Brown wrote:
What I HAVE done (about ten years ago) was to add a second #10 THHN
to my Tee vertical for 160M, spaced roughly 8-10 inches from the
first, wired in parallel top and bottom. The vertical section is 100
ft. The measured result was to approximately double the SWR bandwidth.
So, going from l/d of 12000 to 120 -> that's a big, big difference,
and the change in SWR bandwidth would be expected.
Yes. And this technique is probably older than I am.
I wonder when the theory was developed, as opposed to empirical
observations. I'm sure that empirical observations (1910, 1920) showed
the bandwidth increase (as all of you running a flattop T antenna from
your Alexanderson alternator on Straight Key Night did).
There's a picture of an "amateur station" from 1922 on Wikipedia using a
cage T. So, Jim, I think you're right, unless you're in your late 90s.
https://en.wikipedia.org/wiki/T-antenna
So people did it, probably because it worked better, but I'll bet the
"why" didn't come until the 30s and 40s. Certainly, by Schelkunoff's
and Hallen's papers in 1940s, the theory on the l/a ratio was well
developed.
(OK,plunging down a rabbit hole.. Pocklington, in 1897, derived the
electric and magnetic field equations for a "thin wire" and they include
the length (l) and radius (a). John Strutt (better known as Lord
Rayleigh) complained that Pocklington didn't solve for a ring, so he did
that derivation.
So I guess that folks did know about the "impedance bandwidth" effects
of wire radius (or, at least could show the effects) at the turn of the
19th to 20th century. Whether Marconi, and other radio pioneers,
actually paid attention to Proceedings of the Royal Society or Annalen
der Physik is another question.
Next time someone asks me about how NEC works, I'll just point them to
Pocklington's paper.<grin>
https://archive.org/details/proceedingsofcam9189598camb/page/324
H.C. Pocklington, "Electrical Oscillations in Wires", Oct 25th, 1897,
Proceedings of Cambridge Philosophical Society.
Fascinating stuff in that volume.. Townsend on electrical properties of
gases. Faraday vs Kerr rotation. UV photoelectricity.
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