Loading Coils; was : Vincent antenna
 

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

Sorry, I didn't know you were joking. Some pretty intelligent,
educated people on this newsgroup do not know the answer else
they would never try to use standing-wave current to measure
the phase shift through a loading coil.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

On Nov 30, 10:59 am, Jim Kelley wrote:
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

OK, that leaves us with a difference of opinion, or a difference in
what we are describing. There was an article in "RF Design" maybe 15
years ago now by John Mezak, K2RDX, describing a helical transmission
line model for coils. At the time, he offered free software to
execute the calculations (which also, to me, offered a very practical
way to calculate coil parameters like inductance, effective shunt
capacitance, and first parallel and series self resonances). He later
charged a nominal fee for an improved version of the software, which I
have. For the "100 turn, 10 inch long, 2 inch diameter" coil wound
with 15AWG copper wire, using John's program, I see a variation of
about 2:1 in propagation velocity between 1MHz and 20MHz. Since the
first parallel self-resonant frequency is predicted to be around 8MHz,
it's perhaps not fair to look as high as 20MHz, but even between 1MHz
and 4MHz, I see about 25% change in predicted propagation velocity.

You may say that perhaps John messed all that up terribly, but I don't
think so...and there are other places you can find similar results.
There's an excellent inductance calculator on-line at
http://hamwaves.com/antennas/inductance.html, and though the absolute
value of its prediction of propagation velocity is about 5% different
than Mezak's, they both show very nearly the same percentage change
with frequency.

It might be worth having a bit closer look at, Jim. Perhaps it's just
that you're thinking of a different effect than what these two
programs (and the theory behind them) are modelling.

Cheers,
Tom

Loading Coils; was : Vincent antenna
 

Jim Lux wrote:
4.5 degrees is easy to measure at 4 MHz with a variety of systems.

If at 4 MHz, you measured 4.5 degrees change in
the phase of *standing-wave current* on each side
of a loading coil in a standing-wave antenna system,
would you report that value as the delay through
the loading coil? One glance at the standing-wave
current equation should convince one that is an
invalid measurement technique.

For instance, the change in the phase of the standing-
wave current is ~5 degrees from feedpoint to tip in
a 90 degree long 1/4WL monopole. How can that standing-
wave current possibly be used to measure the delay
through a loading coil in the middle of that antenna?
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

Jim Kelley wrote:
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.)

I've described it before. I used a dual-trace
100 MHz O-scope and estimated the phase angle
between the two traces at about 7% of a cycle.
That phase angle was certainly NOT ANYWHERE
NEAR the 4.5 degrees reported by W8JI.

W8JI measured a 4.5 degree phase shift in the
standing-wave current being used for the
measurement although virtually no phase
information exists in the standing-wave current
phase. W7EL made exactly the same mistake in
his measurements. No wonder the two agree.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

On Nov 30, 10:30 am, "Tom Donaly" wrote:
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

;-) Yeah, I know what (they think) they are writing about; I'm
writing about coils more-or-less in open air, which should match
pretty well with the current discussion. But again, as with so many
of the discussions here, it's not worth getting tangled up in. I just
thought it bears mentioning that there are some coil models available
out there that go beyond simple inductance. Inductors are among the
least ideal components I deal with, and having models that address the
discrepancies has been helpful to me in practical designs. If people
want to argue, rant, get red in the face, ... about how something
works, more power to them, but I've got some designs to work out and
I'd rather be spending time on them. (How small can I make a 1MHz
bandpass filter that has less than a couple dB passband attenuation,
more than 120dB attenuation on 2MHz and 3MHz, and shows distortion
below -140dBc for inputs up to half a watt or so...??)

Cheers,
Tom

Loading Coils; was : Vincent antenna
 

Jim Lux wrote:
Cecil Moore wrote:
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.

Jim, you misunderstood what I was trying to say and that is:
It is impossible to measure a 3 ns delay through a 2"dia,
100T, 10" long coil at 4 MHz because the delay is much longer
than 3 ns. It is closer to 30 ns.

I DID NOT say it is impossible to measure a 3 ns delay at 4 MHz!
I said it is impossible for that coil to exhibit a 3 ns delay
at 4 MHz, therefore 3 ns is not a possible measurement value.
--
73, Cecil http://www.w5dxp.com

tesla coils antennas maxwell Loading Coils; was : Vincentantenna
 

On 30 Nov, 11:01, Jim Lux wrote:
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.

Let me make it quite clear. I was referring to a single coil and
not the feeding arrangement. I used that as a refernce only in
conjunction
with the subject of antenna coils. This single coil, tho resonant,
does
not meet the requirements that Maxwell demands ie equilibrium.
Further study of that coil will show the effect of ground beyond the
coil
which thus involves the system as well as the associated coil for feed
coupling.
Regards
Art Unwin..KB9MZ....xg

Loading Coils; was : Vincent antenna
 

Jim Kelley wrote:

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?

Total current phase is the context of my posting above.
We were talking about total current phase, not total
current amplitude. To be precise, the statement should
read: "That's why total current phase cannot be used
to measure a delay through a coil in a standing-wave
antenna". It is difficult to post context-free English.

W8JI and W7EL both used standing-wave current *phase* to
try to determine the delay through a coil. That is an
invalid measurement concept. If they had used the standing-
wave current amplitude instead to calculate the phase
shift, they would have gotten much closer to a valid
result. But they are arguing about current amplitude
drops which are simply relative phase shifts between
the forward and reflected current.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

On 30 Nov, 12:25, K7ITM wrote:
On Nov 30, 10:59 am, Jim Kelley wrote:





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

OK, that leaves us with a difference of opinion, or a difference in
what we are describing. There was an article in "RF Design" maybe 15
years ago now by John Mezak, K2RDX, describing a helical transmission
line model for coils. At the time, he offered free software to
execute the calculations (which also, to me, offered a very practical
way to calculate coil parameters like inductance, effective shunt
capacitance, and first parallel and series self resonances). He later
charged a nominal fee for an improved version of the software, which I
have. For the "100 turn, 10 inch long, 2 inch diameter" coil wound
with 15AWG copper wire, using John's program, I see a variation of
about 2:1 in propagation velocity between 1MHz and 20MHz. Since the
first parallel self-resonant frequency is predicted to be around 8MHz,
it's perhaps not fair to look as high as 20MHz, but even between 1MHz
and 4MHz, I see about 25% change in predicted propagation velocity.

You may say that perhaps John messed all that up terribly, but I don't
think so...and there are other places you can find similar results.
There's an excellent inductance calculator on-line athttp://hamwaves.com/antennas/inductance.html, and though the absolute
value of its prediction of propagation velocity is about 5% different
than Mezak's, they both show very nearly the same percentage change
with frequency.

It might be worth having a bit closer look at, Jim. Perhaps it's just
that you're thinking of a different effect than what these two
programs (and the theory behind them) are modelling.

Cheers,
Tom- Hide quoted text -

- Show quoted text -

Where can I obtain a copy of Johns program?
TIA
Art

Loading Coils; was : Vincent antenna
 

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

Stand by your guns, Jimmie, you are correct. It's just
that some otherwise intelligent people on this newsgroup
have forgotten everything they ever learned in
Fields&Waves 101.

In an antenna physically shorter than 1/4WL, there is
no way that I know of to get the reflected wave back
in phase with the forward wave at the feedpoint without
the reflected wave making an electrical 180 degree round
trip, i.e. 90 electrical degrees in each direction.
--
73, Cecil http://www.w5dxp.com