H. Adam Stevens wrote:
SNIP

Nobody builds everything I want.
Oh well
73 all

H.

Ah Ha! I see an entrepreneurial [sp?] opportunity for you!!

Build it yourself. If it's good .... you contribute to the economy and
become $$$$$$$$$$$$$$$$$$$$$$$ beyond your wildest dreams.

"Dave Shrader" wrote in message
news:IyU9c.29578$K91.88263@attbi_s02...


H. Adam Stevens wrote:
SNIP

Nobody builds everything I want.
Oh well
73 all

H.

Ah Ha! I see an entrepreneurial [sp?] opportunity for you!!

Build it yourself. If it's good .... you contribute to the economy and
become $$$$$$$$$$$$$$$$$$$$$$$ beyond your wildest dreams.


Building ham gear?
How about "make a tiny profit if you're lucky"?
But then there were guys like Collins, Halligan, Swan.......hmmmmmmmm

73
;^))))))
H.

Tom Bruhns wrote:
o In other words, the SWR circles are useless
for finding the maximum-reactance point on
the impedance curve.

Useless for finding the *exact* point, yes. Quite useful enough for a
lot of ballpark work. For instance, Zs and Zm are usually as close
as EZNEC's prediction of antenna feedpoint impedance is to the
actual real-world antenna feedpoint impedance.

Example: Zm = 1000+j800 ohms

Where did this impedance come from? A full wave dipole's feedpoint
impedance is usually around 5000+ ohms. The value above does not
look like the Xmax impedance between 1/2WL resonance and one-wavelength
(anti)resonance for HF thin-wire dipoles. Such a dipole's Xmax impedance
is around 3000+j2000 ohms.

Does Zm represent the impedance at the Xmax point? It's not the Zmax point.

Ref. Impedance: 50 ohms: SWR = 32.82; Zs for that circle = 821.25+j819.73
Ref. Impedance: 300 ohms: SWR = 5.59; Zs for that circle = 865+j811.31
Ref. Impedance: 600 ohms: SWR = 3.; Zs for that circle = 1000+j800

(The 600 ohm case illustrates that there is no requirement that Zs and Zm differ.)

That's better than I expected out of my ballpark rule-of-thumb, Tom.
Note that Xs is within 2.5% of Xm in all three cases and right on for the
type of transmission line that I use. Zs is within 9.5% of Zm. Looks like
my estimates were twice as accurate as I had imagined. Thanks for proving
my original point.

The purpose for my rule-of-thumb is to get me close enough to achieve
cut-and-try tuning from that point on. That's also the purpose to which
I put EZNEC. In my latest experiment, EZNEC missed the actual resistive
component of the impedance by 125% but that's OK because I am close
enough to know which way to go from there.
--
73, Cecil http://www.qsl.net/w5dxp



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(Tom Bruhns) wrote in message om...
....
o In other words, the SWR circles are useless
for finding the maximum-reactance point on
the impedance curve.

And indeed, why would we look for a reactance arc tangent to an SWR
circle which passes through the maximum-reactance point on the antenna
impedance versus frequency curve, when we already know that
max-reactance point, and know it's at a point on the antenna impedance
curve which is tangent to that reactance arc? Why go looking for a
point that's a rough approximation to what we want when we already
have just what we want?

HOWEVER, for some understanding of why the antenna impedance versus
frequency function has the shape it does, I highly recommend Joseph
Boyer's "Antenna-Transmission Line Analog" articles in, um, March and
April or April and May 1978 "Ham Radio" magazine. I suppose that set
of articles hits really close to what Reg wrote in this thread a while
ago that probably went zinging over the heads of many, and seemed to
be ignored by the target audience... I've been known to supply copies
of those articles to folk who can't find them locally.


We now return you to your regularly-scheduled obfuscation.

Tom Bruhns wrote:
Why go looking for a
point that's a rough approximation to what we want when we already
have just what we want?

The point is that we *DON'T* already have what we want. EZNEC is
only an approximation. Most people cannot measure the feedpoint
impedances of their antennas when it is over a few hundred ohms,
so a rule-of-thumb is helpful. The impedance graph in the ARRL
Antenna Book is helpful.

For any SWR above 5:1, A+jA will be relatively close to the maximum
reactance point. Setting A = Rmax/2 will be relatively close to the
behavior of a thin-wire HF dipole at the maximum reactance point
between 1/2WL and one-wavelength. Exactly what is it that you think
"we" already have?

Tom, exactly what are you trying to prove by picking all those nits?
If I say I weigh about 200 pounds, are you going to argue that I
don't weigh 200 pounds - that I acutally weigh 204.3785 pounds?

And you didn't say where you got the 1000+j800 ohm value for the
maximum reactance point. That looks like an unreasonable value
for a thin-wire HF dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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On Tue, 30 Mar 2004 10:50:43 -0600, Cecil Moore
wrote:

EZNEC is only an approximation.

The Great Approximator is arguing against approximation?
Presumably because its solutions are better. But like dissatisfaction
with the weather, you only have to wait for the change:

Most people cannot measure the feedpoint
impedances of their antennas when it is over a few hundred ohms,
so a rule-of-thumb is helpful.

Ah! Back to approximations without references or (dare I say it?)
that measurement that "most" people cannot perform.

The impedance graph in the ARRL Antenna Book is helpful.

Another approximation and to this point no valid comparisons, merely
testimonial.

what are you trying to prove by picking all those nits?

And then the Great Nit-Picker finds one:

That looks like an unreasonable value

Again, testimonial sans reference or measurement.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

wrote:
EZNEC is only an approximation.

The Great Approximator is arguing against approximation?

On the contrary. Most of our models are approximations.
Exactly what is your agenda in rejecting approximations?
--
73, Cecil http://www.qsl.net/w5dxp



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On Tue, 30 Mar 2004 12:19:34 -0600, Cecil Moore
wrote:

Richard Clark wrote:

wrote:
EZNEC is only an approximation.

The Great Approximator is arguing against approximation?

On the contrary. Most of our models are approximations.
Exactly what is your agenda in rejecting approximations?

On Tue, 30 Mar 2004 10:50:43 -0600, Cecil Moore
wrote:

EZNEC is
only an approximation.

Richard Clark wrote:
wrote:
Richard Clark wrote:
The Great Approximator is arguing against approximation?

On the contrary. Most of our models are approximations.
Exactly what is your agenda in rejecting approximations?

On Tue, 30 Mar 2004 10:50:43 -0600, Cecil Moore
wrote:
EZNEC is only an approximation.

What's the matter, Richard? Can't you answer the question? There is
absolutely nothing wrong with being "only an approximation". That is
not a negative statement. Too bad you have never learned that everytime
you make a measurement, you make an error. There are no 100% accurate
measurements. Everything except cardinal numbers is an approximation.
--
73, Cecil, W5DXP



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On Tue, 30 Mar 2004 17:36:57 -0600, Cecil Moore
wrote:

Richard Clark wrote:
wrote:
Richard Clark wrote:
The Great Approximator is arguing against approximation?

On the contrary. Most of our models are approximations.
Exactly what is your agenda in rejecting approximations?

On Tue, 30 Mar 2004 10:50:43 -0600, Cecil Moore
wrote:
EZNEC is only an approximation.

What's the matter, Richard? Can't you answer the question?
On Tue, 30 Mar 2004 10:50:43 -0600, Cecil Moore
wrote:
EZNEC is only an approximation.