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

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

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

___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

"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"

___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

"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)

"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

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:

"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"