Richard Fry wrote:
"Cecil Moore" wrote
Adding or subtracting loading-coil degrees is what
happens while one is tuning a screwdriver antenna.
At resonance, the screwdriver is electrically very
close to 90 degrees in length.

Note that the electrical length and the physical length
are nowhere near the same value. The electrical length
can be 90 degrees at resonance while the physical length
is only 13 degrees for a 75m mobile antenna.

It may have the reactance of an unloaded ~90-degree, self-resonant
radiator. But in normal applications that doesn't make a screwdriver the
radiational equivalent of that full-sized radiator, because the
radiation resistance of the physically/electrically short screwdriver
whip is less than a full-sized antenna -- and much less on the lower bands.

I agree 100% and have never disagreed. I have already stated
that the radiation characteristics of an antenna depend upon
its *physical* length while the feedpoint impedance depends
upon its *electrical* length. A screwdriver antenna may be
only 13 degrees long *physically* on 75m. Of course, it is
NOT going to radiate like a physical 90 degree antenna. It
is going to radiate more like a 13 degree (short) antenna.

You have apparently misunderstood what I am trying to say.
I have made *zero assertions about radiation patterns* except
to answer your earlier posting on that subject.

A dummy load can have the reactance of a resonant screwdriver, too, but
a dummy load is not a very good antenna. I doubt you would claim that
it is electrically 90 degrees in length, just because it has the same
reactance as an unloaded ~90 degree, self-resonant monopole.

A dummy load's feedpoint impedance is not (Vfor+Vref)/(Ifor+Iref),
i.e. not a virtual impedance, so your comment is irrelevant in
this context. The IEEE Dictionary distinguishes between those
two definitions of impedance, (B) for an antenna, (C) for a
dummy load.

That conclusion applies to a screwdriver antenna system, as well.

Since it is possible to tune a screwdriver antenna to the
270 degree mode, the following will assume the screwdriver
antenna system is used only in the 90 degree mode:

A screwdriver antenna system has radiation characteristics
appropriate for its *physical* length of, e.g. 13 degrees.
A screwdriver antenna system with a low resistive feedpoint
impedance is electrically 90 degrees long because
(Vfor+Vref)/(Ifor+Iref) is resistive. The only way for Vfor
and Vref to be 180 degrees out of phase is for the antenna
to be electrically 90 degrees long. The only way for Ifor
and Iref to be in phase is for the antenna to be electrically
90 degrees long. That's simple wave reflection model physics.

In abandoning the wave reflection model, many people have
abandoned any possibility of understanding what happens
in a standing-wave antenna. Sooner or later, their short
cut methods bite them in the posterior. The W8JI and W7EL
current measurements are an example.

Anyone who never looks for the "missing" phase shifts in
a mobile antenna will never find them. Side 1 of the
argument assumes they are not there. Side 2 of the argument
assumes they are there in the loading coil. Both sides
are wrong. I have gone looking for the "missing" phase
shifts and have found them. Here is a lossless transmission
line example which is *physically 45 degrees long*:

---Z0=600, 22.5 degrees---+---Z0=100, 22.5 degrees---open

What is the impedance looking into the stub? Where are the
"missing" 45 degrees?
--
73, Cecil http://www.w5dxp.com

K7ITM wrote:
On Nov 29, 9:11 am, Jim Kelley wrote:
...
Over the range of a few octaves, propagation delay on the other hand
does not vary to any significant extent as a function of frequency.
Ostensibly, it should be equal to sqrt(LC) series L, shunt C.


Actually, Jim, I do expect it to have considerable frequency
dependence. I think you can find info about this in books that
address the design of travelling-wave tubes.

But...one must be very careful about describing exactly the experiment
or the conditions around a particular scenario. That's why I don't
have much interest in getting involved in this "discussion": it could
well be that much of the difference among all the claims and counter-
claims could be trivially resolved through better communication.

Cheers,
Tom

I don't think they're writing about real transmission lines, Tom. If
they were doing that, there would be no discussion because then it would
be too hard to understand.
73,
Tom Donaly, KA6RUH

Cecil Moore wrote:

...
In a lossless stub, the *total current* is 100%
standing-wave current. There is zero phase shift
in the current from one end of the stub to the other.
That's why total current cannot be used to measure
a delay through a coil in a standing-wave antenna.

Cecil:

Of course you are correct--it was meant to be a joke man, albeit a silly
one ...

Regards,
JS

Cecil Moore wrote:
Jim Lux wrote:

I should think that many hams have things that can measure 3 ns
(1000mm light time), particularly in a repetitive system. That's one
cycle at 300 MHz, or 36 degrees at 30 MHz.


The referenced W8JI 3 nS "measurement" was the delay
in a 2' dia, 100 T, 10" long loading coil on 4 MHz,
i.e. 4.5 degrees.


4.5 degrees is easy to measure at 4 MHz with a variety of systems.

Basic measurement theory says that the phase measurement uncertainty is

uncertainty in radians = 1/sqrt(T * Psig/No)
where T is the integration time, Psig is the signal power, and No is the
noise spectral density (W/Hz)

Let's throw in some numbers..

Psig = 1 mW (1E-3W)
No = -160 dBm/Hz (kTB noise + 14 dB)
T = 10 millisecond

uncertainty = 1/sqrt(1E-2 * 1E16) = 1 / 1E7 = 1E-7 radian

1 degree is about 0.017 radian, so I think you wouldn't have much
problem measuring the phase shift, from a physics standpoint.. all a
matter of experimental technique..

Anyway, there are LOTS of ways to do the measurement, most of which
would require only things that hams have sitting around, with a few
hours of cobbling together.

K7ITM wrote:
On Nov 29, 9:11 am, Jim Kelley wrote:
...

Over the range of a few octaves, propagation delay on the other hand
does not vary to any significant extent as a function of frequency.
Ostensibly, it should be equal to sqrt(LC) series L, shunt C.



Actually, Jim, I do expect it to have considerable frequency
dependence. I think you can find info about this in books that
address the design of travelling-wave tubes.

I can't think of an example of an active (or reactive) device which
doesn't have frequency dependent characteristics. To the extent that
indices of refraction are frequency dependent, propagation velocity
does in fact vary with frequency. If it didn't, we wouldn't see
rainbows. Dielectric constants do indeed have a frequency dependence.
But to first order, at radio frequencies, in amateur applications,
for the purposes of this discussion, and in my opinion, the effect is
less than considerable - particularly if we assume the L and C in
sqrt(LC) are correct at the frequency of interest. ;-)

73, Jim AC6XG

Tom,
May I point out that a Tesla coil is an "antenna" that does not
conform
to Maxwells laws with respect to the adherance to the LC ratio.
The LC ratio is out of balance such that the capacitor is not
of the correct size to store and then return the imposed energy from
the inductive heavy coil which is visually seen as resulting in a
spark.
Regards
Art

Huh...

tesla coils follow all of Maxwells equations quite nicely. Paul
Nicholson did some very nice analysis on this a few years back,
published at a link previously posted.

They're two coupled LC resonant circuits, with the coupling adjusted to
around k=0.2. There are higher order systems with 3 or more resonators,
as well (called Magnifiers in the TC world)

The challenge in spark making is choosing appropriate operating
parameters (coupling, radius of curvature, topload capacitance, etc.) to
optimally promote spark growth.

Cecil Moore wrote:

I measured a ~25 nS delay in a 75m bugcatcher coil.

What did you use to make that measurement? (I hope you don't say you
used a Bird Wattmeter.)

73, ac6xg

Cecil Moore wrote:
Richard Clark wrote:

Cecil Moore wrote:

The referenced W8JI 3 nS "measurement" was the delay
in a 2' dia, 100 T, 10" long loading coil on 4 MHz,
i.e. 4.5 degrees.


Jim's point is that it can be done!


In that particular coil at 4 MHz - no, it cannot be done.

measuring the phase shift between two sinusoidal currents at 4MHz to a
precision of hundredths of a degree is easy. HP sold a box (the 8405
vector voltmeter) that did this decades ago. Actually, they've sold two
different boxes (the 8508A ), both of which I've used. My point was
that you don't even need to go that far, and that most experimentally
oriented hams probably have stuff that can be used to make an improvised
measurement of that accuracy.

I note that the TAPR or N2PK VNAs could easily do the measurement.

The practical challenge is figuring out how to get a current probe that
doesn't perturb the measurement. Optical pickups are one approach. high
impedance probes with resistive leads are another. Both are commonly
used in antenna measurements where you want to measure the fields directly.

One could, of course, also do a near field range type measurement, but
the inversion from measurements at one set of locations to values at
another presumes that you believe Maxwell's equations, which I seem to
think might be at issue among the contenders here.


now, if you said you wanted to measure tenths of a degree at 50 GHz, I'd
say you have a real challenge in front of you

Cecil Moore wrote:


That's why total current cannot be used to measure
a delay through a coil in a standing-wave antenna.

Not even if the frequency is known and there's a standing wave current
loop at one end of the coil and a standing wave current node at the
other end?

73, ac6xg

On Fri, 30 Nov 2007 14:29:12 -0500, "Jimmie D"
wrote:


"Tom Donaly" wrote in message
et...
Cecil Moore wrote:
Tom Donaly wrote:
And, if the total electrical length isn't 90 degrees, you
add a few degrees to the loading coil to make it come out right.
Very ingenious.

Adding or subtracting loading-coil degrees is what
happens while one is tuning a screwdriver antenna.
At resonance, the screwdriver is electrically very
close to 90 degrees in length.

Suuurrrre it is. You've got 90 degrees on the brain, Cecil.
Next, you'll be talking about 90 degree equilibrium.
73,
Tom Donaly, KA6RUH

I must be wrong too which doesnt surprise me.

Are you saying that if I put a center loaded antenna on my trucks tool box,
tune it to reonance at some freqency then the antenna is not electrically 90
degrees or some integer mutilple of 90 degrees in length at that frequency.

Some integer multiple meaning "odd integer multiple" if we are to
continue abusing this implication.

The concept that a resonant antenna could be some other electrical length is
something new to me as I thought this was the defintion of resonance being
equivalent to saying the feedpoint impedance is non reactive.


Hi Jimmie,

Basically the land-mine issue here is the hijacking of the usage of 90
degrees (or any other application of this unit) to describe a resonant
condition. That is because more frequently, and certainly more
appropriately, the usage of degrees is restricted to the physical
dimension as its significance is especially marked in relation to a
simple antenna's directivity. As you anticipate above, the simple
electrical 90 degree observation repeats through an infinite multitude
with a turn of the wheel.

There are posters who visit intermittently, and those who post
frequently that confuse the expressed electrical degrees as also
inheriting the directivity qualities associated ONLY with the physical
dimension expressed in degrees.

This might be observed through the example of a quarterwave antenna.
Its directivity is well known. If some "inventor" were to add a
lumped (or distributed) Z to the same structure, that "inventor" could
easily claim they added (for the sake of argument) 135 degrees to make
the structure exhibit the gain of a 5/8ths wave antenna. Frequently
this charade is carried out with smaller antennas being "elevated" to
full size performance (hence the appeal of the current topic in its
original subject line and the "invention" of adding coils).

With this in mind, you might enjoy how gaming the group is played out
by the more frequent poster(s) insisting on polluting the topic of
directivity with the "electrical" length. The entertainment factor
has been zested up recently by adding the term "equilibrium."

73's
Richard Clark, KB7QHC