Operation of bootstrap coax traps
 

Picking up on a comment in a previous thread on coax traps

The type of trap I am discussing here is one where a coil is formed of
a length of coax, the outer conductor of one coax end is tied to the
inner conductor of the other coax end, and the remaining connections
(outer at one end and inner at the other) form the terminals for the
trap.

I have drawn a diagram of the configuration at
http://www.vk1od.net/lost/coil.gif .

I am unsure of who originated this design, so until I determine that
reliably, I will refer to the design as the "bootstrap" type, as the
inductor is connected in parallel with one end of the transmission
line, and the outer of the other end of the line is connected to the
end of the inductor so as to bootstrap the pair.

I have seen explanations of the operation of this circuit, and those
that I have seen seem unsound.

Let me propose an explanation of how this trap works.

With reference to the diagram, I1 is the current into terminal A and
the inner conductor of the coax. Lets designate V1 as the voltage
beween the inner and outer of the coax at that point.

I2 is the current out of the other end of the coax inner conductor,
and lets designate V2 as the voltage beween the inner and outer of the
coax at that point.

The outer conductor of the coax forms a coil with and equivalent
inductance and series resistance.

The current that flows on the outside of the coax outer conductor is
I1+I2.

V1, I1, V2, and I2 have a relationship given by the common
transmission line equations, given gamma (the complex propagation
constant) for the transmission line, and its length.

The impedance between terminals A and B is given by (V1+V2)/I1.

The values for V1, V2, and I1 can be found by solving the set of
simultaneous equations that describe the system.

It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Owen

--

Operation of bootstrap coax traps
 

a lot of the performance probably depends on the frequency vs length of the
coax. if the frequency is low enough that the delay in the coax is small
then you basically have a capacitor (the inner conductor to the inside of
the shield), in parallel with an inductor (the outside of the shield). it
gets a bit uglier because of capacitance between turns on the outside, but
that is likely much smaller than the internal capacitance. so you end up
with a basic parallel resonant circuit. at high frequencies where the
length of coax is longer than a small fraction of a wavelength that probably
falls apart and would take much more complex analysis to figure out.

"Owen Duffy" wrote in message
...

Picking up on a comment in a previous thread on coax traps

The type of trap I am discussing here is one where a coil is formed of
a length of coax, the outer conductor of one coax end is tied to the
inner conductor of the other coax end, and the remaining connections
(outer at one end and inner at the other) form the terminals for the
trap.

I have drawn a diagram of the configuration at
http://www.vk1od.net/lost/coil.gif .

I am unsure of who originated this design, so until I determine that
reliably, I will refer to the design as the "bootstrap" type, as the
inductor is connected in parallel with one end of the transmission
line, and the outer of the other end of the line is connected to the
end of the inductor so as to bootstrap the pair.

I have seen explanations of the operation of this circuit, and those
that I have seen seem unsound.

Let me propose an explanation of how this trap works.

With reference to the diagram, I1 is the current into terminal A and
the inner conductor of the coax. Lets designate V1 as the voltage
beween the inner and outer of the coax at that point.

I2 is the current out of the other end of the coax inner conductor,
and lets designate V2 as the voltage beween the inner and outer of the
coax at that point.

The outer conductor of the coax forms a coil with and equivalent
inductance and series resistance.

The current that flows on the outside of the coax outer conductor is
I1+I2.

V1, I1, V2, and I2 have a relationship given by the common
transmission line equations, given gamma (the complex propagation
constant) for the transmission line, and its length.

The impedance between terminals A and B is given by (V1+V2)/I1.

The values for V1, V2, and I1 can be found by solving the set of
simultaneous equations that describe the system.

It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Owen

--

Operation of bootstrap coax traps
 

Owen Duffy wrote:
The current that flows on the outside of the coax outer conductor is
I1+I2.

It appears to me that the two currents are designed
to create a transmission line stub in the middle of
an antenna wire. What if I2 is equal in magnitude
to I1 and phase-shifted by 180 degrees? Wouldn't
that cause all the current to flow *inside* the coax
thus eliminating common-mode currents at the trap?
If common-mode currents are eliminated at 'A', wouldn't
that also eliminate common-mode currents at 'B'?
--
73, Cecil http://www.w5dxp.com

Operation of bootstrap coax traps
 

Dave wrote:
a lot of the performance probably depends on the frequency vs length of the
coax. if the frequency is low enough that the delay in the coax is small
then you basically have a capacitor (the inner conductor to the inside of
the shield), in parallel with an inductor (the outside of the shield). it
gets a bit uglier because of capacitance between turns on the outside, but
that is likely much smaller than the internal capacitance. so you end up
with a basic parallel resonant circuit. at high frequencies where the
length of coax is longer than a small fraction of a wavelength that probably
falls apart and would take much more complex analysis to figure out.

This thing is much easier analyzed as a distributed network
transmission line. At the resonant frequency where I2 is 180
degrees out of phase with I1, assuming |I1|=|I2|, common-mode
currents are eliminated so there are none available at 'B'.
On frequencies where I2 is not 180 degrees out of phase with
I1, common-mode currents flow on the outside braid of the trap
and thus continue to flow down the antenna wire at point 'B'.

The key to understanding the operation of this trap is to
realize that current ceases to flow on the outside of the
trap braid at the trap's designed-for frequency.
--
73, Cecil http://www.w5dxp.com

Operation of bootstrap coax traps
 

Owen Duffy wrote:
Picking up on a comment in a previous thread on coax traps

The type of trap I am discussing here is one where a coil is formed of
a length of coax, the outer conductor of one coax end is tied to the
inner conductor of the other coax end, and the remaining connections
(outer at one end and inner at the other) form the terminals for the
trap.

I have drawn a diagram of the configuration at
http://www.vk1od.net/lost/coil.gif .

I am unsure of who originated this design, so until I determine that
reliably, I will refer to the design as the "bootstrap" type, as the
inductor is connected in parallel with one end of the transmission
line, and the outer of the other end of the line is connected to the
end of the inductor so as to bootstrap the pair.

I have seen explanations of the operation of this circuit, and those
that I have seen seem unsound.

Let me propose an explanation of how this trap works.

With reference to the diagram, I1 is the current into terminal A and
the inner conductor of the coax. Lets designate V1 as the voltage
beween the inner and outer of the coax at that point.

I2 is the current out of the other end of the coax inner conductor,
and lets designate V2 as the voltage beween the inner and outer of the
coax at that point.

The outer conductor of the coax forms a coil with and equivalent
inductance and series resistance.

The current that flows on the outside of the coax outer conductor is
I1+I2.

V1, I1, V2, and I2 have a relationship given by the common
transmission line equations, given gamma (the complex propagation
constant) for the transmission line, and its length.

The impedance between terminals A and B is given by (V1+V2)/I1.

The values for V1, V2, and I1 can be found by solving the set of
simultaneous equations that describe the system.

It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Owen,

I have placed another file he

http://www.k6mhe.com/n7ws/TrapMysteries.pdf

that may be helpful.

Wes

Owen

--

Operation of bootstrap coax traps
 

On 18 Nov 2006 08:45:13 -0800, "Wes" wrote:



Owen,

I have placed another file he

http://www.k6mhe.com/n7ws/TrapMysteries.pdf

that may be helpful.

Wes

Thanks Wes. That seems to be the document that serves as the root
reference for most discussion, though someone may have described the
circuit prior to that document.

I searched the net for the paper to no avail, so thanks, it is most
helpful, and very kind of you to fetch it out and scan it.

There are some noteable inconsistencies betweent that document and the
other two you posted.

I also think that there are significant issues in N3GO's paper, more
to follow.

I will do some modelling of the coil configurations for which he
reported measurements.

Owen
--

Operation of bootstrap coax traps
 

On Sat, 18 Nov 2006 16:01:55 GMT, Cecil Moore
wrote:

Dave wrote:
a lot of the performance probably depends on the frequency vs length of the
coax. if the frequency is low enough that the delay in the coax is small
then you basically have a capacitor (the inner conductor to the inside of
the shield), in parallel with an inductor (the outside of the shield). it
gets a bit uglier because of capacitance between turns on the outside, but
that is likely much smaller than the internal capacitance. so you end up
with a basic parallel resonant circuit. at high frequencies where the
length of coax is longer than a small fraction of a wavelength that probably
falls apart and would take much more complex analysis to figure out.

This thing is much easier analyzed as a distributed network
transmission line. At the resonant frequency where I2 is 180
degrees out of phase with I1, assuming |I1|=|I2|, common-mode
currents are eliminated so there are none available at 'B'.
On frequencies where I2 is not 180 degrees out of phase with
I1, common-mode currents flow on the outside braid of the trap
and thus continue to flow down the antenna wire at point 'B'.

I designed a trap using VE6YP's calculator. The trap was designed for
resonance at 7MHz, used Belden 8262 (RG58C/U) on a 50mm dia former. I
assumed Q of the inductor is proportional to f^0.5, and that Q at 1MHz
was 60, which gives a Q of around 160 at 7MHz. I think the Q
assumptions are realistic considering the effect of the braided
conductor and proximity effect of the close space turns.

Analysis of that trap using my model suggests that at resonance (which
is 2% lower than predicted by the calculator) |I1||I2|, |V2| is a
little less than |V1|. The coax is not a 1:1 impedance transformer or
current transformer in any way, shape or form. If one examines the
standing wave on the coax, at trap resonance, a current minimum and a
voltage maximum occur the A end of the coax.


The key to understanding the operation of this trap is to
realize that current ceases to flow on the outside of the
trap braid at the trap's designed-for frequency.

I suggest that at resonance (if that is what you meant), impedance is
a maximum, I1 becomes small, |I2||I1|, and the current flowing on
the outside of the coax (I1+I2) is not zero.

Owen
--

Operation of bootstrap coax traps
 

Owen Duffy wrote:
I suggest that at resonance (if that is what you meant), impedance is
a maximum, I1 becomes small, |I2||I1|, and the current flowing on
the outside of the coax (I1+I2) is not zero.

Sorry, I got phasing coils and traps confused in
my thought processes. A trap is probably about half
of a 180 degree series resonant phase-shifting coil.

Also please note that points 'A' and 'B' are reversed
between your drawing and N3GO's paper.

The main thing to remember is that the forward wave
on the standing wave antenna is reflected by the
trap. For the voltage and current phasing to be
correct at the feedpoint, the trap must present close
to an open-circuit to the forward wave.
--
73, Cecil http://www.w5dxp.com

Operation of bootstrap coax traps
 

On Sat, 18 Nov 2006 03:58:04 GMT, Owen Duffy wrote:

It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Here's a discussion of the coax trap from an old QEX:
http://www.arrl.org/qex/Mueller.pdf

S.T.W.

Operation of bootstrap coax traps
 

Owen Duffy wrote:
It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Is the following information available?

1. Compared to a 468/f CF dipole, what is the length
of the trapped dipole wire between the traps at resonance?

2. Compared to a 468/f CF dipole, what is the
resonant feedpoint impedance of the trapped dipole?
--
73, Cecil http://www.w5dxp.com

Operation of bootstrap coax traps
 

On Sat, 18 Nov 2006 03:58:04 GMT, Owen Duffy wrote:


Picking up on a comment in a previous thread on coax traps

....

I have put some notes together on a draft page proposing a
mathematical model for these traps.

The draft is at http://www.vk1od.net/coaxtrap/index.htm .

This is a different approach to any that I have seen, principally
because it treats the coax section as a transmission line (as I
believe it is). That is not to say it is correct, so if anyone can
identify errors in the document, I welcome their feedback.

I have not constructed and measured the example trap. Construction is
easy, I just don't have equipment to perform high accuracy
measurements of RF complex impedances up to around 100k.

Comments appreciated.

Owen
--

Operation of bootstrap coax traps
 

On Mon, 20 Nov 2006 15:20:30 GMT, Cecil Moore
wrote:

Owen Duffy wrote:
It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Is the following information available?

1. Compared to a 468/f CF dipole, what is the length
of the trapped dipole wire between the traps at resonance?

2. Compared to a 468/f CF dipole, what is the
resonant feedpoint impedance of the trapped dipole?

Cecil, I don't think there is a single answer to either of the
questions, if there is, it probably depends on so many assumptions as
to be even less relevant than the answer to those questions for a
dipole without the traps.

To illustrate the point, I have seen an optimal length / diameter
ratio proposed for the traps, but none of the articles that I have
seen explains what factors were considered in the optimisation, or the
extent of the optimisation. So what does an "optimised trap" mean?

My experience with coaxtraps on low HF is that they are relatively low
L/C and do not result in much shortening of the antenna. If one of the
reasons for trapping an antenna is to reduce its length, then other
trap designs are probably better for that purpose.

Owen
--