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Re: [TowerTalk] Fwd: Fwd: OCFD

To: "Richard (Rick) Karlquist" <richard@karlquist.com>
Subject: Re: [TowerTalk] Fwd: Fwd: OCFD
From: Edward Mccann via TowerTalk <towertalk@contesting.com>
Reply-to: Edward Mccann <edwmccann@yahoo.com>
Date: Wed, 29 Mar 2017 09:25:05 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Gentleman:

For an interesting (albeit empirical) view of the non-exact harmonic 
relationships in a dipole wire, I commend the following link from ON4AA. While 
extolling the OCFD, the article does play into to non-exact harmonic 
discussion currently on the board:

http://hamwaves.com/cl-ocfd/en/index.html

As to the notion of "end-effects", and the evolution of the number 468 vs 492 
in the relationship of real-space vs free-space length, I have long been been 
annoyed at the cavalier response as to "end effects," which if Googled even in 
today's modern world, yields little mathematical justification for the "reduce 
the loa by 3 to 5  %. 

Back in the day at MIT, working for Lan Jen Chu, one of the more interesting 
side trips was an assignment research the origins of the end-effect matter. 
Believe it or not, starts with Lord Rayleigh and investigations to harmonics 
and end effects from organ pipes, some open, some terminated. The next hundred 
years or so saw incredible immersion in the math involved, and led to Max 
Abramham researching the matter as extended from previous French academics 
looking into the notion of vibrating metal rods; some open and some held fast. 
Remember, this was in the day when EM radiation was evolving, albeit rapidly.

In fact, the time line is boggling when you think about it: Maxwell kicks if 
off in 1855, Heaviside reduces notion to calculus in 1873,  Hertz fires up his 
rig in 1891, and Marconi's transatlantic experiment (some suggest they heard 
Southern Hemisphere Atmospherics in Newfoundland and not the signal 
transmitted, but what the hell, it was the guy with good marketing who got out 
there first!)
The point is, all this took only 46 years!

But even then the math continued to be of interest, and continuing to advance  
the principals to mathematics was de rigeur, frequently leading to academic 
word-scuffles between sponsors of various theories (think battle of the 
Matchbox, ARRL, SPC tuner overtaking the Ultimate TransMatch, etc.)

The essence of the mathematical analysis was a niggling term in a complex 
calculus, relating to the accumulation of charge at the end of a radiating wire 
that was terminated (read hooked to an insulator) rather than continuing into 
free space. 

This accumulation of charge (at the end the wire, or on the insulator) was a 
capacitive event (accumulation of charge is what they do, right?) and as such 
had the effect of shortening the physical length of a given wire for a given 
frequency. We now assume this is reasonable at the high-voltage end of the 
dipole, near the insulator.

Anyway, Old Max was caught up in the works of E/M radiation, with an 
authoritative treatise in 1904/1905. Reviews from MIT and Yale follow at the 
links below, as well as a link to the book.

http://www.ams.org/journals/bull/1908-14-05/S0002-9904-1908-01601-X/S0002-9904-1908-01601-X.pdf

http://www.ams.org/journals/bull/1905-11-07/S0002-9904-1905-01236-2/S0002-9904-1905-01236-2.pdf

https://ia601409.us.archive.org/32/items/theoriederelekt04fpgoog/theoriederelekt04fpgoog.pdf

Max was a big-time contributor to Annalen Physik back in the day, and stayed 
involved with this world until he succumbed to the chase to define unified 
field theory, and bumped into Einstein.

We don't hear much about Max these days, and clearly the modern math, has 
eclipsed the need to mathematically explain the accumulation of charge at the 
end of a radiating wire.

One of the then-hot topics of the day was published (pre-internet !) and 
subject to Peer Review (duh-does it exist outside certain circles? Not even 
much in Newington CT !):

Zur Theorie der Strahlung und des Strahlungsdruckes
AuthorsAnnalen Physik, Vol 319, Issue 7, 1904, pages 236-287.
http://onlinelibrary.wiley.com/doi/10.1002/andp.19043190703/abstract

While this is more than you cared to know about end effects, even to you 
physicists, I thought it might go well with your morning coffee to expand a bit 
on a topic that has not much of an interested audience left, since it's only a 
matter of shortening the Antenna by three to five percent. What the hell.

PS-Nice to see topics like this pop up from time to time. Keep 'em coming.

73,
Ed McCann
AG6CX



Sent from my iPhone

> On Mar 28, 2017, at 7:18 PM, Richard (Rick) Karlquist <richard@karlquist.com> 
> wrote:
> 
> 
> 
>> On 3/28/2017 2:54 PM, jimlux wrote:
>> 
>> because it's not "really" an end effect or capacitance. The actual
>> phenomenon is that  X goes through zero at a frequency where the dipole
>> is not exactly a half wavelength long.  There's a lot of ways you can
>> analyze around it or conceptualize it, but they're just mental models.
>> 
>> 
>> Another way to think about it is that the propagation velocity down the
>> wire is not c, but slightly slower (because the wire has inductance and
>> free space capacitance, and you can calculate the prop velocity as
>> sqrt(L/C))
> 
> The trouble with this description is that it doesn't explain why the
> third harmonic resonance of a dipole is greater that 3X the fundamental
> (half wave) resonance.  OTOH, the "end capacitance" concept neatly
> explains it.  Three times as much wire, but no additional end effects.
> 
> 73
> Rick N6RK
> _______________________________________________
> 
> 
> 
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