"aunwin" wrote in message
news:aqK1c.176934$jk2.646180@attbi_s53...
Why must only series circuits be considered for radiators?.

The last discription I saw of a quarter wavw antenna was that of a paralell
circuit. Isnt that basically how a capacity hat shortens an antenna, by
increasing the paralell capacitance

What is it about parallel circuits that make them
unsuitable?

Who says they are not.


Is stagger tuning a parallel circuit ?

This question being out of context with the other questions seems to
indicate you really dont know what stagger tuning means so I dont know how
to reply.

Regards
Art

Jimmy wrote:
"The last description I saw of a quarter wave antenna was that of a
parallel circuit. Isn`t that basically how a capacity hat shortens an
antenna, by increasing the parallel capacitance?"

Parallel or series hardly makes any difference.

The 1/4-wave antenna is essentially a 1/2-wave antenna with the missing
1/4-wave piece replaced by a ground reflection. Terman illustrates
current distribution in a doublet on page 866 of his 1955 edition. He
says:
"These current distributions are those that would be obtained by
applying the exciting voltage in series with the wire at a current loop,
or to one end of the wire."

The series representation is conventional and comes from the distributed
nature of resistance, inductance, and capacitance along the antenna
wire.

From the generator or transmission line`s point of view, it may be more
convenient to view the antenna load as a parallel resonant circuit.
Parallel or series circuit, they are mathematically interchangeable by
using conversion formulas which appear in various books including the
ARRL Antenna Book. Use whichever form you like.

There is a difference between a length of wire and a tank circuit. The
wire has multiple harmonically related resonances. The tank circuit does
not. It has only one resonance.

Jimmy also wrote:
"Who says they are not (parallel circuits suitable to model an
antenna)?"

Yes, Art Unwin, who says they are not?

Jimmy also wrote:
"This question (is stagger tuning a parallel circuit?) being out of
context with the other questions seems to indicate you really don`t know
what stagger tuning means so I don`t know how to reply."

Art for years has hijacked threads to advertise a tuned loop conjoined
with a dipole. One of his claims is that the loop is tuned to one
frequency and the dipole is tuned to a different frequency ergo a
broadband antenna is produced. You must guess between the lines to make
sense of what Art says.

Best regards, Richard Harrison, KB5WZI

On Sun, 7 Mar 2004 21:18:46 -0600 (CST),
(Richard Harrison) wrote:

Jimmy wrote:
"The last description I saw of a quarter wave antenna was that of a
parallel circuit. Isn`t that basically how a capacity hat shortens an
antenna, by increasing the parallel capacitance?"

Parallel or series hardly makes any difference.

Part of this debate has ignored that all resonant circuits can be
analyzed as both parallel and series. That is, barring your and my
observations.

To force the parallel resonant observation upon the quarterwave
vertical, all that need be done is to move the drive from the base to
the tip. The same current distribution will be observed, the same
radiation characteristic will persist, and as such nothing has really
changed.

This may raise hosannas from Art in that he has been redeemed by this
move - but at a cost. Moving that drive to that point necessarily
brings a lead that is a quarterwave long to accomplish this mission.
We then find ourselves in a situation where the solution has become
part of the problem. Do we really have a drive at the top, or another
radiator? Art, I am sure, would dismiss this necessary lead as "not
part of the antenna" and would close the books before the audit is
complete. It would be in fact the classic folded dipole. To escape
that and maintain the mystery of driving from the top, we would have
to accept unipolar RF sources (soon to be patented).

However, if we were to return to Jimmy's question/observation of the
top hat; that structure resides at the point we speculatively drove
(the distal tip), and with respect to its own contribution looking
back towards ground, it sees an entirely different circuit topology
than does the drive at the base. This is not exactly the same
situation as moving the drive. The top hat does not grace a full
quarterwave vertical as it would be redundant to that mission. Such
an addition would end up instead throwing the design into a quasi
3/8ths tuning, or such, to dubious purpose. Thus the analysis becomes
murky (for further debate suitable to efficiency per unit length). We
can shorten the quarterwave by small intervals and find the top hat
appears to replace that missing length; but as we shorten, the system
becomes capacitively reactive and we hardly need more.

One of the language problems with the name Capacity Top Hat, and the
expectation of adding more capacitance is that the short antenna is
already excessively capacitive. Logically, the addition of more
capacity does not lead to resonance. The purpose of a top hat
transcends notions of resonance to answer problems of radiation
characteristics. Resonance, as always, is answered through other
devices (inductors) that reside there to serve that same problem of
radiation.

After 10 years of reading a spectrum of discussion, I have never,
ever, read any post that purported a mission to build a 100pf top hat.
I have never read anyone ask how big a hat was needed to resonate
such-and-such inductance. The structure is often too big to qualify
as a lumped capacitor and calling it by this name is a convention, not
a reality.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
One of the language problems with the name Capacity Top Hat, and the
expectation of adding more capacitance is that the short antenna is
already excessively capacitive. Logically, the addition of more
capacity does not lead to resonance.

Yet, we can usually add enough top hat metal to bring the antenna
system to resonance. Must be your uncertainty principle at work. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Clark wrote:
"One of the language problems with the name Capacity Top Hat, and the
expectation of adding more capacitance is that the short antenna is
already excessively capacitive."

Yes, but that is an incomplete description. The short antenna has an
excess of capacitive REACTANCE. It can be tuned to resonance by
increasing the capacitance between its ends.

Best regards, Richard Harrison, KB5WZI

Richard Clark, KB7QHC:

Part of this debate has ignored that all resonant circuits can be
analyzed as both parallel and series. That is, barring your and my
observations.

To force the parallel resonant observation upon the quarterwave
vertical, all that need be done is to move the drive from the base to
the tip. The same current distribution will be observed, the same
radiation characteristic will persist, and as such nothing has really
changed.


Uh, huh, NOT!

The top hat does not grace a full
quarterwave vertical as it would be redundant to that mission. Such
an addition would end up instead throwing the design into a quasi
3/8ths tuning, or such, to dubious purpose.

Uh, huh.
Quasi 3/8 tuning (with 1/8 radials) provides 50 ohm impedance, no need for
matching junk, lowers tha angle and provides increase in gain. Dubious? Not to
me.

Logically, the addition of more
capacity does not lead to resonance.

Oh no?

Yuri, K3BU

On 13 Mar 2004 14:09:45 GMT, oUsama (Yuri Blanarovich)
wrote:
Logically, the addition of more
capacity does not lead to resonance.

Oh no?

Ah, such a bonus round!

How many pF capacitance in your top hat? :-)

73's
Richard Clark, KB7QHC

Richard Clark wrote:
"How many pF capacitance in your top hat?"

How many degrees in your vertical?
What`s the ground system?
What`s the periphery of your vertical?
What`s the surge impedance of your vertical?
What other loading are you using ?

ON4UN works examples in the 2nd edition of "Low-Band DXing". His
examples happen to have slightly more than 100 pF when there`s no
loading coil. I`ve seen other top loading capacitance values of 100-500
pF.

ON4UN`s capacitance hat, as used with a 40-foot vertical pipe on 160
meters which has a 166-microhenry loading coil at the top, has 43 pF.

XL must equal Xc in the antenna circuit because the antenna must be
resonant to maximize current and radiation.

The 19th edition of the ARRL Antenna Book says on page 6-36:
"For estimating the capacitance of a T antenna made of wires, an
approximation is to use 6 pF per meter for vertical wires, and 5 pF per
meter for horizontal wires." With parallel wires, the total capacitance
must be discounted when the wires are close together.

Best regards, Richard Harrison, KB5WZI

Even if one knows the capacitance of a top-hat it's of no use unless one
also knows the value of the inductance associated with it.

If the dimensions of a top-hat are tediously experimentally varied until the
antenna is tuned to resonance then one still has no idea either of the
capacitance or inductance. Not that the values would be of any use to
anybody after the job is done. And in all likelihood the experimental
procedure would not result in an optimum configuration.

What is needed are the means of *predicting* top-hat capacitance even before
construction materials are obtained. Optimum construction, or suitable in
some sense, could then be chosen.

Readers may wish to be reminded, from given dimensions program TOPHAT2
computes the performance of top-capacitance-loaded vertical antennas, not
necessarily very short as for mobile operation. In the process various data
of interest are produced including capacitance of the top-hat.

The top-hat consists of N radial wires optionally surrounded by a wire ring.
As N is increased capacitance increases until it is the same as a disk of
the same diameter. Capacitance also depends to a limited extent on height
above ground.

For good measure the program also computes L and C values of the base
matching L-network to 50 ohms.

Download in a few seconds from website below self-contained program TOPHAT2
and run immediately.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

On Thu, 18 Mar 2004 09:04:40 -0600 (CST),
(Richard Harrison) wrote:

Richard Clark wrote:
"How many pF capacitance in your top hat?"

How many degrees in your vertical?
What`s the ground system?
What`s the periphery of your vertical?
What`s the surge impedance of your vertical?
What other loading are you using ?

ON4UN works examples in the 2nd edition of "Low-Band DXing". His
examples happen to have slightly more than 100 pF when there`s no
loading coil. I`ve seen other top loading capacitance values of 100-500
pF.

ON4UN`s capacitance hat, as used with a 40-foot vertical pipe on 160
meters which has a 166-microhenry loading coil at the top, has 43 pF.

XL must equal Xc in the antenna circuit because the antenna must be
resonant to maximize current and radiation.

The 19th edition of the ARRL Antenna Book says on page 6-36:
"For estimating the capacitance of a T antenna made of wires, an
approximation is to use 6 pF per meter for vertical wires, and 5 pF per
meter for horizontal wires." With parallel wires, the total capacitance
must be discounted when the wires are close together.

Best regards, Richard Harrison, KB5WZI

Hi Richard,

I didn't expect any actual, literal, quantitative response from Yuri,
nor from many others than yourself for that matter.

I would offer that this "capacitive" top hat is another one of those
matters taken on faith, and when that faith is disturbed, then heads
bury into the sand.

For one, accepting that a capacitor that encompasses significant
wavelength dimensions is a travesty of logic in its own right. Trying
to maintain that logic is humorous by the contortions of the faithful.

Some of your numbers above are very suspect on the simple computation
of capacitance if we allow that it is done for DC or low frequency
(like 60Hz) AC. Specifically:
ON4UN works examples in the 2nd edition of "Low-Band DXing". His
examples happen to have slightly more than 100 pF when there`s no
loading coil. I`ve seen other top loading capacitance values of 100-500
pF.
This must contain some rather elaborate, undisclosed presumptions that
sorely tax real implementations. I've dwelt on this waiting for
someone to hit the trip wire.

This all began with the innocent observation, obtained through
nebulous sources:
"The last description I saw of a quarter wave antenna was that of a
parallel circuit. Isn`t that basically how a capacity hat shortens an
antenna, by increasing the parallel capacitance?"

It is tempting to equate the large top hat structure to the plate of a
capacitor. It serves such a metaphor so well by abstraction. That
is, a lot of conductive surface (or at least by the suggestion of the
skeleton of a disk) suspended above earth (presumably the other plate
to complete this circuit).

The first and obvious problem is that it is too big to qualify as just
a capacitor. The neophyte typically ignores the caution of not
treating large structures as lumped elements (or vise-versa).

If we were to dismiss this warning entirely, that is proceed as the
neophyte, and employ the DC or low AC analysis of a significantly
sized hat, what would its capacitance be? However, this is getting
the cart before the horse. To this point scribblers to this forum
really don't even consider how much capacitance (still talking this DC
or low AC stuff) a monopole offers. As I have a prospective design
for 40M in mind, I will start it off with a 2.54cM tube, 11.1M tall.
I will then decimate its length to add top hats to return the
structure to its former resonant frequency (7.1MHz). Right out of the
gate at its full height, the whip shows all of 4.99pF to earth around
it. I will crop it 25% to 8.4M (4.80pF); and 45% to 6.15M (4.58pF);
and 67% to 3.66M (4.21pF).

Do we use a solid disk, or a skeleton? The practical problem is no
one builds a solid disk for HF. To obtain that several hundred
picoFarads elevated, say, 10M above earth, would lead to a nightmare
sized disk. Hence surface area plummets and so does this capacitance.
However, for the sake of argument, we will maintain the diameter, but
approximate it through a skeleton of 16 spokes of #8 wire (even here,
far more aggressive than many neophyte's efforts).

Lop off 25% of that radiator and resonance shifts to another, higher
frequency. We can add a top hat to accomplish this, and for that
missing 25% we will find a top hat of 12.5 cM diameter will do this
job. Now how much capacitance does that represent? In this case
0.011pF. How much is missing? In this case 0.19pf about 20 times
more than what the hat offers.

Lop off 45% of that radiator and resonance shifts to an even higher
frequency. We can add a larger top hat to return to that original
frequency, and for that missing 45% we will find a top hat of 2 M
diameter will do this job. Now how much capacitance does that
represent? In this case 0.24pF. How much is missing? In this case
0.41pf (about 2 times more than what the hat offers).

Lop off 67% of that radiator and resonance shifts to an even higher
frequency. We can add a very much larger top hat to return to that
original frequency, and for that missing 67% we will find a top hat of
7 M diameter will do this job. Now how much capacitance does that
represent? In this case1.37pF. How much is missing? In this case
0.78pf (about 2 times less than what the hat offers).

Given the quid pro quo, the numbers reveal a very strong nonlinearity
for a resonance equation that is solidly ratio metric. That is to
say: halve the inductance, then double the capacitance. This can be
observed to occur NOWHERE except by accident at one point (I will
leave this to the student to discover that propitious combination).

This logic of "capacitance" grows even more absurd if one simply takes
Reggie's former blighted thread about the inductance of a dipole and
employs the low frequency capacitance and inductance to discover that
by those values his 40M dipole resonates at 95MHz (unless it is
hanging 4 inches off the ground perhaps).

Giving such values to large structures is a fantasy.

73's
Richard Clark, KB7QHC