"Jerry" wrote in
:

....
Try this one: www.k1jek.com This is a dipole that comes in lengths
of 73' and 140' respectively. The short one covers 80-10, the longer

Ah, the Cobra! An interesting antenna for exploration of the myth that
linear loading is essentially lossless, see my analysis of NEC models of a
Cobra design at http://www.vk1od.net/cobra/index.htm .

Owen

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

...
Try this one: www.k1jek.com This is a dipole that comes in lengths
of 73' and 140' respectively. The short one covers 80-10, the longer

Ah, the Cobra! An interesting antenna for exploration of the myth that
linear loading is essentially lossless, see my analysis of NEC models of a
Cobra design at http://www.vk1od.net/cobra/index.htm .

Owen

Says "Page not found"!


JWO

On Wed, 25 Apr 2007 22:45:33 -0400, "Jerry"
wrote:


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

...
Try this one: www.k1jek.com This is a dipole that comes in lengths
of 73' and 140' respectively. The short one covers 80-10, the longer

Ah, the Cobra! An interesting antenna for exploration of the myth that
linear loading is essentially lossless, see my analysis of NEC models of a
Cobra design at http://www.vk1od.net/cobra/index.htm .

Owen

Says "Page not found"!


JWO


Works okay for me. Nice paper ( I stole the NEC deck).

Danny, K6MHE

Danny Richardson wrote in
:

....
Works okay for me. Nice paper ( I stole the NEC deck).

Hi Danny,

You will see that the FR card has a symbolic variable for the frequency,
you will need to feed it a float value. (I used a PERL script to run a
series of models by tailoring the basic NEC deck, executing it, and
summarising the results.)

Differently to many of the commercial Cobras, my model used 2mm dia
copper and wider spacing. Most seem to use smaller, even very small
spacings where proximity effect becomes significant, smaller conductors,
and possibly steel cored copper clad conductors.

IMHO, the interesting aspect of the article is the loss mechanism of
linear loading, it makes sense once you see the current magnitudes and
phase and realise this thing at some frequencies carries large conductor
currents for low net current moment.

I have been exploring a 13m unloaded vertical with 2 radials for each of
80, 40, 30, & 20 metre bands, and there is a different but similar (!!!)
effect where at some frequencies the radials carry large individual
currents (larger in magnitude than the current in the vertical) with a
peak in loss at these frequencies, see
http://www.vk1od.net/multibandunload...al/13mEV-1.gif .

Owen

On Thu, 26 Apr 2007 03:34:29 GMT, Owen Duffy wrote:

Danny Richardson wrote in
:

...
Works okay for me. Nice paper ( I stole the NEC deck).

Hi Danny,

You will see that the FR card has a symbolic variable for the frequency,
you will need to feed it a float value. (I used a PERL script to run a
series of models by tailoring the basic NEC deck, executing it, and
summarising the results.)

[snip]

Owen

No problem Owen, I'll use MultiNec which very easily allows me to run
"test cases" using variables with no programming required on my part.
G.

Danny

Danny Richardson wrote in
:
....
No problem Owen, I'll use MultiNec which very easily allows me to run
"test cases" using variables with no programming required on my part.

Ok. There are other tools that allow symbolic variables, and of course NEC
allows specification of a range of frequencies on the FR card.

The main reason for using PERL to build, run and sumamrise is the summarise
stage. At that point I merge the NEC output with real transmission line
models, L-tuner models etc... and without wading through 5MB of NEC output
by hand. The system view is a bigger picture than the NEC output.

Owen

Jerry wrote:
"Owen Duffy" wrote:
http://www.vk1od.net/cobra/index.htm .

Says "Page not found"!

Worked for me.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
Jerry wrote:
"Owen Duffy" wrote:
http://www.vk1od.net/cobra/index.htm .

Says "Page not found"!

Worked for me.
--
73, Cecil http://www.w5dxp.com

Musta been a temporary thing. Worked shortly after I tried it again.

BTW, for whatever shortcomings Joe's Cobra has, I've had excellent results
and been complimented both on the ham bands and on some other
military/non-amateur frequencies I am authorized from 2-22 MHZ. So long as
it works! I'll probably do something else later, but at the time, the Cobra
was the best considering personal considerations and lot limitations. And,
actually, I am not one of those who must have the biggest signal going--just
so you can hear me well enough to do the job!

73

Jerry

"Jerry" wrote in
:


"Cecil Moore" wrote in message
...
Jerry wrote:
"Owen Duffy" wrote:
http://www.vk1od.net/cobra/index.htm .

Says "Page not found"!

Worked for me.
--
73, Cecil http://www.w5dxp.com

Musta been a temporary thing. Worked shortly after I tried it again.

BTW, for whatever shortcomings Joe's Cobra has, I've had excellent
results and been complimented both on the ham bands and on some other
military/non-amateur frequencies I am authorized from 2-22 MHZ. So
long as it works! I'll probably do something else later, but at the
time, the Cobra was the best considering personal considerations and
lot limitations. And, actually, I am not one of those who must have
the biggest signal going--just so you can hear me well enough to do
the job!

Jerry, it seems to me that the Cobra is a compromise, and we each make
our own judgements about the trade-offs that are acceptable. Of course,
those individual considerations make recomendations like "it works great
for me, it is the best for you" rather worthless.

Some of the claims about some antennas (including the Cobra) seem
unrealistic, but that doesn't condemn the antennas, it just means people
building or buying them based on those claims have been mislead.

It is my view that understanding the behaviour of the antennas helps you
to make an informed judgement about the trade-offs in your own situation,
and it allows you to get the best out of the antenna as installed.

Modelling suggests that the Cobra Jnr has a couple of weaknesses, one is
the spikes in loss at some frequencies (an artefact of linear loading),
most but not all of which are outside amateur bands; and the second is
that the "window" of low loss around 80m is narrow and might be worth
checking that as installed, that window is centred over your most
desirable operating frequncies. These effects scaled for the double size
are more pronounced with the full sized Cobra.

An effect similar to the second point above exists also with the G5RV,
and I have suggested a procedure in-situ tuning for optimising 80m
performance. If I had a full sized Cobra and intended using it on 160m,
or a Cobra Jnr on 80m, I certainly would think about doing similar.

Owen