Trap antenna
 

HI,

I picked up two coils that could be a trap for the CWA-840 COMET
antenna. (They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.
Could someone tell me how they works ?
In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather longer
that 1/4 wave. (10.1m).
Why ?

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun

Trap antenna
 

"béo-master" wrote in message
. fr...
HI,

I picked up two coils that could be a trap for the CWA-840 COMET antenna.
(They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.
Could someone tell me how they works ?
In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather longer
that 1/4 wave. (10.1m).
Why ?

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun


All coils are self-resonant at some frequency. Wind it right and off you
go!

"Sal"

Trap antenna
 

On 10/20/2011 5:28 PM, Sal wrote:
wrote in message
. fr...
HI,

I picked up two coils that could be a trap for the CWA-840 COMET antenna.
(They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.
Could someone tell me how they works ?
In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather longer
that 1/4 wave. (10.1m).
Why ?

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun


All coils are self-resonant at some frequency. Wind it right and off you
go!

"Sal"



Good point, Sal. I hope the OP has a GDO to tell him the self-resonant
frequency. If not, there are other ways.

John, KD5YI

Trap antenna
 

http://www.w8ji.com/traps.htm

Trap antenna
 

"W5DXP" wrote in message
...

http://www.w8ji.com/traps.htm
-
Cool!

Trap antenna
 

béo-master wrote in
. fr:

HI,

I picked up two coils that could be a trap for the CWA-840 COMET
antenna. (They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.

That is a rather simplistic characterisation of the trap. If you were to
measure its impedance over a wide range of frequency, the data would
give a better idea of the extent of any parallel capacitance (and there
has to be some, even if only stray capacitance. Somethimes the
capacitance is between some metal tubes that make the end supports for
the trap.

Note that measuring the impedance of traps is very challenging.

Could someone tell me how they works ?

The trap introduces some reactance and resistance that is intended with
appropriate conductor lengths, to give a modestly low VSWR at the
frequencies of interest.

The traditional explanation is that they are resonant at one of the
frequencies of operation, and act like a switch, cutting off current to
the outboard legs. Of course, current would flow in the outboard legs
eve if you used a physical switch.

That explanation is appealing, but limits your options. There are
advantages is making the resonant frequency of a trap fall outside the
bands of interest.

In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather
longer that 1/4 wave. (10.1m).
Why ?

See above.

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun

The trap is probably self resonant below 40m... but that is just a guess
from experience of having designed such things.

Owen

Trap antenna
 

Thanks a lot for your answers.
I have now most of the explainations.
Thanks for the links. (I had already read some)

Trap antenna
 

On 10/20/2011 09:19 AM, béo-master wrote:
HI,

I picked up two coils that could be a trap for the CWA-840 COMET
antenna. (They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.
Could someone tell me how they works ?
In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather longer
that 1/4 wave. (10.1m).
Why ?

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun

This is not a trap antenna:

1)- At 40 meters the coil works like a medium impedance insulator,76uH
represents 5300 ohms.

2)- At 80 meters, the antenna works like a vertical mobile antenna, the
coil resonates with the 2.3m section.

I made several of these antennas years ago, and the 40 meter section is
always longer than in a simple 40 meter dipole.
--
Alejandro Lieber LU1FCR
Rosario Argentina

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

Trap antenna
 

On 10/21/2011 10:00 PM, Owen Duffy wrote:
wrote in
. fr:

HI,

I picked up two coils that could be a trap for the CWA-840 COMET
antenna. (They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.

That is a rather simplistic characterisation of the trap. If you were to
measure its impedance over a wide range of frequency, the data would
give a better idea of the extent of any parallel capacitance (and there
has to be some, even if only stray capacitance. Somethimes the
capacitance is between some metal tubes that make the end supports for
the trap.


or C from the winding of the coil to the surrounding tubing.

Or the "self C" of the coil (viz Medhurst's formulae for self C of an
inductor)

There's a goodly bit of empiricism in trap design. You can use basic
physics to calculate, but there's enough non-idealness in most
construction techniques that the calculations get you to a starting
point. (e.g. you could model the transition from the antenna element to
the coil in the trap with a lot of detail, or you could just build one
and try it, then adjust)



Note that measuring the impedance of traps is very challenging.

Could someone tell me how they works ?

The trap introduces some reactance and resistance that is intended with
appropriate conductor lengths, to give a modestly low VSWR at the
frequencies of interest.

The traditional explanation is that they are resonant at one of the
frequencies of operation, and act like a switch, cutting off current to
the outboard legs. Of course, current would flow in the outboard legs
eve if you used a physical switch.

That explanation is appealing, but limits your options. There are
advantages is making the resonant frequency of a trap fall outside the
bands of interes



Here's a (not very realistic) example of how it might work

Say you want an antenna that covers both 10m and 15m. You start with a
10 meter resonant antenna. Now hook a trap which has very high
impedance at 10m on the end of it. Since it's high Z, not much current
flows, so it doesn't change the 10m behavior very much. But, you also
want it to work at 15m. So you figure out how long an additional
element you need to put "outboard" of the trap to get the system to
resonate. Typically, the trap is a parallel LC, so below resonance, it
looks somewhat inductive. Since a short element looks capacitive, the
added length for 15m will be shorter than it would be without the trap.


There's a whole raft of design approaches about where you put the
resonance relative to the frequencies of use, and how you want the
current distribution to work, and then there's all the "what's practical
to make and manufacture" and "tolerance to misadjustment/weather/aging".

You can get a pretty good start with a modeling code like NEC, but
eventually, you've got to go out and start fiddling in real life,
because almost all traps are not precisely modeled by NEC for a variety
of reasons.



In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather
longer that 1/4 wave. (10.1m).
Why ?

See above.

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun

The trap is probably self resonant below 40m... but that is just a guess
from experience of having designed such things.

Owen

Trap antenna
 

___Original Message_________________________________________
From: Jim Lux
Date: Mon, 24 Oct 2011 Time: 10:21:41

Say you want an antenna that covers both 10m and 15m. You start with a
10 meter resonant antenna. Now hook a trap which has very high
impedance at 10m on the end of it. Since it's high Z, not much current
flows, so it doesn't change the 10m behavior very much. But, you also
want it to work at 15m. So you figure out how long an additional
element you need to put "outboard" of the trap to get the system to
resonate. Typically, the trap is a parallel LC, so below resonance, it
looks somewhat inductive. Since a short element looks capacitive, the
added length for 15m will be shorter than it would be without the trap.


There's a whole raft of design approaches

Hi béo-master

Here is a simple trap antenna design tool that I have successfully used
a few times:

http://www.k7mem.150m.com/Electronic_Notebook/antennas/shortant.html

--
73
Ian, G3NRW

The AIM4170 Antenna Analyzer:
http://homepage.ntlworld.com/wadei/aim4170.htm

Trap antenna
 

Jim Lux wrote in news:j846r5$7h3$1
@news.jpl.nasa.gov:

There's a goodly bit of empiricism in trap design.

And a goodly bit of misinformation in some of the traditional ham sources.

I have had an interest in the so called coax traps, and just in the last
week or so, make some headway with some good measurments of the underlying
inductor formed by the coil of coax shield. There were some surprises. For
those interested, see http://www.vk1od.net/antenna/coaxtrap/index.htm .

Short of performing Medhurst style measurements on a range of inductors,
there is no way to be sure that the effects observe apply generally, or
what a more general model might be.

Owen

Trap antenna
 

very interesting
thanks a lot

Trap antenna
 

"Ian Wade G3NRW" wrote in message
...

73
Ian, G3NRW

The AIM4170 Antenna Analyzer:
http://homepage.ntlworld.com/wadei/aim4170.htm

Say, after following the link, above, I just took a look at that AIM4170 web
site and it looks like it's more trouble to get it up and running than most
things. A lot of the steps are left to the imagination ... or they assume a
whole lot about what potential buyers already know.

Anybody want to toss me a bone in the form of reassurance that it's easier
than it looks? Cables, adapters,serial-to-USB ( a known PITA), etc.

Thanks in advance,
"Sal"

Trap antenna
 

___Original Message_________________________________________
From: Sal
Date: Wed, 26 Oct 2011 Time: 19:51:49

http://homepage.ntlworld.com/wadei/aim4170.htm

Say, after following the link, above, I just took a look at that AIM4170 web
site and it looks like it's more trouble to get it up and running than most
things.


Sal

I assume you are talking about the AIM Forum, he

http://aim4150.proboards.com/index.cgi?board=analyzer

Yes, there are many questions about AIM4170 operation there, but look
closely at the dates of the messages. Most of them are now very old. The
software has settled down very nicely now, and the instrument is indeed
very easy to set up and use.

To find out all the information you need, download the latest version of
the software (currently in AIM_846.zip), from he

http://www.w5big.com/prog_update.htm

The download contains the AIM software (which you can run in demo mode
without the AIM hardware), plus the manual.

--
73
Ian, G3NRW

The AIM4170 Antenna Analyzer:
http://homepage.ntlworld.com/wadei/aim4170.htm

Trap antenna
 

On 10/26/2011 12:37 AM, Owen Duffy wrote:
Jim wrote in news:j846r5$7h3$1
@news.jpl.nasa.gov:

There's a goodly bit of empiricism in trap design.

And a goodly bit of misinformation in some of the traditional ham sources.

I have had an interest in the so called coax traps, and just in the last
week or so, make some headway with some good measurments of the underlying
inductor formed by the coil of coax shield. There were some surprises. For
those interested, see http://www.vk1od.net/antenna/coaxtrap/index.htm .

Short of performing Medhurst style measurements on a range of inductors,
there is no way to be sure that the effects observe apply generally, or
what a more general model might be.


There is a fair amount of literature on L and C (and loss) for shielded
inductors, which is what a lot of traps look like.. the ones which use
the C to the surrounding tube, anyway, like in the 4,5,6-BTV. Some
papers have generic cookbook-ey design equations which might be useful
(although I don't have any citations off the top of my head).

For a bare coil of the appropriate L/D ratio, Medhurst will get you in
the right starting place.

The problem would be things like manufacturing variability, if you're
copying (or writing instructions) one-off design. Stuff like 'how thick
is the enamel/polyurethane/PVC insulation'.

Sort of like the measurements of Z and loss for zipcord. The dielectric
properties aren't controlled in manufacturing, so what you measure on
brand X, 16 Feb 2001 may have little or no relation to what you measure
on brand X, 20 Oct 2010 vintage.

Trap antenna
 

"Sal" wrote in :

....

Say, after following the link, above, I just took a look at that
AIM4170 web site and it looks like it's more trouble to get it up and
running than most things. A lot of the steps are left to the
imagination ... or they assume a whole lot about what potential buyers
already know.

Sal, I bought an AimUHF, and it worked fine out of the box.

I suspect that the problem that most hams encounter with analysers and
VNAs is that the knowledge of basic complex numbers, AC circuit theory,
transmission lines, and antenna systems does not 'come in the box'.

If acquiring such a thing is the stimulus for expanding knowledge on
those topics using the box as a learning aid, that is great. But to
many, they are safer to just treat it as a VSWR measuring device.

I am staggered by the popular advice offered to newbies who ask 'why is
my VSWR high' on eHam and QRZ to borrow an analyser. The advice appears
to offered mainly by people who don't understand the instruments, and
possibly have never used them effectively.

A classic example is the advice to connect the instrument at the shack
end and tune for resonance, for as everyone knows, "an antenna just
ain't gonna work any good unless it is resonate (sic)".

The getting of knowledge just isn't a priority in a world of instant
gratification.

Owen

Trap antenna
 

Jim Lux wrote in
:

On 10/26/2011 12:37 AM, Owen Duffy wrote:
Jim wrote in news:j846r5$7h3$1
@news.jpl.nasa.gov:

There's a goodly bit of empiricism in trap design.

And a goodly bit of misinformation in some of the traditional ham
sources.

I have had an interest in the so called coax traps, and just in the
last week or so, make some headway with some good measurments of the
underlying inductor formed by the coil of coax shield. There were
some surprises. For those interested, see
http://www.vk1od.net/antenna/coaxtrap/index.htm .

Short of performing Medhurst style measurements on a range of
inductors, there is no way to be sure that the effects observe apply
generally, or what a more general model might be.


There is a fair amount of literature on L and C (and loss) for
shielded inductors, which is what a lot of traps look like.. the ones
which use the C to the surrounding tube, anyway, like in the
4,5,6-BTV. Some papers have generic cookbook-ey design equations
which might be useful (although I don't have any citations off the top
of my head).

For a bare coil of the appropriate L/D ratio, Medhurst will get you in
the right starting place.

The problem would be things like manufacturing variability, if you're
copying (or writing instructions) one-off design. Stuff like 'how
thick is the enamel/polyurethane/PVC insulation'.

Sort of like the measurements of Z and loss for zipcord. The
dielectric properties aren't controlled in manufacturing, so what you
measure on brand X, 16 Feb 2001 may have little or no relation to what
you measure on brand X, 20 Oct 2010 vintage.

All noted.

Many readers will recall my interest over more than a decade in
predicting the effective RF resistance of the outside surface of a
braided coax shield, especially when it forms a solenoid... as in the
coax traps.

The last round of measurements by VK2KRB were most interesting, because
they strongly suggested that Q was not proportional to root of f as
Medhurst and predecessors observed for round copper wire (and of course,
R was higher than for an equivalent sized round copper conductor).

Jim, interesting that you mention ZIP cord. I have seen a number of
articles recently discussing the TL characteristics of ZIP cord, and
again many readers will recall Jack Smith's measurements published here
about 10 years ago.

I recently put a new TL calculator up, it uses input parameters of Ro,
vf, k1, k2 to solve problems similarly to the older TLLC. It is at
http://www.vk1od.net/calc/tl/atllc.htm . I wrote an article with some
examples of using it at http://www.vk1od.net/calc/tl/atllcEx.htm .
Example 2 plots Jack's data on ZIP attenuation from back then. A
stunning set of measurements, and statistically tighter than I have seen
from any one else. Nevertheless, I see a wavelike shape to the error
between actual and the model, a growing sinusoid that prompts the
question of why, was it some common mode effect and radiation.

Owen

Trap antenna - and AIM4170
 

Dear Group: An echo: the AIM4170 is the greatest thing since sliced bread
(or NEC) for serious antenna people. I use one with an inexpensive- very
small, slow, lap-top to great effect.

The device is also ideal for measuring real components. The ability, if you
buy a small set of high quality resistors, to calibrate out the effects of
fixtures is a delight to use. I have a GR bridge. It has long been the
standard. But my goodness is its use a lot of work.

Note that the device can tell the sign of the phase and I have found that it
works well in the presence of strong signals. Indeed, one feature is that
the device may be asked to scan a band of frequencies and display the
stronger signals found. It is also very easy to calibrate the internal
frequency reference to WWV. All of this with only one moving part - the
on/off switch.

73, Mac N8TT

"Owen Duffy" wrote in message
...

"Sal" wrote in :

....

Say, after following the link, above, I just took a look at that
AIM4170 web site and it looks like it's more trouble to get it up and
running than most things. A lot of the steps are left to the
imagination ... or they assume a whole lot about what potential buyers
already know.

Sal, I bought an AimUHF, and it worked fine out of the box.

I suspect that the problem that most hams encounter with analysers and
VNAs is that the knowledge of basic complex numbers, AC circuit theory,
transmission lines, and antenna systems does not 'come in the box'.

If acquiring such a thing is the stimulus for expanding knowledge on
those topics using the box as a learning aid, that is great. But to
many, they are safer to just treat it as a VSWR measuring device.

I am staggered by the popular advice offered to newbies who ask 'why is
my VSWR high' on eHam and QRZ to borrow an analyser. The advice appears
to offered mainly by people who don't understand the instruments, and
possibly have never used them effectively.

A classic example is the advice to connect the instrument at the shack
end and tune for resonance, for as everyone knows, "an antenna just
ain't gonna work any good unless it is resonate (sic)".

The getting of knowledge just isn't a priority in a world of instant
gratification.

Owen


J. C. Mc Laughlin
Michigan U.S.A.
Home:

Trap antenna
 

"Ian Wade G3NRW" wrote in message
...

I assume you are talking about the AIM Forum, he

http://aim4150.proboards.com/index.cgi?board=analyzer

Yes, there are many questions about AIM4170 operation there, but look
closely at the dates of the messages. Most of them are now very old. The
software has settled down very nicely now, and the instrument is indeed
very easy to set up and use.

To find out all the information you need, download the latest version of
the software (currently in AIM_846.zip), from he

http://www.w5big.com/prog_update.htm

The download contains the AIM software (which you can run in demo mode
without the AIM hardware), plus the manual.


The AIM4170 Antenna Analyzer:
http://homepage.ntlworld.com/wadei/aim4170.htm

Thanks, Ian,

I did look at the forum briefly, but my reference was to this page
http://www.arraysolutions.com/Products/AIM4170.htm and one
or two links out. There were fleeting references to several operating
systems versions, as well as talk about USB-to-COM-port adaptation,
which scares me. (Never tried it; numerous horror stories are extant.)

I gleaned no real knowledge as to what I needed to buy to make it
work with both my WinXP box in the shack or my WinVista laptop.

I'll grab the software, load it and take it for a drive. It's not that I'm
some
Chicken-S kid who needs a lot of hand holding. (I have seven computers,
running everything from DOS 5.0 to Vista, with Mac OS 10.5 and Ubuntu
Linux thrown in.) I just don't want to spend $700+ for a leap into the
abyss.

Thanks for the advice.

"Sal"
(KD6VKW)

Trap antenna
 

"Owen Duffy" wrote in message
...
"Sal" wrote in :

...

Say, after following the link, above, I just took a look at that
AIM4170 web site and it looks like it's more trouble to get it up and
running than most things. A lot of the steps are left to the
imagination ... or they assume a whole lot about what potential buyers
already know.

Sal, I bought an AimUHF, and it worked fine out of the box.

I suspect that the problem that most hams encounter with analysers and
VNAs is that the knowledge of basic complex numbers, AC circuit theory,
transmission lines, and antenna systems does not 'come in the box'.

If acquiring such a thing is the stimulus for expanding knowledge on
those topics using the box as a learning aid, that is great. But to
many, they are safer to just treat it as a VSWR measuring device.

I am staggered by the popular advice offered to newbies who ask 'why is
my VSWR high' on eHam and QRZ to borrow an analyser. The advice appears
to offered mainly by people who don't understand the instruments, and
possibly have never used them effectively.

A classic example is the advice to connect the instrument at the shack
end and tune for resonance, for as everyone knows, "an antenna just
ain't gonna work any good unless it is resonate (sic)".

The getting of knowledge just isn't a priority in a world of instant
gratification.

Owen

Thanks, Owen,

I understand the world of complex numbers and vector impedance
well enough. My concern is whether I can make the thing work with
what appeared to me to be a dearth of hookup instructions.

How did you know what cable(s) to order? Are you operating with
a software COM port or do/does your computer(s) come with a
hardware COM port?

Anecdote:
Right after Field Day, for which I am the club Chairman, I followed
a fellow club member's suggestion to try to get a network logging
program running in preparation for next year. No dice. Hours
wasted.

I followed many conflicting or wrong instructions; the best I managed
was to get the two computers to each acknowledge the existence of
the other -- but no more. No sharing of information and darn little
help from the maker of the software, who assumes we are are all
networking engineers.

Moral:
So, with the bitter taste of failure still fresh, I'm not really ashamed
to act shy around this next (possible) installation.

As I told Ian, I'll try the software and see if it radiates warmth. It
needn't be hot ... just not ice cold. :-|

"Sal"

Trap antenna
 

"Sal" wrote in :


....

Thanks, Owen,

I understand the world of complex numbers and vector impedance
well enough. My concern is whether I can make the thing work with
what appeared to me to be a dearth of hookup instructions.

How did you know what cable(s) to order? Are you operating with
a software COM port or do/does your computer(s) come with a
hardware COM port?
The AIMuhf connects to the computer by USB, just a mini USB IIRC, and I
think they supplied a cable.

....
As I told Ian, I'll try the software and see if it radiates warmth.
It needn't be hot ... just not ice cold. :-|

The software is a very important element.

In my experience, the software with these kinds of things is
amateurish... but usable.

I also have a TecTec VNA, but I avoid using it if I have something else
that will do the job, and the AIMuhf is usually better if a one port VNA
is up to the task.

You will see plots from both of those in the article
http://www.vk1od.net/antenna/7MDipole/7MDipole02.htm .

There are other ham grade VNAs, I haven't researched them deeply, some
may be kits or homebrew that leave you challenged to find some of the
parts.

Owen

Trap antenna
 

"Owen Duffy" wrote in message
...
"Sal" wrote in :


...

Thanks, Owen,

I understand the world of complex numbers and vector impedance
well enough. My concern is whether I can make the thing work with
what appeared to me to be a dearth of hookup instructions.

How did you know what cable(s) to order? Are you operating with
a software COM port or do/does your computer(s) come with a
hardware COM port?
The AIMuhf connects to the computer by USB, just a mini USB IIRC, and I
think they supplied a cable.

...
As I told Ian, I'll try the software and see if it radiates warmth.
It needn't be hot ... just not ice cold. :-|

The software is a very important element.

In my experience, the software with these kinds of things is
amateurish... but usable.

I also have a TecTec VNA, but I avoid using it if I have something else
that will do the job, and the AIMuhf is usually better if a one port VNA
is up to the task.

You will see plots from both of those in the article
http://www.vk1od.net/antenna/7MDipole/7MDipole02.htm .

There are other ham grade VNAs, I haven't researched them deeply, some
may be kits or homebrew that leave you challenged to find some of the
parts.

Owen

Thank you, Owen. That web page was all very instructive.

"Sal"

Trap antenna
 

On 10/27/2011 1:04 PM, Owen Duffy wrote:
Jim wrote in
:

On 10/26/2011 12:37 AM, Owen Duffy wrote:
Jim wrote in news:j846r5$7h3$1
@news.jpl.nasa.gov:

There's a goodly bit of empiricism in trap design.

And a goodly bit of misinformation in some of the traditional ham
sources.

I have had an interest in the so called coax traps, and just in the
last week or so, make some headway with some good measurments of the
underlying inductor formed by the coil of coax shield. There were
some surprises. For those interested, see
http://www.vk1od.net/antenna/coaxtrap/index.htm .

Short of performing Medhurst style measurements on a range of
inductors, there is no way to be sure that the effects observe apply
generally, or what a more general model might be.


There is a fair amount of literature on L and C (and loss) for
shielded inductors, which is what a lot of traps look like.. the ones
which use the C to the surrounding tube, anyway, like in the
4,5,6-BTV. Some papers have generic cookbook-ey design equations
which might be useful (although I don't have any citations off the top
of my head).

For a bare coil of the appropriate L/D ratio, Medhurst will get you in
the right starting place.

The problem would be things like manufacturing variability, if you're
copying (or writing instructions) one-off design. Stuff like 'how
thick is the enamel/polyurethane/PVC insulation'.

Sort of like the measurements of Z and loss for zipcord. The
dielectric properties aren't controlled in manufacturing, so what you
measure on brand X, 16 Feb 2001 may have little or no relation to what
you measure on brand X, 20 Oct 2010 vintage.

All noted.

Many readers will recall my interest over more than a decade in
predicting the effective RF resistance of the outside surface of a
braided coax shield, especially when it forms a solenoid... as in the
coax traps.

The last round of measurements by VK2KRB were most interesting, because
they strongly suggested that Q was not proportional to root of f as
Medhurst and predecessors observed for round copper wire (and of course,
R was higher than for an equivalent sized round copper conductor).

Jim, interesting that you mention ZIP cord. I have seen a number of
articles recently discussing the TL characteristics of ZIP cord, and
again many readers will recall Jack Smith's measurements published here
about 10 years ago.

I recently put a new TL calculator up, it uses input parameters of Ro,
vf, k1, k2 to solve problems similarly to the older TLLC. It is at
http://www.vk1od.net/calc/tl/atllc.htm . I wrote an article with some
examples of using it at http://www.vk1od.net/calc/tl/atllcEx.htm .
Example 2 plots Jack's data on ZIP attenuation from back then. A
stunning set of measurements, and statistically tighter than I have seen
from any one else. Nevertheless, I see a wavelike shape to the error
between actual and the model, a growing sinusoid that prompts the
question of why, was it some common mode effect and radiation.


Interesting..
So the loss is about 0.12 dB/meter at 10 MHz, compared to, say, RG-58 at
0.04 dB/meter. That's a huge difference, especially since the center
conductor on the RG-58 is probably smaller than the zip cord (what was
the wire size? I think you cited 1.22 mm diameter? AWG 16? or AWG 18?)

Trap antenna
 

Jim Lux wrote in
:

....

Interesting..
So the loss is about 0.12 dB/meter at 10 MHz, compared to, say, RG-58
at 0.04 dB/meter. That's a huge difference, especially since the
center conductor on the RG-58 is probably smaller than the zip cord
(what was the wire size? I think you cited 1.22 mm diameter? AWG 16?
or AWG 18?)


The sizes came from Jack. 1.2mm is about #16.5.

The loss model suggests that dielectric loss is significant, even at
10MHz. Yes, I think you are correct that the inner conductor of RG58 is
fractionally smaller. IIRC, about 80% of copper loss in coax is in the
inner conductor, so the ZIP cord is at the disadvantage of having a
smaller 'other conductor' compared to the shield of the coax. There may
be some further degradation due to proximity effect. (Of course, ZO
favours the ZIP line.)

It seems that the copper loss is close to four times what might be
expected of round copper conductor. I don't know whether the conductors
were tinned, and what the effect of stranding was.

Another possible source of loss is radiation as I mentioned in the
earlier posting.

I note that Belden ceased supply of its 75 ohm twin lines, though I
think they used PE dielectric. Their #13 (8210) seems to have higher
conductor loss than accounted for by round copper conductors with normal
skin effect.

I can't help but be suspicous that proximity effect is a significant
part of the reconciliation gap.

Owen