On Oct 21, 8:32*am, Mike Coslo wrote:
After taking a good look at the loading coil, its apparent that there
isn't much choice. The bottom of the coil is attached to the lower mast,
and a four pronged plate that the tap wire is attached to at the same
junction. So unless no tap is used, some portion will be shorted/
bypassed or the like.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?
--
73, Cecil, w5dxp.com

On 10/21/10 10:55 AM, Cecil Moore wrote:
On Oct 21, 8:32 am, Mike wrote:
After taking a good look at the loading coil, its apparent that there
isn't much choice. The bottom of the coil is attached to the lower mast,
and a four pronged plate that the tap wire is attached to at the same
junction. So unless no tap is used, some portion will be shorted/
bypassed or the like.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?


Kinda my original question.

Intuition tells me that ideally - in order of preference:

1. the entire unused portion of the coil should just disappear.

2. A shorting sleeve that renders the unused portion of that loading
coil as a fatter part of the mast.

3. What I have now, a #12 wire from the top of the bottom part of the
mast to the spot that I tuned the antenna. The bottom of the coil is
attached to the same point on the bottom mast.

1. is impossible without having separate replaceable tuning coils. Crazy
inconvenient.

2. This would be the world's fattest screwdriver antenna.

3. This becomes the question? Is this worth worrying about? And testing
would be interesting for each frequency to determine which ones benefit
from shorted/non shorted operation.

Which now leads me to ask, what would be a good way to set up such an
experiment? I guess if follows on that what exactly is the phenomenon
that I would be witnessing? A transformer effect in unshorted condition
certainly would be a problem even for my transmitting equipment?

- 73 de Mike N3LI -

Mike Coslo wrote in
:

On 10/21/10 10:55 AM, Cecil Moore wrote:
On Oct 21, 8:32 am, Mike wrote:
After taking a good look at the loading coil, its apparent that
there isn't much choice. The bottom of the coil is attached to the
lower mast, and a four pronged plate that the tap wire is attached
to at the same junction. So unless no tap is used, some portion will
be shorted/ bypassed or the like.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?


Kinda my original question.

Intuition tells me that ideally - in order of preference:

1. the entire unused portion of the coil should just disappear.

Yes, but even better, a new coil optimised for the band. Not an original
thought though!


2. A shorting sleeve that renders the unused portion of that loading
coil as a fatter part of the mast.

Mike, some thoughts.

A word explanation of what my simple model draws out quantitatively for
an ideal coil (meaning insignificant distributed capacitance)...

Just one turn shorted / open...

If you short just one turn at the end of the coil, a current is induced
in the shorted turn. The current depends on the flux coupling factor k
and the number of turns in the rest of the coil. The loss depends on the
current squared and resistance... so whilst the current squared might be
high, resistance would be relatively low. Nevertheless, if k is high,
then the current induced in that shorted turn will increase total loss
significantly. The effect of a large current in the shorted turn is to
reduce the inductance of the combination, the higher the current (high
k), the greater the reduction in inductance.

If on the other hand, that one turn was left open a voltage is induced in
the open turn. The voltage depends on k and the number of turns in the
rest of the coil.

More turns shorted / open...

If more turns are shorted, the induced current is lower, and R is higher,
the net effect is that additional loss is reduced. Inductance is reduced,
but at a slower rate than intially.

If more turns are left open, the voltage induced in the unused turns
increases and could be many times the voltage developed across the used
turns. A first approximation is k times the turns ratio times voltage
developed across the used turns.

Shorting each of several adjacent turns...

This approach assures the highest circulating current in the shorted
turns, assuring the worst loss at all tappings.

Screwdrivers...

The approach taken in some screwdrivers uses a very low resistance
shorted turn (the tube) to make contact at the tapping point, and it
protects the unused coil turns (now inside the tube) from flux. Done
properly, that might well be a fairly good solution... I haven't tried to
measure it.

k...

Avoiding high k provides finer granularity of inductance adjustment and
lower loss when few turns are shorted... but of course lower k means a
physically larger coil for the same inductance, and potentially higher R
which increases loss.


Owen

Owen Duffy wrote:
Mike Coslo wrote in news:i9n3jq$ds62$1
@tr22n12.aset.psu.edu:

...
So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.

I wrote some notes based on a simple model of an air cored single layer
solenoid, they are at
http://www.vk1od.net/tx/concept/TappedCoil/index.htm .

I saw that yesterday and it looks good (a figure of the equivalent T
model would help)

The model suggests that shorting the unused turns is a poorer solution
when the flux coupling factor is relatively high, and a very small number
of turns are shorted. Poorer both because of loss and the granularity of
L adjustment.

In that situation, the voltage induced in open unused turns is not very
high, whereas it can be extreme in cases where most of the turns are
unused.


But in that situation, the voltage across the unshorted turns is likely
to be low, because you've got low reactance, right? So the net effect is
small.

Jim Lux wrote in
:

Owen Duffy wrote:
Mike Coslo wrote in news:i9n3jq$ds62$1
....
The model suggests that shorting the unused turns is a poorer
solution when the flux coupling factor is relatively high, and a very
small number of turns are shorted. Poorer both because of loss and
the granularity of L adjustment.

In that situation, the voltage induced in open unused turns is not
very high, whereas it can be extreme in cases where most of the turns
are unused.


But in that situation, the voltage across the unshorted turns is
likely to be low, because you've got low reactance, right? So the net
effect is small.

Hi Jim,

It depends what you take as a reference. A rough estimate of the voltage
across the unused turns is the turns ratio (unused /used) time the flux
coupling factor, which in practical applications is likely to be in the
range 0.2 to 0.5.

If the application was a valve amp pi coupler, the voltage impressed
across the used turns could be a couple of thousand volts, and on a 10m
tap, the voltage across the unused turns could easily be 5 to 10 times
that.

In the case of a loading coil for a multiband whip, the current in the
used turns is probably similar from band to band, but the used turns are
lower for higher bands and so the flux cutting the unused turns is lower,
but there are more unused turns. In this application, insulation problems
are probably less severe than the PA with a bandswitch.

To my mind, the interesting thing is why shorting some turns is 'ok', and
then to understand that doesn't make it 'ok' in all scenarios. For
instance, someone taking a valve amp pi coupler and implementing it on a
single powdered iron core changes the situation significantly.

Re the Tee circuit, yes, but it takes a while to draw it up... not nearly
as quick as grinding a few numbers and creating a graph! I have a backlog
of hand drawing scanned into articles to redraw already.

Owen

On 10/20/2010 03:04 PM, Owen Duffy wrote:
Mike wrote in news:i9n3jq$ds62$1
@tr22n12.aset.psu.edu:

...
So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.

I wrote some notes based on a simple model of an air cored single layer
solenoid, they are at
http://www.vk1od.net/tx/concept/TappedCoil/index.htm .

The model suggests that shorting the unused turns is a poorer solution
when the flux coupling factor is relatively high, and a very small number
of turns are shorted. Poorer both because of loss and the granularity of
L adjustment.

In that situation, the voltage induced in open unused turns is not very
high, whereas it can be extreme in cases where most of the turns are
unused.

So, a combination of methods may be optimimum, depending on the flux
coupling factor, voltage withstand, granularity of variation of L, etc.

Owen

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns add.


--
Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:
http://1fcr.com.ar

Alejandro Lieber wrote
in :

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns
add.

Hello Alejandro,

I think the graphs in the article demonstrate that there is a simple
analytical solution, giving the appropriate parameters.

There are issues about the applicability of the model to the real world,
but the model does show trends about what is desirable and undesirable
practice. Wheeler's formula for inductance has its shortcomings
(Wheeler's formula doesn't take into account wire diameter for instance),
but it is (IMHO) sufficiently accurate to expose some of the
'interesting' effects that prompted your initial question.

The model is based on basic electricity and magnetism, stuff that isn't
so appeallng to new age hams.

Owen

Owen Duffy wrote:
Alejandro Lieber wrote
in :

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns
add.

Hello Alejandro,

I think the graphs in the article demonstrate that there is a simple
analytical solution, giving the appropriate parameters.

There are issues about the applicability of the model to the real world,
but the model does show trends about what is desirable and undesirable
practice. Wheeler's formula for inductance has its shortcomings
(Wheeler's formula doesn't take into account wire diameter for instance),
but it is (IMHO) sufficiently accurate to expose some of the
'interesting' effects that prompted your initial question.


I think that for the purposes of looking at the effect of shorting/not
shorting turns, Wheeler is more than adequate. the key is the fact that
it encapsulates the difference between the Nturns^2 (zero length
solenoid with all turns coincident, fully coupled) and Nturns (zero
coupling) for dimensions that are "practical" for ham use.

And the other interesting thing is that the graph of inductance vs
length for coils of interest here is that it looks pretty linear (and
the fact that the coil stock vendors refer to "uh/inch" kind of confirms
that)

Jim Lux wrote in
:

I think that for the purposes of looking at the effect of shorting/not
shorting turns, Wheeler is more than adequate. the key is the fact
that it encapsulates the difference between the Nturns^2 (zero length
solenoid with all turns coincident, fully coupled) and Nturns (zero
coupling) for dimensions that are "practical" for ham use.

And the other interesting thing is that the graph of inductance vs
length for coils of interest here is that it looks pretty linear (and
the fact that the coil stock vendors refer to "uh/inch" kind of
confirms that)

An interesting bit of trivia for the models I created is that the flux
coupling coefficient doesn't vary much with turns for a given diameter
and coil pitch.

When you make relatively large diameter coils of fine pitch, k is higher,
and that creates the conditions for higher loss in shorted turns.

The implication for long loose coils is that k is low, mutual inductance
is low, inductance approaches a constant L per unit length etc.

Sensibly, most air cored solenoids operate in the midrange.

Owen